Traction in a tractor tire

1. Lecture
on
Traction and related definitions, traction theory , tyre types, water
ballasting, wheel slippage, mechanics of a rigid wheel (traction and
towed)
By-
Er Kishan Kumar Patel
PhD 1 st year (FMPE)
SV COLLEGE OF AGRICULTURAL ENGINEERING AND TECHNOLOGY &
RESARCH STATION
INDIRA GANDHI KRISHI VISHWAVIDYALAYA, RAIPUR
2019-20

2. Traction
Traction.
The process by which a tractor develops tractive force and overcomes
motion resistance to produce desired motion.
Traction is the force in the direction of travel, developed by the traction
device medium (soil) and transferred to the vehicle.
The power developed in engine, finally goes to wheels or tracks, which
move the tractor with or without an attached load.
During the movement of wheels, some slip occurs, which causes
reduction in speed. The slippage also occurs with increasing load.

3. Traction can be increased by:
1. Using rubber tyres with grooves
2. Placing tyre chains
3. Using ribbed treads on track links
and
4. Putting lugs, cleats or grousers on
the wheel rim.

4. Some important definitions related to traction
It is the ratio of the total force output of the traction device in the
direction of travel to the dynamic weight on the traction device.
It is the force required in the direction of travel to overcome the
resistance of motion.
The force developed on the tractor interface by the traction device as a
result of applied torque from the power source.
Tractive force
Coefficient of Traction
Rolling Resistance

5. The efficiency of the tractive device is converting the axle input power
into output power, the term tractive efficiency (TE) has been defined as
TE = Output power/ Input power x100
Basically tractive efficiency is converting the axle torque into net traction
It is the ratio between rolling resistance and dynamic weight.
Any force imposing resistance against desired motion.
The depth to which the traction device penetrate into the soil measured
normal to the original, undisturbed surface.
Several factors lower the tractive efficiency; among these are steering,
rolling resistance, slip, and friction in and deflection of the traction
device.
Traction efficiency
Coefficient of Rolling Resistance
Sinkage
Motion resistance

6. Rim pull is a term which is –used to designate the tractive force between
the rubber tyres of the driving wheel and the surface on which they
travel. The coefficient of traction is high enough to eliminate tyre
slippage. The maximum rim pull is a function of the power of the engine
and the gear ratios between the engine and the driving wheels.
It is the relative movement of the wheel or track in the direction of travel
for a given distance under load and at no load condition. It can be
calculated by the formula:
Wheel slip, percent = N1 – N0 / N1 × 100
Where,
N1 = number of revolutions of driving wheels or sprockets for a given
distance under load and;
N0 = number of revolutions of the driving wheels or sprockets for the
same distance at no load
Rim Pull
Wheel Slip or Track Slip

7. Mechanics of a rigid wheel

8. Above figure illustrates one of several alternative methods of
describing the forces acting on a wheel.
The figure is divided into three distinct force states: braked, driven, and
driving.
The transition point between the braked and driven force states is the
towed wheel condition.
A towed wheel is unpowered: axle torque is zero neglecting bearing
friction.
The transition point between the driven and driving force states is the
self-propelled wheel condition.
For a self-propelled wheel, pull is zero with the applied torque simply
overcoming the motion resistance of the wheel.

9. The curves presented in Figure represent a given soil strength, tire size,
and load.
As soil strength increases, the curves move upward to the left, as soil
strength decreases, they move downward to the right.
In Figure, both the axle torque and pull are plotted as functions of
wheel slip.
These reactions develop from soil stresses resulting from slippage
(motion loss) of the wheel.
Slip is defined as
Where,
S - wheel slip
Va - actual travel speed
Vt - theoretical wheel speed = r
r - rolling radius of wheel on hard surface
 - angular velocity of wheel

10. A towed wheel is an unpowered wheel and axle torque is considered to
be zero . The towed force of a towed pneumatic tire is generally
dependent upon load, size and inflation pressure, as well as soil strength.
For soils that are not very soft and tires that are operated at nominal tire
inflation pressures, the towed force can be predicted from-
Where
FT = towed force of wheel
W= dynamic wheel load
B = unloaded tire width
D = unloaded tire diameter.
Cn= wheel numeric
Cn = B D CI/W
CI = cone index
TOWED WHEEL

