CHAPTER ONE
1.0
INTRODUCTION
1.1
BACKGROUND OF STUDY
Most food substances have its origin in plants, although Some foods are obtained
directly from plants, and others from animal. But even animals that are used as food
sources are raised by feeding them food which are derived from plants. Grains have
been cultivated for thousands of years and they’ve been a major component of the
human diet. Additionally, any food made from wheat, rice, oats, cornmeal, barley or
another cereal is a grain product. These refined grain products are used in the making
of bread, pasta, oatmeal, breakfast cereals, tortillas, and grits. Cereal grain is a staple
food that provides more food energy worldwide than any other type of crop. Corn
(maize), wheat, and rice – in all of their varieties – account for 87% of all grain
production worldwide "Production STAT". 2008. The various types of grains
include the following including wheat, oats, rice, corn (maize), barley, sorghum, rye,
and millet.
Grains are defined as a small, hard, dry seeds, with or without its fruit layers,
harvested for human or animal consumption. Babcock, P. G., ed. 1976. The two main
types of commercial grain crops are cereals such as wheat and rice, and legumes
such as beans and soybeans. After being harvested, dry grains are more durable than
other staple foods such as starchy fruits (plantains, breadfruit, etc.) and tubers (sweet
potatoes, cassava, and more). Hence this durability has made grains well suited to
industrial agriculture, since they can be mechanically harvested, transported by rail
or ship, stored for long periods in silos, and milled for flour.
1
Why process grains?
In layman’s language, processing refers to the act of converting any material from
one form to another. In agriculture, crop processing machinery is used to transform
grains and other produce from their raw form to the refined and edible form. Crop
processing also aims at preparing crops for convenient transportation, storage, and
also for market and feeding to livestock. Based on latest technologies, these
machines not only reduce time of operation but also saves the labor cost. It has been
observed that grains tend to last from season to season, hence we can’t digest them
raw. Grains must be flaked, cracked, puffed, popped or grounded before being
consumed. And with the usage of mechanical devices in crop processing it reduces
the quantity of the wastage to a greater degree.
There are various reasons why grains need to be processed. One of which is that it
adds value to the grain which results in the development of new products or the
improvement of existing ones. Processing allows grain to be mixed with
supplements, thereby increasing its digestibility, and affects its palatability,
digestion and passage rates. Hence there is a high demand for grains to processed.
Also in a developing country like Nigeria where most raw materials and finished
flour product is being imported due to the unavailability of refined grained products,
this lead to inflation of the price of similar food product that depends strictly on
refined grain produce. hence the need for such grains to be cultivated and processed
locally so as to reduce the cost food products.
One of the major type of processed grain that is widely used in our present society
is flour. Flour is a powder made by grinding uncooked cereal grains or other seeds
or roots (like cassava). It is the main ingredient of bread, which is a staple food for
many cultures, making the availability of adequate supplies of flour a major
2
economic and political issue at various times throughout history. Wheat flour is one
of the most important ingredients in Nigeria and other various North African
cultures, it is the defining ingredient in their styles of breads and pastries.
Nigeria is an important market for grains with a big population. Its government is
moving to achieve greater levels of self-sufficiency. Nigeria is a huge export market
for wheat with a very high demand for wheat flour for the production of bread,
noodles, pasta, crackers and biscuits (cookies) etc. Hence, the processing of grains
is one of the most important processing activity that needs to be carried out in the
agricultural sector of Nigeria. This will in turn help to boost the nation economy and
serves as an avenue for job creation. This will in turn influence the prices of some
processed food which requires grains as one of its major constituent.
1.2
STATEMENT OF PROBLEM
In Nigeria today despite the giant strides in modernization other areas still remain
quite backwards in its production and food processing industries. There’s a high
demand for the supply of wheat-based food, the cassava flour import has increased
over the years and with present state of the economy there is a steady rise in cost of
such refined grain products. Hence this has resulted to the emerging need for these
refined grain product such as flour to be produced locally.
There are various traditional or indigenous way of processing grains in our rural
areas, one of which is, by pounding the dried grains in a mortar with a pestle and
sieving it with a screen, but this can no longer meet the demand for the refined grain
flour. Although there have been some mechanized methods adopted by small scale
farmer and food processing unit, but this method has proven to be time and energy
consuming and does not result in high quality grade produce and are bulky and
therefore avoided by the indigenous manufacturers. Some of these machines only
3
provide on function, either grinding or sieving, therefore are not feasible, since some
part of the process still needs to be carried out manually.
