The main sources of power for agricultural machines, i.e., the diesel engine for self-propelled machines and the electrical motor for many stationary machines used on the farmstead. Pull-type machines must receive propulsion and rotary power from the tractor. Power is transmitted from the tractor to the machine by means of traction, power-take-off drives (PTO), and/or by fluid power. Rotary power is also transmitted by means of belts and chains. Topics related to rotary power transmission are presented here.

1. Adama, Ethiopia
Biniam Zewdie G/Kidan *
•Haramaya Institute of University
P.O.Box:138; Dire Dawa, Ethiopia
•Mobile: +251910408218/+25191582832
•E-mail: nzg2001nzg@gmail.com/zewdienico@gmail.com

3. The main sources of power for agricultural machines,
i.e., the diesel engine for self-propelled machines and
the electrical motor for many stationary machines used
on the farmstead.
 Pull-type machines must receive propulsion and
rotary power from the tractor.
 Power is transmitted from the tractor to the machine
by means of traction, power-take-off drives (PTO),
and/or by fluid power.
 Rotary power is also transmitted by means of belts
and chains.
 Topics related to rotary power transmission are
presented here.

4. Figure: Drive system for an industrial
application employing a belt drive, a gear

5. • Belts, ropes, chains, and other similar
elastic or flexible machine elements are
used in conveying systems and in the
transmission of power over comparatively long
distances.
• It often happens that these elements can be
used as a replacement for gears, shafts,
bearings, and other relatively rigid power-
transmission devices.
• In many cases their use simplifies the design of

6. • In addition, since these elements are elastic
and usually quite long, they play an important
part in absorbing shock loads and in damping
out and isolating the effects of vibration.
• This is an important advantage as far as machine
life is concerned.

7. • Crowned pulleys are used for flat belts, and
grooved pulleys, or sheaves, for round and
V belts.
• Timing belts require toothed wheels, or
sprockets. The timing belt does not stretch or
slip and consequently transmits power at a
constant angular-velocity ratio.
• In all cases, the pulley axes must be
separated by a certain minimum distance,
depending upon the belt type and size, to

9. Other characteristics of belts are:
They may be used for long center
distances.
• Except for timing belts, there is some
slip and creep, and so the angular
velocity ratio between the driving and
driven shafts is neither constant nor
exactly equal to the ratio of the pulley
diameters.

10. Types of Belt: -Light, medium and Heavy drives:
• Light drives transmit small powers at belt speeds up to 10
m/s like in agricultural machines and small machine tools.
• Medium drives are used to transmit medium power at belt
speeds over 10m/s and up to 22 m/s in machine tools.
• Heavy drives are used to transmit power above 22m/s like in
compressors and generators

12. CHARACTERISTICS OF FLAT BELT MATERIALS

14. The flat belts connecting pulleys can be arranged in
a number of ways depending upon the purpose of the
drive. The arrangements are described below :
• Open Belt Drive An endless belt goes around two
pulleys one of which is driving and another driven.
Tight side of belt, as a rule, is kept lower so that
the catenary action of the top portion is utilised in
increasing angle of lap of the pulleys. In this
arrangement, as shown in Figures (a) and (b), the
two pulleys rotate in the same direction.
• The centre line of the pulleys could be horizontal,
vertical and inclined to horizontal.

16. Compound belt drive is
used when power is
transmitted from one shaft to
another through number of
pulleys

19. Cross or Twist Belt Drive
• Belt running on two parallel shafts but crossing
itself, as shown in Figure (c) would cause the
pulleys to rotate in the direction opposite to each
other.
• In such an arrangement excessive belt wear would,
occur because the belt will rub on itself.
• This wear is minimised if centre distance rip larger
than twenty times the width of beIt.
• The speed of this type of drive must not exceed 15
m/s and for belts wider than 200 mm the idler
pulley must be used to reduce rubbing.
• Capacity of such belt drive is only 75% of an equivalent
open belt drive if the ratio of pulley diameters is less than
3:1. If, however, this ratio is higher the capacity of drive is

21. Similarities between open belt drive and
closed belt drive
• In both the cases, driving and driven shafts
must be parallel.
• Both can transmit power and motion for
substantially large distances, as compared
to gear or chain drive.
• Slip can occur in both the cases. So they
are non-positive drive.

