CEMA Bucket Elevator Book
Chapter 1
General
Chapter Lead: D. Warren Knapp
Contacts and References
Name
Company
E-Mail
D. Warren Knapp
Screw Conveyor Corp
wknapp@screwconveyor.com
Chris Tarver
Maxi Lift
ctarver@maxilift.com
Jeff Gerhart
Martin Sprocket & Gear
jgerhart@martinsprocket.com
DRAFT HISTORY
Draft Number
1
2
Date
September 3, 2012
June 13, 2013
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Second Draft
June 13, 2013
GENERAL DESCRIPTION
Bucket elevators (legs) are the most efficient means of elevating free flowing material
such as grain, pelleted and soft ingredients, finished feeds, coal, aggregate. Except for
sticky materials which will not discharge completely from buckets.
There are two major types of bucket elevators; centrifugal discharge and gravity discharge
(continuous elevators). Both types use buckets or cup mounted on belt or chain. Bucket
elevators of the centrifugal discharge are normally used in the feed industry and most are
of belt type. However pellets and other friable materials may be best handled in
continuous bucket elevators that operate at low speeds. The continuous buckets are
discharged by gravity on the back of the preceding bucket while passing over the head
pulley, thus reducing breakage caused by the centrifugal force discharge of a centrifugal
elevator. Bucket elevators usually require the least amount of horsepower for vertical
conveying of any conveying system.
The bucket elevator has been in existence for many centuries. The ancestors of the
modern day bucket elevator first appeared at about 230 BC. These devices were
predominantly used for elevating water by the use of pots attached to an endless rope. It
is believed that the water used for the famous Hanging Gardens of Semiramus was
brought up to a height of 300 feet by this means. A remarkable achievement, having
regard to the fact, that modern elevators rarely work to heights greater than 150 - 200
feet. Since inception in 230 BC, the bucket elevator has gone through a period of
evolution. There was a flourish of activity in elevator design and patents between 1850
and 1930. Since that time there has been very little new work or mathematically
supported designs developed. There are few in depth texts on bucket elevator design
and/or proper application. This is true for all bucket elevators, not only the feed and
grain legs.
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As referenced above, there are two major types. The centrifugal and the continuous with
about 15 sub-types and numerous variations of each subtypes.
The centrifugal type bucket elevator is by far the most dominant in the feed industry. This
machine is the heart of most feed mills. It is rare to find a feedmill which has any fewer
than five bucket elevators (legs).
The statement that the bucket elevator is the heart of most feed production facilities may
seem prejudiced, in light of the fact this document is generated by the manufactures of
bucket elevators. It is not in way prejudice, just enthusiastic about the proper design of
bucket elevators.
As an example of the importance of the bucket elevator in the movement of materials the
following is offered for consideration.
Commonly, at least one bucket elevator is employed in each of the following areas:
receiving, grinding, mixing, pelleting, storage and load out. Commonly feedmill designers
and operators give a large amount of consideration to the selection of hammer mills,
mixers, pellet mills, liquid additions, and baggers. But, how much consideration is given to
the bucket elevator, the device supplying each of these machines with the required
material for continuous production or how they effect the material being elevated? The
most plant manager or owner is responsible for six basic areas: production, quality, cost,
safety, housekeeping, and employee relations. The bucket elevator is a key function in
each one of these areas. You will have poor production, low quality, increased cost, safety
problems, bad housekeeping, and unhappy employees if you have an improperly
designed or misapplied bucket elevator. Your production costs increases if the bucket
elevator continually requires maintenance or constantly causes damage to your finished
goods. You will have safety problems if your bucket elevator does not comply with all of
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the state, federal, and insurance safety requirements. You will have housekeeping
problems if your bucket elevator has improper connections and joints allow dusting which
is not only a safety but also a housekeeping problem. You may agree that the bucket
elevator influences these areas; but you may not believe a bucket elevator affects
employee relations. How many have seen a happy employee after they have just finished
digging out a plugged elevator?
It is a common belief that a bucket elevator (leg) is a simple machine. It is nothing more
than sheet metal housing with two pulleys and a piece of belt with some attached buckets.
This is not the true. The bucket elevator though simple in appearance is quite complex if
properly designed. There are a number of old “rules of thumb” and common beliefs in
designing a bucket elevator. Some are correct and some are incorrect. For the most part
the basic mathematics for these rules of thumb have been forgotten over the years.