Also, the existing mills such as the attrition mill, the hammer mill used by some
industries show some inefficiency. Such inefficiencies are:
i.
Inability to produce uniform grind of the refined grain flour.
ii.
Time taken to crush material to the size of the screen as in the hammer
mill.
iii.
Contamination of refined grain flour due to multi-purpose nature of the
mill, particularly in non-specialized production processes
To solve this problem a machine is needed that can process the major function of
grinding, sieving and storage. It has to be affordable and reasonably small in size, to
allow use by average indigenous farmers in grain processing. Without these,
unrefined grains will have little value in the commodity market.
1.3
OBJECTIVE OF STUDY
The main objective of this project is to design and fabricate a modified milling
machine which combines both impact and shearing milling action coupled with a
pneumatic conveying and clarifying action. The combined action is intended to lead
to efficient milling of grains into fine powder. Unlike the normal hammer mill, it
does not use a screen classifier; rather it employs air classifier in which the fine
product is carried in the air-stream through the blower’s chamber. Also, less time is
required for pulverization and due to the air-tight nature, dust spillage is minimized.
The air circulating in the machine helps to cool the processed flour which improves
the Quality of the flour produced.
4
This project has the following specific objectives
i.
To develop a compact and portable in size allowing for easy transportation
and less installation space.
ii.
To design a mill pulveriser, which will be cost effective, and reasonably
affordably.
iii.
To fabricate a mill which will be easy to operate and maintain incorporated
to vibration and noise.
iv.
To design a pulveriser which will reduce the time of production and easy
to operate and maintain.
v.
To develop a fully integrated machine which will be able to pulverize
various grains and transport them to a storing unit for further processing
and packaging.
1.4
JUSTIFICATION OF STUDY
Engineering and technological advancement in our society has helped improve the
standard of living in our present day world. Grain processing plays a key role in the
socio-economic development of most west African countries. In Nigeria refined
grains flour are used for the production of bread, noodles, pasta, crackers and biscuits
(cookies). This has contributed to Nigeria’s wheat market which was estimated at
just under $1 billion in U.S. exports as of 2010. Also the Demand for other grains
such as corn, sorghum, millet is also very high but, there is a Production deficit of
processed grains locally which results to the importation of various refined grains.
However, Nigeria has a potential to increase its production through the application
of improved processing methods and better marketing (Gourichon H, 2013).
5
In most food processing industries, Conventional hammer mills that are extensively
employed in the processing of solid minerals and grains suffers from a number of
weaknesses that greatly hamper their productivity, efficiency and effectiveness.
These weaknesses include the following
i.
The conventional hammer mill cannot produce materials whose particle
size is less than 400µm. For the most commonly processed food minerals
like grains, tubers, cassava, etc. The particle sizes produced are relatively
large and they cannot be directly used for processing grains as flour to
make bread, biscuit and foo-foo for local consumption [Beintema and
Stads, 2004], [Eyo, 2008].
ii.
The fineness of the particles produced depend on the hole size of the screen
sieve employed. Large particles can block the holes of the sieve screen
thereby, reducing the output of the hammer mill.
iii.
To maintain the output, the screen sieves are continuously changed. Hence,
it requires the acquisition of a lot of expensive accessories which cannot
be produced locally.
iv.
Excessive dust particles are usually released into the atmosphere where
hammer mills are operating. This constitutes a health hazard for the human
operators and environmental pollution for the surrounding plants, animals
and human communities.
The defects and shortcoming of currently used hammer mills have meant that most
hammer mill operators and owners in Nigeria are running their business at marginal
profit levels. This is because virtually all the hammer mills being utilized are of old
designs and pulverization is not achieved in these systems.
6
1.5
SCOPE OF STUDY
This project will detail the process of manufacturing an integrated mill pulveriser
for grain processing as carried out by the students involved in the project work. It
will consider the steps of manufacturing and possible limitations and draw backs
that may arise as a result of the production and manufacturing processes. It will also
consider the design that should be incorporated into the design of the blower so as
to obtain maximum efficiency during pulverization. The mill pulveriser is basically
used for agricultural application and other various area of manufacturing where the
need is of high importance.
