12-1
Agricultural and Biosystems Engineering Department
Iowa State University
12/07/06
Carl J. Bern
Chapter 12
Mechanical Grain Conveying
INTRODUCTION
Material handling is a unit operation which changes the spacial location of material without changing its form
except incidentally (Pinches, 1958). Material handling operations with grain involve many types of grain
conveying devices. These types of mechanical grain conveying devices will be discussed in this chapter:




Belt conveyors
Flight conveyors
Bucket conveyors
Screw conveyors
These devices all find wide use in agriculture, and are very interesting from an engineering point of view.
Important design factors
Important design factors of material handling equipment are:











Capacity
Safety
Reliability
Original cost
Operating cost
Maintenance
Simplicity of design and fabrication
Product damage
Cleanability
Pollution (usually noise and dust)
Power requirements
The importance of each factor depends on the application. Cleanability is important for a seed conveyor when
seed left in the conveyor will be mixed with the next grain conveyed. It is of no importance in a farm design
where the only material handled is corn for feed. Product damage is important in a conveyor loading grain to be
marketed since an increase in fine material could result in a lowered value. It has a low priority for a conveyor
loading a grinder.
Energy considerations
As noted above, power requirement (energy) is an important consideration in the design of a conveyor. Much
more attention has been given to this design aspect since the so-called energy crisis of 1973. The energy input to
a conveyor is used for two things:


To operate the conveyor
To lift material
12-2
The quantity of energy expended operating a conveyor is dependent on the conveyor design and is something to
consider as conveyors are compared. The energy expended in lifting material represents an increase in potential
energy of the material mass and is not dependent on conveyor design. If the conveying path is horizontal, this
component is zero. If the conveying path slopes down, this energy input is negative, meaning there can be an
output of energy from the conveyor. In some cases when this output exceeds conveyor requirements, the
conveyor produces net energy which can be used for other purposes.
The "perfect" conveyor
The hypothetical perfect conveyor is one which moves material without friction losses. In this conveyor, the
energy to operate the conveyor is zero. No actual conveyor can operate without friction. However, it is useful to
compare actual conveyors with a perfect conveyor doing the same job.
Power for a perfect conveyor
Figure 12-1 shows forces moving a particle during conveying from point 1 to point 2 along a frictionless surface.
Figure 12.1 Forces on particle being conveyed along a frictionless surface path.
Summing forces tangentially, we obtain:
F cos  = mg sin 
where
(12-1)
F = conveying force on particle
m = particle mass
g = acceleration of gravity
In order to move the particle from point 1 to point 2, the work required is
2
W   Fcosds
1
(12-2)
where ds is an infinitesimal distance along the frictionless surface. Since ds sin θ = dy,
y2
W   mgdy  mg(y 2  y1 )
y1
(12-3)
12-3
Note that Equation 12-3 represents the work necessary to convey the particle in the absence of friction. It is thus
the energy required by a "perfect" conveyor. Note also that the work required is independent of the route taken
between point 1 and point 2.
For a continuous flow of material, power required by the perfect conveyor is:
P
mg(y2  y1 )
t
where:
P
mg/t
= conveyor power
= mass flow rate
Comparing conveyors
To judge among various conveying methods as to their energy requirement, each conveyor type will be used in
the design of a hypothetical conveying system and compared for energy requirement to an impossible "perfect"
system which requires no energy to operate the conveyor. Example 12-1 will describe this system and illustrate
the computation procedure.
Example 12-1
Corn (45 lb/ft3) is to be moved at a rate of 140 000 lb/h from the bottom of a 4-ft-deep pit to discharge 1 ft above a
20-ft-diameter bin having a loading hole 27 ft above ground level. Compute power (hp) and energy per unit grain
mass (hp h/ton) required assuming a "perfect" conveyor.
The total lifting height is: 4 + 27 + 1 = 32 ft.
P=
(32 ft) (140 000 lb) (h) (min hp)
 2.26hp
(h) (60min) (330 00 ft lb)
E=
(2.26 hp) (h) (2000 lb)
hp h
 .0323
(140,000 lb) ton
ton
Power and energy necessary for the "perfect" conveyor, which requires only power necessary to lift the material,
is 2.26 hp h/ton.
Energy efficiency of the conveyor can be defined as a ratio of the increase in potential energy of the material to
energy input. In conventional units, the equation is:
Ec 
(Lh) (Q) (100)
(hp) (33 000) (60)
where
Ec = energy efficiency,
hp = power from conveyor motor, hp
Lh = lift height, ft
Q = mass flow rate, lb/h
(12-5)
12-4
The factor 33 000 converts hp to ft lb/min; the factor 60 converts hours to minutes. For the example,
Ec 
(32) (140 000) (100)
 100%
(2.26) (33 000) (60)
Conveyor types, it will be seen, will fall into a high energy requirement group and a low energy requirement
group. Those types which slide grain on a surface as it is conveyed will be in the high group because of friction
losses. Conveyors which carry material on anti-friction bearings will be in the low group.
Gravity
Flow of grain by gravity can be utilized where slopes are adequate for reliable flow of material. Table 12-1 lists
spout or flow slopes for material flow.
Table 12-1. Minimum angles for material flow (MWPS 1983).
(Material)
Spout angle or floor slopes, degrees
grain, dry
grain, wet
pellets
meal
37
45 (minimum)
45
60
Table 12-2 lists grain flow rates for clean, dry grain flowing through a round tube from a dead stop. This would
be the condition existing when the tube discharges through a gate from a bin opening.
Table 12.2 Grain flow rates through tubes (Ditzenberger, 1980.)
Tube diameter,
inches
6
8
10
12
14
16
18
20
22
24
26
Corn
1,686
3,000
4,679
6,741
9,178
11,907
15,168
18,632
22,654
26,963
31,641
Soybeans
Flow rate, bu/h
2,023
3,600
5,615
8,089
11,014
14,396
18,202
22,356
27,185
32,356
37,969
Wheat
2,580
4,590
7,159
10,313
14,042
18,355
23,207
28,507
34,660
41,534
48,411
BELT CONVEYORS
A belt conveyor consists of an endless moving belt which supports and moves material. Figure 12.2 illustrates the
components of a belt conveyor. The belt is usually fabric-reinforced rubber. It is carried on idlers fitted with antifriction bearings. On the top (load) side of grain conveyors, these idlers are usually arranged to trough the belts
and thus increase the allowable load cross-section (Figure 12-3a). Return idlers under the belt carry the belt flat
and can be installed at longer spacings than the load-carrying idlers (Figure 12-3b).
12-5
Some portable belt conveyor designs eliminate carrying idlers by running the loaded belt inside a metal tube.
During return, the belt is carried on return idlers under the tube. The tube is the main structural component of the
conveyor, and also covers the loaded conveyor. This design may be less expensive to build, but power
requirements will be higher due to the sliding friction of the belt.
Most designs drive (apply power to the belt) at the head pulley since this prevents the return side of the belt from
being tensioned due to load.
History
Flat belt conveyors were in use in U.S. industry by 1840 to carry clay, sawmill refuse, and stone. In 1876, the
North Central Railroad elevator in Baltimore was equipped with a 30-in rubber-belt grain conveyor which ran at
550 ft/min. Ball or roller bearings were in common use on belt conveyors by 1920 (Hetzel and Albright, 1941).
Although conveyor configurations remain similar to early designs, vast improvements in belts, bearings, and
drives have been made through the years.
Figure 12-2. Nomenclature of components of a typical belt conveyor (CEMA, 1979).
Figure 12-3. Belt conveyor idlers (CEMA, 1979)
12-6
Loading and unloading
Belt conveyor loading is normally done through a feed chute located just ahead of the tail pulley. Closely spaced
idlers in this region prevent excessive belt deflection due to dynamic loading forces (Figure 12-2).
The simplest discharge arrangement consists of discharge over the head pulley (Figure 12-4a). A discharge chute
may be necessary to direct flow after it leaves the end of the belt (Figure 12-2).
Discharge along the run of a belt conveyor is difficult. A plow is one way to discharge over the side of the belt
(Figure 12-4b). The plow, held solid above the belt, pushes grain off the side of the belt. The plow is attached to
the conveyor frame and can be designed to be movable along the belt. It may not be usable on troughed belts.
A tripper (Figure 12-4c) is a device which lifts the belt and its contents high enough so that material can be
discharged over a belt pulley and then allowed to flow down a gravity chute to a pile beneath either side of the
belt. Various tripper designs allow flow on the belt past the tripper and even movement of the tripper by belt
power. A moving tripper allows formation of a continuous pile of triangular cross section below the belt.
Figure 12-4. Belt discharge methods (CEMA, 1979)
12-7
General characteristics of belt conveyors
We will note here the general characteristics of belt conveyors. Some will be explained more in the design
section.
Belt width
Belt widths range from 18 to 96 inches. The most economical design is usually one which uses the narrowest
possible belt running up to its highest allowable speed.
Belt speed
Maximum belt speeds range from 50 to 1000 ft/min. Speed is limited by the tendency of material to blow off the
belt, by belt slippage on the drive pulley as centrifugal force acts on the belt, and by the dangers of belt damage as
large sharp lumps are loaded.
CEMA, 1979 recommends maximum belt speeds listed in Table 12-3 for belts carrying grain or other free
flowing, nonabrasive material.
Table 12-3.
#Non-steel
Recommended maximum belt speeds and belt weight for grain and
other free-flowing, nonabrasive material (CEMA, 1979)
Belt width, in.
Max. belt
speed, ft/min.
Approx. belt weight
lb/ft #
18
24
30
36
42
48
54
60
72
84
96
500
700
700
800
800
1000
1000
1000
1000
1000
1000
3.5
4.5
6
9
11
14
16
18
21
25
30
cable belts for material in 30 to 74 lb/ft3 range
Power requirement
Power requirement is comparatively low since the load is carried on anti-friction bearings. Since there is no
sliding of material during movement, power is independent of product moisture content.
Incline
Incline is limited by the repose characteristics of the material being moved. Since the belt is smooth, material will
tend to roll down if the incline is too great. The limit for a smooth, slick material such as hulled or polished rice is
8 degrees. A fibrous, interlocking material like wood chips can be conveyed at a 27-degree incline.
12-8
Recommended maximums for grain are in the range from 8-18 degrees. Recommended maximums for specific
materials are listed in Appendix A, Table A-2.
The limitation on incline is one factor limiting use of portable belt conveyors for grain. The belt conveyor must
be quite long to discharge into grain storage structures. Some specialized designs employ rubber flights molded
into the belt surface. These flights reduce the tendency for material to roll down, and allow steeper belt runs.
Capacity
A very wide range of capacities is possible with belt conveyors. A capacity of over 300 000 bu of corn per hour is
theoretically possible (96 in. belt, maximum speed). No other material handling method can approach such a
capacity. As a result, belt conveyors find wide use in applications such as grain elevators where high capacities
are required.
Product damage
There is practically no damage to material while being conveyed on a belt conveyor since there is little relative
motion between the material and the belt. There may be product damage occurring during loading and unloading.
Noise
Noise level comparatively is low since a belt conveyor has none of the usual sources of high conveyor noise
(scraping of surfaces, high-speed fans, impact of particles).
Distance
Conveying distance is unlimited. Belt conveyor systems can be designed like pipelines for dry material. A belt
conveyor system has been proposed to carry corn 250 miles east from Storm Lake, IA to a Mississippi river barge
terminal. Although technically feasible, the conveying costs were projected to be higher than rail car rates and so
the system was not built (Des Moines Tribune, 1972).
Investment cost
Belt conveyors are comparatively high in cost and designed for long life and heavy service.
Enclosure
Belt conveyors are not inherently enclosed and unless there is a reason to add the expense of enclosure (dust
containment, weather protection) they are usually left open.
Combined operations
Unit operations such as weighing, sorting, or spraying can be carried out during belt conveyor transit.
Belt conveyor design
Methods will be presented here for preliminary designs of belt conveying systems. Procedures for estimating belt
size, speed and power requirements will be explained.
12-9
Load cross section
The volume capacity of a belt conveyor is the product of belt speed and load cross section. Figure 12-5 shows
dimensions used to compute load cross section for a troughed belt. When material is loaded on the belt it falls to
its filling angle of repose with the horizontal, but then slumps to a circular profile ABC which has a center at D.
Figure 12-5. Area of belt conveyor load cross section (CEMA 1979).
At the sides of the load cross section, the top surface of the material meets the belt at angle , henceforth called
the surcharge angle. As the conveyor belt passes over successive carrying idlers, material on the belt is agitated
and the cross section assumes a more flattened shape. Since lines AD and CD are perpendicular to the material
top surface, angle ADC is 2 . Material is loaded to within c inches of the belt edge.
