In this article we will discuss about:- 1. Meaning of Traction 2. Traction Theory 3. Mechanics of Tractor Chasis.
Meaning of Traction:
Traction is the force in the diction 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.
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.
Traction Device:
It is a device for propelling a vehicle using the traction forces from the supporting surface.
Coefficient of 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.
Tractive Efficiency:
It is the ratio of output power to input power, usually expressed in percent.
Tractive efficiency is affected by several factors such as:
1. Tyre inflation pressure
2. Soil condition
3. Wheel size
4. Speed of travel
5. Slope of the land
6. Height of the hitch and
7. Shape and size of lugs.
Rolling Resistance:
It is the force required in the direction of travel to overcome the resistance of motion.
Coefficient of Rolling Resistance:
It is the ratio between rolling resistance and dynamic weight.
Wheel Slip or Track Slip:
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:
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.
Traction Theory:
Pneumatic tyres are used for common tractors. When such a tractor moves over soil, it has to overcome the rolling resistance R as shown in Fig. (10.30).
Where,
F = force of traction in the direction of travel.
P = pull used for useful work and
R = rolling resistance.
In order to calculate pull P and rolling resistance R. Bekker has suggested following equations:
F = A [C + ρ tan φ]
or, F = AC + W tan φ
Where,
A = area which will shear off
ρ = soil pressure
C = cohesion of soil and
φ = angle of internal friction of soil
W = Aρ = vertical load over the shearing area.
It is evident that tractive force is usually dependent upon vertical force and the area of shearing.
For a track-type tractor, the soil pressure p is given by:
ρ = W/BL
Where, B is the width of each track and L is the length of track in contact with the soil.
For a pneumatic tired wheel, the contact area with the soil is an ellipse and for that uniform soil pressure ρ is given by:
= ρ W/0.78 BL
Rolling Resistance:
The rolling resistance due to soil compaction can be approximated by assuming that the energy required to overcome the rolling resistance is equal to the work done in deforming the soil. The rolling resistance R can be obtained by the relationship.
Where,
n = coefficient of wheel sinkage
Kc = cohesive modulus of soil deformation
Kφ = friction modulus of soil deformation
B = width of each track
W = vertical load over the shearing area and
L = length of track in contact.
This equation is derived by assuming a flat plate as in a crawler tractor. This equation is approximately correct also for rubber tired tractors. When the rolling resistance and tractive force are calculated, the useful pull P can be obtained by the equation P = F – R.
Mechanics of Tractor Chasis:
Mechanics of tractor chasis involves a number of forces but to understand the main principle a simple approach is followed, considering the main forces as:
1. Gravitation
2. Soil reaction
3. Traction force and
4. Drawbar pull.
Referring to (Fig. 10.31) and considering tractor as a free body, the algebraic sum of all forces acting parallel or perpendicular to the direction of motion as well as the algebraic sum of moments about any point must be separately zero.
∑H = 0, ∑V= 0, ∑ M = 0
H is the horizontal force, V is the vertical force, M is the moment and a is angle between pull (P) and horizontal,
H = F-P cos α = 0 … (1)
Where, F is frictional force
V = R1 + R2 – W-P sin α = 0 … (2)
Taking moments about the point A
∑M = WX1 – Y1 P cos α – SP sin α – R1 X2 = 0 … (3)
Solving equation (3) for R1
The stability of the tractor in the vertical plane is determined by R1 and the tractive capacity by R2
Centre of Gravity:
It is the point on the tractor body at which its weight may be considered as acting.
Exact location of the centre of gravity can be determined by the following methods:
(a) Suspension
(b) Balancing method and
(c) Weighing method
The above three methods do not consider the movement of oil in the crankcase and transmission, fuel in tank and weight of operator.
(a) Suspension Method:
The tractor is suspended from any convenient point, strong enough to carry its weight. The centre of gravity will be in the vertical plane through the point of suspension. By repeating this operation with another point of suspension another plane may be located and its intersection with the previous vertical line will determine the centre of gravity.
(b) Balancing Method:
This method is used for track type tractor. A plumb bob and a large timber equal in length to the overall width of the tractor and about 15 cm or more thick are required. If the tractor is driven so that the lug point positions of the two tracks match and it is then driven slowly on the timber, so that it can be balanced over the two lug points. The centre of gravity will be in the vertical plane though the lug points. By backing the tractor on the timber, another plane may be located.
(c) Weighting Method:
For a 4 wheeled tractor, weighing method is commonly used for locating its centre of gravity.
Referring the Fig. 10.31, a vertical plane containing the centre of gravity can be determined by the following equation:
X1 = R1X2/W
If the front wheels of the tractor are raised, a distance Y2 and R2 determined (Fig 10.32) another plane containing the centre of gravity, may be determined by:
X1’ = R1 ‘X2 ‘/W
R1’ is the weight of the tractor on the front wheels which is measured. The intersection of two planes located at distance of X2 and X2’ locates the centre of gravity of the tractor.