1.6
LIMITATION OF STUDY
The main constraint in this project can be said to be the availability of necessary
funding. Due to the fact, that this is a manufacturing project, the procurement of raw
materials, metal sheets, motor, suction blades and other components necessary for
the completion of the machine requires considerable funds.
7
CHAPTER TWO
2.0
2.1.
LITERATURE REVIEW
A REVIEW ON GRAIN PROCESSING
The use of grinding machine is one of the oldest and simplest methods of processing
agricultural raw material alternative to the traditional methods of grain processing
using stone, mortar and pestle. However, in considering the food processing
industry, it has been characterised by a labour and production output shortage due to
the lack of support by the government and the ideology on the pursuit of white collar
jobs by most Nigerian graduates. Nigeria now due to the failing of its agricultural
sector now depends on mostly imported refined grains to meet up with the high
demand and shortage of processed food. Although there have been recent
developments which has led to the Mechanization of most field activities in
agriculture, and to a certain extent it has overcome some basic challenges faced by
small scale farmers and local food processing industries. The advancement of
science and technology reduces complexities from many agricultural processes.
Modern technologies in area of irrigation, plantation, harvesting, has made the entire
agricultural cycle more economical and easier than ever. This led to the fabrication
of various machineries such as feed mill, grain dryers, rice huller, winnowing
machine, threshing machine etc. These machines are co Mechanization was not only
used to replace human labour but also increased productivity and the areas covered.
Over the years there have been various modification made on theses equipment so
as to increase its output efficiency and specialization. Thus the development of a
mill pulveriser. This system consists of a conventional hammer mill integrated with
a blower and a sieve. Which aids the pulverization by making only fine grounded
grain particles is being covey through its passage unit and stored.
8
A mill pulveriser is a modification of a hammer mill that utilizes air flow to separate
various particles. The development of a mill pulveriser incorporates the design of a
hammer mill and a blower. A hammer mill is a type of crusher, which can be used
for grinding rock, forage, grains or other large size particles into smaller pieces by
the repeated blows of little hammers. It is designed to convert larger pieces of
material into smaller particle sizes.
2.2
HISTORY OF HAMMER MILL
The invention of hammer mill dates back to the 4 th century around Zhao dynasty
period in china (1050 BC -221 BC). The invention, involved conversion of rotary
water wheel energy to linear trip hammer energy. The trip hammer evolved out of
the use of mortar and pestle which in turn gave rise to the tilt-hammer and then trip
hammer device. This device was used in the pounding and polishing of grains, which
help reduce the labour of pounding manually with hands and arms. In Chinese
mythology Fu Hsi was said to have invented the pestle and mortar, which is so
useful, and later on it was cleverly improved in such a way that the whole weight of
the body could be used for treading on the tilt-hammer, thus increasing the efficiency
ten times. Afterwards the power of animals donkeys, mules, oxen, and horses was
applied by means of machinery, and water-power too used for pounding, so that the
benefit was increased a hundredfold. (Caves Books Ltd 1986).
Although Chinese trip hammers in China were sometimes powered by the more
efficient vertical-set waterwheel, the Chinese often employed the horizontal-set
waterwheel in operating trip hammers, along with recumbent hammers. The
recumbent hammer was found in Chinese illustrations by 1313 AD, with the
publishing of Wang Zhen's Nong Shu book on ancient and contemporary (medieval)
metallurgy in China. There were also illustrations of trip hammers in an
encyclopedia of 1637, written by Song Yingxing (1587–1666).
9
This gave rise to future designs which has evolved over the years. A patent was first
issued in 1830 for a machine with a wooden box containing a cylindrical drum,
revolving at 350 revolutions per minute, on which hammers were attached. The
machine was designed to shatter any rock fed into the box. This machine never went
into commercial production, but is considered as the forerunner to the hammer mill.
The hammer mill has gone on to become the most widely used crusher utilizing highvelocity impacts to break rocks.
However, while initially designed to crush rocks, the hammer mill was adapted for
grinding grain for livestock feed. It was discovered that many types of forage benefit
in nutritive value after being broken down.
The Gehl company produced the first grain grinding hammer mill in the 1920s. It
dominated the market for 30 years, during which time it also developed a portable
truck mounted mill.
Developing a Screenless Hammer Mill
In 1990, Carl Bielenberg of Appropriate Technology International (ATI) began
developing a screenless hammer mill. His prototype separated flour from larger
particles through an opening in the circumference of the grinding chamber. Flour
passed through the opposite side of the rotating blades while the larger pieces
continued inside the chamber.