The area of load cross section is, thus, the sum of area Ab (the trapezoid) and areas As, the surcharge. Distance l
is estimated as follows:
l = 0.371 (b) + 0.25
where
(12-6)
l and b are as defined in Figure 12-5.
The surcharge angle is a property of the material being conveyed and is 5 to 20 degrees less than the filling angle
of repose. See Table 12-4. Flowability is the fourth characteristic of the material code from Table A-1 in
Appendix A. For example, wheat has material code 47LC25N (from Table A-2). The 2 indicates a free-flowing
material (Table 12-4).
The load cross section as defined here exists in a vertical plane. The effective load cross section of inclined belts
decreases as the cosine of the angle of conveyor slope since this cross section is measured in a plane normal to the
belt. The actual loss of capacity is usually very small.
12-10
Table 12-4. Flowability - Angle of surcharge - Angle of Repose (CEMA) 1979.
For grain, a 20-degree troughed belt with three equal-length rolls is common. Load cross sections along with
volume capacities for this type of belt are listed in Table 12-5.
Belt capacity
Belt capacity is the product of belt speed and load cross section. An example problem will illustrate the
computation procedure:
Example 12-2
Compute the capacity (bu/h) of a 36-in. belt conveyor running at 285 ft/min and carrying wheat (1 bu = 1.245 ft3).
From Table A-2, wheat has a code of 47C25N. The fourth character (2) indicates a 10 degree surcharge angle
(Table A-1, Table 1204). From Table 12-5, load cross section is 0.596 ft2 and capacity at 100 ft/min is 3579 ft3/h.
(3579 ft 3 ) min (285 ft) bu
 8192 bu/h
h (100 ft) min (1.245 ft 3 )
(12-7)
12-11
Table 12-5.
Belt
Width
(Inches)
18
24
30
36
42
48
54
60
72
84
96
0°
.089
.173
.284
.423
.588
.781
1.002
1.249
1.826
2.513
3.308
Load cross section and capacity for 20-degree troughed belt, three equal rolls (CEMA, 1979).
5°
.108
.209
.343
.509
.708
.940
1.204
1.501
2.192
3.014
3.967
At- Cross Section of Load
(ft2)
Surcharge Angle
10°
15°
20°
.128
.147
.167
.246
.283
.320
.402
.462
.522
.596
.684
.774
.828
.950 1.074
1.099 1.260 1.424
1.407 1.613 1.822
1.753 2.009 2.270
2.560 2.933 3.312
3.519 4.030 4.551
4.631 5.302 5.986
25°
.188
.359
.585
.866
1.201
1.592
2.037
2.537
3.701
5.085
6.687
30°
.209
.399
.649
.960
1.332
1.765
2.258
2.812
4.102
5.635
7.411
0°
537
1041
1708
2538
3533
4691
6013
7498
10961
15079
19850
5°
653
1258
2060
3057
4250
5640
7225
9006
13155
18089
23806
Capacity at 100 ft/min
(ft3/h)
Surcharge Angle
10°
15°
20°
769
886
1005
1477
1698
1924
2414
2772
3137
3579
4107
4645
4972
5703
6447
6594
7560
8544
8444
9678 10935
10552 12057 13621
15364 17599 19876
21119 24186 27309
27787 31816 35921
25°
1128
2155
3511
5196
7210
9552
12223
15223
22210
30511
40128
Power requirement
The power requirement of a belt is estimated by use of this equation:
hp 
where hp =
Te =
V=
(Te) (V)
(33 000)
power to drive pulley, hp
effective tension, lb
belt speed, ft/min
Te is effective tension at the drive pulley which must be supplied by the drive. The torque supplied to the drive
pulley shaft is the product of Te and the drive pulley radius.
Ordinarily, to compute hp, V is known and Te is estimated by summing tensions necessary to run the conveyor
and lift the material. For a basic straight-line belt conveyor of the type commonly used for grain movement, Te
can be estimated by use of this emperical (adaped from CEMA, 1979):
Te = L (0.00068Wm + 0.05 Wb + .58) + Wm (0.035L + H) + 225
where
(12-9)
L = conveyor length, ft (use pulley-to-pulley centerline distance)
Wm= weight of material, lb per ft of belt length
Wb = weight of belt, lb per ft of belt length
H = vertical distance material is raised (+) or lowered (-)
The equation can estimate effective tension for a straight-line belt conveyor with no accessories (plows, trippers
for example) operating at 32 F or above.
The drive pulley is connected to a source of shaft power (usually an electric motor) through a drive assembly.
Since drive pulley speed is slower than electric motor shaft speed (usually 1725 r/min), the drive is designed to
reduce speed by means of gears, chains or belts. The efficiency of each reduction is about 93% and drives using
two reductions are most common. Lost power is dissipated as heat from drive components.
30°
1254
2394
3897
5765
7997
10592
13552
16876
24617
33813
44466
12-12
An example will illustrate power computation:
Example 12-3
A 36-in belt conveyor runs at 285 ft/min and carries wheat at a rate of 245 ton/h. The conveyor is 200 ft long and
lifts the wheat 40 ft. Compute necessary motor power output.
Wm 
(245ton) min h (2000 lb)
 28.66 lb/ft
h (285ft) (60min) ton
Wb = 9 lb/ft (Table 12-3)
Substituting into Equation 12-9:
Te = 200 (0.00068 (28.66) + 0.05 (9) + 0.58) + 28.66 (0.035 (200) + 40) + 225
Te = 1781.9 lb
Substituting into Equation 12-8:
hp 
(1781.9) (285)
 15.38 hp
33 000
Assuming each of two speed reductions is 93% efficient, the motor must deliver:
(15.38)
 17.78 hp
(0.93) (0.93)
Belt conveyor application
Belt conveyors are best suited for low slope, heavily used, high capacity, stationary applications demanding high
reliability. At the high end of their capacity range there may be no alternative conveying method available.
FLIGHT CONVEYORS
Flight conveyors consist of one or two endless flexible drive lines (chains, belts, cables) to which flights are
attached. Flights drag along material as the drive line is pulled in a circuit. There are many variations in
agriculture and industry.
As is the case with belt conveyors, half of the drive line is inactive and is continually pulled back to the loading
point empty. (Some circuit-type conveyors may have a drive line more than half active.)
General characteristics of flight conveyors
It is difficult to generalize about characteristics here since flight conveyors are so varied.
Speed
Flight conveyors travel at drive line speeds from 25 to 300 ft/min. Speeds in the range from 100 to 200 ft/min are
most common. Higher speeds accelerate wear and may increase product damage.
12-13
Power
Power requirement is high (higher than belt conveyor, other things being equal) because the drive line, flights and
material are all dragged along a surface. This dragging also makes noise. Some designs use plastic liners on
flights or on interior conveyor surfaces to reduce friction and noise.
Incline
Allowable incline depends on the flight conveyor type. Some are designed for horizontal use only. Others may
operate at extreme slopes or even vertically.
Product damage
Some grain damage occurs in flight conveyors because of rubbing action and possible pinch points between
conveyor components. However, damage is usually less than with screw or pneumatic systems.
Design parameters for conventional flight conveyors
Figure 12-6 is a double chain, portable flight conveyor also known as a farm elevator. Load is carried on top in
the open, with the drive line return below. Flights are rectangular. This type of flight conveyor is very versatile
and, with little or no modification, can be used for grain, feed, ear corn, forage, and even bundles of shingles.
This type of conveyor is inexpensive, often noisy, and will have a long life of intermittent use since it is needed
only a few hours per year. Although driving from the bottom sprocket is not desirable (more chain and bearing
stress), it is often done because of the difficulty of transferring power to the discharge end of this type of
conveyor. It will be used as an example for design computations for flight conveyors.
Figure 12-6. Double chain, portable flight conveyor (farm elevator).
12-14
Typical design parameters
This type of conveyor is operated with chain speeds between 25 and 300 ft/min. Flight spacing is about equal to
flight width and flight height is about 40% of flight width.
The theoretical volume capacity is given by Equation 12-10.
C = (V) (h) (w) (12-10)
where C = theoretical volume capacity
V = drive line speed
h = flight height
w = conveyor width (flight length)
Equation 12-10 neglects the volume of the flight and chain and assumes slug flow of grain. To consider flight
volume, multiply Equation 12-10 by (s-t)/s, where t is flight thickness and s is the flight spacing.
At 100% of theoretical volume capacity, the conveyor is full to the flight depth. The conveyor will operate at
various fractions of theoretical volume capacity depending on conveyor slope and the repose characteristics of the
material conveyed. Henderson and Perry, (l976) list the percentages shown in Table 12-6. A conveyor with an
enclosed conveying chamber will have less effect of slope on its capacity.
Table 12-6. Flight conveyor approximate volume capacity (Henderson and Perry, 1976).
Incline, degrees
0
20
30
40
Approx. % of theoretical capacity
115
77
55
33
Power requirement
Power requirement of a flight conveyor can be estimated by Equation 12-8. Te is now defined as:
Te = 1.1 (force to slide drive line + force to lift drive line
up + force to slide material + force to lift material
up + force to slide drive line - force to lift drive line going down)
(12-11)
The drive line consists of the chain and flights. The description assumes the conveyor slopes up toward the
discharge end. In this case, gravity force on the return side of the drive line subtracts from the turning effort.
The added 10% is to account for friction in sprocket bearings.
In terms of conveyor parameters, the equation is:
Te = (1.1)L(Wc(Fc cos  + sin ) + Wm(Fm cos  + sin ) + Wc(Fc cos  - sin ) + h2(0.044))
where
Te = turning effort, lb
(12-12)
L= conveyor length, ft
Wc = weight of chain and flights, lb/ft
h = average depth of material in conveyor, in.
Fc = kinetic friction coefficient of chain and flights on conveyor floor (Table A-3)
12-15
 = conveyor slope, degrees
Wm= weight of material on conveyor, lb/ft
Fm = kinetic friction coefficient of material on conveyor floor (Table A-3)
The term 0.044 h2 is an empirical factor to account for grain friction on conveyor walls (Rexnord, 1980). It may
be negligible for open, top-load conveyors.
The equation can be simplified to:
Te = (1.1)L(2Wc Fc cos  + Wm (Fm cos  + sin ) + h2(0.044))
If Wc is not known, it can be approximated by:
(12-13)
Wc = 0.0024 (total weight of material on conveyor, lb), lb/ft
(12-14)
This equation, adapted from Rexnord, 1980, assumes Wc to be a function of both conveyor length and weight of
material per unit length of conveyor.
From Table A-3 it can be seen that Fm varies from one grain to another and usually increases with moisture
content. Power requirement of a flight conveyor is, thus, influenced by grain moisture. An example will illustrate
use of the equations.
Example 12-4
Estimate the capacity (tons/h) and motor power requirement for this flight conveyor carrying dry corn:
Flights are 12 in. long. Spacing equals length and height is 40% of length. The drive line weight is 3 lb/ft. All
conveyor parts are steel. Table A-2: Bulk density = 45 lb/ft3
tan 
23.1
 0.58
40
 = 30
L=
40
 46.19 ft
cos 30
Effective volume capacity is calculated using Equation 12-10 and a value from Table 12-6:
0.55(C) = (0.55)
capacity =
(125 ft) (0.4 ft) (1 ft) 27.5 ft 3
=
min
min
(27.5ft3 ) (45 lb) (ton) (60 min)
tons
 37.13
3
h
min ft 2000 lb h
12-16
Wm 
h
(27.5ft3 ) (45 lb) (min)
lb
 9.9
ft
min ft 3 125 ft
Wc  (0.0024)
9.9lb46.19 ft
lb
 1.1
ft
ft
(27.5 ft 3 (min) (12in)
 2.64 in
min 125 ft 1 ft 1 ft
Substituting into Equation 12-13:
Friction coefficients are obtained from Table A-3 in the Appendix.
Te = (1.1) (46.18) (2(3) 0.57 cos 30 + (9.9) 0.27 cos 30 + sin 30) + 2.64 (0.044))
Te = 299.4 lb
hp 
(299.4) (125)
 1.13 hp
(33000)
Assume the drive reduces speed in two steps, each with an efficiency of 0.93.
motor power required 
1.13
 1.31 hp
(0.93) (0.93)
Application of conventional flight conveyors
Conventional flight conveyors are inexpensive simple machines. They are best suited to intermittant use, low
volume applications where power requirement is not an important factor and suitability for a variety of materials is
important.
