final hit 300. batsie
TRANSCRIPT
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HIT 300 (DESIGN AND INNOVATION PROJECT)
REDESIGN AND IMPROVING THE MODEL OF A ROTARY
GRAIN DE-WEEVIL
BY
BATSIRAI WEBSTER MASHAVA
REG. NUMBER: H0810835T
PROGRAM: B.TECH. FOOD PROCESSING TECHNOLOGY (HONOURS)
SUPERVISOR: MR. W. GWALA
DATE: 28 JUNE 2011
This document is submitted in partial fulfillment of the requirements of the course HIT 300offered in Part III of a Bachelor of Technology Degree program in Food Processing Technology(Honours).
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Dedications
This work is dedicated to all those who played a role in this work with particular emphasis
directed to my parents and friends.
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Acknowledgements
It is with great honour and privilege to have had an opportunity of carrying this design project.
Many persons have given their utmost support and encouragement and the few names singled out
here represent all those without whose indispensable support, this phase could not have been
successful.
Many thanks go to Mr W. Gwala, project supervisor and lecturer, Eng. M. Mukurunyorowa,
project fund facilitator, and G.M.B whose guidance and dedicated support cannot be
overemphasised.
I would like to thank the Almighty God for his providence.
Great appreciation and thanks go to my colleagues for their assistance and support during the
design process.
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Abstract
This project is a contribution towards improving grain cleaning preparation by efficiently
removing weevils at an industrial level making it easy and less time consuming. It is aimed at
redesigning and improving the rotary grain de-weevil by increasing capacity, cleaning efficiency
and reducing resident time comparing to manual cleaning.
Design principles were employed to come up with detailed illustrative diagrams of the design.
The materials used in the design was mild steel in place of stainless steel for the purpose of
producing the prototype since the material was readily available and easy to acquire unlike the
stainless steel.
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Contents
Dedications ........................................................................................................................................... 2
Acknowledgements ............................................................................................................................... 3
Abstract ................................................................................................................................................. 4
List of tables…………………………………………………………………………………………………………………………………………6
Executive summary ................................................................................................................................. 10
1.1 Rationale ....................................................................................................................................... 10
1.2 Aim ................................................................................................................................................ 11
1.3 Objectives ...................................................................................................................................... 11
1.4 Benefits ......................................................................................................................................... 11
1.5 Delimitation .................................................................................................................................. 11
1.6 Limitations ..................................................................................................................................... 12
LITERATURE REVIEW ............................................................................................................................... 14
2.4.0 Grain Weevils ............................................................................................................................. 19
2.4.1 Appearance ................................................................................................................................ 19
2.4.2 Life Cycle .................................................................................................................................... 20
2.4.3 Type of Damage ......................................................................................................................... 20
2.4.4 Maize Weevil Resistance ............................................................................................................ 21
2.5.0 Machine Design .......................................................................................................................... 21
2.5.1 Engineering Material .................................................................................................................. 21
2.5.2 Material Type ............................................................................................................................. 22
2.6.0 Electric Motors ........................................................................................................................... 23
2.6.1 Determining Motor Loads .......................................................................................................... 23
2.6.1.1 Reasons to Determine Motor Loads ....................................................................................... 23
2.6.2 Input Power Measurements ...................................................................................................... 24
2.6.3 Torque of Shafts ......................................................................................................................... 25
2.7.0 Frustums .................................................................................................................................... 27
2.7.1 Volumetric Formula of a Frustum .............................................................................................. 28
2.7.2 Surface Area Formula of a Conical Frustum............................................................................... 28
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2.7.3 Conical Frustum Slant Angle ...................................................................................................... 29
2.8.0 Safety, Health and Environmental Issues .................................................................................. 30
2.8.1 Safety ......................................................................................................................................... 30
2.9.0 Existing Grain Cleaning Machines .............................................................................................. 30
2.9.1 The Clipper ................................................................................................................................. 30
2.9.2Fanning-mills or winnowing-machines ....................................................................................... 31
2.9.3 Rotary Cleaners .......................................................................................................................... 32
2.9.4.1 Function: ................................................................................................................................. 33
DESIGN METHODOLOGY ......................................................................................................................... 35
3.1 Data Gathering .............................................................................................................................. 35
3.2.0 Experimental Procedures ........................................................................................................... 36
3.3.0 Parameters Measured ............................................................................................................... 36
3.3.1 Determination of the grain size ................................................................................................. 36
3.3.3 Determination of time ............................................................................................................... 37
3.3.4 Determination of Shaft diameter ............................................................................................... 37
3.3.5 Determination Feed Rate ........................................................................................................... 37
3.3.6 Determination of Cleaning Efficiency ........................................................................................ 37
3.4.0 Design Process ........................................................................................................................... 38
3.4.1 Research Design ......................................................................................................................... 38
Chapter 4 ..................................................................................................................................................... 40
4.0 Research Design Process ................................................................................................................... 41
4.1 Design Brief ................................................................................................................................... 41
4.1.0 Product Design Specifications .................................................................................................... 41
4.2.1 Capacity ...................................................................................................................................... 42
Rotary sieves’ internal dimensions (capacity dimensions)................................................................. 42
4.2.2 Height of the machine................................................................................................................ 43
4.2.3 Orifice diameters of the sieves .................................................................................................. 44
4.2.4 Width and Length of the machine ............................................................................................. 44
4.3.0 Alternative Concept Design ....................................................................................................... 44
4.3.1 Alternative Solution No. 1 .......................................................................................................... 45
4.3.1.1 Principle of Operation: ............................................................................................................ 45
4.3.2 Alternative Solution No. 2 .......................................................................................................... 47
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Dimensions of the Second Alternative................................................................................................ 47
4.3.1.2 Principle of Operation ............................................................................................................. 48
4.4.0 Concept Evaluation (Comparison of the two Alternatives) ....................................................... 49
4.4.1 Evaluation based on capacity ..................................................................................................... 50
4.4.3 Evaluation based on Capital Cost ............................................................................................... 50
4.4.4 Evaluation based on Maintenance Cost..................................................................................... 51
4.4.5 Floor Space Consumption .......................................................................................................... 51
4.5.0 Detailed Design .......................................................................................................................... 51
4.5.1 Dimensions of the machine ....................................................................................................... 51
4.5.2 Machine parts ............................................................................................................................ 51
4.5.3: Calculating the torque of the shaft ........................................................................................... 52
4.5.4 Calculating the forces acting on the shaft: ................................................................................ 52
4.5.5 Calculation of the shaft diameter .............................................................................................. 55
4.5.6 Calculation of the Maximum Revolutions per minute (rpm) that could be attained under load
............................................................................................................................................................ 57
4.5.7 Calculation of Power required Under Load ............................................................................... 57
4.5.7 Cost Evaluation .......................................................................................................................... 58
Chapter 5 ..................................................................................................................................................... 59
5.0 Calculation and Discussion ................................................................................................................ 60
5.1 Calculation of Speed required for Efficient Cleaning .................................................................... 60
5.2 Calculating the required power to sustain this speed under a load. ............................................ 60
5.3 Calculating Capacity of the design ................................................................................................ 61
Chapter 6 ..................................................................................................................................................... 63
6.0 Conclusion and Recommendations................................................................................................... 64
Bibliography ................................................................................................................................................ 65
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List of tables
Table 1 NEMA‟s Service Factor at Synchronous Speed (RPM) for drip proof motors ..…24
Table 2 Sieve dimensions …………………………………………………………………...42
Table 3 Gradient angle of the frustum …………..……………………………………….42 Table 4 Height of the design ……………………………………………………43
Table 5a Alternative no. 1 dimensions …………...……………………………………….44
Table 5b material costing of Al No.1 ……………………………………………………47
Table 6a Alternative no. 2 dimensions ……………………………………………………47
Table 6b material costing of Al No.1 ……………………………………………………49
Table 7 Alternative evaluation and analysis ……………………………………………………50
Table 8 Recommend values of Km and Kt ……………………………………………………55
Table 9 Details of the existing machine …..........................................................................60
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Chapter 1
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EXECUTIVE SUMMARY
1.1 Rationale
Food Incorporated is a company located in Waterfall and it is under Super Sands industrial
sector. It was founded in 2003 and it is owned by Mr. Mukunyorowa. The company is involved
in the mass production of maputi, cooking oil and soy chunks. The company has primarily two
prime raw materials in operation, which are maize and beans. The company receives an average
of 60 - 80 tonnes of maize per week during the harvesting season and 42 - 49 tonnes per week is
popped into maputi. Grain cleaning operation done at this company is manual which has
therefore lead to various problems with weevil infestation on the maize grain. The problem with
these weevils is associated with failures in popping out of the maputi. The efficiency of manual
cleaning at Food Incorporated is substantially below production capacity (12%) that is 250 Kgs
per hour in which it can be increased mechanically by 42% to meet maximum production
capacity per day. This cleaning operation is done by three people, which means its both labour
intensive and time consuming.