Initial tests produced a larger, courser material than conventional hammer mills. He
developed a series of improvements, but was incapable of designing a machine that
could perform to the standards of conventional mills. He presented his machine to a
series of MIT students in hopes that they could produce a more successful machine.
An MIT student named Amy Smith headed the project. Her redesigned mill was
capable of running continuously without clogging. It was also capable of being
10
manufactured in a small village workshop. This second aspect was extremely
important to Smith. After discovering the failings of conventional hammer mills, she
was determined to develop a machine that would be useful for third world countries.
Many African women used the hammer mill to grind grain, but its screen was prone
to breaking. Screens cannot be produced locally and are expensive to replace. So
Smith dedicated her research to “the African women who cannot afford the 5 cents
it takes to mill a kilogram of flour and thus spend hours performing the backbreaking labour necessary to prepare food for their families. Hence the development
of screenless hammer mill also known as a mill pulveriser
2.3 What is a Mill?
A mill is a device that breaks solid materials into smaller pieces by grinding,
crushing, or cutting. Such comminution is an important unit operation in many
processes. There are many different types of mills and many types of materials
processed in them. Historically mills were powered by hand (e.g., via a hand crank),
working animal (e.g., horse mill), wind (windmill) or water (watermill). Today they
are usually powered by electricity. The grinding of solid matters occurs under
exposure of mechanical forces that trench the structure by overcoming of the interior
bonding forces. After the grinding the state of the solid is changed: the grain size,
the grain size disposition and the grain shape.
Milling could also be referred to the process of breaking down, separating, sizing,
or classifying aggregate material. For instance, rock crushing or grinding to produce
uniform aggregate size for construction purposes, or separation of rock, soil or
aggregate material for the purposes of structural fill or land reclamation activities.
11
Grinding may serve the following purposes in engineering:
i.
increase of the surface area of a solid
ii.
manufacturing of a solid with a desired grain size
iii.
pulping of resources.
For the purpose of our study we will be considering majorly mill pulveriser. Which
is the most effect type of mill for achieving high quality refined grains.
A conventional mill pulveriser is a device consisting of a rotating head with freeswinging hammers, which reduce rock, grains or similarly hard objects to a
predetermined size through a perforated screen [Sci-Tech Dictionary, 2003].
A mill pulveriser commonly known as a screenless hammer mill is like a regular
hammer mills, is used to pound grain. However, rather than a screen, it uses air flow
to separate small particles from larger ones. Conventional hammer mills in poor and
remote areas, such as many parts of Africa, suffer from the problem that screens
break easily, and cannot be easily bought, made or repaired. Thus regular hammer
mills break down and fall into disuse. The screenless hammer mill uses air flow to
separate small particles from larger ones, rather than a screen, and is thus more
reliable. The screenless hammer mill is claimed to be 25% cheaper and much more
energy efficient than regular hammer mills, as well as more reliable.
Hammer mills pulveriser are widely utilized in the agricultural, wood, mining and
chemical industries. The vast majority of Nigerians live in the rural areas
[Okpala,1990] and are predominantly engaged in agriculture[Anifowoshe,1990].
Nigeria is blessed with equatorial [Morgan and Moss, 1965], tropical [Clayton,
1958], guinea [Jones, 1963], sudan [Olajire, 1991] and sahel [Pande, et al, 1993]
climatic zones; thus making her suitable for the profitable cultivation and production
12
of a wide variety of grains [FAOSTAT,2004]. The farmers rely on ancient and
antiquated methods that are inefficient for storing the grains and processing [Biewar,
1990], [Igbeka and Olumeko, 1996], [Adejumo and Raji, 2007], and thus lead to
large storage losses due to rodents, damp, fungi and natural decay [Agboola, 1992],
[Agridem, 1995]. Furthermore, the grains and other similar farm products in their
unprocessed states are bulky, difficult to transport and fetch very low prices in the
market [Dixon, et al, 2001], [Taylor, et al, 2006]. This is a major cause of poverty
amongst rural farmers [Killick, 1990], [Umoh, 2003] that encourages rural-urban
migration [UNEP, 2006]. Thus, to guarantee access to food [Simon, et al, 2003],
[Sanchez, et al, 2005], reduce rural-urban migration and encourage sustainable
development [Barber, 2003], [Altieri, 2004], the processing of agricultural products
like grains or solid minerals like clays and feldspars into more valuable products by
the use of hammer mills must be encouraged and fostered.