En masse conveyor
The en masse conveyor is a type of flight conveyor which moves grain in slug flow (en masse) rather than in
discrete elements between flights. Figure 12-7 is a cutaway view of an en masse conveyor. Load is carried on the
bottom with return on the top.
Figure 12-7. En masse flight conveyor (Huss and Schlieper, Inc. 1981).
12-17
The enclosed box design retains dust, protects grain from weather, and allows long spans without additional
support. Low-height flights, which operate submerged, plus the chain move a layer of grain along the conveyor
floor. Grain above is carried along in a continuous stream filling the chamber up to the level of the return track
supports. Metal-to-metal sliding contact is avoided in some models by use of ultra high molecular weight
polyethelene (UHMWP) wear bars (as shown) or as conveyor liners. The drive line rests on UHMWP inserts or
rollers on the return. During loading, grain falls through the return drive line. Discharge is under the drive
sprocket, or at any intermediate point. Ease of employing any number of intermediate discharges is an advantage.
In this form the en masse conveyor is intended for no-incline or low-incline applications. Slope limits are usually
in the 5- to 10-degree range
Modification of the flight design allows the en masse conveyor to be used for inclined or even vertical
applications. Figure 12-8 shows en masse conveyor flight designs for various applications. Conveyors for higher
inclines have a solid partition between the load and the return sections of the conveyor and grain bears against all
four walls during conveying. Some designs limit incline to 45 or 60 degrees. Others allow vertical application.
Portable models in the configuration of farm elevators are also available. These portable conveyors are driven
from the discharge end through a shaft extending along the conveyor to a PTO or electric motor drive near the
ground.
Speed, power and capacity
En masse conveyors are designed for drive line speeds of 100 to 275 ft/min. Table 12-7 illustrates the range of
capacities available with en masse conveyors.
Conveyor size listed is the width x height of the conveyor box cross section in inches. Grain is assumed to flow at
drive line speed in a slug the width of the conveyor and about 65% of its height.
Conveyor capacities up to nearly 100 000 bu/h and lengths to 400 ft make this conveyor type appropriate for
many high capacity applications.
12-18
Figure 12-8. En masse conveyor flight configurations (Buhler-Maig (1983).
12-19
Table 12-7. Typical en masse conveyor horizontal capacity (Huss and Schieper, Inc. 1981).
CAPACITY CHART - UNITS PER HOUR
Conv
Size1
12x22
UNITS
1
50
75
CU. FT
68.4
3420
5130
BU.
54.7
2730
4100
18x22
CU. FT 106.3
5310
7970
BU.
85.0
4250
6370
24x22
CU. FT 144.2
7210 10810
BU.
115.3
5760
8650
30x22
CU. FT 182.1
9100 13650
BU.
145.6
7280 10920
18x28
CU. FT 136.9
6840 10260
BU.
109.5
5470
8210
24x28
CU. FT 186.7
9330 14000
BU.
149.3
7460 11200
30x28
CU. FT 236.3
11810 17720
BU.
189.0
9450 14170
36x28
CU. FT 286.0
14300 21450
BU.
228.8
11440 17160
42x28
CU. FT 335.7
16780 25170
BU.
268.5
13420 20140
48x28
CU. FT 385.4
19270 28900
BU.
308.3
15410 23120
54x28
CU. FT 435.1
21750 32630
BU.
348.0
17400 26100
1width x height of box cross section, in.
100
6840
5470
10630
8500
14420
11530
18210
14560
13690
10950
18670
14930
23630
18900
28600
22880
33570
26850
38540
30830
43510
34800
125
8550
6840
13280
10630
18020
14420
22760
18210
17110
13690
23330
18670
29530
23630
35750
28600
41960
33570
48170
38540
54380
43510
Speed ft/min
150
175
10260 11970
8200
9570
15940 18600
12750 14880
21630 25230
17300 20180
27310 31860
21850 25490
20530 23950
16420 19160
28000 32670
22400 26130
35440 41350
28350 33080
42900 50050
34320 40040
50350 58740
40280 46990
57810 67440
46240 53950
65260 76140
52210 60910
200
13680
10940
21260
17000
28840
23070
36420
29130
27380
21900
37340
29870
47260
37800
57200
45760
67140
53710
77080
61660
87020
69610
225
15390
12310
23910
19130
32440
25950
40970
32770
30800
24640
42000
33600
53160
42530
64350
51480
75530
60420
86710
69370
97890
78310
250
17100
13680
26570
21260
36050
28840
45520
36420
34220
27380
46670
37340
59070
47260
71500
57200
83920
67140
96350
77080
108770
87020
An example will illustrate power computation for an en masse conveyor.
Example 12-5.
An en masse conveyor is to be used to convey dry corn a distance of 200 ft along a 10 degree incline at a rate of
600 000 lb/h. The conveyor is to be steel with UHMWP flights and chain wear plates. Specify the conveyor and
estimate the power requirement assuming a dual reduction drive.
600000 lb ft 3
ft 3
 13333
h 45 lb
h
From Table 12-7, a 12 x 22 conveyor running at between 175 and 200 ft/min will handle this capacity.
13333 - 11970 x

13680 - 11970 25
x  19.9
Conveyor speed is 175 + 19.9 = 194.9 ft/min.
Power is estimated by Equations 12-12 and 12-13.
Weight of material on conveyor =
275
18810
15040
29230
23380
39650
31720
50070
40060
37640
30110
51340
41070
64980
51980
78650
62920
92310
73850
105980
84780
119650
95720
12-20
(600 000) lb min h 200 ft
 10,262 lb
h (194.9) ft (60min)
Wc = 0.0024 (10 262) = 24.6 lb/ft
Fc = 0.2 (Table A-3)
Wm = 51.31 lb/ft
Fm = 0.27
h
(13333 ft 3 ) min h (12 in)
 13.7in.
h (194.9 ft) (60 min) (1 ft) ft
Te = (1.1) (200) (2(24.6) (0.2) cos 10 + 51.31 (0.27 (cos 10) + sin 10) + 0.044 (13.7)2 )
Te = 8910.4 lb
hp 
(8910.4) (194.9)
 52.63 hp
(33 000)
motor power 
52.63
 60.8 hp
(0.93) (0.93)
Application of en masse conveyors
En masse conveyors are often specified for heavy use applications for grain in permanent installations. Often
distances are short and an overhead span is crossed.
Cable-flight conveyor for feed
Figure 12-9 is a cable drive conveyor for feed using disc-shaped molded plastic flights pulled through a metal
tube. The tube can be routed through a complicated path and if the cable makes a circuit, there may be little idle
return conveyor. One model uses a 60-mm inside tube diameter and a cable speed of 100 ft/min. Circuit lengths
up to 2000 ft are allowable. Throughput is about 2 tons/h.
Figure 12-9. Cable-drive flight conveyor
12-21
BUCKET CONVEYORS
Bucket conveyors are vertical belt conveyors with buckets bolted on to carry the load. They offer for vertical
conveying many of the desirable features of belt conveyors. (The drive line can be chain, but belts are used in
most grain conveyors.)
Figure 12-10 is a side view of a bucket conveyor type commonly used for grain. Common terminology of
conveyor parts and dimensions is also included. The figure shows a dual-leg conveyor. This means the up and
the down sides are in separate enclosures (legs). A single-leg type has the entire belt in one enclosure. The entire
bucket conveyor is sometimes called a leg. A motor drives the head pulley. Takeup adjustment is at the foot
pulley. It can be by bolts (as shown) or by gravity from weights hung on the shaft.
Figure 12-10. Bucket conveyor (Bloome et al. 1978).
12-22
Types of bucket conveyors
The three common belt conveyor types vary in the way material is discharged. Figure 12-11 illustrates these
types. The centrifugal discharge type is discharged by centrifugal action as loaded buckets pass over the head
pulley. The head section must be specially designed for proper discharge. This will be discussed more later.
Most grain conveyors are centrifugal discharge and all discussion following this section will be about that type.
Positive discharge conveyors employ an idler below the head pulley. As the drive line (which may be chain in
this case) runs around the idler, each bucket is inverted over a discharge spout, causing positive discharge. This
type conveyor runs at lower speeds and is used for light, fluffy, or fragile materials or those tending to stick in
buckets.
Figure 12-11. Bucket conveyor types (Thomas Conveyor Co., 1980)
Continuous conveyors have buckets placed as close as possible (continuously) on the belt. During discharge,
material flows over the preceding bucket whose front and projecting ends form a chute to direct material into the
discharge spout. This conveyor type is used for heavy abrasive and lumpy materials like cement, crushed stone,
and clinker.
Another design (not shown) uses hanging buckets which allow vertical, angled, and horizontal belt routing.
General features
Bucket conveyors have low power requirements since load is carried in buckets supported by antifriction
bearings. Power and capacity are not affected by grain moisture content. Their noise level is relatively low.
Bucket conveyors are reliable, relatively trouble free, and have a long service life. On farms they often are the
common section of a closed-loop handling system. Horizontal conveyance in such a system is accomplished by
angled gravity spouting from the bucket conveyor discharge. In grain elevators and other related industries,
bucket conveyors are the preferred method of vertical grain movement. Alternatives include vertical screw
conveyors and pneumatic systems, both of which have higher power requirements and a greater potential for grain
12-23
damage. During actual grain lifting, practically no damage occurs in a bucket conveyor. However, loading and
unloading operations have a potential to break kernels. This will be discussed more later.
Bucket conveyors can be categorized by belt speed into high speed (450 to 1000 ft/min) and low speed (under 450
ft/min). High speed designs are most common for the high capacity conveyors in grain elevators. Low-speed
bucket conveyors are common on farms.
Conditions for centrifugal discharge
A Centrifugal discharge conveyor must be designed with the proper combination of belt speed, head pulley
radius, head section shape, and bucket shape for proper discharge. Figure 12-12 shows the forces acting on grain
in a conveyor bucket as it rounds the head pulley.
Figure 12-12. Forces on grain during centrifugal discharge.
The effective force on the grain is the resultant of the grain force, W which always acts down and C, the
centrifugal force which always acts out along a radius from the head pulley centerline. When the resultant force
on a kernel points out through the bucket opening, the kernel will leave the bucket. Centrifugal force on a mass is
given by Equation 12-15.
C
W(Vt) 2
g r (3600)
where C = centrifugal force, lb
W = weight, lb
Vt = tangential velocity, ft/min
g = acceleration of gravity = 32.2 ft/s2
r = effective radius of mass, ft (usually measured to a point halfway across the bucket
projection)
(12-15)
12-24
Hetzel and Albright, 1941 recommend that for centrifugal discharge of grain, C = W. If this condition exists the
resultant on the kernel will be zero when the cup is directly above the head pulley center line. After that, the
resultant force will have a direction out of the bucket and discharge will begin.
The speed for C = W will be referred to as the critical speed. If the equation C = W is combined with equation
12-15 and simplified, the result is:
Vt  60 gr
(12-16)
For the conveyor,
Vt = 2 r N
(12-17)
where N = pulley speed, rev/min
Combining 12-16 and 12-17, we obtain:
N
(12-18)
54.19
r
where N is now the critical speed. Note that the radius here is the effective mass radius and not the pulley radius.
Belt speed can be computed by revising Equation 12-17:
Vb = 2 rp N
(12-19)
where: Vb = belt speed, ft/min
rp = pulley radius, ft
Buckets
Figure 12-13 shows the bucket shape and size designation. Buckets are made of fabricated metal (usually steel),
cast metal, or of a non-metallic material. Non-metallic buckets (polyethelene, urethane, poly vinyl chloride)
reduce drive line stresses because they are much lighter than metal buckets. However, one manufacturer cautions
against their use in combustible environments because of their ability to retain static electrical charge and produce
sparks (Rexnord, 1980).
The radius of the grain in the bucket will vary by the length B in the figure. An example will illustrate use of the
equations.
Figure 12-13. Bucket size designation (Bloome et al., 1978)
12-25
Example 12-6.
A bucket conveyor is designed for centrifugal discharge and the head pulley is to operate at critical speed. The
head pulley is 18 in. in diameter, with a 0.5-in. belt thickness and a 6-in. bucket projection. Compute the correct
head pulley speed and belt speed.
r
(9  0.5  3)
 1.042 ft
12
N
Vb 
54.19
1.041
 53.09 rev/min
2 π(9) (53.09)
 250.18 ft/min
12
Designs in use for grain conveying vary considerably from the critical speed condition for centrifugal discharge.
