Threshing—It is the process of detaching the kernels from the ears/pods/ or panicles by a combination of impact and rubbing action. It is accomplished either by treading the harvested crop under the feet of man or hooves of animals, and/or beating the harvested crop with stick or striking the harvested crop on hard and rough surface or using mechanical thresher.

1. BAHIR DAR UNIVERSITY
BAHIR DAR INSTITUTE OF TECHNOLOGY (BiT)
FACULTY OF MECHANICAL AND INDUSTRIAL
ENGINEERING
Course Title: Agricultural Machinery
Technology(AEng5161)
Course Title: Agricultural Machinery
Technology(AEng5161)
Chapter-4
Threshing Machine
By
Solomon Tekeste
(Lecturer)
7/13/2018 1

2. Threshing
• Threshing—It is the process of detaching the kernels
from the ears/pods/ or panicles by a combination of
impact and rubbing action.
• It is accomplished either by treading the harvested
crop under the feet of man or hooves of animals,
and/or beating the harvested crop with stick orand/or beating the harvested crop with stick or
striking the harvested crop on hard and rough
surface or using mechanical thresher.

3. Method of threshing
• Hand beating
• Trampling/ treading with a bunch of animals
• Manual threshers (Pedal thresher, Hands driven
threshers)
• Stationary Mechanical threshers• Stationary Mechanical threshers
• Combine Harvesters

4. Threshing in Ethiopia
• Mainly threshing in Ethiopia is done in a traditional way,
by trampling with a bunch of animals and/or by beating
the crop with stick. This method is usually tedious and
time consuming.
• In addition exposed the grain for contamination with
inert matter such as soil, small stones, weed seed, straw,
and immature and unfilled grains.
These extraneous materials will degrade the final grain• These extraneous materials will degrade the final grain
quality and market value of the produce.
• High threshing losses occurred due to some grains
remain unthreshed, some grains scatter around, some
grains stick to the mud floor and cannot be recovered,
and cracking and breakage due the hoof pressure of
trampling animals.
• Therefore, in this system quantitative and qualitative
postharvest losses are often very high.

5. Stationary/Mechanical Threshers
Parts of Mechanical Thresher
• A mechanical thresher consists of the following
parts:
– Feeding device
(chute/tray/trough/hopper/conveyor)
Threshing cylinder (hammers/spikes/rasp
(chute/tray/trough/hopper/conveyor)
– Threshing cylinder (hammers/spikes/rasp
bars/wire loops/syndicator)
– Concave (woven wire mesh/punched sheet/welded
square bars)
– Blower/aspirator
– Sieve shaker/straw walker

6. Mechanical Thresher

7. The threshing process/principle
• In mechanical threshers threshing operation is
accomplished by a rotating cylinder and a concave
grate.
• As the cylinder rotates, crop is forced through the
gap between the concave and the cylinder and is
subjected to impact and rubbing action that causesubjected to impact and rubbing action that cause
grains to be detached.
• Mechanical threshers are also equipped with cleaning
unit to separate the straw from the grain. The
cleaning unit mainly consists of sieve mechanism and
blower. The sieve can be reciprocating or rotary type.

8. • Separating process/ Mechanism:-The main separation
of the grain from the straw is through the concaves.
The loose grains mixed with straw as it leaves the
cylinder are separated by rotating straw rack. These
racks may consist either of one piece or of several
sections which alternately move with a slight elliptical
action to pitch the straw rewarded with each
movement
• Cleaning process/mechanism:-The function of cleaning• Cleaning process/mechanism:-The function of cleaning
unit is to remove chaff or other foreign matters from
the grain. This is done by passing un-cleaned grains
over a series of oscillating sieves and screens through
which a current of air is forced/blowed by a
fan/blower. Different types of sieves and screens are
available for different kinds of crops.

9. Threshing Unit
Types of threshing cylinders and associated concaves
• In mechanical threshers, there are four primary
types of threshing cylinders and associated concaves.
These include:
rasp-bar cylinder and concave,– rasp-bar cylinder and concave,
– spike-tooth cylinder and concave
– angle-bar cylinder and concave, and
– wire loop cylinder and concave.

10. Rasp-bar cylinder
• The rasp-bar cylinder
consists of a number of
corrugated steel bars
that are mounted on
cylinder.
• The concave is made of• The concave is made of
parallel bars that are
held together by
parallel curved bars

11. Spike-tooth cylinder
• The spike-tooth cylinder
has spikes on the bars
• The concave has matching
spikes,
• The threshing action in• The threshing action in
this design is that of
tearing and shredding.
Compared to other
cylinders, there is less
damage to the grain.

12. Spike-tooth cylinder

13. Angle-bar cylinder
• The angle-bar
cylinder is made of
helical rubber-
coated angle irons
• The concave is also• The concave is also
rubber-coated.
• The angle-bar design
is commonly used for
crops such as clover
and alfalfa seed.

14. Wire loop cylinder

15. Threshing performance
• The performance of threshing mechanisms is measured by
threshing efficiency, separation efficiency, the amount of
grain damage, percentage of scattering loss, percentage of
untreshed grain and the amount of straw breakup.
• Threshing performance parameters are affected by the
following factors:following factors:
– Design factors: cylinder diameter, concave length, number
of rasp bars, cylinder-concave gap(fixed);
– Operating parameters: cylinder speed, cylinder-concave
gap(adjustable), material feed rate;
– Crop condition: crop moisture content, crop maturity, crop
type.