GRAIN PROCESSING EQUIPMENTS
In many countries worldwide, grain processing is of economic importance,
producing flour and cereal from the refined grain. In addition, with success small
farming and micro food processing unit has evolved over the years in developing
countries and utilises the use of some basic equipment’s. Grain processing
equipment’s used in refining grains may be classified by speed, as follows
i.
Low Speed
ii.
Medium Speed
iii.
High Speed
13
Low Speed mill
A low speed mill utilizes the principle of impact and attraction. An example of a low
speed mill is the Ball and tube mills. A ball mill is a pulveriser that consists of a
horizontal rotating cylinder, up to three diameters in length, containing a charge of
tumbling or cascading steel balls, pebbles, or rods. A tube mill is a revolving cylinder
of up to five diameters in length used for fine pulverization of ore, rock, and other
such materials; the material, mixed with water, is fed into the chamber from one end,
and passes out the other end as a slurry.
Both types of mill include liners that protect the cylindrical structure of the mill from
wear. Thus the main wear parts in these mills are the balls themselves, and the liners.
The balls are simply "consumed" by the wear process and must be re-stocked,
whereas the liners must be periodically replaced.
The ball and tube mills are low-speed machines that grind the grain with steel balls
in a rotating horizontal cylinder. Due to its shape, it is called a tube mill and due to
use of grinding balls for crushing, it is called a ball mill, or both terms as a ball tube
mill. The grinding in the ball and tube mill is produced by the rotating quantity of
steel balls by their fall and lift due to tube rotation. The ball charge may occupy one
third to half of the total internal volume of the shell. The significant feature
incorporated in the mills design is its double end operation, each end catering to one
elevation of a boiler. The system facilitated entry of raw grains and exit of pulverized
fuel from same end simultaneously. This helps in reducing the number of
installations per unit.
14
Medium Speed mill
This type of mill consists of two types of rings separated by a series of large balls,
like a thrust bearing. The lower ring rotates, while the upper ring presses down on
the balls via a set of spring and adjuster assemblies, or pressurised rams. The material
to be pulverized is introduced into the centre or side of the pulveriser (depending on
the design). As the lower ring rotates, the balls to orbit between the upper and lower
rings, and balls roll over the bed of grain on the lower ring. The pulverized material
is carried out of the mill by the flow of air moving through it. The size of the
pulverized particles released from the grinding section of the mill is determined by
a classifier separator. If the refined grain is fine enough to be picked up by the air, it
is carried through the classifier. Coarser particles return to be further pulverized.
Vertical spindle roller mill
Similar to the ring and ball mill, the vertical spindle roller mill uses large "tires" to
crush the grain seeds. These mills are usually found mostly in large food processing
plants. Raw grains and minerals are fed through a central feed pipe to the grinding
table where it flows outwardly by centrifugal action and is ground between the
rollers and table. Hot primary air for drying and grain transport enters the wind box
plenum underneath the grinding table and flows upward through a swirl ring having
multiple sloped nozzles surrounding the grinding table. The air mixes with and dries
grains in the grinding zone and carries pulverized grain particles upward into a
classifier.
Fine pulverized grains exit the outlet section through multiple discharge pipes
leading to the storage unit, while oversized grain particles are rejected and returned
to the grinding zone for further grinding. Pyrites and extraneous dense impurity
material fall through the nozzle ring and are ploughed, by scraper blades attached to
15
the grinding table, into the pyrites chamber to be removed. Mechanically, the vertical
roller mill is categorized as an applied force mill. There are three grinding roller
wheel assemblies in the mill grinding section, which are mounted on a loading frame
via pivot point. The fixed-axis roller in each roller wheel assembly rotates on a
segmentally-lined grinding table that is supported and driven by a planetary gear
reducer direct-coupled to a motor. The grinding force for grain pulverization is
applied by a loading frame. This frame is connected by vertical tension rods to three
hydraulic cylinders secured to the mill foundation. All forces used in the pulverizing
process are transmitted to the foundation via the gear reducer and loading elements.
The pendulum movement of the roller wheels provides a freedom for wheels to move
in a radial direction, which results in no radial loading against the mill housing
during the pulverizing process.