One way to compare different designs is to compute the C/W ratio. C/W = 1 if the conveyor operates at critical
speed. A survey of some manufacturer's specifications showed variations from C/W = 0.71 to C/W = 5.8. The
low ratio design will not begin to discharge until the bucket is well past the top of the pulley. The high ratio
design will begin to discharge before it reaches the top. In each case, head section geometry must be designed to
accommodate resulting grain trajectories. One design in use (not recommended for grain) uses a belt speed of
1000 ft/min and C/W = 17.1. The discharge chute extends horizontally from the top of the head section.
If the head pulley speed is much slower or faster than the head section is designed for, grain will miss the
discharge chute and fall down the down leg causing a condition known as "back legging." Back legging damages
grain, cuts capacity, and wastes power.
Loading buckets
Conveyor buckets are loaded in the foot (or boot) section. A designer aims for a feed system which fills buckets
to a high percent of their capacity with minimum power consumption, grain damage, and dust generation. All of
the conveyors shown in Figure 12-11 are loaded into the up leg. This is the preferred loading method for farm
size conveyors. If grain is introduced above the foot pulley shaft center, buckets are filled as they move vertically.
Spillage into the foot section is minimized. If it is necessary for the system, grain can be loaded on the down side
(or on both the up and down sides as shown in Figure 12-10). Grain loaded on the down side is subject to
centrifugal force as it is swept under the foot section by the buckets. The practice of making the foot pulley
smaller than the head pulley may increase centrifugal emptying forces to a point where capacity is cut and grain
damage and dust generation increase. Ditzenberger, 1980, recommends that the foot pulley diameter never be
smaller than 66% of the head pulley diameter so that these problems are avoided.
Some high speed designs can be satisfactorily loaded only on the down side. At high belt speeds, grain must be
introduced with a velocity component in the same direction as bucket movement, as is the case with a downangled spout into the down leg. Loading on the up side results in reduced bucket filling and increased power.
High-speed machines develop positive air pressure in the foot section as air "carried" down by buckets is
displaced by grain. Ventilation pipes can be fitted to route this air to the head section which operates at negative
pressure for the same reason.
12-26
Capacity
Bucket conveyor capacity depends on belt speed, bucket volume, bucket spacing, and the percent of fill attained
by the bucket. Capacity tables usually assume buckets are filled to 85% of full. In designing a line of bucket
conveyors, manufacturers often select a combination of belt speed and head pulley diameter which will give
proper centrifugal discharge. With this combination held constant, capacity for different models is varied by
varying bucket spacing or belt width (bucket length).
Table 12-8 shows bucket conveyor capacities for different farm applications. Capacities larger than 5000 bu/h are
seldom required on farms. Grain trade applications may require capacities over 60 000 bu/h.
Table 12-8. Bucket conveyor capacities and applications (MWPS-1978).
CAPACITY
bu/h
500-700
1000-1200
1500-2000
2500-3000
COMMENTS
APPLICATIONS
Well suited to wet and dry grain
handling on continuous flow dryer
1 - Small farm needs
2 - Feed making only, with separate elevators
for receiving wet grain
3 - As wet and dry grain elevator on
continuous flow dryer.
Well matched to 6” augers. Gravity 1 - Small and medium farms, feed and/or cash
spouts: 6”
grain.
2 - Small batch dryers, and layer or batch-inbin drying methods on small to medium
farms.
Well matched to load-unload rates on 1 - Medium to large farms, feed and cash
many mechanized batch dryers
grain.
2 - Load-unload on batch and batch-in-bin
Maximum size for 6” gravity spouts.
drying systems.
3 - Primary leg in a continuous flow or batch
Maximum size for 8” horizontal augers
drying setup.
in 25% corn.
Matched to 8” overhead augers in dry 1 - Large farms, feed and cash grain.
grain.
2 - As load-unload on large batch and batch-inbin dryers.
Gravity spouts: 8”
3 - .As primary leg in two-leg installations on
continuous flow dryers.
Figure 12-14 is a nomograph showing capacities resulting with different combinations of belt speed, bucket size,
and bucket spacing. Bucket sizes are given by nominal bucket length x projection in inches (See Figure 12-12).
Bucket volumes listed assume buckets are filled to line x-x on Figure 12-13 and are typical for the bucket sizes
listed.
12-27
Figure 12-14. Bucket conveyor capacity nomograph (Bloome et al., 1978).
Different brands with the same nominal dimensions may vary + or - 15% from the listed volumes. Conveyor capacity
assumes buckets are filled to 85% of volume. One bushel in Figure 12-14 is 1.245 ft3. In selecting a bucket size - bucket
spacing combination, be sure bucket spacing exceeds bucket height (the smaller number) by at least an inch.
An example will illustrate use of Figure 12-14.
Example 12-7.
A bucket conveyor runs with a belt speed of 440 ft/min and uses 9x6 buckets. What bucket spacing is needed for a
capacity of 3000 bu/h?
Line CD is drawn from 440 ft/min to 3000 bu/h. It crosses the diagonal solid line at E, which is called the turning point.
Now a line is extended from F, the 9x6 volume, through E to G, a bucket center-to-center spacing of 8.3 in.
The same result can be obtained by computation:
(440 ft) (200 x 0.85 in 3 ) ft 3 bu h (60 min) (12 in.)
min bucket (1728 in 3 ) (1.245 ft 3 ) (3000 bu) h ft
= 8.3 in/bucket
12-28
Power requirements
Power requirements for bucket conveyors are usually estimated by computing the necessary lifting power and
then adding a component to account for friction losses. Equation 12-19 was adapted from Bloome et al., 1978.
hp 
1.1 (C) (BD) (h)
C

(33 000) (60)
(2490)
(12-19)
where hp = power required, hp
C = conveyor capacity, ft3/h
BD = material bulk density, lb/ft3
h = lift height (distance between conveyor shaft centers), ft
Example 12-8 illustrates use of the power equation.
Example 12-8.
Estimate the motor power required for the conveyor of Example 12-7, assuming speed is reduced in two steps, the
material conveyed weights 45 lb/ft3, and the lift height is 50 ft.
C
(3000 bu) (1.245 ft 3 )
 3735 ft 3 /h
h bu
hp =
(1.1) (3735) (45) (50) 3735

 6.17 hp
(33 000) (60)
2490
motor power =
6.17
 7.13 hp
(0.93) (0.93)
Bucket conveyor applications
Bucket conveyors are well suited for high-rate vertical conveyance applications which find heavy use. In this type
of situation, there may be no realistic alternative method. If a vertical auger or pneumatic system is an alternative,
the bucket conveyor is the best choice where heavy use causes its high ownership cost and low operating costs to
add to the lowest total cost.
For farms, well planned systems designed around legs are hard to match for convenience. The tall leg becomes a
status symbol and landmark. However, its high investment cost is sometimes hard to justify. Because of its
intermittent use pattern through the year, other more energy-intensive conveyors which cost less to buy are
ultimately cheaper. The height of the leg necessitates wires for support. Wind and lightning can cause damage.
Maintenance of the head section is difficult.
SCREW CONVEYORS
A screw conveyor consists of a helicoid or screw or auger which moves material as it rotates within a tube or
trough. It is one of the oldest and, at first sight, simplest of the mechanical conveying devices. Archimedes is
credited with using a screw conveyor to pump water from ships over 2200 years ago. For this reason, it is
sometimes referred to as the Archimedean screw. It has been in continual use for countless conveying tasks since
12-29
that time. Its simple appearance is deceiving. Its operating characteristics are far more complex and hard to
predict than those of any of the other mechanical conveying devices.
In some references, including those of the American Society of Agricultural Engineers, a screw conveyor is called
an auger. The terms will be used synonymously here.
Screw conveyor terminology
Screw conveyor terminology has been standardized by the American Society of Agricultural Engineers. Figure
12-15 shows the hand of the helicoid flighting (called "helicoid" from now on). The hand convention corresponds
to that of a screw fastener.
Figure 12-15. Hand of screw conveyor helicoid (ASAE, 1983a).
Figure 12-16 shows the dimensional specifications of a screw conveyor. The illustration shows a portable or
transport type screw conveyor. The terminology also applies to a fixed machine or a portable unit without a
wheeled chassis.
12-30
SECTION 3 – DIMENTIONAL
SPECIFICATIONS
3.1 Auger length: The length of the tube assembly including
any intake but not including any intake hopper or head drive
components (dimension A).
3.2 Intake length: The length of the visible flighting with
the control gate (if unit is so equipped) in the full open position
(dimension B).
3.3 Transport angle: The angle included between the auger
tube and the ground when the unit is in the lowest
recommended transport position and with hitch on ground
(dimension C).
3.4 Maximum operating angle: The angle included
between the auger tube and the ground when the unit is in the
highest recommended operating position, and with the hitch on
the ground (dimension D).
3.5 Auger Size: The outside diameter of the auger Tub
(dimension E).
3.6 Reach at maximum height: The horizontal distance
from the foremost part of the under carriage to the center of the
discharge end when the unit is at the maximum recommended
operating angle with hitch on ground (dimension F).
3.7 Maximum lift height: The vertical distance form the
ground to the lowest point of the discharge (excluding down
spout attachments) when the unit is raised to the maximum
recommended operating angle and with the hitch on the ground
(dimension G).
3.8 Transport height: The vertical distance from the ground
to the uppermost portion with the unit in the lowest transport
position and with the hitch on the ground (dimension H).
3.9 Eave clearance: The vertical distance from the ground
to the foremost component of the undercarriage when the unit
is at the maximum raised height (dimension J)
3.10 Discharge length: The total length of conveying from
the outer end of the exposed flighting assembly at the intake to
the centerline of the discharge (dimension K).
Figure 12-16 Screw conveyor dimensional specifications (ASAE, 1983b).
Pitch and flighting terminology for some of the more common helicoid configurations are shown in Figure 12-17.
The single flight, standard pitch is the most common configuration and is also the one we will be discussing at
greatest length.
12-31
Figure 12-17. Pitch and flighting terminology (Thomas Conveyor Company, 1980).
12-32
Typical specifications
Typical specifications needed for power and capacity computations are listed in Table 12-9 for typical farm-type
conveyors. Note that the nominal conveyor size is the outer tube diameter. For industrial horizontal conveyors
(Section 12.4.5), it is usually the helicoid diameter. Industrial conveyors usually have larger shaft sizes and much
lower maximum speeds.
Table 12-9. Typical farm type screw conveyor specifications
Nominal Conveyor
diameter,
in
4
6
8
10
12
Tube inside
diameter,
in
3.90
5.88
7.85
9.80
11.80
Helicoid
diameter,
in
3.37
5.13
7.25
9.00
11.00
Shaft diameter,
in
0.84
1.40
1.50
2.38
2.88
Max
speed,
rev/min
875
650
500
350
350
General features
Screw conveyors are simple, compact machines. They are usable at any angle of inclination and for many bulk
materials. Besides conveying, they can be used (sometimes simultaneously) for metering or feeding, heating,
cooling, mixing, and even digging applications. Chapter 13 describes auger feeders for pneumatic conveyors.
Chapter 6 describes application of acid preservative during conveyance in a screw conveyor. They are inherently
enclosed and can be made dust tight with suitable modifications to the feed and discharge sections. There is no
idle conveyor return.
Power requirements are relatively high because material is moved by sliding and is continually mixed. Grain
damage can be a problem because of pinch points created between the auger tube and flighting. Pinch points (and
other features) can be dangerous to operators. This is discussed later in this chapter. Some screw conveyors are
noisy.
Life (in actual use time) is relatively short because of abrasion of helicoid and tube surfaces by the conveyed
material. On farms, screw conveyors used occasionally will last many years. Purchase cost is relatively low
because of the machine's compact and simple design. Operating cost is relatively high because of the high power
requirement. Design for portability is easy because the tube can serve as a structural member.
Principle of operation
The operating principle of a horizontal screw conveyor is obvious. Material resting on the bottom of the tube is
pushed along in somewhat the way a snow plow pushes snow off a road. In this case, the plow is continuous and
the road slopes toward the center. Material plowed far enough to the side rolls back to the center, only to again
contact the plow (helicoid) which keeps coming. The effect is conveyance along the helicoid center line and also
mixing. The operation takes place regardless of the helicoid rotational speed, although as speed is increased,
dynamic effects will come into play. The material will be thrown rather than pushed.
In a vertical screw conveyor, material will not move up the conveyor unless a certain critical rotational speed is
exceeded. This critical speed is the speed at which material travels neither up nor down. If the helicoid is turning
above critical speed, material in the conveyor is accelerated in a circular motion. Centrifugal force moves it out
against the tube wall, or against other material to slide up the inclined helicoid surface as the helicoid rotates.
12-33
Material slides on both the helicoid and tube wall and moves in a spiral motion up until its discharge from the
conveyor. At angles intermediate between 0 and 90 degrees, there is a transition from the horizontal mode to the
vertical mode of operation.