Though these rules of thumb are considered to be proper for finial design they are not, at
best they are guidelines to start the design. Some of the more popular these rules of
thumb or opinion are:
1. A slow belt speed means less damage and a better discharge. Not true belt speed
alone is not a controlling factor. It is relevant factor only when considered in conjunction
with pulley diameter, bucket projection, bucket angle and the properties of the material
being conveyed.
2. A bucket elevator will operate the same for all grain and or products.
3. If you want more capacity just speed the leg up.
Designing a bucket elevator for application requires the following information:
a.
Type of material handled
b.
Capacity
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c.
Particle size (nominal) Max & Min & %
d.
Moisture content
e.
Hydroscopic
f.
Free flowing or abrasive
g.
Friable
h.
Fibrous
i.
Will it compact
j.
Temperature of the material
k.
Density and pounds per cubic foot
1.
Corrosive
m.
High fat or oil content
n.
Does the boot need to be self-cleaning
o.
How is the elevator being fed
p.
To what does the elevator feed
q.
What are the surrounding environmental conditions
r.
s.
Federal and local codes and regulations
Is there any special federal regulation such as FDA, which must be satisfied
Possibly, the best way to explain designing for applications is by example.
Selection of a Receiving Leg for an Up Grade or New Mill
First, the type of material to be elevated. It is common for this machine to encounter
corn, milo, barley, oats, wheat, corn screenings, pellets, dust, brewer's grain, malt hulls,
alfalfa pellets, wheat mids, bakery by-products, cottonseed hulls, beet pulp, soybean
meal, cottonseed meal, meat meal, corn and wheat meal, limestone, phosphates, salt,
and urea. In answering the above questions for the receiving leg, it becomes quite
obvious the particle sizes are all over the board. So is the moisture content, and some are
hydroscopic, free flowing, abrasive, fibrous, and some will compact. Most of the
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materials in will be at ambient temperatures. Density or pounds per cubic foot will vary
anywhere from 12 to 100 PCF. Corrosive? Yes. High fat and oil content? Yes. How is the
material being fed? Generally controlled by gravity and gate or conveyor. How is it
discharging? Usually to a spout or a series of spouts.
Whole grains are the predominant material handled, and should be the base for the
capacity. However, the leg should be reviewed for the capacity requirements of each of
the products it will be expected to handle. Since the bucket elevator is a volumetric
device, and elevates material based on cubic feet per hour, it cannot discriminate
between one hundred (100) pound material per cubic foot and 15 pounds per cubic foot
material.
The product weight variation will effect the selection of the receiving bucket elevator. If
a 50 TPH, full line feed plant, has a daily requirement for 325 tons of whole grains
consisting of corn, milo, barley, oats, and wheat, and a requirement for 24 tons per day of
cottonseed hulls, with the whole grain requirement being 38% of the total daily
requirements, and cottonseed hulls 3%.
325 tons of whole grain is equal to 14,444 cubic feet. 38% of 8 operating hours is 3, divide
the 14,444 by 3, the minimum capacity is 4,751 cubic feet per hour. Considering the
cottonseed hulls, at 12 pounds per cubic foot, a 24 ton per day requirement equals 4,000
cubic feet, 3% of 8 operating hours is .24 hour. To elevate 4,000 cubic feet of cottonseed
hulls in .24 hour the capacity is 16,000 cubic feet per hour. I have stated these two
examples to show the extremes of variation. As we all know, in any operation regardless
of the duration of operating hours, 100% efficiency is not achieved. Of course, it is
possible to extend the elevating and receiving hours to have minimum quantity of
ingredients in overhead storage. However, to maintain this quantity you must at least
replenish it at the rate it is being consumed. To make a true and exact determination of
the capacity required for a receiving unit, you must take into additional considerations
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such as percentage of ingredients that will be transferred through this unit, other uses for
this particular bucket elevator, and allotted time for the receiving operation. Over the
years, some general rules of thumb have been developed in sizing the receiving system
for feed mills. These are merely rules of thumb and should be used as guidelines. The
receiving elevator should have an average handle rate equal to 3 to 4 times your average
production rate. If you have 50 TPH production rate, the minimum size receiving unit
should have a nominal capacity of 150 to 200 tons per hour based on 40 cubic foot
material. Additionally, the nominal capacity of a bucket elevator should be considered to
be 75% of its theoretical maximum volume, which would equal a 13,333 CFH. As you see
this rule of thumb gets you close, but if our requirement for cottonseed hulls is correct
and receiving is limited to 8 hours then the capacity should be 16,000 CFH.