Calculating with the company‟s monthly delivery which adds to approximately 240 tonnes
subtracting with the monthly production of 168 tonnes, which means 72 tonnes are vulnerable of
weevil destruction and damage. This possible since the female corn weevils lay between 36 and
254 eggs which in just 14 days the larva hatch and begin to grow, within 21days the larvae grows
and repeat the reproductive process.
The rotary grain cleaner found at G.M.B consists of a single cylindrical rotating sieve. The
machine is only capable of remove a fraction of weevils, smaller impurities and does not remove
any impurities bigger than the grain such as stones and stalks. The capacity of the machine is 1100kg/h and its cleaning efficiency is 87.2% which is not a sufficient percentage of maize grain
cleaning at Food Incorporated. According to G.M.B‟s statistical maize grain reception (of about
100 -200 tonnes per week, such a machine is very slow in its returns. Regularly G.M.B process
30 tonnes of maize grain per day with that compromised cleaning efficiency.
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1.2 Aim
To improve the rotary grain cleaner by redesigning a machine that will increase its
capacity and cleaning efficiency in relation to time.
1.3 Objectives
To increase the daily production of maputi from 6 tonnes per day to more than 10 tonnes
per day at Food Incorporated.
To increase the capacity and cleaning efficiency of the current rotary de-weevil to meet
the requirements of 10 tonnes per day.
1.4 Benefits
The project was to insist Food Incorporated, GMB and farmers to improve their grain
quality prior to storage and processing, and it was a platform to the nation in creating
room for employment and bringing about the power of innovation to the nationalindustry.
This project will help the designer to partially fulfill his Bachelor of technology degree in
Food Processing Technology and to gain valuable knowledge and experience in the
design process and subject under study.
1.5 Delimitation
This project is rational and it will limit itself to the conceptual design and producing a
working product model by including a blower and another sieve on the current machine.
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1.6 Limitations
The project is done concurrently with internship therefore becomes strenuous and energy
consuming.
Acquiring information from the Internet was also limiting since some most websites
require registered membership in order to access information.
The project was limited by the time allocated to it hence the prototype for
demonstrations‟ sake is still in progress.
The project‟s working prototype was limited to tools and to material unavailability
especially sieves.
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Chapter 2
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LITERATURE REVIEW
2.1 Introduction
The highest ever standards for grain quality, required by both customers and governmental
agencies, have sharply increased the importance of precision grain cleaning methods for today's
Markets. Fines, dust and dirt must be removed to much more critical tolerances of less than ….
Thus to meet this need grain cleaning methods were improved from generation to generation.
According to Brennan (2006) defined “Cleaning” as the phase or phases of the post-harvest
system during which the impurities mixed with the grain mass are eliminated.
Clean grain has a higher value than grain that is contaminated with straws, chaff, weed seeds,
soil, rubbish, and other non-grain materials. There are three main importance of grain cleaning
and these are:
i. Cleaning improve the ability of the grain to be safely stored, reduce unwanted materials
(dockage) for milling, and improve milling output and quality.
ii. Cleaning is essential for protection of process equipment as well as the final consumer,
for example, it is essential to remove sand, stones or metallic particles from beans/maizeprior to milling to avoid damaging the machinery.(Brennan, 2006)
iii. Impurities lower the quality of the product and are also a focal point for potential
infestation during storage. Dirty grain can attract certain insects or fungus that might
damage the grain. Therefore cleaning is greatly required.
2.2 Grain Cleaning Process
All food raw materials are cleaned before processing. Increased mechanization in harvesting and
subsequent handling has generally led to increased contamination with mineral, plant and animal
contaminants. (Brennan, 2006)
There are a number of cleaning methods available, classified into dry and wet methods, but this
project focuses on dry cleaning methods.
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2.2.1 Dry Cleaning Methods
The main dry cleaning methods are based on screens, aspiration or magnetic separations. Dry
methods are generally less expensive than wet methods and the effluent is cheaper to dispose of,
but they tend to be less effective in terms of cleaning efficiency. Precautions may be necessary to
avoid the risk of dust explosions and fires (Brennan, 2006).
2.2.2 Winnowing
Winnowing is an agricultural method developed by ancient cultures for separating grain from
chaff, which uses wind or a fan to remove the light elements from the grain. It is also used to
remove weevils or other pests from stored grain (Harrison, 1922). Wind winnowing method
involves tossing the grain into the air and letting the wind carry off the lightest impurities. Such
impurities include unfilled grains, chaff, weed seeds, and straw can be removed from the grain
by using wind (Harrison, 1922).
This method has its disadvantages, labour intensive, time consuming and it does not eliminate
the heavier impurities, nevertheless it simple and no energy is required. Fig 1.1, illustrates how
winnowing is done.
Fig 1.1 wind winnowing
Advances have been made to improve the wind winnowing operation. Techniques included using
a winnowing fan (a shaped basket shaken to raise the chaff) or using a tool (a winnowing fork or
shovel) on a pile of harvested grain (Thompson, 1991).
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Mechanical winnowers use a fan and several superimposed reciprocating sieves or screens are
now used in many countries. Winnowers are simple machines that consist mainly of a hopper to
receive the grain, a fan and a set of sieves. These can be manually powered or motorized and
have capacities from 100kg to 3 tons per hour (www.knowledgebank.irri.org). Fig 1.2 shows a
winnowing machine
Fig 1.2 winnowing machine
The rotary winnowing fan is used to crank a fan to produce the air stream (Thompson, 1991).