Critical speed
The critical speed of a screw conveyor is defined as the speed at which a single particle in the conveyor will travel
in a circular motion with no vertical movement up or down. The critical speed is dependent on conveyor and
material parameters. We can derive an expression for the critical speed by summing the forces on a single
particle.
Figure 12-18 is a view looking down on a particle within a vertical screw conveyor. The helicoid is turning at 
rad/s.
 = helicoid speed, rad/s
ro = radius of particle path
c = centrifical force
m = particle mass
Figure 12-18. A particle within a vertical screw conveyor.
The particle, at radius ro, is rotating at helicoid speed and is thus subjected to centrifugal force, C. Figure 12-19 is
view AA of Figure 12-18, with the helicoid unwound to form an upward sloping surface.
12-34

g
Ft
K

= angle of helicoid in
= acceleration of gravity
= kinetic friction coefficient between particle and tube
= helicoid force on particle
= angle between helicoid force and normal line to helicoid surface
Figure 12-19. Horizontal view of particle on helicoid surface.
The force C, acting normal to the tube wall, produces the friction force CFt against the tube wall. Seed weight,
mg, acts down. Helicoid force K can be resolved into normal component K(cos), a normal force, and K(cos )Fh,
the friction force. Note then that:
tan  = Fh
(12-21)
where Fh = static friction coefficient between particle and helicoid.
At speeds above or below critical speed, Fh drops to the lower kinetic value since motion between the helicoid
and particles established.
12-35
Figure 12-20. Polygon of forces at critical speed.
Figure 12-20 shows the polygon of forces on the particle. The polygon is closed at critical speed. At this
condition:
tan (90     ) 
mg
C Ft
g tan (   )
ro Ft
Wc 
(12-22)
(12-23)
where Wc = critical speed, rad/s.
Nc 
30

g tan (   )
ro Ft
(12-24)
where Nc = critical speed, rev/min
The equation indicates that critical speed will be lowered by increasing Ft and/or decreasing Fh.
Vierling and Sinha, 1960, state that with force feeding (by, for example, a horizontal feeding screw), a vertical
screw can convey material when operating at critical speed. It is also important to note that this analysis assumes
no interactions with other particles. Such interaction would mean different friction factors and possibly different
helicoid slopes since slope increases toward the center of the helicoid.
12-36
Theoretical capacity
The theoretical capacity of a screw conveyor is the product of the free cross sectional area and the speed of
advance along the conveyor. The greatest possible distance of advance is one pitch length per revoltuion.
Theoretical capacity is, thus:
Ct 
Ct 
where Ct
Dh
Ds
P
N
 (Dh 2  Ds 2 ) in 2 (Pin) (N rev) ft 3
4 rev min 1728 in 3
(12-25)
(Dh 2  Ds 2 ) PN
2200
= theoretical capacity, ft3/min.
= diameter of helicoid, in.
= diameter of shaft, in.
= pitch length, in.
= rotational speed, rev/min.
The equation neglects helicoid thickness and assumes no leakage of material around the edges of the helicoid.
Note that helicoid diameter rather than tube inside diameter is used.
The ratio of actual capacity of a screw conveyor to theoretical capacity is the volumetric efficiency. It is
commonly expressed as a percent. This variable will be discussed more later.
Important operating parameters
Many grain and conveyor parameters have important influences on the operation of screw conveyors. We will list
all that are usually considered important, and then define some of the most important relationships.
Parameters having important influences on screw conveyor power and capacity include (not in order of
importance):














material particle size
material bulk density
material flowability
material-to-tube friction
material-to-helicoid friction
conveyor intake length and geometry
conveyor length
conveyor speed of rotation
conveyor diameter
tube-to-helicoid clearance
helicoid pitch length
number of helicoids on shaft
conveyor outlet geometry
conveyor angle of inclination
The other mechanical conveyors studied do not have nearly so many parameters having large effects on power
and capacity.
12-37
Many of these parameters have interacting effects. In other words, the effect on power or capacity of changing
parameter A may be different for different levels of another parameter, B.
In the following discussions, parameters not mentioned are assumed to be held constant.
Intake length
The intake length is the length which the helicoid protrudes from the tube if the conveyor is loaded from a hopper
or a mass of grain. It is often specified in helicoid diameters. The general effect on capacity of increasing the
intake length can be predicted from intuition. Capacity must be zero with zero intake length. Capacity increases
at a decreasing rate as intake length is increased. This is illustrated in Figure 12.21.
Figure 12-21. Effects of intake length on screw conveyor capacity (Rehkugler, 1967).
The expected effect is shown most clearly for 10 degrees, 300 rev/min. In this instance, there is an interaction of
exposed screw speed, and inclination in their effects on capacity. Increasing the speed enhances the effect of
increasing exposure length; increasing the inclination makes capacity less sensitive to intake length.
Any feature which changes the flow pattern or grain pressure in the conveyor intake region will change conveyor
capacity. Hopper geometry and fill level are important. Placement of intake guards can also have large effects.
An intake guard meeting ASAE Tentative Standard ASAE S361.1T (ASAE, 1983c) reduces capacity about 17%,
compared to the unguarded condition (Sevart et al., 1984). Vertical conveyor capacity can be increased by force
feeding of grain to the intake through a horizontal screw conveyor. Vertical screw conveyors for unloading ships
have helicoid flighting welded to the outside of the tube above the intake region. This tube is rotated in a
direction which causes the flighting to force grain down to the intake and thereby increase capacity.
White et al., 1962 compared six different conveyor inlet configurations (Figure 12-22).
12-38
Figure 12-22.
Performance of 6-in. screw conveyor under different inlet conditions (White et al., 1962).
Most modifications resulted in lower capacity than the usual 2-diameter exposure of standard-pitch helicoid. The
only arrangement to give a higher capacity was a 2-diameter exposure of double helicoid auger.
Power requirement per unit length of conveyor increases with increasing exposure length. The rate of increase is
very rapid at first since more helix is being turned and more grain is being moved. Beyond two diameters, the
power increase is less and is due mainly to powering the helicoid against the friction of the grain mass. Many
screw conveyors are designed with a 2-diameter exposure length.
Slope and rotational speed
Figures 12-23 and 12-24 show the effects of slope and speed on capacity and power. At any speed, capacity goes
down almost linearly with slope and, in a vertical position, is usually 30 to 40% of the horizontal value. Power
goes up with rotational speed at any slope.
Power is at a maximum at slopes in the 40- to 60-degree range. It is lower at greater and lesser slopes. Several
effects cause this relationship. Capacity is changing with slope, as is the vertical distance of conveyance.
Capacity increases with rotational speed up to a point where centrifugal force on the grain in the intake region
apparently prevents further increases and may cause a decrease in capacity.
12-39
Figure 12-23.
Capacity, slope, speed relationships for a 4-in. screw conveyor carrying 56.5 lb/bu wheat
(Millier, 1958).
Figure 12-24.
Power, slope, speed relationships for a 4-in. screw conveyor carrying 55.5 lb/bu wheat (Millier,
1958).
12-40
Conveyor power per unit length and conveyor capacity are not influenced by conveyor length.
Figure 12-25 shows the effect of incline and rotational speed on volumetric efficiency. The volumetric efficiency is the fraction
of theoretical capacity carried by the conveyor. The figure shows experimental results for a 1.5-in. standard pitch conveyor with
an intake length of 2 diameters. The conveyor carried dry millet.
Figure 12-25. Volumetric efficiency versus speed for various angles of inclination (Roberts and Willis, 1962)
Moisture content
Unlike the previous three conveyor types, screw conveyor power and capacity are significantly influenced by
product moisture content. Other things equal, power goes up and capacity goes down as moisture is increased.
Most tables and equations assume dry grain, meaning not over 15% moisture. An extension engineer's rule-ofthumb says conveyor capacity will be halved and power doubled when grain is wet (over 20% moisture).
Table 12-10 shows capacity and power for a 6-in. screw conveyor carrying wet (25% and dry (14%) corn. Speed
and slope are seen to interact with moisture content in their effects on power and capacity.
Discharge
Discharge geometry can have large effects on power and capacity. Axial discharge out the conveyor end seldom
presents any problems. Radial discharge through an opening and chute can result in compaction of material and
reduction of capacity if the opening is too small or configured incorrectly. Precise requirements for discharge
dimensions were not found in the literature.
12-41
Table 12-10. Effect of corn moisture on conveyor performance (White et al., 1962).
Comparison of performance data for a 6-inch screw conveyor handling 14 and 25 percent moisture shelled corn
(wet basis); 12 inches exposed helix at the screw inlet.
Auger
Corn
speed
moisture
rev/min percent
200
14
25
bu/min
9.9
6.2
hp/10’a
.28
1.37
Angle of elevation of screw conveyor
22.5°
45°
67.5°
bu/min hp/10a bu/min hp/10’a bu/min hp/10’a
9.2
.41
8.3
.44
6.7
.44
5.3
1.40
4.7
1.31
3.4
.97
0°
90°
bu/min hp/10’a
4.6
.32
2.6
.32
400
14
25
18.1
11.6
.56
1.84
16.8
10.3
.82
1.89
14.2
8.5
.88
1.78
11.5
6.7
.83
1.45
8.6
5.0
.70
.70
600
14
25
25.2
15.8
.84
2.32
23.4
13.7
1.22
2.34
19.4
11.3
1.28
2.27
15.1
8.6
1.16
1.92
12.4
6.8
1.05
1.09
800
14
25
29.4
18.3
1.07
2.80
27.6
15.8
1.54
2.85
22.8
12.9
1.62
2.75
18.0
9.7
1.46
2.44
14.8
7.9
1.32
1.55
aHorsepower
is that required at auger drive shaft. Horsepower loss in drive train must be added to determine the
total horsepower requirement of the conveyor.
Design of industrial horizontal screw conveyors
Horizontal screw conveyors in sizes from 6 to 24 in. diameter are used in applications similar to those for belt
conveyors and en masse conveyors. Capacities attainable are not as high as for these other conveyors. A 24-in
horizontal screw conveyor turning at its maximum speed of 100 rev/min can move grain at a rate of about 13 000
bu/h when loaded to 45% of theoretical capacity.
Procedures for sizing conveyors and for estimating power requirements are presented here for standard flighting
conveyors carrying grain.
Sizing horizontal screw conveyors
In the design procedure presented in CEMA, 1980, horizontal screw conveyor maximum speed and degree of
loading are governed by material characteristics. This is illustrated in Table 12-11. An example will illustrate the
procedure.
Example 12-9.
Specify the size and speed of a horizontal screw conveyor to move corn at a rate of 500 000 lb/h.
From Table A-2 the material code for shelled corn is 45C25.
The conveyor, then, must move:
500 000
ft 3
 11111
45
h
12-42
Table 12-11. Horizontal screw conveytor capacity (CEMA,1980).
12-43
From Table 12-11, a material class of C-25 allows a 45% fill (first group). The percent of fill must be governed
by the auger loading procedure. It is not self regulating by the auger. The capacity will require use of a 24-in.
screw conveyor. The required speed is computed as follows:
11111 ft 3 h 1 rev
rev
 67.75
3
min
h (164 ft ) min
The 24-in screw conveyor turning at 67.75 rev/min will move 500 000 lb of corn per hour.
Power requirement of horizontal screw conveyors
To estimate total power requirement, the power to overcome conveyor friction is added to the power to transport
material (CEMA, 1980):
hpf 
LN Fd
500000
hpm 
CL (BD) (Fm)
1 000 000
hp = (hpf + hpm) Fo
where: hpf
L
N
Fd
hpm
C
BD
Fm
hp
Fo
(12-26)
(12-27)
(12-28)
= power to overcome conveyor friction, hp
= conveyor total length, ft.
= conveyor rotational speed, rev/min.
= empirical diameter factor (Table 12-12)
= power to transport material, hp
= capacity, ft3/h
= bulk density of material as conveyed, lb/ft3
= empirical material factor (Table A-2)
= power required at conveyor shaft, hp
= empirical small motor overload factor (Figure 12-26)
Total length L is limited by the torque which can be transmitted through shafts and couplings. Computation
procedures for this are not presented here. Factor Fd, Fm, and Fo are all empirically derived. Fd (Table 12-12) is
proportional to the conveyor weight per foot. Fm has been formulated from experience and has no measurable
relation to any material physical property. Fo causes larger motors to be applied to small (under 5.2 hp)
installations. The increased motor size here has proven effective in avoiding stalling due to minor overloads or
choke conditions.