Depending on the required grain fineness, there are two types of classifier that may
be selected for a vertical roller mill. The dynamic classifier, which consists of a
stationary angled inlet vane assembly surrounding a rotating vane assembly or cage,
is capable of producing micrometre-fine pulverized grain particles with a narrow
particle size distribution. In addition, adjusting the speed of the rotating cage can
easily change the intensity of the centrifugal force field in the classification zone to
achieve grain fineness control real-time to make immediate accommodation for a
change in fuel or boiler load conditions. For the applications where a micrometrefine pulverized grain is not necessary, the static classifier, which consists of a cone
equipped with adjustable vanes, is an option at a lower cost since it contains no
moving parts. With adequate mill grinding capacity, a vertical mill equipped with a
static classifier is capable of producing grain fineness up to 99.5% or higher <50
mesh and 80% or higher <200 mesh, while one equipped with a dynamic classifier
16
produces refined grain fineness levels of 100% <100 mesh and 95% <200 mesh, or
better.
2.4 Mill pulveriser utilization in Nigeria
These machines were originally designed and manufactured in Britain and the
United States of America in the early 1930’s (Lynch and Rowland, 2005). They were
brought into Nigeria by the tin mining companies in Jos and were copied by local
artisans. Since then, there have been no significant improvements in their design or
method of operation. The lack of innovation in the areas of design and operating
principles of hammer mills has constituted the greatest hindrance facing the growth
of solid minerals and grains processing industries in Nigeria. Although it is known
that highly efficient, economical and fast speed models of hammer mills and
pulveriser are being utilized in the chemical, powder, nuclear and food or grain
processing industries, a recent search on the internet revealed that there is no single
drawing of any type, model or prototype available on the web. This implies that
manufacturers of this equipment regard their designs as proprietary. Moreover, there
are no published articles describing the working principles of these new designs.
Thus, a successful growth and development of the solid minerals and grains
processing sector of the Nigerian economy would depend largely on the design and
fabrication of indigenous machines and equipment [Spangenberg, 2002]. These
would be machines and equipment whose technology, maintenance, replacement,
upgrading, efficiency and reliability are well understood and undertaken locally
without the need for minimal contribution from any foreign expert or technology.
This paper reports on the design and construction of a hammer mill with end suction
lift capability and hopes that the commercialization and widespread application of
the device will contribute significantly to the growth of the grains processing
industries in Nigeria.
17
The problems experienced by the refined grain production include the following:
(a) Design faults
(b) Construction faults
(c) Difficulty of financing
(d) Operational problems due to incorrect screen size or poor maintenance and
(e) Organizational problems arising from the differences of approaches and lack of
coordination.
All these aspects need to be taken into account. Therefore, the general design was
based on the process of allowing a strong and durable metallic object inform of
hammers to beat any material that obstruct its way during operation, thereby
resulting into breakage of the material which can also be referred to as size reduction
in comminution operation. This usually occurs in an enclosed chamber called the
crushing chamber. From which it falls through a sieve located directly under the
crushing chamber which is then transported with the aid of a suction compressor fan
which will help to transport the crushed grains into a storage unit. The physical and
mechanical properties of the mineral to be crushed were studied as this would help
immensely in the design of various components of the rotor. The engineering
properties and some other parameters are the main factors considered before design
of the machine.
18
CHAPTER THREE
3.0
RESEARCH METHODOLOGY
3.1 INTRODUCTION
Design is the transformation of concepts and ideas into useful machinery (Bernard
et al., 1999). The procedures in the design and construction of the modified cassava
milling machine are explained. Theoretical design and material selection:
The materials for the construction of the modified cassava milling machine are: the
shaft, pulley, belt, electric motor, the bearing, the mild steel plates, mild steel angle
bars and mild steel cylindrical tube. These materials were selected based on the
power requirement in the milling of dried cassava chips to flour. By mere feeling, it
was found that cassava chips when dried to moisture content of 5% (wb) can be
crushed into powder with the human fingers. Thus, the power required for its milling
is low.
Hammer mills for fine pulverizing and disintegration are operated at high speeds.
The rotor shaft may be vertical or horizontal, generally horizontal (Perry and Don,
1998). The shaft carries hammers, sometimes called beaters. The hammers may be
T-shaped element, bars, or rings fixed or pivoted to the shaft or to disks fixed to the
shaft. The grinding action results from impact and attrition between lumps or
particles of the material being ground, the housing and the grinding elements. It also
consists of a heavy perforated screen (Henderson and Perry, 1982) which can be
changed. Though it is a versatile machine and its hammer wear does not reduce its
efficiency, yet the power requirement is high and it does not produce uniform grind.