2.2.3 Screen and Shifting
Smaller materials such as weed seeds, soil particles and stones can be removed by sieving the
grain through a smaller sized screen (2.5mm or less sieve opening). (Hanson, 1985)
Screens are essentially size separators based on perforated beds or wire mesh . Larger
contaminants are removed from smaller food items: e.g. straw from cereal grains, or pods and
twigs from peas. This is termed „scalping‟, see Fig. 1.3 a. alternatively, „dedusting‟ is the
removal of smaller particles, e.g. sand or dust, from larger food units, see Fig. 1.3 b (Brennan,
2006).
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Fig 1.3 Screening of dry particulate material: (a) scalping, (b) dedusting
2.3.0 The Maize Grain
Maize is originally from Mesoamerica, it is very adaptable to different weathers, and nowadays
its consumption is worldwide. In fact, the maize is the most widely grown cereal crop. In the
global production of cereals crops, the maize ranks first after rice (paddy) and wheat. Likewise,
corn is the leading cereal crop in Zimbabwe in terms of volume and value. It is a staple grain
mainly dominant and cheap (FAO, 1992)/ (Mangelsdorf and Reeves, 1939).
The corn ranges from white to dark brown or purple in color but white and yellow are most
common. According to Coleman and Larkin (1999), the botanical name for maize is Zea mays
and it belongs to the grass family (Gramineae).
The maize kernel consists four main principle parts:
i. Bran/Hull (Outer layer) – It constitutes approximately 5-6% of the kernel. It is important
due to the presence of vital minerals, fiber and vitamins. The hull is the pericarp.
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ii. Germ – Covers 10-14% of the kernel. It contains high crude oil and oil soluble vitamins
iii. Endosperm – It mainly constitute starch and proteins; and it covers 80-85% of the kernel
iv. Tip-cap
Fig 3, illustrate the structure of the maize kernel.
Fig 2 Maize kernel, source Illinois (1964)
The approximate chemical compositions of maize kernel are 9.5% protein, 2.2% fiber, 4.3% ash,
33% crude oil and 12% moisture. (Burge and Duensing, 1989)
Maize is grown seasonally and to that effect storage is important to encourage availability
through the year. Therefore cleaning is considered to be one of the most important procedures
required to improve the shelf-life of maize especially against weevils.
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2.4.0 Grain Weevils
Weevils are beetles that are attracted to many grain crops such as wheat, rice, barley and corn
that are grown. Weevils and insects degrade the grain coat integrity and endosperm. Their
damaging effects are largely due to their ability to reproduce quickly and then spread throughout
the corn in a matter of days. (Longstaff, 1981)
The maize weevil‟s scientific name is Sitophilus zeamais. The maize weevil is similar to the
rice weevil in appearance, except that the insects are longer, adults reaching a length of 3-
3.5mm. (ento.psu.edu)
2.4.1 Appearance
The maize adult weevil is a small snout beetle which varies in size, averaging between 1/8- to
3/16-inches long. It varies from dull red-brown to nearly black and is usually marked on the back
with four light reddish or yellowish spots. The maize weevil has fully developed wings beneath
its wing covers and can fly readily. The thorax is densely pitted with somewhat irregularly
shaped punctures, except for a smooth narrow strip extending down the middle of the dorsal
(top) side. (Longstaff, 1981) Fig 3.1 illustrates a detailed maize weevil appearance.
Fig 3.1 Maize weevil (Sitophilus zeamais)
An egg hatches in a few days into a soft, white, legless, fleshly grub which feeds on the interior
of the grain kernel. The grub changes to a naked white pupa and later emerges as an adult beetle.
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The maize weevil is slightly larger than the rice weevil and has more distinct colored spots on
the forewings. It is a stronger flier than the rice weevil. The habits and life cycle are similar to
the rice weevil.
2.4.2 Life Cycle
The female maize weevil bores a hole into a kernel of grain and lays an egg, sealing the opening
with gelatinous material. Full development of the weevil takes place within the kernel of grain
and is not discernible from the outside. The development phase of the grain weevil depends on
temperature, normally taking minimum life cycle of 28 days. Females can lay more than 200
eggs under favorable conditions. (Longstaff, 1981)
The entire life cycle from egg to adult may be as short as one month during the summer or as
long as 5 months during the winter, being very dependent upon the temperature. There are
usually 4 generations per year with adults living between 7-8 months.
2.4.3 Type of Damage
These weevils are very destructive grain pests. Weevils develop as larvae within the grain
kernels. The weevil (presumably) chews a small hole on the ear of the seed emitting a
pheromone which will attract other maize weevils to the area. They subsequently lays eggs in the
resulting cavity. The larva bores throughout the seed and pupates there, the adult emerging after
it has matured. Since weevils have the ability to reproduce quickly, they frequently cause almost
complete destruction of grain in elevators or bins, where conditions are favorable. Infested grain
will usually be found heating at the surface, and it may be damp, sometimes to such an extent
that sprouting occurs. (Derera et al, 2001)
Corn is a favorite host of the maize weevil, and can become infested in the field as well as in
storage. Like many other stored grain pests, Sitophilus species tolerate or prefer low moisture
levels in their food. Fig 3.2 shows an affected maize grain with weevils
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Fig 3.2 Destructed grains by weevil
2.4.4 Maize Weevil Resistance
According to Perez-Mendoza (1999), the increasing occurrence of insecticide resistance and
increasing environmental concerns about the use of chemical insecticide mean that alternative
control methods are required. Studies have been carried out focusing on the grain factors
contributing to maize weevil resistance and inheritance mechanism of resistance.
The tactic for control of stored products pests in commercial facilities in Mexico are almost
completely depend on pesticides but in recent years(1991 to 1992), managers of commercial bins
have reported control failures of these chemicals. It was thought to be less insecticide applicationbut the most likely reason was due to insecticide resistance in the target weevil (Perez-Mendoza,
1993 and Subramanyam, 1995).
The factors contributing to weevil resistance have been studied extensively and include
concentration of phenolic compounds in pericap tissue, grain protein content, grain hardness and
completeness/ tightness of husk cover (Classen et al, 1990).
2.5.0 Machine Design
2.5.1 Engineering Material
The selection of a material for a machine part or structural member is the most important job for
a designer before the final dimensions of the designed part are decided. According to Jindal
(2010), technical properties of material can be classified into three:
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i. Physical properties, such as density, homogeneity, thermal expansion coefficient and
composition.
ii. Technical properties, such as machinability, forgeability, castability, drawability and
defromability.
iii. Mechanical properties, such as strength, toughness, hardness, corrosion resistance, impart
strength, fatigue (a progressive and localized structural damage that occurs when a
material is subjected to cyclic loading [Stephens, 2001]) and creep resistance.
The design of a machine part depends upon the strength and stiffness of the material. Stresses
developed in the machine part and the deflection in members depends on the dimensions of the
member the various types of rigidity, namely torsional rigidity, flexural rigidity and stiffness.
(Jindal, 2010)
In many cases the frictional property of the material also plays an important role in the selection
process, since the coefficient of friction should be low for bearing materials and high for brake
lining.
2.5.2 Material Type
Most of food processing machinery is made up food grade material that is not capable of
imparting any side effects on the processed food, for example, iron rusts therefore using it on
moist food products it owes contamination. Hence each material used is of advantage or
disadvantage to the food industry. Most of food processing machines are made up of stainless
steel or galvanized material. Stainless steel does not react with food either those with moisture or
not.
List of Food Processing material
i. Stainless steel
ii. Aluminium
iii.