12-44
Table 12-12. Conveyor diameter factor, Fd (CEMA, 1980)
Screw Diamter Factor, Fd
Screw
Diameter
inches
4
6
9
10
12
Figure 12-26.
Fd
12.0
18.0
31.0
37.0
55.0
Screw
Diameter
inches
14
16
18
20
24
Fd
78.0
106.0
135.0
165.0
235.0
Small-motor overload factor (CEMA< 1980)
12-45
An example will illustrate use of the equation
Example 12-10.
The conveyor of Example 12-9 is 50 ft long. What is its power requirement?
Substituting into equations 12-26, -27, -28:
hpf 
(50) (67.75) (235.0)
 1.59 hp
500 000
hpm 
(11111) (50) (45) (0.4)
 10.0 hp
1 000 000
From Figure 12-26, Fo = 1 since hp is greater than 5.
hp = 1.59 + 10.0 = 11.59 hp
Assuming a double reduction, motor power required is:
motor power 
11.59
 13.40 hp
(0.93) (0.93)
Among the three conveyor types used for mechanical conveying (belt, flight, screw), the screw conveyor is often
chosen for relatively short runs (less than 50 ft) and/or where processing is done during conveyance (heat transfer
or mixing, for example). Its initial costs would probably be lowest and its operating cost would probably be
highest.
The design procedure shown here tends to be quite conservative and most applicable to grain elevator and
industrial processing applications. An indication of this can be seen in the recommended maximum speed for the
6-in. conveyor. Table 12-11 lists it at 60 to 165 rev/min, depending on the material class code. Farm augers of
this size run at speeds form 263 to 625 rev/min.
Power and capacity of screw conveyors
Because of the number of important variables affecting power and capacity of inclined screw conveyors, no
system of easy-to-use prediction equations is available. Reliance on tables of empirical information is a common
design procedure.
Performance tables
Table 12-13 lists capacities and speeds for a line of screw conveyors. Table values assume horizontal operation
with dry corn at 90% of theoretical capacity.
12-46
Table 12-13.
Approximate screw conveyor capacities in bu/h for horizontal operation with dry grain
(Hutchinson, 1983).
AUGER
diameter, in
PULLEY
diameters, in
rev/min
CAPACITY PER
100 rev/min AT 90%
LOAD
60
60
60
60
NET CAPACITY
bu/n
4
2.5
2.5
2.5
2.5
-5
- 8
- 10
- 12
875
547
437
365
525
328
262
219
5
3
3
- 7
- 8
750
656
90
90
675
590
6
3
3.5
3.5
5
PTO
- 12
- 12
- 15
- 12*
438
510
429
263
625
240
240
240
240
240
1051
1224
1029
631
1500
8
3
- 12
3.4 - 15
5
- 12*
PTO
438
397
263
540
480
480
480
480
2102
1905
1262
2592
10
3
- 15
5
- 12*
PTO
350
263
320
1200
1200
1200
4200
3156
3840
12
3
- 15
5
- 12*
PTO
350
263
320
2000
2000
2000
7000
5260
6400
*Reducer Drive
Capacity decrease for angle of operation:
20% at 45° (unless pressure fed)
50% at 90° (unless pressure fed)
Capacity decrease for 25% moisture grain: 40%
Tables 12-14, 12-15, 12-16, and 12-17 are the results of the classic experimients of White et al., 1962. They show
the effect of angle of elevation, speed, diameter, exposure length, and grain type on capacity and power
requirement. These are the most often quoted tables for screw conveyor characteristics.
12-47
Table 12-14.
Auger
speed
rev/min
200
400
700
1,180
a
Performance data for a 4-inch nominal diameter screw conveyor handling shelled corn (bushel
weight: 56 pounds); moisture content 13.2 to 14.2 percent wet basis (White et al., 1962).
Length of
exposed helix
at intake
inches
6
12
18
24
bu/hr
140
150
150
150
0°
hp/10 fta
.11
.12
.13
.14
Angle of elevation
45°
bu/hr
hp/10 fta
110
.13
120
.15
120
.17
120
.18
bu/hr
40
60
70
80
90°
hp/10 fta
.10
.11
.12
.13
6
12
18
24
270
290
290
300
.23
.29
.33
.38
180
220
240
240
.25
.29
.32
.36
90
130
150
160
.19
.24
.26
.27
6
12
18
24
410
470
480
480
.33
.43
.51
.60
280
350
380
380
.40
.52
.64
.76
160
220
250
270
.29
.41
.47
.49
6
12
18
24
490
650
740
770
.41
.63
.85
1.08
320
460
530
560
.61
.81
1.01
1.21
200
310
360
380
.46
.67
.79
.88
Horsepower is that required at auger drive shaft. Power loss in drive train must be added to determine the total
power required for the conveyor.
12-48
Table 12-15.
Performance data for a 4-inch nominal diameter screw conveyor handling soybeans (bushel
weight - 54.5 to 56.0 pounds); moisture content 11.0 to 11.2 percent wet basis (White et al.,
1962).
Angle of elevation of screw
Auger
speed
rev/min
300
500
700
900
1,100
Intake
exposure
inches
6
12
18
24
bu/hr
210
215
220
220
0°
hp/10 fta
.15
.16
.21
.21
6
12
18
24
330
340
350
360
6
12
18
24
22.5°
bu/hr hp/10 fta
180
.21
190
.22
190
.25
190
.26
45°
bu/hr hp/10 fta
150
.22
160
.24
160
.26
170
.27
67.5°
bu/hr hp/10 fta
100
.17
140
.23
150
.25
150
.26
90°
bu/hr hp/10 fta
80
.17
110
.19
120
.21
130
.24
.23
.27
.35
.38
280
300
310
310
.31
.39
.45
.48
230
260
270
280
.34
.43
.47
.49
170
200
230
250
.32
.40
.43
.46
130
160
180
220
.27
.33
.35
.41
420
450
470
500
.28
.37
.49
.53
360
400
410
435
.41
.54
.63
.66
290
350
380
380
.45
.60
.66
.71
210
270
290
330
.41
.57
.60
.65
170
210
240
290
.37
.47
.49
.58
6
12
18
24
465
520
570
640
.33
.47
.62
.69
400
470
520
550
.49
.67
.81
.87
330
410
460
470
.55
.74
.85
.93
240
310
350
400
.51
.71
.77
.84
200
250
300
350
.45
.60
.63
.73
6
12
18
24
490
600
690
780
.38
.55
.77
.84
420
530
610
650
.56
.78
1.00
1.06
340
460
530
540
.64
.86
1.03
1.14
265
320
390
450
.60
.82
.92
1.01
220
280
340
400
.55
.71
.81
.92
aHorsepower
is that required at auger drive shaft. Horesepower loss in drive train must be added to determine the
total horsepower requirement of the conveyor.
12-49
Table 12-16.
Performance data for a 6-inch nominal diameter screw conveyor handling shelled corn (bushel
weight -54 to 56 pounds); moisture content 14.5 percent wet basis (White et al., 1962).
Angle of elevation of screw
Auger
speed
RPM
200
400
600
800
Intake
exposure
inches
6
12
18
24
bu/hr
590
590
620
630
6
12
18
24
970
1090
1170
1190
6
12
18
24
6
12
18
24
aHorsepower
0°
hp/10fta
.20
.28
.32
.44
22.5°
bu/hr
hp/10fta
520
.30
550
.41
570
.43
590
.50
bu/hr
370
500
510
550
45°
hp/10fta
.33
.44
.47
.55
67.5°
bu/hr
hp/10fta
280
.31
400
.44
430
.45
470
.54
.35
.56
.74
.97
850
1010
1070
1110
.52
.82
.92
1.13
650
850
940
1010
.60
.88
1.02
1.18
480
690
720
830
.57
.83
.92
1.07
380
520
560
660
.46
.70
.80
.92
1210
1510
1650
1700
.49
.84
1.17
1.47
1050
1400
1500
1570
.72
1.22
1.42
1.74
820
1160
1270
1440
.82
1.28
1.52
1.80
590
910
1010
1140
.77
1.16
1.42
1.60
490
740
800
920
.64
1.05
1.23
1.40
1320
1760
1990
2140
.58
1.07
1.57
1.95
1100
1660
1790
1910
.86
1.54
1.96
2.32
890
1370
1510
1740
.95
1.62
2.08
2.39
640
1080
1220
1360
.92
1.46
1.94
2.12
540
890
1000
1100
.77
1.32
1.64
1.89
bu/hr
220
280
310
350
90°
hp/10fta
.25
.32
.36
.40
is that required at auger drive shaft. Horsepower loss in drive train must be added to determine the
total horsepower requirement of the conveyor.
12-50
Table 12-17.
Performance data for a 6-inch nominal diameter screw conveyer handling soybeans (bushel
weight - 54 to 56 pounds); moisture content 11 to 12 percent wet basis (White et al., 1962).
Angle of elevation of screw
Auger
speed
rev/min
200
Intake
exposure
inches
6
12
18
24
400
600
800
0°
bu/hr hp/10 fta
490
.30
500
.40
520
.50
540
.60
22.5°
bu/hr hp/10 fta
410
.41
430
.53
500
.60
520
.67
bu/hr
320
360
440
470
45°
hp/10 fta
.41
.57
.66
.68
67.5°
bu/hr hp/10 fta
240
.38
290
.50
360
.60
390
.64
bu/hr
180
220
240
290
90°
hp/10 fta
.34
.40
.45
.52
6
12
18
24
880
990
1110
1180
.52
.84
.98
1.36
710
830
1030
1040
.71
1.14
1.18
1.62
570
690
880
900
.77
1.20
1.29
1.63
400
540
740
800
.70
1.04
1.23
1.54
310
390
460
560
.60
.79
.95
1.14
6
12
18
24
1080
1350
1620
1690
.68
1.20
1.45
2.13
890
1130
1510
1520
.96
1.61
1.74
2.52
700
930
1280
1320
1.07
1.71
1.94
2.51
510
710
1050
1100
1.00
1.48
1.88
2.32
390
500
660
790
.87
1.10
1.47
1.76
6
12
18
24
1180
1610
1980
2020
.78
1.51
1.93
2.93
960
1310
1840
1850
1.12
1.98
2.29
3.43
740
1080
1530
1640
1.28
2.10
2.54
3.48
550
820
1230
1320
1.22
1.84
2.44
3.24
420
640
810
1000
1.10
1.50
1.98
2.56
aHosepower
is that required at auger drive shaft. Horsepower loss in drive train must be added to determine the
total horsepower requirement of the conveyor.
Estimating capacity and power of inclined screw conveyors
Most inclined screw conveyor designs are based on tests of power and capacity since these parameters are very
difficult to estimate. A rational computational procedure will be presented here. This procedure can be used for
initial estimation of power and capacity. Because of the complexity of the problem, it is much less reliable than
procedures for any of the other conveyor types.
Assumptions
The procedure assumes a steel standard-pitch single helicoid conveyor loaded from a hopper or grain mass by two
diameters of exposed helicoid.
Procedure
1.
Compute conveyor size and speed using specifications from Table 12-9, along with Equation 12-25.
Estimate volumetric efficiency using Figure 12-25. This will require a trial and error procedure. If grain
moisture is over 17%, use half of the volumeteic efficiency predicted from Figure 12-25.
2.
Compute hpf and hpm using Equations 12-26 and 12-27. If grain moisture is over 17%, double Fm.
12-51
3.
Compute the lifting power, hpl, using Equation 12-29:
hpl 
where hpl
C
BD
h
Fl
4.
C (BD) (h) (Fl)
(33 000) (60)
(12-29)
= power to lift material, hp
= capacity, ft3/h
= bulk density as conveyed, lb/ft3
= lift height = L sin , ft
= approximate lift factor = 4 for moisture < or = 17%, or = 8 for moisture > 17%
Add power components:
hp = hpf + hpm + hpl
(12-30)
and apply drive efficiency factor.
Example 12-11.
Specify nominal size, speed, and power for a 30-ft-long screw conveyor to move 25% moisture corn at a 45degree incline at a rate of 500 bu/h.
1. Try a 6-in conveyor. Substituting into Equation 12-25:
Ct 
((5.13)2  (1.4)2 ) (5.13)N
 0.0568N
2200
(500 bu) (1.245 ft 3 ) h
ft 3
 10.373
h bu (60 min)
min
Assume volumetric efficiency at 0.5(0.62) = 0.31
10.375
 0.0568N
0.31
N = 589.2 rev/min.
This is below the 650 rev/min maximum speed, so it is acceptable.