16. Factors affecting threshing performance
Concave length
• It increases asymptotically with concave length up to a
certain point. Increasing concave length beyond this point
does not increase threshing efficiency and might even
decrease it under certain conditions.
• Increasing the concave length increases the separation
efficiency but at a diminishing rate.
• Increasing concave length tends to increase grain damage• Increasing concave length tends to increase grain damage
slightly.
Cylinder diameter
• Increasing the diameter of the conventional threshing
cylinder increases threshing losses at a rate of about 0.9%
for each 7.5 cm increase in the diameter.
• Increasing cylinder diameter reduces grain damage.

17. Factors affecting threshing performance
Cylinder speed
• Cylinder speed is one of the most important variables
affecting threshing efficiency. Cylinder speed varies between
150 to 1500 rev/min for different crops. The cylinder speed
is determined by the crop type and condition. Wet, hard-to-
thresh conditions require higher speeds, but grain damage
increases as cylinder speed is increased.
• Grain separation efficiency increases with cylinder speed.
• Cylinder speed has the most profound effect on grain damage• Cylinder speed has the most profound effect on grain damage
during threshing, as increasing cylinder speed increases
damage exponentially.
Feed rate
• Threshing losses increase with material feed rate.
• Increasing the feed rate has a negative effect on the
separation efficiency.
• Increasing feed rate provides more cushioning that may
reduce grain damage.

18. Factors affecting threshing performance
Cylinder speed
• Cylinder speed is one of the most important variables
affecting threshing efficiency. Cylinder speed varies
between 150 to 1500 rev/min for different crops. The
cylinder speed is determined by the crop type and
condition. Wet, hard-to-thresh conditions require highercondition. Wet, hard-to-thresh conditions require higher
speeds, but grain damage increases as cylinder speed is
increased.
• Grain separation efficiency increases with cylinder speed.
Feed rate
• Threshing losses increase with material feed rate.
• Increasing the feed rate has a negative effect on the
separation efficiency.

19. Factors affecting threshing performance
Cylinder-concave gap
• Another factor affecting the quality of threshing is the
cylinder-concave gap. If the gap is too large, the crop is not
threshed completely. Too narrow a gap results in excessive
power and grain damage.
• Increasing the cylinder-concave clearance tends to reduce the
separation efficiency
Moisture content
• Moisture content also affects threshing efficiency. Generally,
the crop becomes hard to thresh at higher moisture content
and as a result the threshing losses become higher. Also, if
the crop is not fully mature and if there is a lot of green
material in the crop, threshing becomes difficult and losses
increase.
• Increasing grain moisture increases grain damage, however at
very low moisture content the kernels tend to crack and
increase grain damage.

20. Threshing efficiency (Te%)
• Threshing efficiency is the percentage of the
threshed grains calculated on the basis of the total
grains entering the threshing mechanism.
• Where CTG = Clean threshed grain
• SpL = Separation loss
• ScL = Scattering loss
• TL = Total threshing loss
• UT = unthreshed grain

21. Separation efficiency
• The separation efficiency of the threshing cylinder is
defined as the percent of grains separated through
the concave grate of a thresher, to the total grain in
the crop entering the threshing mechanism.

22. Grain damage
• Grain damage refers to mechanical damage to grain
during the process of threshing. It includes broken
kernels, kernels with skin damage, and kernels with
internal damage. Mechanical damage to grain results
in poor germination, poor storability, and poor
processing characteristics.

23. Table 1 Typical cylinder peripheral speeds (rasp-bar or spike-tooth)
and clearances for various crops
Crop Peripheral Speed,
m/s
Mean Clearance (rasp-
bar cylinders), mm
Alfalfa 23–30 3-10
Barley 23–28 6–13
Edible beans 8–15 8–19
Beans for seed 5–8 8–19
Clovers 25–33 1.4–6Clovers 25–33 1.4–6
Corn 13–22 22–29
Flax 20–30 3–13
Grain sorghum 20–25 6–13
Oats 25–30 1.5–6
Peas 10–15 5–13
Rice 25–30 5–10
Rye 25–30 5–13
Soybeans 15–20 10–19
Wheat 25–30 5–13

24. • Depending on flow of material in the threshing units,
mechanical threshers are divided in to two; these are;
– Radial/Tangential Threshing Unit
– Axial Threshing unit

25. • Depending of feeding of material to the threshing
cylinders

26. The Current Advancement of Threshing Operations
• In the current technological advancement threshing
operation is carried out in combine harvester where
harvesting, threshing, winnowing, cleaning the
threshed crops is done in a single operation by a
single unit of machine called combine harvester.
• A modern grain combine performs many functional• A modern grain combine performs many functional
processes. These are gathering and cutting (or in case
of windrows, picking up), threshing, separation, and
cleaning.

27. Process Diagram of a Combine Harvester

28. Combine Harvester
• There are two main kinds of combines, conventional
types and rotary types. Either of these types may be
self-propelled or pulled by a tractor and powered by
the PTO drive.

30. Reference
1. Ajit K. Srivastava et. al. 2012. Engineering Principles
of Agricultural Machines. Second Edition. American
Society of Agricultural and Biological Engineers.
2. D. N. Sharma and S. Mukesh. 2010. Farm Machinery
Design Principles and Problems. Second Edition.
Pusa Agri-Book Service, IARI, New Delhi.Pusa Agri-Book Service, IARI, New Delhi.
7/13/2018 30