Common types available in the industry include the Imp Pulveriser, the Mikro
Pulveriser, the Fitz Mill, etc. Another class of size reduction machines is the Ring
19
roller mills. They are equipped with rollers that operate against grinding rings (Perry
and Don, 1998). Pressure is applied with heavy springs or by centrifugal force of the
rollers against the ring. Either the ring or the rollers may be stationary. The grinding
ring may be in a vertical or a horizontal position. Ring-roller mills also are referred
to as ring roll mills or roller mills or medium-speed mills. Ring-Roller mills are more
energy efficient than hammer mills. The energy to grind grain to 80% passing 200
mesh was determined as: hammer mill-22hp/ton; roller mill- 9hp/ton (Luckie and
Austin, 1989). Common types available include the B/W Pulveriser and the Roller
Mill. The third class available is the Attrition Mills. The disc attrition which is
sometimes called the Burr mill consists of a set of two hard surfaced circular plates
pressed together and rotating with relative motion (Onwualu et al., 2006). Stones are
replaced by steel disks mounting interchange metal or abrasive grinding plates
rotating at higher speeds, thus permitting a much broader range of application. They
are used in the grinding of tough organic materials, such as wood pulp and corn grits
(Perry and Don, 1998). Grinding takes place between the plates, which may operate
in the vertical or horizontal plane. The material is fed between the plates and is
reduced by crushing and shear. Though the power requirement is low, operating
empty may cause excessive burr wear and a lot of heat is generated during shearing
action. The objective of this study is the development of a modified milling machine
which combines both an impact and shearing milling action with a pneumatic
conveying and clarifying action. The combined action is intended to lead to lead to
efficient milling of cassava into fine powder. Unlike the normal hammer mill, it does
not use a screen classifier; rather it employs air classifier in which the conceptual
design was based on the principle of design by analysis (Nortion, 2006). The
methodology was to introduce special features into the hammer mill pulveriser so
that certain lapses noticed in the convectional hammer mill is reduced to a bearable
level.
20
3.2 LIST OF PARTS
The major components of the new hammer mill are
i
Inlet tray,
ii
Throat,
iii
Magnetic chamber,
iv
Rotor,
v
Crushing chamber,
vi
Hammer mill body,
vii
Hammers/Beaters,
viii
Screen,
ix
Bearings,
x
Blower/ fan compresor
xi
Discharge tank,
xii
Table or stand,
xiii
Mechanical drive,
xiv
Pulleys.
Inlet tray/Hopper: This is the pathway through which the material to grinded will be
pour into the hammer mill. The inlet tray was fabricated with a 3mm thickness metal
plate (Mild steel). The tray was braced on the sides by 1inch by inch angle iron of
the same dimension. Inside the tray we have a gate which is used in regulating the
flow of feed into the crushing chamber of the hammer mill.
Throat: This provides the passage for the material to be grind into the crushing
chamber. This was also fabricated with 3mm thickness metal plate.
21
Rotor: This is shaft of 30mm diameter that is holding 3 circular discs of diameter
90mm and is these circular discs that are carrying the hammers/beaters.
Crushing chamber: This is unit houses the rotor that holds the beaters and the screen
for sieving.
Hammer mill body: This was made of 3mm thickness plate with dimension 400mm
length & 215mm width & height 420mm. The hammer mill body is made in such a
way that it can easily be assembled & disassembled.
Hammers/Beaters: The hammers/beaters are a rectangular 3mm thickness metal that
does the grinding of material. It is 85 x 30mm in dimension with a drill hole of 12mm
at 30mm interval from both ends.
Screen: The screen act as a sieve for grinded materials before it will be finally
discharged. It was fabricated with 6mm thickness metal plate with many drilled hole
which will act as the sieve for the grinded material.
Bearings: The bearings provide sliding motion between the main shaft and the shaft
holding the hammers.
Discharge: This is the section through which the grinded material will be passed out
it will also be made with a 3mm thickness metal sheet.
Table or stand: This is the platform on which the whole machine is mounted. It was
made with mild steel I-beam. It was made of 2 inches by 2 inches’ angle iron. It is
the base to which the hammer mill body and the prime mover is bolted
Mechanical drive: A 5.5Hp, 3600rpm petrol engine was used as the prime mover of
the machine through belt transmission.