2.
hpf 
(30) (589.2) (18)
 0.636 hp
(500 000)
hpm 
(500) (1.245) (30) (45) (0.8)
 0.672 hp
(1 000 000)
(500) (1.245) (45) (30) (sin 45) (8)
 2.40 hp
(33 000)
3.
hpl 
4.
hp = (0.636 + 0.672) 1.85 + 2.40 = 5.54 hp
12-52
A 6-in. conveyor turning at 589 rev/min and a power input of 5.54 hp is required. Table 12-10 estimates a 6-8in
conveyor turning at 389 rev/min and requiring 5.26 hp is required.
SAFETY CONSIDERATIONS
As was noted in Section 12.1.1, safety is an important consideration in grain conveyor designs. Since there is
some overlapping among conveyor types, they will be discussed in a separate section.
On-farm conveyors
Table 12-18 shows some statistics for farm machinery accidents. The statistics show that elevators are the most
dangerous machines on the farm, in terms of accidents per million hours of exposure. Also, not-fatal elevator
accidents are more severe than accidents with other farm machines, in terms of days lost per accident.
The statistics do not differentiate among types of conveyors, so screw conveyors, flight conveyors, and possibly
bucket conveyors are included. In the period from 1978 through 1982 farm screw conveyors outsold farm flight
conveyors by over 8 to 1 (Implement and Tractor, 1983). It can then be assumed that most elevators are screw
conveyors.
There are a number of ways to be injured by a portable screw conveyor on wheels (a transport auger):
1. The intake region presents a pinch point between the tube and the turning helicoid and a rotating
shaft for possible entanglement.
2. The auger tube can fall upon failure of the hydraulic or cable-actuated lift mechanism.
3. The entire machine can tip sideways.
4. The tube assembly can contact overhead electrical lines during transport.
5. Many screw conveyors are power take off (PTO) driven. Entanglement in the PTO shaft is, thus,
possible.
Table 12-18 Farm machinery accident statistics.
Tractor
Corn picker
Wagon
Baler
Combine
Elevator
Accident frequency per million man
hours use
Michigan
Ohio
8.4
7.4
48.6
62.3
71.9
51.0
106.4
---112.0
90.1
573.6
981.5
Average days lost per
non-fatal accident
58
22
76
5
209
340
References: Doss and Pfister, 1972 and National Safety Council, 1974.
The ASAE has established a tentative standard for auger conveying equipment. ASAE Tentative Standard ASAE
S361.1T (Safety for agricultural auger conveying equipment) is Appendix B. The purpose of the Standard is to
establish safety recommendations which will minimize the possibility of injury during normal operation of auger
conveying equipment used to convey agricultural materials on farms. The standard specifies intake guard
dimensions (hazard 1 above), winch and cable requirements (hazard 2), lateral stability requirements (hazard 3),
and PTO guarding (hazard 5).
12-53
Bucket conveyor safety considerations
In grain elevators, bucket conveyors are the most common known location of primary explosions (see Figure 1217). Friction in bucket conveyors ranks next to "cutting and welding" and "unknown" as an ignition source of
primary explosions in grain elevators (see Figure 12-28).
Johnston, 1979 describes this likely scenario of the start of a fire or explosion:
1.
2.
3.
4.
5.
Material stops leaving the conveyor and the belt and buckets plug and jam.
The drive motor increases its torque output and belt slippage begins.
At the slippage point, the belt rapidly heats up, begins to melt, and lubricates further slippage.
The belt begins to burn and spreads burning embers within conveyor.
Since grain elevator bucket conveyors routinely contain dust concentration exceeding the minimum
explosive concentration, explosion and/or fire can result.
The scenario can be avoided by a control system which can detect blockage conditions and shut down feeding
conveyors, and can detect belt slippage and shut down the conveyor when a certain level of slippage occurs.
Figure 12-27. Locations of primary explosions in grain elevators (Johnston, 1979).
12-54
Figure 12-28. Ignition sources of primary explosions in grain elevators (Johnston, 1979).
GRAIN BREAKAGE IN CONVEYORS
The general topic of grain breakage is discussed at greater length elsewhere. Breakage in specific mechanical
conveyors will be discussed here.
The importance of grain breakage during conveying differs with circumstances. Gentleness to grain is not a very
valuable characteristic for a conveyor carrying grain to a grinder. The breakage of any grain destined for
livestock feed (about 80% of Iowa corn) does not decrease its value directly. Lowered storability and handling
the fines may be problems with livestock feed.
Breakage of grain may result in lowered market value and lowered value as a feedstock for milling.
Grain breakage in various handling operations
Fiscus et al. 1971a and b carried out series of experiments with corn, soybeans, and wheat (all dry) to determine
breakage resulting from various operations.
Grain breakage in free-fall drop tests
Gravity conveyance can damage grain because of impact at the end of a fall. Fiscus et al. 1971a measured grain
velocities after discharge from 8- and 12-in. orifices (Figure 12-29). Velocities of dry corn, wheat, and soybeans
differed little and all data was pooled to compute the regression lines shown on the graph. The free fall line is the
velocity attained by a particle accelerated by gravity but not subject to air resistance. Since kernels within the
stream do not react with the air like individual kernels, the stream attains velocities higher than the terminal
velocity of individual kernels at about a 50-ft drop height. Kernel velocity exceeded zero at zero drop height due
to motion within the grain bin.
12-55
Figure 12-29. Grain velocity versus drop height (Fiscus et al., 1971a).
Breakage was measured after grain impacted upon grain in a bin. Breakage was the percent weight of particles
passing through 0.159 x 0.159-in. screen openings (corn) and through 0.158 x 0.5-in. screen openings for
soybeans. Wheat breakage was much lower and was not reported. The breakage relationships are shown in
Figure 12-30. Breakage is seen to be an exponential function of velocity.
12-56
Figure 12-30. Grain breakage versus velocity (Fiscus et al. 1971a)
Other tests showed that grain falling on grain is damaged less than grain falling on concrete. Note that breakage
is much worse for corn than for soybeans, that breakage is higher for lower moisture, and for lower grain
temperatures.
Many different devices and methods have been tried in effects to avoid high velocity impact after gravity
conveyance. Stephens and Foster, 1977 tried various flow retarders on a 50-m inclined grain spout carrying 11- to
19-% moisture corn at temperature of 4 to 11 C. Damage was the weight percent of fines passing through a 4.76mm (12/64-in) round hole screen. Table 12-19 shows results. The retro-air employed a 2.2-kW fan which forced
air up the tube against grain flow.
12-57
Table 12-19. Corn breakage per handling (Stephens and Foster, 1977)
Flow retarder
Retro-air
No retarder (control)
Spout retarder
Cushion box
Spout retarder and cushion box
Breakage increase
per handling, %
% of control
3.64
107
3.41
100
3.22
94
2.83
83
2.65
78
The spout retarder is a cone-shaped device installed near the end of the spout. Inside, a 5-L bucket fills with grain
and then continually spills over as grain continues to impact on its opening. The cushion box employs the same
principle, but grain makes a 45 degree direction change through the device. All devices except the retro air
reduced corn breakage to some extent.
Two other findings from this study are worth noting. Drying treatment had a greater effect on breakage than did
flow retarder action. Breakage for all tests averaged 5.87% per handling for corn dried in a batch dryer with 90 to
100 C air, 2.66% per handling for corn dried in a bin with 50 to 60 C air, and 0.92% per handling for corn dried in
the field. Also, breakage is approximately cumulative. About the same breakage will occur each time corn is
dropped.
Bucket conveyor breakage
In another series of tests, Fiscus 1971b measured breakage in a bucket conveyor. Breakage for wheat was defined
as the weight percent passing through 0.065 x 0.25-in screen openings. Corn and soybean breakage were
measured as described earlier. Table 12-20 shows the results.
Several points can be noted from the results:






Corn had by far the highest breakage, followed by soybeans, and then wheat.
Breakage went up with decreasing grain moisture.
Breakage went up with decreasing grain temperature.
There was no difference in breakage between the two bucket styles used.
Half full buckets increased breakage 0.2 points for corn, compared to full buckets. There was no
difference for other grains.
Feeding corn on the up leg side increased breakage 0.3 points compared to feeding on the down leg
side. There was no difference for other grains.
12-58
Table 12-20. Bucket elevator percent breakage (Fiscus et al. 1971b)
Grain
Moist, %
Temp, °F
Test wt, lb/bu
TEST CONDITION
Belt
Boot
Bucket
speed, feeding loading
fpm
method
650 Front
½ Full
650 Front
Full
940 Front
½ Full
940 Front
Full
650 Back
½ Full
650 Back
Full
940 Back
½ Full
940 Back
Full
650 Front
½ Full
650 Front
Full
940 Front
½ Full
940 Front
Full
650 Back
½ Full
650 Back
Full
940 Back
½ Full
940 Back
Full
13.3
43
54.3
Corn
12.7
15.1
85
28
54.4
54.2
Bucket
style
Nu-Hy
Nu-Hy
Nu-Hy
Nu-Hy
Nu-Hy
Nu-Hy
Nu-Hy
Nu-Hy
Link
Link
Link
Link
Link
Link
Link
Link
Average
14.8
84
53.6
Soybeans
10.8
12.6
58
43
57.8
57.9
Spring wheat
10.9
12.9
28
36
61.1
61.1
Winter
Wheat
11.5
48
63.5
0.11
0.11
0.17
0.15
0.11
0.11
0.12
0.33
0.15
0.17
0.18
0.22
0.15
0.16
0.18
0.19
0.16
0.13
0.13
0.12
0.13
0.11
0.12
0.10
0.20
0.11
0.14
0.10
0.08
0.13
0.10
0.18
0.20
0.13
Mean percent breakage
3.18
2.74
2.89
2.68
2.21
1.64
2.01
2.67
2.95
2.81
3.03
2.36
2.48
2.26
2.67
1.98
2.54
1.03
0.68
1.06
0.95
1.03
0.81
0.90
0.82
1.06
0.79
0.96
0.89
0.67
0.65
1.38
0.92
0.91
1.17
0.92
1.30
0.80
0.78
0.28
0.41
0.29
1.00
0.35
0.39
0.82
0.24
0.20
0.79
0.83
0.66
0.30
0.21
0.33
0.26
0.20
0.19
0.24
0.29
0.21
0.18
0.32
0.35
0.38
0.22
0.28
0.31
0.27
0.42
0.22
0.43
0.53
0.32
0.22
0.42
0.51
0.37
0.37
0.46
0.40
0.29
0.30
0.68
0.51
0.40
0.43
0.25
0.34
0.37
0.27
0.24
0.27
0.28
0.36
0.32
0.40
0.33
0.24
0.24
0.33
0.26
0.31
0.11
0.16
0.15
0.16
0.17
0.14
0.12
0.11
0.15
0.13
0.13
0.12
0.12
0.12
0.12
0.12
0.13
Grain breakage in screw conveyors
The screw conveyor has long had a reputation of being a major cause of grain damage. Grain damage can occur
by crushing between the helicoid and the tube wall, and by abrasion against the tube and helicoid surfaces.
Hall and Sands, 1970, conducted tests using multiple passes through a 12-ft-long nominal 6-in screw conveyor.
Inside tube diameter was 5.875 in. Helicoid diameter was 5 in. Corn was at 13% moisture. Fines were defined as
material which would pass through a 16/64-in square hole. Figure 12-31 shows results. Fines production is much
higher for partial auger loading and for higher drying temperatures, and is much greater at higher auger speeds.
12-59
Figure 12-31. Cumulative fines production in a screw conveyor (Hall and Sands, 1970).
12-60
PROBLEMS
12-1.
What percent increase in volume capacity could be expected if a 24-in., 225 ft/min. belt conveyor
designed for hulled rice is fully loaded with flaxseed?
#12.2.
Compute the load cross section of a 48-in., 20-degree troughed belt carrying a material with a surcharge
angle of 17 degrees.
12-3.
Specify belt width and speed needed to carry dry navy beans at 10 000 lb/min. (Use the smallest belt
possible).
12-4.
Design a 20-degree troughed belt conveyor to do the conveying job outlined in Example 12-1. Assume
pulley centers must be 1 ft below the loading point and 1 ft above the discharge point. Thus the top
pulley center is 39 ft above the bottom pulley center. Use the narrowest belt which will carry the
volume. Drive is double reduction. Specify conveyor length, width, belt speed, hp, hp x h/ton and
energy efficiency.
12.5.
A 48-in, 20-degree troughed belt conveyor is carrying corn down a 10-degree decline to a loading dock
fully loaded and at maximum speed. At what conveyor length will the required motor output be
theoretically zero?
12.6.
Assuming a motor efficiency of 75%, compute the power dissipated from the motor and from the drive
in Example 12-3. Express in kW.