11. The variations of driving wheel performance is based on the
consideration that the normal tire inflation pressures in agricultural tyres
produce tire deflections of approximately 20 per cent.
Therefore, traction prediction equation can be given as for net pull, slip,
and load -
Where,
FP = wheel pull
W = dynamic wheel load
e = base of natural logarithms
Cn = CIBD/W
CI, B,D and S as stated before.
DRIVING WHEEL

12. Ballasting
Ballasting
The normal load on traction wheels can be increased by putting water
or air inside the pneumatic tyre-tube.
Tyre ballasting is where the tubeless tractor tyre or inner tube is filled
with water in order to provide extra weight to stabilise a tractor.
Filling water in the tube is water ballasting, and filling sand in the tube
is sand ballasting. Generally the tube is filled up to maximum of 75% by
the ballasting material and rest with the air at the rated air pressure.
It can also be used to lower the centre of gravity of the machine which,
in turn, makes it more stable in situations where the ground is uneven.

13. Water ballasting procedure
Agricultural tyre valves are usually of the ‘air and water type’.
This is done by using the correct air/water adapter on the end of the
water hosepipe and airline.
You need to position the tractor tyre with the air/water valve at the top
of the tyre, and then fill the tyre or inner tube with water.
Once the tyre has been fitted on the tractor or machine, the tyre is
deflated to a low level of pressure, around 7psi or 0.5 bars.
It’s a good idea to add anti-freeze to the water in case temperature
drop below freezing, and then only ballast the tyre up to a maximum of
75% of the tyre volume, which is usually to the level of the tyre valve
and thus which allows the air to escape.

14. Finally, finish inflating the tyre with air and adjust the pressure as
required.
As only 25% of the tractor tyre or inner tube is filled with air, you
should regularly check the air pressure in the tyre, we suggest once per
month.

15. Tractor tyres
A tire forms a torus of a complex flexible reinforced composite material surrounding a steel hub
and filled up with air under pressure.

16. The agricultural tire must perform the following functions:
1. Support the vehicle and associated loads at some low level of
ground pressure,
2. Absorb shock loads and cushion the vehicle against minor surface
irregularities,
3. Provide traction (and braking), .
4. Provide for steering and directional stability,
5. Resist the abrasive action of the various surfaces on which it is
expected to operate.
TIRE FUNCTION

17. Tyre types
(a) Hard surfaces such as roads
(b) Normal agricultural work, dry soil
(c) Soft, wet agricultural soils
(d) Lawns, low sinkage is required
(e) Dry soil, heavy loads as in earth
moving
(f) Saturated puddle soils

18. The bias ply tyre construction utilizing rubber-coated layers known as
plies composed of textile cords, usually nylon and sometimes Kevlar.
The plies layered diagonal from one bead to the other bead at about a
30 degree angle.
 One ply is set on a bias in one direction as succeeding plies are set
alternately in opposing directions as they cross each other and the ends
are wrapped around the bead wires, anchoring them to the rim of the
wheel.
The layers of plies are them covered with more rubber to form the
tread of the tyre.
Bias ply tyres are sometimes called cross-ply tyres.
Bias /cross ply tyres

20. The radial is a type of tyre that is constructed with rubber coated,
reinforcing steel wire belts that are assembled parallel and run from side
to side, bead to bead at an angle of 90 degrees to the circumferential
centreline of the tyre.
 They constrict the radial ply cords and stabilize the tread area.
This makes the tyre more flexible which reduces rolling resistance to
improve fuel economy.
Then numerous rubber coated steel belts are then constructed into the
"crown" of the tyre under the tread to form a strong stable two-stage unit.
Radial tyres

22. Tractor tyres

23. References and suggested readings
http://ecoursesonline.iasri.res.in/mod/page/view.php?id=126223
http://www.agricultureinindia.net/machinery/farm-tractor/traction-and-traction-theory-used-
in-tractors-agriculture/16191
https://www.bigtyres.co.uk/blog/tractor-tyre-
ballasting.html#:~:text=Tyre%20ballasting%20is%20where%20the,where%20the%20ground%20
is%20uneven.
Tractors and their power units by John B. Liljedahl , Paul K. Turnquist, David W. Smith and
Makoto Hoki