Coated or painted ironiv. Cast Iron (good at absorbing vibration)
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2.6.0 Electric Motors
An electric motor converts electrical energy into mechanical energy. Most electric
motors operate through interacting magnetic fields and current-carrying conductors to generate
force. Electric motors are found in applications as diverse as industrial fans, blowers and pumps,machine tools, household appliances, and disk drives. They may be powered by direct
current (e.g., a battery powered portable device or motor vehicle), or by alternating current from
a central electrical distribution grid or inverter. Electric motors may be classified by the source of
electric power, by their internal construction, by their application, or by the type of motion they
give. (Donald, 1978)
In an electric motor the moving part is called the rotor, and the part that stands still is called
the stator. Magnetic fields are produced on poles, and these can be salient poles where they are
driven by windings of electrical wire. A shaded pole is used in some motors and contains an
inductor to delay the phase of the magnetic field for that pole. A commutator is a device for
varying the direction of flow of current to coils depending on the angle of rotor (Edwin, 1897).
2.6.1 Determining Motor Loads
To compare the operating costs of an existing standard motor with an appropriately-sized energy
efficient replacement, you need to determine operating hours, efficiency improvement values,
and load. Part-load is a term used to describe the actual load served by the motor as compared to
the rated full-load capability of the motor. Motor part-loads may be estimated through using
input power, amperage, or speed measurements.
2.6.1.1 Reasons to Determine Motor Loads
Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency isusually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range
of 5 to 10 hp; meaning that the peak efficiency is at 7.5 hp. A motor‟s efficiency tends to
decrease dramatically below about 50% load. Overloaded motors can overheat and lose
efficiency. Many motors are designed with a service factor that allows occasional overloading
(Branan, 2005)
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According to Branan (2005), Service factor is a measure of periodically overload capacity at
which a motor can operate without overload or damage. The NEMA (National Electrical
Manufacturers Association) standard service factor for totally enclosed motors is 1.0. A motor
operating continuously at a service factor greater than 1 will have a reduced life expectancy
compared to operating at its rated nameplate horsepower.
Table 1 NEMA’s Service Factor at Synchronous Speed (RPM) for drip proof motors.
Power(HP)
Service Factor - SF
Synchronous Speed (RPM)
3600 1800 1200 900
1/6, 1/4, 1/3 1.35 1.35 1.35 1.35
1/6 1.25 1.25 1.25 1.25
¾ 1.25 1.25 1.15 1.151 1.25 1.15 1.15 1.15
1 1/2 and up 1.115 1.15 1.15 1.15
Therefore, 1hp motor with a service factor of 1.15 can operate at:
1 HP x 1.15
= 1.15 HP
without overheating or otherwise damaging the motor if rated voltage and frequency are supplied
to the motor. Insulation life and bearings life are reduced by the service factor load.
2.6.2 Input Power Measurements
With measured parameters taken from hand-held instruments, an equation can be used to
calculate the three-phase input power to the loaded motor. You can then quantify the motor‟s
part-load by comparing the measured input power under load to the power required when the
motor operates at rated capacity (Bollen, 1999).
Where: Pi = Three-phase power in kW
V = RMS voltage, mean line-to-line of 3 phases; I = RMS current, mean of 3 phases, and PF =
Power factor as a decimal.
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According to Fairchild (2004), The power factor of an AC electric power system is defined as
the ratio of the real power flowing to the load to the apparent power in the circuit, and is a
dimensionless number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50%
pf). Real power is the capacity of the circuit for performing work in a particular time. Apparent
power is the product of the current and voltage of the circuit.
Where: = Input power at full-rated load in kW
hp = Nameplate rated horsepower, and
= Efficiency at full-rated load.
Where: Load = Output power as a % of rated power
Pi = Measured three-phase power in kW, and Pir = Input power at full-rated load in kW.
Calculating the three phase power and the input power at full rated load, therefore, the load of
the electric motor can be determined using equation 3.
2.6.3 Torque of Shafts
Torque is the tendency of a force to produce rotation about an axis or pivot, also called as
moment or moment of force. Just as a force is a push or pull, a torque can be thought of as a
twist. Torque consists of force acting on distance. In relation to the design, the torque of the
electric motor will determine the power that can be used by a machine under full load (Khurmi,
2005).
Therefore torque can be calculated by the following formulas:
Where T = Twisting moment (or torque) acting upon the shaft,
J = Polar moment of inertia of the shaft about the axis of rotation,
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τ = Torsional shear stress, and
r = Distance from neutral axis to the outer most fibre
= d / 2; where d is the diameter of the shaft.
And for a round solid shaft, polar moment of inertia,
Therefore substituting equation (5) into (4), hence
Where , and
d = the diameter of the shaft.
The twisting moment (T ) may be also obtained by from power transmitted in a shaft (Khurmi,
2005).
Where N = rpm
Every shaft that is under horizontal load undergoes both twisting and bending moments.
According to Khurmi (2005), when the shaft is subjected to a bending moment only, then themaximum stress (tensile or compressive) is given by the bending equation.
Where M = Bending moment,
I = Moment of inertia of cross-sectional area of the shaft about the axis of rotation,
= Bending stress, and
y = Distance from neutral axis to the outer-most fiber.
But when the shaft is subjected to combined twisting moment and bending moment, then the
shaft must be designed on the basis of the two moments simultaneously. Various theories have
been suggested to account for the elastic failure of the materials when they are subjected to
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various types of combined stresses. The following two theories are important from the subject
point of view:
1. Maximum shear stress theory or Guest's theory. It is used for ductile materials such as
mild steel.
2. Maximum normal stress theory or Rankine‟s theory. It is used for brittle materials such
as cast iron.
The equivalent twisting moment may be defined as that twisting moment, which when acting
alone, produces the same shear stress (τ) as the actual twisting moment. This moment can be
denoted by ,which is the equivalent bending moment,
* √ +
Where Km = Combined shock and fatigue factor for bending, and
Kt = Combined shock and fatigue factor for torsion.
T = torque M = bending moment
The values of and are represented in table 8 in section 4.5.5
And also,
Where Bending stress (tensile or compressive) induced due to bending moment.
2.7.0 Frustums
In geometry, a frustum (plural: frusta or frustums) is the portion of a solid (normally
a cone or pyramid) that lies between two parallel planes cutting it. There are several types of
frustum, namely conical frustum, pyramidal frustum and square frustum, illustrated in Fig 5
below. These entire type shares the same volumetric formula, but the surface area formula is
different for each. (Boyer, 1991)
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(a) Pyramidal frustum (b) conical frustum (c) square frustum
Fig 5 Frustums
2.7.1 Volumetric Formula of a Frustum
The general formula for any frustum shape is:
V = .........................................................................................................Eq.11
where: π is 3.14159… and a, b are the radii of the two bases.
This formula was derived by Hero of Alexandra in the 10th AD. (Boyer, 1991)
2.7.2 Surface Area Formula of a Conical Frustum
The surface area of a right circular cone frustum shown in Fig 5 (b) is:
A = π ( )………………………………………….…Eq.12
where a and b are the base and top radii respectively. The conical frustum is of more concern to
the ideal design since the rotary sieves have a frustum shape.
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2.7.3 Conical Frustum Slant Angle
The slant angle of a conical frustum is obtained from getting the slant height first through the
application of Pythagoras theorem. Bisecting the conical frustum from the center a triangle is
formed and the slant height is obtained by applying the Pythagoras theorem. (Boyer, 1991)
Pythagoras Theorem
………………………………………………………………………………...Eq. 13
Where s is the slant height, a is the height and b is the base.