If the feed rate to the elevator does not have a variable control to limit the flow of
material the horsepower would be based on 16,000 CFH @ 100 PCF. On a 150’ discharge
height elevator this would mean a 200 HP drive. If you restrict the flow rate on minerals
to 50% of the grain products, it would require a 100 HP drive.
The capacity requirement for the mash elevator, since in the United States, most feed
mills operate with a batch mixer, is not always obvious. If in a production facility using a
single 5-ton horizontal ribbon mixer with drop bottom, having established a 5 minutes
cycle time, which provides 1/2 minute for scale discharge, 31/2 minutes for mixing, 1/4 of
a minute mixer discharge, and 3/4 of a minute contingency, with a surge beneath the
mixer having a capacity equal to one batch. The mash elevator must empty the surge
prior to the next discharge of the batch mixer. The minimum capacity for this machine
would be cycles per hour X tons per cycle or 60 tons, based on 35 #/cuft material. or 3500
cubic feet per hour. It is generally considered prudent to be able to empty the lower surge
in a time frame equal to the mixing cycle, or 3 1/2 minutes in this case. The bucket
elevator would have a minimum requirement of 4,900 cubic feet per hour. Thus, in
selecting a machine for this area, I would recommend a unit, which has a nominal
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operating capacity of 5,000 cubic feet per hour. The same logic should be applied to all
elevators through out the mill.
Thus far we have only determined that the operating capacity of the bucket elevator. We
have not defined any of the functional requirements of this machine. Each one of the
three areas, which we have discussed, has a different functional requirement. The
receiving leg, although quite cumbersome in sizing for capacity, can be designed similar to
that of a standard grain leg. This machine usually falls into the category of a high-speed
centrifugal discharge.
At this point one should discuss some of the design requirements which is often
overlooked by the user, and is not governed by any regulatory agency, and often not
considered by manufacturers.
The centrifugal bucket elevator has a minimum operating speed and a maximum
operating speed. The optimum operating speed will vary with respect to the variation in
product or commodity being handled. Basically the principal criteria of bucket unloading
or discharge is on the up leg side of a vertical bucket elevator, the load is in a straight
uniform motion and acted upon by a constant gravity force. This force is equal to mass
times gravity. As the bucket begins to move around the head pulley an additional
centrifugal force appears which is represented by the equation F=MVo2/R or CF=WV2/qR.
Where M is the mass of load in the bucket in kilograms, Vo is the velocity of the center of
gravity of the bucket load in meters per second, and R is the radius of rotation (distance
from the center of gravity of the bucket to the center of the pulley). As a bucket revolves
on the pulley, the resultant of the force of gravity and the centrifugal force varies in
magnitude and direction. When the resultant force has rotated out of the bucket plane
the sheer plane lies wholly inside the bucket indicating all remaining material in the
bucket is now subjected to a force that in time will eject it. The optimum speed for
discharge is affected by the weight of the material, the angle repose of the material, the
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coefficient of friction between the material and the bucket or cup, and the actual shape of
the bucket. The grain and feed organizations have for the most part, adopted a standard
bucket configuration originally introduced by B.I. Willer “Calumet” and K.I. Willis Style “CC”
The acceptance of this style bucket has established in the industry an average angle
between the base of the cup and the lip for a high speed grain or feed bucket of 30°.
Using the 30° average for a metal cup, the average angle repose of 28° 22 minutes,
(Department of Agriculture average for the angle repose (emptying) for various grains),
and a simplified formula, it is relatively easy to determine reasonable belt speeds for
grain bucket elevators. This formula is velocity in feet per minutes equals 88.56 times the
simplification and results in an answer near to the optimum operating speed obtained
from more rigorous calculations.
In addition to designing a bucket elevator to operate at the proper speed for the buckets
(cups) being used and the material handled, it is also necessary to take into consideration
the shape of the discharge hood or bonnet, and the discharge opening. All of these items
are crucial factors in the design of a bucket elevator. Proper consideration of all of these
items will result in a machine which operates efficiently, with little or no down legging or
back-legging, and minimal damage to the material.