22. Open Belt Drive Cross Belt Drive
belt proceeds from top of one pulley
to the top of other pulley without
crossing.
belt proceeds from top of one pulley to
the bottom of other pulley and thus
crosses itself.
Driving shaft and driven shaft
rotate in same direction.
Driving shaft and driven shaft
rotate in opposite direction.
Contact angle (or wrap angle) between
the belt and pulley is comparatively
small (always below 180º).
Contact angle between the belt and
pulley is comparatively large (always
above 180º).
Length of the open belt is
smaller as compared to cross
belt.
For the same pulley diameter and same
centre distance between driver and
driven shaft, longer belt is required in
cross belt drive.
Here belt remains in same plane in
every rotation during its operation.
Here belt bends in two different planes
in every rotation during its operation.
Here belt does not rub with
itself. So belt life increases.
Here belt rubs with itself and
thus life of the belt reduces.
Open belt drive is suitable when
driving and driven shafts are in
horizontal or little bit inclined.
Cross belt drive can be advantageously
applied for horizontal, inclined and
vertical positions of driving and driven

23. Quarter-Twist Drive
• In this arrangements two shafts are at right angle to
each other. The chance of slipping can be avoided
by providing larger face width of pulley.
• For this arrangement, illustrated in Figures (d) and
(e), pulley face width exceeds the belt width by at
least 40%
Belt Drive with an Idler Pulley
• When centre distance between the shafts is short,
the angle of lap on smaller pulley becomes small.
'This smaller angle of lap does not permit high belt
tension.
• If it is not possible to increase belt tension by any other means
to the desired value, then an idler pulley, as shown in Figure
(f) is used to increase the belt tension.
• This drive has a disadvantages in the sense that belt

24. Belt Drives with many Pulleys
• Figures (g) and (h) show an arrangement in which
a number of pulleys are driven by a single belt.
• In such an arrangement either all the pulleys could
rotate in the same direction [its in Figure (g)] or
some may rotate in the direction opposite to that of
others [as in Figure (h)].

25. The advantages of V-belt drives may be summed up below
:
a) Compact layout. The minimum centre distance is 1/2(D, + D2)
+ 3h, where h is the height of belt section. A V-belt drive is
accommodated in a machine.
b) High velocity ratios up to 10 are permissible.
c) Noiseless operation.
d) The drive can take up shocks, especially at starting.
e) The reversal of direction whereby the tight side can be on top
does not change power.
f) The angle of inclination of centre line does not affect
power.
g) The V-belt is not affected by heat, moisture and dust.
h) Though power capacity of one belt is not very high, several of
them run on the same set of pulleys and increase power of
drive. The drives can be made to have variable speed drives.

26. The V-belt drive has a few distinct drawbacks as
compared to flat belt drive. These are stated as under :
a) V-belts have lower life than flat belts mainly because
of high bending stress that is caused due to higher
ratio of belt depth to diameter of pulley than that in
case of flat belt drive and also because of high
frequency of stress variation.
b) The design of pulleys for V-belt drives is more
complicated than for flat belt drives.
c) V-belt drives cannot be used for larger power.
d) The centre distance in case of V-belts may not be
large.
The V-belt drives are used quite widely and within their power
range (i.e. up to 1100 kW), employment of V-belt drives is second

27. THE DESIGNER MUST DO THE FOLLOWING:
 Choose the type of power transmission elements to be used:
gears, belt drives, chain drives, or other types. in fact, some
power transmission systems use two or more types in series
to optimize the performance of each.
 Specify how the rotating elements are arranged and how the
power transmission elements are mounted on shafts.
 Design the shafts to be safe under the expected torques and
bending loads and properly locate the power transmission
elements and the bearings. it is likely that the shafts will have
several diameters and special features to accommodate
keys, couplings, retaining rings, and other details. the
dimensions of all features must be specified, along with the
tolerances on the dimensions and surface finishes.
DESIGN CONSIDERATION

28.  Specify suitable bearings to support the shafts and
determine how they will be mounted on the shafts
and how they will be held in housing.
 Specify keys to connect the shaft to the power
transmission elements; couplings to connect the
shaft from the driver to the input shaft of the
transmission or to connect the output shaft to the
driven machine; seals to effectively exclude
contaminants from entering the transmission; and
other accessories.
 Place all of the elements in a suitable housing that
provides for the mounting of all elements and for
their protection from the environment and their
DESIGN CONSIDERATION

29. Design Consideration
of Belt
• Speed of the driving and
driven shaft
• Speed reduction ratio
• Power to be transmitted
• Positive drive requirement
• Space available
• Service condition
• Shaft layout
• Center distance between
shaft

30. Design Consideration of Belt Terminology
• Belt Forces: Main forces acting in the belt
drive are given by
α = Arc of belt contact, rad
θ = angle of v-belt

43. Drawbacks of belt drives are:
1. Endless belts usually
cannot be repaired when
they break. They must be
replaced.
2. Slippage can occur, particularly if
belt tension is not properly set
and checked frequently. Also,
wear of belts, sheaves, and
bearings can reduce tension,
which makes re-tensioning
necessary.
3. Adverse service environments
(extreme temperature ranges,
high moisture, oily or chemically
filled atmospheres, etc.) can
damage belts or cause severe
slipping.
Advantages of belt drives include:
1.No lubrication is required,
or desired.
2.Maintenance is minimal
and infrequent.
3.Belts dampen sudden
shocks or changes in
loading.
4.Quiet, smooth operation.
5.Sheaves (pulleys) are
usually less expensive
than chain drive
sprockets and exhibit
little wear over long

44. THE END
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