Therefore the slant height s is
……………………………………………………………………………….Eq. 14
Obtaining the slant height from equation 11 makes it easy to obtain the slant angle by applying
the cosine rule. According to David (2003), in a triangle ABS in which the side opposite each
vertex is denoted by the corresponding lower case letter illustrated in Fig 6 below. Then thecosine rule entails:
……….…………………………………………………………Eq.15
B
.a s
S b A
Fig 6 a triangle
This rule can be rearranged to give:
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………………………………………………………………………….Eq. 16
2.8.0 Safety, Health and Environmental Issues
2.8.1 Safety
Safety has become a matter of paramount importance these days in design. Machines must be
designed to serve mankind, not to harm it. Industrial regulations ensure that the manufacturer is
liable for any damage or harm arising out of a defective product. Use of a factor of safety only in
design does not ensure its overall reliability. According to R.S Khurmi, G.K Gupta (2005) some
machines are dangerous to operate, especially those which are speeded up to insure production at
a maximum rate. Therefore, any moving part of a machine which is within the zone of a worker
is considered an accident hazard and may be the cause of an injury. It is, therefore, necessary that
a designer should always provide safety devices for the safety of the operator. The safety
appliances should in no way interfere with operation of the machine
2.9.0 Existing Grain Cleaning Machines
2.9.1 The Clipper
The Clipper is known as a grain cleaning farm machine that is made up wood, several removable
sieves and a fan. The blast within the machine is generated with attached pulleys on the fan
blades with crossed belts. This machine is both manual and automated. When one simply want to
use power, the cog wheel must be substituted by a pulley. Operations done by the use of power
require the shaker shaft to have a speed of 390 to 400 revolutions per minute. This operation will
be valid to secure a strong air blast to blow out all light, chaffy kernels, leaving only the plum,
heavy kernels for seed. (Michigan, 2003)
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Fig 5 the Hand Powered Clipper.
1. Shoe, with two receiving grooves for screens. The screens are securely held in place by a
compressing rod.
2. Straw spout, for carrying off particles, straw, etc.
3. Screening spout, for removing sand, fine weeds, etc.
4. Vertical Air shaft, through which the draft passes upward, carrying off chaff, dust, etc.,
through dust hood
5. Grain box, which will hold 5 bushels. It has a lifting board at the end which allows the
contents to be easily scooped into sacks
6.
Dust hood, through which chaff, dust and worthless matter are discharged. All light,
imperfect and foreign grain and seeds are discharged at the opening.
7. Fan, with iron arms, fitted on turned steel shafting, equipped with core pulley. This
allows changing the speed of the Fan, which is necessary to property clean grain and
seeds of varying weights.
2.9.2Fanning-mills or winnowing-machines
These clean coffee, grain, etc., from chaff, dirt, and other light impurities. The apparatus shown
in Fig. 5.1 is designed for hand-use. Multiplying gear is placed between the crank-handle and bar
shown. The sieves are vibrated by means of a crank disk-rod and bell-crank. The grain is fed in
at tile top, and passes through the sieves, the uppermost of which is coarse, while the lower ones
are of varying degrees of fineness, the object being to distribute the fanning duty, by arresting the
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motion of the grain, so that the coarser impurities pass out between the upper and the finer ones
between the lower sieves.
Figure 5.1 Fanning mill
2.9.3 Rotary Cleaners
Rotary Cleaners are a versatile screen cleaner specifically designed for efficient cleaning using arotating sieve joined to. According to Alvan Blanch,
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2.9.4 Universal Screen Grader/Cleaner KUT
Universal screen graders type KUT are designed for removal of admixtures and impurities from
stream of basic material by aid of screens and air stream. They are namely used for post-harvest
cleaning and pre-cleaning of grain. Universal screen grader is a machine capable to clean all
sorts of cereals and seeds, like poppy, e.g. poppy seed, buckwheat, maize, beans, but also dried
herbs, spices, tobacco, salt, all that according to the type of sieves used. The machine is suitable
for all cases of sieving and sorting even when the crops are frequently changed. All parts coming
in contact with the sorted material can be easily cleaned. (Brenan, 200
2.9.4.1 Function:
The machine is fitted with pair of sieves, which ensure separation of coarse (big) and fine (small)
impurities according to the mesh in the sieves. The screens are cleaned by silicone balls (good
jumping even at low temperatures). The vibrations are created by electric vibrators. And the
casing with the sieves is cushioned by springs. Screens casing is of frame modular construction.
Connected air channel enables separation of light impurities. Pair of sieves is included into the
machine delivery, the size of mesh is recommended to the customer according to the type of crop
for the best result. Fan and cyclone separator are delivered as optional accessories. Universal
screen graders KUT can be widely used in food and agriculture industry.
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Chapter 3
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DESIGN METHODOLOGY
The chapter highlights the systematic framework that was used, research design, sampling
techniques, methods of data collection, analysis and presentation and limitation of the research.
The objective of this chapter was to come up with a method that can be used to design an
economically feasible and more efficient grain cleaner.
3.1 Data Gathering
In this design of an efficient de-weevil roto-fanning machine, various sets of data were collected.
This information was Food Incorporated basis of cleaning procedures and G.M.B‟s grain
cleaning plant. Information on weevils‟ infestation was obtained from G.M.B and other various
sources to be highlighted in this chapter. The data was divided into two classes that are primary
and secondary data.
Primary data
This is the type of data that is obtained directly from the field and this data. In the project
primary data was gathered using experiments, brain-storming, questionnaires and interviews.
Experiments conducted included determining the rate of rotation for maximum sieving action,
determining capacity per hour and determining the size of the grain. Interviews and
questionnaires were also conducted in order to get information concerning production from the
personnel.
Secondary data
This is the type of data that is collected from secondary sources such as records, journals,internet and books, in this particular project secondary data concerning the sales of juice was
used in order to evaluate if the project was going to be profitable or not to the company. Another
source of data that was obtained from secondary sources was total mixing time.
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3.2.0 Experimental Procedures
Twenty kilogram of maize grain samples was taken randomly from a heap of received sacks and
fed into the hopper manually. Feeding time, cylinder speed, blower speed and sieve reciprocation
speed and frequency were recorded experimentally. Approximate feed rate was computed as
weight of crop fed into the machine per unit time in kg/hr. Grain output was expressed in kg/hr
by recording time taken in cleaning operation and weight of grain recovered.
Feed rate of sieve was obtained by the output from the oscillating frustum sieve. Cleaned grains
in each collector box were collected in a transparent polyethylene bag and labeled to be
analyzed. Sample collected in each box was weighed with an electronic weighing balance.
Weight of cleaned grain was recorded for each box. The difference gave weight of impurities.
This was used to determine cleaning efficiency.
The impurity discharge section chaff was collected and weighed using electronic weighing
balance to determine the corresponding loss or grain cleaning efficiency.
3.3.0 Parameters Measured
3.3.1 Determination of the grain size
From the samples, 10 maize grains were selected at random for determining the physical axial
characteristics. For each grain, three linear dimensions were measured, that is length (L), width
(W), and thickness (T) using a micrometer screw gauge (Sheffied S 139 Br) reading to 0.01
mm. After determining all the three linear dimensions, a geometric mean diameter de was
determined as given by Mohsenin (1980)
de = (abc)1/3
,where, a,b,c = diameters/ dimensions along three axes (all in mm).