Loading of the buckets is equally important as unloading. The loading of the buckets is
affected as greatly by the belt speed as the discharge. In the true interpretation of the
formulas and what effects the discharge is speed, it is not belt speed but angular velocity
of the centroid at the cup. The angular velocity is greater than the belt speed. Practices
employed earlier in the United States, used boot pulleys considerably smaller than the
head pulley, this has an adverse effect on the loading. If you have a belt speed of 573
FPM a 42" diameter head pulley, and a 24" diameter boot pulley, the angular velocity of
the boot pulley will be considerably larger than that of the head pulley. If 8" projected
buckets were used the velocity of the centroid at the head pulley would be 682 FPM,
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conversely, the velocity at the tail pulley would be 764 FPM. This differential in velocity
of the centroid has a drastic effect upon the loading of the buckets in the boot section. In
order for material to enter the cup while the buckets are in angular contact with the
pulley, it is important that the same consideration be given to the boot design as that of
the head. As should be obvious by now, the design of a bucket elevator is far from a
simple procedure and best left to companies or individuals who are well experienced in
this area and have a proven track record.
All of the information I have reviewed thus far can easily be altered by changing cups to a
European design, such as the low profile, or going to the continuous which I mentioned
earlier. The continuous bucket elevator is designed to handle friable material like pellets.
This unit uses a bucket with a V configuration, having a front lip angle of 40° nominal.
These buckets on close centers and the discharge is affected by the material sliding in a
trough created by the bucket proceeding. These machines generally operate at a belt
speed of approximately 200 FPM. There are no adverse centrifugal forces as required in
the discharging of a centrifugal elevator. The continuous bucket elevator allows a gentle
filling and discharge. It does require a greater distance between pulley centerline and the
throat of the discharge. The advantage of using a continuous discharge bucket elevator
versus the centrifugal is a decrease in breakage. The centrifugal elevator generates
velocities at discharge point of cup 42% greater than the belt speed.
USDA research data indicates that high-speed centrifugal elevator damage as much as 3.5%
per handling with an average of 1.1%. Friable pellets are subject to 1.1% to 3.5% breakage
as a result of handling in a centrifugal bucket elevator. In a 5O TPH facility, this is the
equivalent to .6 to 1.8 tons per hour. If this can be reduced by the proper selection of a
bucket elevator by 50%, a savings of 600 to 1750 tons per year based on 8 hour a day
operation, 5 days a week, 50 weeks a year. Over 10 years this becomes a large cost of
production.
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The continuous bucket elevator offers possibilities of greatly reducing the breakage that
would be normally encountered with a centrifugal elevator normally used as a pellet leg.
The advantage of a continuous leg is slower belt speed; it does not discharge by the
centrifugal force and therefore, applies less damage to the product being handled. The
disadvantage is higher initial cost. This design has been used for many years in industrial
application handling materials considered to be friable and require extreme delicate
handling. You may want to consider this type of elevator in the future.
Today everyone is aware of the state, federal, and insurance requirements for safety in
the work place. These regulatory agencies have established safety requirements of
bucket elevators used in the feed and grain industry. Two of these agencies are OSHA and
NFPA. In reading documents from these agencies you will encounter to phrases “it shall
be” and “should be”. “It shall be” is telling you to do it that way. “It should be” is tell you
that it is highly recommended that you do it that way. There is a fine line difference
between these two agencies. If you state or local codes has adopted the OSHA codes then
it is a law to which you must comply. NFPA makes recommendations only and have no
bearing as to enforcement or law unless the NFPA standard has been adopted by
reference or by transcription. Since most states have adopted the federal OSHA codes I
will address this topic for that position and limited to bucket elevators in grain handling
facilities. This is covered in OSHA Standards for General Industry (29 CFR PART 1910)
section 1910.272
1.
Elevator legs must be of dust tight construction of noncombustible material.
2.
Exterior legs when used shall be provided with an explosion relieve panel
constructed in the vertical casing where maximum possible explosion relieve area in the
head.
3.
Elevator legs or portions of elevator legs which are located inside shall have the
maximal practical explosion relieve area through the roof directly to the outside. (Small
exterior legs that are used as part of the processing equipment are exempt).
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Boot section shall be provided with adequate doors for clean out of the entire
boot and for inspection of the boot pulley and leg belt.
5.
The use of plastic as lining material shall be limited to impact points and wear
surfaces only.
6.
Inspection doors shall be provided in the head section to allow complete
inspection of head pulley, lagging, the leg belt, and a discharge throat of the bucket
elevator.
7.
Leg head section between the upper and down casings shall be hoppered at an
angle of not less than 45°.