22
Pulleys: Two pulleys were used for this machine which was the driver and the driven
pulleys
respectively. The driver pulley is mounted on the mechanical drive engine while the
driven pulley is mounted on the rotor of the hammer mill machine.
3.3 DESIGN CONSIDERATIONS.
Determination of the Shaft Speed
The transmission system used is belt transmission via a pulley (specifically v-belt
selection) using a mechanical drive petrol engine of 3600rpm with pulley of diameter
130mm (D1) and the diameter on that of the rotor is 198mm (D2).
23
Thus to calculate the shaft speed, the following parameters are used:
………………………(1)
Where
N1 = revolution of the smaller pulley, rpm.
N = revolution of the larger pulley, rpm.
This shaft speed is only obtained when there is no slip condition of the belt over the
pulley. When slip and creep condition is present, the value (3600 rpm) is reduced by
4%
Determination of the Belt Contact Angle The belt contact angle is given by
……………………… (2)
[Hollowenko et al, 2004]
Where R = radius of the large pulley, mm r = radius of the smaller pulley, mm
The angles of wrap for the pulleys are given by
……… (3)
……… (4)
[ Hollowenko et al, 2004]
[Hollowenko et al, 2004]
24
Where
α1 = angle of wrap for the smaller pulley, deg
α 2= angle of wrap for the larger pulley, deg
μ a/sin ½
Comparing the capacities, e
θ of the pulley,
Where μ = coefficient of friction between the belt and the pulley = 0.25
(assumption);
= angle of groove ranges from 300to 400. Assume
= 400 (Joseph E. Shigley,
Choles R.Mischke: Mechanical Engineering Design, 2001) and
o
Using μ = 0.25; Θ = 40
0.25 x 3.04/sin20
For the smaller pulley e = 9.22
0.25 x 3.04/sin20
For the larger pulley e
= 10.68
Since that of smaller pulley is smaller, the smaller pulley governs the design.
2.1.3. Determination of the Belt Tension
25
The belt tension is given below [Khurrmi and Gupta, 2007]
Maximum Tension in belt
……………………………(5)
Centrifugal Tension in belt
…………………………… (6)
……………………………(7)
To get tension in slack side using the relationship below
………………………………(8)
Where
T1 = the tension in the tight side of belt, N
T2 = the tension in the slack side of belt, N
S1 = the maximum permissible belt stress, MN/m
The allowable tensile stress for leather belting is usually 2-3.45MPa [3] Let
2.4MPa = 2.4
26
M = mass per unit length of belt
Let = belt density = 1000kg
for leather belt
A = area of belt,
B=Belt breadth= 12.5mm T=belt thickness= 8mm
v = linear velocity of belt
V=
N = speed of motor =
36006rpm, d= diameter of motor pulley=130mm
=
centrifugal force acting on the belt
2.1.4. Determination of the Torque and Power Transmitted to the Shaft Power
required by the shaft is given by
…………………………(9)
1hp = 0.75kw
Maximum power of petrol engine is 5.5hp and power required by engine is 3.23hp
so 2 hp was an appropriate selection.
Torque at the main shaft is given by
……………………(10)
27
Determination of the Hammer Weight
……………………………(11)
It can be seen that the action of the weight of hammer shaft on the main shaft is
negligible.
Determination of the Centrifugal Force Exerted by the Hammer
Centrifugal force exerted by the hammer can be calculated as given by:
……………………… (12)
Hammer Tip speed
……………………… (13)
The angular velocity of the hammer is given by
……………………… (14)
The centrifugal force on the hammers, Fh, is given by
………………… (15)
Where,
= centrifugal force
= number of hammers
= mass of each hammer
= radius of hammer
= angular velocity of hammer
28
Assuming inelastic impact between the hammers and material, the velocity of
material, Vm, given by
........................................(16)
.
Where
=
velocity of material being milled
= mass of material being milled
= number of material impacted
The minimum width of hammer, wh, to withstand the centrifugal force at impact is
given by
............................. (17)
Where
= width of hammer
= diameter of hammer
= thickness of hammer
= working stress on hammer
Determination of the Hammer Shaft Diameter
The bending moment on the shaft is given by
29
…………………… (18)
30
31
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34