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This procedure was done with regards to determine the size of the sieves to be used. Therefore
measurements of all geometric mean diameter where averaged to obtain the maximum and
minimum average sieve diameter.
3.3.2. Bulk Density of Samples
A 20 liter bucket container with an initial weight of 0.9 kg was used. The sample, which filled
the container, was weighed using electronic mettler balance (Sarturious 2355, maximum 160g, d
0.001d). The bulk density of the samples were determined using the method given by
Mohsenin(1980).
,where: m = mass of grain, chaff or straw (kg)
v = volume of container (m3)
3.3.3 Determination of time
Crop feeding times in seconds were measured with stopwatch. A stopwatch was used to
determine the measurement of air velocity in meters per second.
3.3.4 Determination of Shaft diameter
Shaft diameter was determined by considering the shear stress of the material, torque required to
turn the shaft under a load, load in the frustum per unit time and revolutions per minute of the
shaft, shown in section 4.5.3.
3.3.5 Determination Feed Rate Approximate feed rate was computed as weight of crop fed into machine per unit time (hr). This
was done by computing a time/mass table for the samples.
3.3.6 Determination of Cleaning Efficiency
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Cleaning efficiency (Purity) was obtained by the procedure detailed by
,where: η =cleaning eff iciency, %= weight of pure grain at the outlet, kg = weight of contaminant (MOG), kg
3.4.0 Design Process
The basic five-step process usually used in a problem-solving can also work for design problems.
Since design problems are usually defined more vaguely and have a multitude of correct
answers, the process may require backtracking and iteration. Solving a design problem is a
contingent process and the solution is subject to unforeseen complications and changes as it
develops.
The five steps used for solving design problems are:
1. Define the problem
2. Gather pertinent information
3. Generate multiple solutions
4. Analyze and select a solution
5. Test and implement the solution
3.4.1 Research Design
The mapping of this design was done through design models. The research design initially
followed two design models in coming up with the best evaluated model to use. The design
processes were according to Hertz (2007) and Norman (1995) and in this project Hybrid design
process model was used, taking into account the weaknesses and strengths of each model over
the other. Fig 6 shows the flow diagram of the design process:
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Fig 6 A Hybrid Design Process Model
According to Hertz (2007) the stage of concept design in the design process, an attempt is made
by the designer to come up with a possible solution to the design problem. The design
specifications were taken into account in coming up with the solution. But, but Norman (1995)
model was voted to be more effective, it gave the designer the room to come with alternative
model which was an effective stage in the designing process. Norman (1995) model allowed the
designer to produce a prototype and finally the evaluation. Norman (1995) model reflected a
weakness in not including costing, but this problem was addressed by Hertz (2007). Therefore
the weakness and strengths of each of the models is accounted for by an alternative model known
as the Hybrid design model.
Design Brief
Product Design Specification
(PDS)
Alternative Concept Design
Detail Design
Working Prototype
Testing and Evaluation
Costing
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Chapter 4
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4.0 RESEARCH DESIGN PROCESS
4.1 Design Brief
The constraints at Food Incorporated production capacity per day do not mean its weekly maizeand soy beans delivery rate. Due to this situation the grains are vulnerable to weevil attack and
degradation. Therefore to meet such a problem an improved design of the existing grain cleaning
machine is required. The design should increase Food Incorporated‟s cleaning capacity from 6
tonnes to 10 tonnes per day. The existing design has a single and 4mm orifice rotating sieve and
it only removes particles smaller than the grain. This rational design is therefore going to add
another sieve (which can either be rotary or vibratory) and a blower on the existing machine to
remove anything bigger and smaller than the grain.
4.1.0 Product Design Specifications
According to Hertz (2007), the specification is a statement of the characteristics that a design
must possess in order to be a solution to the human need, and it is more properly known as a
product design specification (PDS). Its aim is to ensure that the subsequent design and
development of a product meets the needs of the user.
The design must remove both bigger and smaller particles than the grain. The main concerns are
on the thorough removal of weevils and dust particulates. The first frustum, 12mm orifice
rotating sieve should allow the grain to pass through and remove any particles bigger than the
grain such as ….., the blower should blow off weevils, dust and anything lighter and the second
frustum, 4mm orifice rotating sieve should remove any smaller particles than the grain including
remain weevils.
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4.2.1 Capacity
Rotary sieves’ internal dimensions (capacity dimensions)
Table 2
Sieves required dimensions
Dimension mm
Length 1000
Internal Diameter, a 300
Diameter, b 200
The rotating sieves have a conical frustum shape.
Calculations
Total volume of the rotating screens V = ………..………………Eq.11
=
=
=198 967 534.7
=1 989 675.347
Total sieving surface area A =π √ ..Eq. 12
π √
= 5.075 Therefore the volume of the two rotating sieves is 0.796 with a total sieving surface area of 5.075 Table 3
Required gradient angel of the frustum
Dimensions mm
Height a 1000
Base b 100
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The base b was obtained by taking a common dimension from both sides which was 400mm and
it was subtracted from 600mm. 200mm was the remaining portion but since bisecting the
frustum would make two triangles (the same as the one in Fig 6) therefore each triangle would
be 100mm.
Calculation
From Fig 6, the slant can be calculated:
Slant height …………………………………………………………….Eq.14
s =
s = 1004.988mm (3d.p)
, therefore the gradient angle B of the slant s can be obtained by cosine rule.
…..………………………………...……..………………………………..Eq.16
4.2.2 Height of the machine
Table 4
Required height dimensions
Dimensions mmHeight from ground tothe final grain discharge
700
Diameter a 600
Diameter b 400
Space between rotatingsieves (clearance height)
500
Top height allowance 200
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Working
Total height of the machine = the sum of all dimensions
= 700+600+400+500+200
=2400mm
The total height of the design is 2.4 meters.
4.2.3 Orifice diameters of the sieves
The machine is consisted of sieves one above the other. The first sieve was made to have an
orifice of 12mm to allow the grain to pass through it and any particle above this diameter is
discharged at the end of the oscillating sieve. The second sieve was made to have an orifice of
4mm to allow anything smaller than the grain to pass through including weevils.
4.2.4 Width and Length of the machine
The machine‟s proposed width was to be about 1 meter and the length was to be 1.65 meters.
4.3.0 Alternative Concept Design
According to Hertz (2007), the third step of design is called conceptualization. At this stage
designer considers the alternative concepts to find the best possible design to solution problem.
The selection of concepts is a decision making process which is carried out in terms of trade –
offs. The success of designer at this stage depends largely on selection of evaluation criteria that
must be relevant to performance specifications of product including also the cost.
Different alternatives were considered and evaluated in order to come up with the best possible
design. Each alternative had its own costing though the main objective in all of these alternatives
was to increase the machine‟s capacity.
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4.3.1 Alternative Solution No. 1
The first alternative was to design a shaker beneath the rotating sieve with an electric
motor driving a centrifugal shaft and a blower. In trying to increase capacity and reducing
residence time of the existing machine at the same shaft angle, the conical frustum rotary
sieve was designed instead. Designing a conical frustum increased the design‟s capacity
from 0.503 to 0.796.