#12-7
Trajectories of material discharged from a belt conveyor must be known for proper design of discharge
chutes. Write a computer program which will print coordinates of (or plot) the trajectory of top and
bottom particles discharged from the end of a horizontal belt conveyor. Load thickness is 100 mm.
Material leaves the belt at the initial point of tangency of the belt with the pulley (directly above pulley
center). Assume this point to have coordinates (0.0). Belt speed is 3 m/s. Neglect air resistance.
Follow the trajectory for 1 s in 0.2-s increments.
12-8.
A belt conveyor is to be designed for loading corn on ships. In the loading operation, any one of the 3
grains is to be conveyed at a rate not less than 3.5 x 106 lb/h up a slope. The belt should have the
volume capacity to carry the lightest grain at the specified rate, and the power necessary to carry the
heaviest grain at the belt's volume capacity. The loading point is 80 ft below the discharge point.
Specify belt slope, width, speed, and power required at motor output shaft.
#12-9
When material having no velocity component in the direction of conveyance is loaded on a belt conveyor,
power is required to accelerate the material up to belt speed. This power is not usually recovered. Compute
the power necessary to accelerate the load in problem 12-3.
12-10
Shelled corn is to be conveyed 1000 ft horizontally at a rate of 10,000 bu/h, by a 20-degree troughed
belt conveyor. (Assume the corn weighs 45 lb/ft3).
(a)
(b)
(c)
12-11.
Compute minimum belt size and speed necessary.
Compute power required by conveyor.
What power is required from the drive motor, assuming two speed reductions?
A double chain all steel flight conveyor is needed to carry dry barley (45 lb/ft3) at a rate of 300 000 lbs
per hour up along a 30-degree slope for a distance of 82 ft. The conveyor is to operate at maximum
allowable speed. Flight dimensions follow normal proportions. Assume chain and flights weight 10
lb/ft and the drive reduces power in two steps. Compute flight dimensions and power requirement
12-61
12-12
A standard dimension (w = s, h = 0.4w) all steel flight conveyor carries a grain having a zero angle of
repose and a bulk density of 720 kg/m3. Drive line speed is 60 m/min. Friction coefficient for grain on
metal is 0.3. Drive efficiency is 90%. Write a computer program which will print specific conveyor
energy per unit mass of grain per unit lift height (kJ/(kg x m)). What angle should the conveyor be
operated at for minimum specific conveyor energy?
12-13.
Design a conventional flight conveyor to do the conveying job outlined in Example 12-1. Assume
sprocket centers must be 1 ft below the loading point and 1 ft above the discharge point. Assume
conveyor is standard design, all steel, and operates at 200 ft/min. and 40 degree slope. Specify conveyor
width, flight height, hp, hp x h/ton, and energy efficiency.
12.14.
An en-masse conveyor is needed to convey soybeans at a rate of 800,000 pounds/hour 100 ft
horizontally. The conveyor is to be steel with UHMWP flights and chain wear plates. Select the
smallest conveyor size which will do the job. The conveyor is to be powered by an electric motor
through a double speed reduction. Specify conveyor size, conveyor speed, and motor hp required.
12-15.
A bucket conveyor having a 0.5-in-thick belt and a 5-in bucket projection is to run at critical speed with
a head pulley speed of 45 rev/min. Compute head pulley diameter and belt speed.
#12-16. A bucket conveyor is to operate at critical speed with a belt speed of 500 ft/min. Bucket projection is 6
in and belt thickness is 0.5 in. Compute head pulley diameter and speed.
#12-17. Compute centrifugal force, as a percent of kernel weight, for kernels at the inner wall, center, and outer
wall of the bucket of problem 12-15.
12-18.
A bucket conveyor is equipped with 10 x 5 buckets at a spacing of 10 in, and is lifting corn (45 lb/ft3) to
a height of 60 ft.
(a)
What should be the belt speed for a capacity of 2500 bu/h?
1. Answer using Figure 12-14.
2. Calculate answer.
(b)
12-19.
Calculate motor power required, assuming two speed reductions. Use calculated speed.
Design a bucket conveyor to do the job outlined in Example 12-1. Specify bucket size, belt speed,
bucket spacing, head pulley diameter, motor HP.
#12-20. Compute the tangent of the slope angle and length in units of diameters per revolution for the outer edge
of a standard-pitch helicoid when its axis is vertical.
12-21
Design a bucket conveyor to move dry corn a height of 40 feet at a rate of 1500 bushels/hour. Specify
bucket size, belt speed and bucket spacing. Compute motor hp requirement, assuming two speed
reducers. Compute energy per unit grain mass (hp•hour/ton), and energy efficiency (%).
12-62
12-22.
Dry soybeans are being conveyed in a 6-in, 60-ft-long, standard-pitch farm-type screw conveyor
operating at 650 r/min and set at a 45-degree angle. Intake exposure is 18 in. (Use tables 12-9 and 1217.)
Compute:
(a)
Power required.
(b)
Capacity, bu/h.
(c)
Percent of theoretical capacity.
12-23.
Using Figures 12-23 and 12-24, define a relationship between energy per unit mass per unit vertical
distance versus slope for 500 rev/min operation. Use W x h/kg x m as the energy unit. At what slope is
the conveyor most efficient?
12-24.
A horizontal (industrial) screw conveyor is needed to carry cottonseed flakes a distance of 22 ft at a rate
of 85,000 lb/h. Specify conveyor diameter, speed, and power required at conveyor input shaft.
12.25.
Estimate maximum possible capacity (bu/h) and power required for a 20-ft, 12-in standard-pitch vertical
farm-type screw conveyor handling dry corn.
12-26.
Design a screw conveyor to do the conveying job outlined in Example 12-1. Assume the conveyor
extends from 1 ft below the loading point to 1 ft above the discharge point at an angle of 45 degrees.
The drive is double reduction. Specify conveyor length, diameter, speed, power required, hp x h/ton,
and energy efficiency.
12-27.
Locate a portable farm screw conveyor (on wheels) on a farm or dealer's lot. Assume that you have
been asked, as part of a product liability case, to ascertain if this auger is in compliance with Sections
4.1, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.1, 5.3 of ASAE S361.2 (Safety for Agricultural Auger Conveying
Equipment). See Appendix B.
Identify auger completely (brand, size, model, serial number, age, location, etc.)
State date of examination.
Explain your procedure for determining compliance for each Section
State clearly your conclusions.
For lateral stability, do a static analysis assuming the auger is in transport position and is horizontal. For
the crank force test, stand on a bathroom scale while cranking and record force changes as you crank
slowly.
12-28.
Write an equation for corn breakage as a function of drop height for 13% moisture corn at 40 F falling
from an 8-in. orifice. Compute the expected breakage for 60 ft drop.
12-29.
The grain ladder concept has been proposed as a means of reducing corn breakage. With the grain
ladder, grain drops in 23-ft steps rather than dropping the total bin depth. Compare damage due to one
92-ft drop to damage in 4 23-ft drops.
12-30.
One million bushels per year of corn at 13% moisture is to be dropped 100 ft from an 8-inch orifice onto
pile (drop height stays constant). A grain ladder is to be built to allow corn to make multiple short drops
instead of one big drop. Each "rung" of the ladder has an annual ownership cost of $500. Each point of
breakage decreases corn value by 0.2c/bu. How many rungs should the ladder have to minimize total
cost? (0 rungs = 1 drop, 1 rung = 2 drops, etc)
12-63
12-31
Design a screw conveyor to move dry corn a height of 40 ft at a rate of 1500 bushels/hour. Use Table
12-16. Assume an intake exposure of 18 inches. Specify angle of elevation and rotational speed.
Compute motor hp requirement, assuming one speed reducer. Compute energy per unit grain mass
(hp•hour/ton) and energy efficiency (%).
12.32
An 8-inch auger is set up to convey corn at a 45° angle of elevation at 600 rev. per minute. Compute its
% of theoretical capacity with corn at 14% moisture and with corn at 25% moisture. Assume a 1.5-inch
diameter shaft size.
12-33
What standard electric motor horsepower size is needed to power a 6-in auger 25 feet long, handling dry
corn at a 45 degree angle of elevation? Repeat for wet corn.
#Relatively difficult.
12-64
REFERENCES
ASAE 1983a. Auger flighting design considerations. ASAE Engineering Practice: EP3899. American Society
of Agricultural Engineers, St. Joseph, MI.
ASAE 1983b. Terminology and specification definitions for agricultural aguer conveying equipment. ASAE
Standard ASAE S374. American Society of Agricultural Engineers, St. Joseph, MI.
ASAE 1983c. Safety for agricultural auger conveying equipment. ASAE Tentative Standard ASAE S361.1T.
American Society of Agricultural Engineers, St. Joseph, MI.
Bloome, P., S. Harp, J. Garton. 1978. Bucket elevators. OSU Extension Facts No. 1106. Oklahoma State
University, Stillwater, OK.
Buhler-Maig. 1983. Chain conveyor design and applications manual. Buhler-Maig, Minneapolis, MN.
CEMA 1979. Belt conveyors for bulk materials. Second edition. CBI Publishing Company, Inc., Boston, MA.
CEMA, 1980. Screw conveyors. CEMA Book No. 350. Conveyor Equipment Manufacturer's Association,
Washington, DC.
Des Moines Tribune 1972. Belt conveyor across northern Iowa. December 17, 1972.
Ditzenberg, D. 1980. Elevator design, spouting and distribution for small grains. Presented at Greater Iowa
Grain Elevator and Processing Society meeting, March 11, Boone, IA.
Doss, H.J. and R.G. Pfister. ca 1972. Farm machinery use study. Agricultural Engineering Dept., Michigan
State University, East Lansing, MI.
Fiscus, D.E., G.H. Foster, H.H. Kaufamann. 1971a. Grain stream velocity measurements. Trans. of ASAE
14(1):162-166.
Fiscus, D.E., G.H. Foster, H.H. Kaufmann. 1971b. Physical damage of grain caused by various handling
techniques. Trans. of ASAE, 14(3):480-485, 491.
Hall, G.E., L.D. Sands. 1970. Operating a screw conveyor with minimum damage to corn. Illinois Research
12(2):14-15.
Henderson, S.M., R.L. Perry. 1976. Agricultural process engineering. Third edition. AVI Publishing Co.,
Westport, CT.
Hudson, W.G. 1954. Conveyors and related equipment. Third edition.
John Wiley and Sons, Inc., New York.
Huss and Schlieper, Inc. 1981. Kleen Flo drag conveyors. Huss and Schlieper Inc., Decatur, IL.
Hutchinson Division, 1983. Hutchinson 1983 catalog. Hutchinson Division, Lear Siegler, Inc., Clay Center, KS.
Implement and Tractor. 1983. Shipments of farm machines and equipment. Implement and Tractor. 98(13):50.
12-65
Johnson, J.A. 1979. Grain elevator monitoring systems. In R.C. Gordon(ed.) A practical guide to elevator
design. National Grain and Feed Association, Washington, DC.
MWPS 1983. Structures and environment handbook. MWPS-1, Fifth edition, Midwest Plan Service, Ames, IA.
National Safety Council 1974. Accident facts. Nation Safety Council, Chicago, IL.
Pinches, H.E. 1958. Materials handling: farm production integrator. Agricultural Engineering 39(9):517.
Rexnord Inc. 1982. Rexnord catalog R80. Rexnord Inc. Milwaukee, WI.
Rboerts, A.W. and A.H. Willis. 1962.
Mechanical Engineers. 176(8):165-187
Performance of grain augers.
Proceedings of the Institution of
Sevart, J.B., B.L. Klausmeyer, J.E. Wray. 1984. Designing a safer grain auger inlet guard. ASAE Paper MCR84-196. American Society of Agricultural Engineers, St. Joseph, MI.
Stephens, L.E., G.H. Foster. 1977. Reducing damage to corn handled through gravity spouts. Trans. of ASAE.
20(2):367.
Thomas Conveyor Company. 1980. Bucket elevators catalog BE-980. Thomas Conveyor Company, Fort
Worth, TX.
Thomas Conveyor Company. 1981. Thomas screw conveyor engineering guide SC-581. Thomas Conveyor
Company, Fort Worth, TX.
Vierling, A., G.L. Sinha. 1960. Investigations into the process of conveying by vertical screw conveyor. Fordern
V. Heben (10(8):587-592 (in German). NIAE Translation No. 95, Journal of Agr. Eng. Res. 5(4):445-451.
White, G.M., L.A. Shaper, I.J. Ross, G.W. Isaacs. 1962. Performance characteristics of enclosed screw
conveyors handling shelled corn and soybeans, Research bulletin No. 740. Purdue University, Lafayette, IN.