8.
The leg shall be driven by an individual motor and drive train capable of handling
the full rated capacity of the elevator without overloading. (Multiple legs on a line shaft
are exempt).
9.
Elevator legs shall be provided with an automatic mechanical or electro
mechanical device which cuts off the power to the motor and activates an alarm in the
event the belt slows down. Feed to the elevator leg shall be stopped or diverted. (Again
small auxiliary legs are exempt).
10.
The elevator leg shall be designed to maximize traction between pulleys and the
belt.
11.
All spouts intended to receive grain or dry ingredients discharge directly from the
elevator shell, be designed and installed to handle the full rated elevating capacity of the
largest elevator leg feeding such spout.
12.
All bins or other receptacles which are fed by the elevator leg which are not
designed with automatic overflow systems, shall be equipped with devices to shut down
equipment or with high level indication devices with visual or audible alarms.
13.
Where drive assemblies involve the use of drives, they shall be static conducting.
14.
Where drive belt is used, the drive train shall use the design with sufficient service
factor to stall the drive without slipping.
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Roller or ball bearings shall be used and they shall be located outside of the
machinery enclosures where bearings will be less exposed to dust and where they are
more accessible for inspection.
16.
Engines, motors, driven equipment used in confined Class II Group G Division 2
Operating areas shall be equipped with safety devices to reduce potential fire and
electrical shock hazard. Federal Register December 1987 Rules and Regulations 49625
Section 4910.272 Grain Handling Facilities.
17.
Inside bucket elevators means an elevator that has more than 20% of the total leg
height above grade or ground level inside a grain elevator structure.
18.
Bucket elevators with casings that have an outside and pass through the roof of
rail or truck dump sheds with remainder of the leg outside the grain are not considered
inside legs.
19.
Bucket elevators shall not be jogged to free a choked leg.
20.
All belts purchased after March 30, 1988 shall be conductive; such belts shall have
a surface electrical resistance not to exceed 300 Megaohms.
21.
Not later than April 1, 1991 all bucket elevators shall be equipped with means of
access to the head pulley section to allow inspection of the head pulley, belt, legging,
discharge throat.
22.
The boot section shall be provided with a means of access for clean out of the boot,
for inspection of pulley and belts.
23.
Not later than April 1, 1991 the employer shall mount bearings externally to leg
casings or provide vibration monitoring temperature monitoring or other means to
monitor the conditions of bearings mounted inside or practically inside the leg casing.
24.
Not later than April 1, 1991 the employer shall equip bucket elevators with a
motion detection device which will shut down the bucket elevator when the belt speed is
reduced by no more than 20% of the normal operating speed.
25.
Not later than April 1, 1991 the employer shall equip the bucket elevators with a
belt alignment monitoring device which will initiate an alarm to employees when the belt
is not tracking, provide a means to keep the belt tracking properly such as a system that
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provides constant alignment adjustment and the above information does not relate to
facilities having a permanent storage capacity of less than 1 million bushels provide the
daily visual inspection is made and the bucket movement and tracking of the belt.
26.
The above applicable requirements are not enforced on operational fire and
explosion suppression systems capable of protecting at least the head and boot section of
the bucket elevator or the bucket elevators which are equipped with pneumatic or other
dust control methods that keep dust contamination in the elevator at least 25% below the
lower explosion limit at all times during operation.
In case you are considering the installation of dust control systems to avoid any of these
particular safety requirements, testing has indicated that 55 grams per cubic meter
or .055 ounces per cubic foot is the minimum concentration required for an explosion of
mixed grain dust. If the conditions are ideal, it could be as low as 20 grams per cubic
meter or .02 ounces per cubic foot. The interpretation of the new regulation would be the
dust must be less than .75 x .02 oz. per cubic foot or .015 oz/cuft. The application of dust
control to a bucket elevator must conform to the American Standards of Industrial
Ventilation and should be in compliance with such publications as Dust Control for Grain
Elevators, National Grain and Feed Association, as well as several other applicable
references.
The above information will be defined in greater detail in the following chaprers.
Reference Text
NFPA 61B
NFPA 61C
NFPA 61D
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NFPA 69
Prevention of Grain Elevator and Mill Explosions (NRC)
Feed Manufacturing Technology I, II, III
Conveying Machines, MIR Publishers Moscow
ASAE Paper No. 69-840, 69-853
GEAPS, March 11, 1980
Link Belt
W. D. Sweet