This model of a design was to achieve the increase of capacity and the removal of
weevils and any undesired materials.
Dimensions of the first alternative
Table 5a
Total height 2000mm
Total width 1000mm
Length 1800mm
Total capacity 0.796
4.3.1.1 Principle of Operation:
The product is fed to the machine by means of a screw conveyor, and then enters the intakesprout where the grain are exposed to blowing process before they enter to the front part of thefirst sieve basket. Blowing serves as a pre-cleaning procedure but removing light material asdust, weevils, chaff and unhealthy small grains. The grains are directed to rotating sieve wherethey are distributed across the whole sieve width. On the first sieve deck, impurities smaller thanthe grain including the grain are separated and moved out of the machine. Any impurities largerthe grain, are removed at the end discharge of the rotary sieve.
The grain from the first sieve deck move to the second sieve deck where impurities smaller thancereal grain are separated. This process in the design is achieved through shaking of the sieve bya vibratory electric motor, shown in fig 6. These impurities are collected in the bottom of thesieve basket from where they are discharged. The overtails from the second sieve deck representthe good grain which leaves the sieve separator clean.
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First angle of the first alternative design
Fig 6: alternative design no. 1
Average Sieving Surface Area Calculation
From equation 9, the sieving surface area of the conical frustum was 5.075. The shaker sieveis 1000mm x 1000mm, therefore its surface area is:
Surface Area = length x width
= 1 x 1
= 1
Hence the average total surface area =
= 3.038
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Material Costing
Table 5b
materials Dimensions (mm) Prices ($)
Chanel iron 50 * 100 * 9 200 150.00Angle Iron 60 x 40 x 12 000 120.00
1st Screen (sieve) 2 200 x 1 000 diameter -12 250.00
2nd Screen 1 000 x 1 000 diameter – 4 150.00
2mm stainless steel sheets 40 x 2 500 96.00
Shaft 60 x 2 500 65.00
4 x bearings 40 and 20 55.00
Electric motor with a gearbox(3 x 400v/18kw)
750.00
Shaker electric motor (1hp) 560.00
Flat bar 12 x 30 x 5 30.00
Blower 350.00Bolts and nuts 12 x 60 x 81 10.00
Centrifugal shaft and pulley 20 x 150 45.00
Belt 320 12.00
Sub total 2 643.00
Labour 650.00
Tax @ 15% 97.50
Total 3 390.50
4.3.2 Alternative Solution No. 2
The second design was to design a machine with two rotating sieve and a blower. Thisdesign consists of two rotating shafts in connection by means of a belt running on twopulleys of different sizes. In trying to increase capacity and residence time the cylindricalrotating sieve was substituted by two conical frustum rotating sieves with the samedimension as that of alternative one.
This model of the design was to reduce manufacturing costs and still achieve an increasedrate of weevil removal and capacity.
Dimensions of the Second Alternative
Table 6 a
Total height 2400mm
Total width 1000mm
Total Length 1850mm
Capacity 0.796
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First angle of the second alternative design
Fig 7: alternative design no. 2
4.3.1.2 Principle of Operation
The maize product is fed to the machine by means of a screw conveyor, and then enters the
intake sprout and the grains enter the front part of the first sieve basket. The grains are directed
to rotating sieve where they are distributed across the whole sieve width. On the first sieve deck,
smaller impurities and the grains are sieved through while any impurities larger the grain, are
removed at the end discharge of the rotary sieve.
Unlike alternative no. 1, the sieved grain enters a chamber of blowing where all lighter
impurities are removed including a small fraction of weevils. The grains from the blowing
section are directed to the second sieve deck where impurities smaller than cereal grains are
separated. This process is achieved through rotation of the sieve in the with the same velocity as
the first sieving basket.
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Average Sieving Surface Area Calculation
Unlike alternative one, the total surface of the sieve is the same since the design consists two
rotary sieves though with different orifice sizes.
Hence, the average sieving surface area =
= 5.075
Material Costing
Table 6b
materials Dimensions (mm) Prices ($)
Chanel iron 50 * 100 * 9 200 150.00
Angle Iron 60 x 40 x 12 000 120.00
1st Screen (sieve) 2 200 x 1 000 diameter -12 250.00
2nd Screen 1 200 x 1 000 diameter – 4 170.00
2mm stainless steel sheets 40 x 2 500 96.00
2 x Shaft 60 x 2 500 125.00
4 x bearings 40 65.00
Electric motor with a gearbox(3 x 400v/18kw)
750.00
Flat bar 12 x 30 x 5 30.00Blower 350.00
Bolts and nuts 12 x 60 x 81 10.00
Belt 920 22.00
Sub total 2 138.00
Labour 500.00
Tax @ 15% 75.00
Total 2 713.00
4.4.0 Concept Evaluation (Comparison of the two Alternatives)
In comparing the two possible evaluations, several parameters were considered. These included
the following:
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Table 7
Parameters Current Machine 1st Alternative 2
nd Alternative
Capacity 900 – 1 300 kg/hr 1 200 – 1 800kg/hr 1 200 - 2000kg/hr
Capital Costs $2 300.00 $3 390.50 $2 713.00
Maintenance Cost Moderate Very high Moderate
Floor spaceconsumption
2.154
Average SievingSurface Area
4.66
4.4.1 Evaluation based on capacity
Fig 8: indicating the average capacity (kg/hr) of grain cleaning
The capacity of alternative number one has a lower range than that of alternative number two this
because of the resident time and the average sieving surface area which appears to be less in
alternative number one, illustrated in Fig 8.
4.4.3 Evaluation based on Capital Cost
Alternative design number one have high capital cost than alternative design number two this is
because of the shaker that requires an extra electric motor to move the centrifugal shaft for
shaking to occur.
0
500
1000
1500
2000
2500
current machine alternative no. 1 alternative no. 2
Average Capacity (kg/hr)
Average Capacity (kg/hr)
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4.4.4 Evaluation based on Maintenance Cost
The maintenance cost of alternative number one is almost twice than of alternative number two.
The main route of having a high maintenance cost in alternative one is because the design
consists of a complex mechanism system that needs regular inspections, for example check of
loose bolts and belt. The machine is not easy to dismantle and it is labour intensive in terms of
dismantling.
4.4.5 Floor Space Consumption
Alternative design number two occupies greater floor space then of alternative number one this is
because of its structural arrangements of parts, i.e. the shaker is supposed to overlap the
oscillating/rotating sieve.
4.5.0 Detailed Design
4.5.1 Dimensions of the machine
The machine dimensions were determined bearing in mind the space available on the floor and
height of the small plant. Food Incorporated‟s plant total height is 4m, and the available area for
the machine installation is 2.2. The machine height is 2,4m, width 1,0m and length is 1,65m.
4.5.2 Machine parts
Electric motor – electric motor on the design consists of a gearbox to promote speed
variations at different feed rates. The AC electric motor of the design has a service factor
of 1.15, a power of 2 hp and maximum RPM of 3 600.
Shaft – the design consists of two shafts with a length of 2.2 meters and a diameter of
35mm (which was determined through bending moments and torque shown in the next
sections). The shafts are in connection by means of chain running on sprocket pulleys to
avoid slipping and to maintain a constant rotation between the shafts. The shafts were
made of mild steel with a mass destiny of ⁄ (Khurmi, 2005).
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Blower – the design consists of a blower responsible in blowing off dust, weevils and any
other material that is light.
Stainless Frustum Sieve – the sieves of the machine are made from stainless with a
thickness of 2mm. The design contains two sieves with different orifice diameter; with
the top sieve having an orifice diameter of 12mm while the bottom sieve, 3mm. Sieve
metal plates of a length by width of 2000 by 1000 mm were used. The 2000mm length
side was winded into a frustum producing diameters 600mm by 400mm.
The reason for using stainless was that it does not react with the product and at the same
it does not react with any metal to cause rusting as in iron which reacts with water.
Sieve Frame – the sieve frame was made up of mild steel plates. It was made in way to
allow an ease in sieve exchange and firmly hold the sieve during rotation.
4.5.3: Calculating the torque of the shaft
When the shaft is subjected to a twisting moment (or torque) only, then the diameter of the shaft
may be obtained by using the torsion equation (Khurmi, 2005)
which can then reach this by substitution of J equation (5):
Where
Hence,
4.5.4 Calculating the forces acting on the shaft:
The weight of a 1 meter frustum sieve with a thickness of 2mm made up of stainless with 12 mm
orifice was determined by:
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From chapter two, stainless steel has a mass density of 7.80g/ . Therefore calculating using
the volume:
Mass = volume ( x mass density (g/ )
= 1 563.381 x 7.80
= 12 194.3718g
= 12.194kg
The mass of the stainless = 12.194kg which is equivalent to 121.94Netwons (N), (taking g =
10)
Considering the weight of the frame which was supposed to hold the frustum had a mass of 8kg
which is equivalent to 80N. Therefore the total load acting on the shaft in respect of the sieve and
the frame was 201.94N but this load was not evenly distributed since at one side of the frustum
sieve has a greater diameter than the other. Hence considering a diameter ratio to determine load
distribution: (mm)
Getting the total ratio: = 2 + 3
= 5Therefore distribution of weight at the ends can be considered: ⁄ ⁄ ………………… (ii)
Considering that the frustum holds 15kg (150N) at any given static time and assuming that the
load is uniformly distributed over the shaft therefore:
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Fig. 9: Analysis of the Shaft under Load.
Resolving moments of inertia shown in Fig 9 – load diagram:
Weight distribution of the frustum was developed from the ratio (i) above:
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Weight at the 400mm: ⁄
Weight at the 600mm:
⁄
Hence,
Therefore reaction forces and acting on the shaft are 120.48N and 169.69N respectively.
4.5.5 Calculation of the shaft diameter
To determine the diameter of the shaft a formula of equivalent bending moments was used(khurmi, 2005).
[ √ ]
Table 8 Recommended values for K m and K t ., source
Nature of load
1) Stationary shaft
i) Gradually applied loadii) Suddenly applied load
1.01.5 to 2.0
1.01.5 to 2.0
2)
Rotating shafti. Gradually applied orsteady load
ii. Suddenly applied loadwith minor shocks only.
iii. Suddenly applied loadwith heavy shocks
1.5
1.5 to 2.0
2.0 to 3.0
1.0
1.5 to 2.0
1.5 to 3.0
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And also,
Where
Bending stress (tensile or compressive) induced due to bending moment.
Substituting equation (i) and (10) into (9), hence: * √ +
* +
Where and are given in table 8 and in this particular case and , M is
the highest resultant bending moment (B.M.) shown in Fig. 9 on the moment diagram (N-m). In
this case; and (for En 8 mild steel), hence:
* +
Therefore the required shaft diameter size to with all of these forces is 33.141 mm but in this
design a diameter of 35 mm was adopted.
Now from equation 3, the torque can be calculated by substituting d with the adopted shaft
diameter:
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, hence the required torque for the shaft was . But since the design consists
of two shafts, therefore it means 9.6487 is the amount of torque found in each
shaft assuming that the loads any give time are at equilibrium.
4.5.6 Calculation of the Maximum Revolutions per minute (rpm) that could be attained
under load
Making RPM the subject of formula:
Data from the type of electric motor required on the design then,
Power =3HP, service factor = 1.15 and (from section 4.5.5)
Hence under load the maximum possible speed is 150.3 rpm.
4.5.7 Calculation of Power required Under Load
Where P = Power (watts)
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N = revolutions per minute (rpm)
Therefore the maximum power required to sustain the shafts‟ load is 15.18Kw at a speed of 150.3 rpm.
4.5.7 Cost Evaluation
The cost evaluation of the design is represented in table 6b.
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Chapter 5
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5.0 CALCULATION AND DISCUSSION
5.1 Calculation of Speed required for Efficient Cleaning
Table 9: Details of the existing rotating cleaning
Capacity (kg/hr) RPM HP Feed Rate(kg/min)
Cylinder size(diameter xlength) mm
1100 14.2 1.5 18 400 x 1000
The existing machine achieves maximum cleaning efficiency of 94.3% grain cleaning process
(that is the removal of dust and weevils) when the oscillating sieve is rotating at a speed of 14.2
rpm. Considering the design‟s electric motor (2HP), it is directly proportional to the related
output (rpm) of the existing machine, therefore to determine the speed at which cleaning process
is most efficient:
Hence by simple proportion of HPs in relation to speed,
Therefore to achieve maximum de-weevil and cleaning of grains in the design a speed of 18.9
rpm is required.
5.2 Calculating the required power to sustain this speed under a load.
Adopting the equation 9 in section 4.5.6:
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Where
, and
The required power consumption of the design theoretically agrees with the need. Considering
electric rates issued in Zimbabwe by Z.E.S.A (Zimbabwe Electricity Supply Authority), its total
rating in summer including customer charge, distribution service and electricity service is
$0.4266 kWh, Therefore for the design to suit Food Incorporated‟s side of gain this means:
, where (9) represents the working hours per day and (31) number of days in a month.
Considering that at present, three people are involved in the manual cleaning of the maize grain
while the design machine only requires one person to hang and remove the sack at the grain
discharge outlet. Each worker receives a salary of $200.00 therefore eliminating two them the
resultant profit of company prior to machine existence is $180.98.
5.3 Calculating Capacity of the design
Assuming that the shaft of the machines is at the same angel.
If the feed rate of the existing machine is at 18kg/min, therefore to obtain the capacity of the
machine at any given time several factors need to be considered. Angle at which flow is going
through, for example in the design when the shaft is at horizontal, the frustum sieve creates its
own angel of (section 4.2.1) which can influence rate of flow.
Using simple proportion and the assumption,
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Where x is an approximate feed rate of the design machine, hence:
Multiplying the feed rate by 60mins to get the capacity per hour,
Further multiplying the capacity by 9hours (normal working hours per day),
Therefore:
Applying an error of , hence
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Chapter 6
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6.0 CONCLUSION AND RECOMMENDATIONS
The set objective of increasing the machine‟s capacity was theoretically attained and at same
time fulfilling the set objective of increasing Food Incorporated‟s production capacity from 6
tonnes per to more than 10 tonnes per day.
The cleaning efficiency as an set objective was unmeasurable since the prototype is still in
progress. The available spacing in the plant is sufficient for installation of the machine and
creating a well safe environment for operation.
The time to submit the project did not give any room for prototype complete therefore results had
to be calculated. The University‟s Food Department should consider the strenuous of coming up
with a detailed project when students are on attachment by increasing time.
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