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General

Module 1. Tractor Mechanics

Module 2. Traction

Module 3. Introduction to Transmission System

Module 4. Clutch System

Module 5. Gear Box

Module 6. Differential and Final drive

Module 7. Brakes

Module 8. Steering system

Module 9. Hydraulics

Module 10. Power Transmission

Module 11. Human Factors

## Lesson 4. Analysis of tractor in static conditions

**Analysis of a tractor:- Level surface and horizontal pull**

Analysis of the tractor is done to measure the weight transfer coefficient due to pull exerted by a tractor while pulling an implement.

Fig. represents the various forces present on a tractor operating on a level horizontal surface with pull being horizontal and parallel to the direction of motion.

On the basis of the assumptions listed above, gravitation may be conveniently and satisfactory represented as shown in fig.1 by the weight W_{1}, supported by the rear wheels, and the weight W_{2}, supported by the front wheels, when the drawbar pull is zero. Likewise, the soil reaction can, for the purpose of this approximate analysis, be resolved into three components R_{1}, R_{2}, and F.

If the tractor is considered as free body, the algebraic sum of all forces acting parallel to the motion must equal zero:

F – P = 0 (1)

Likewise, the algebraic sum of all forces acting perpendicular to the direction of motion must equal zero:

R_{1} + R_{2} – W_{1} – W_{2} = 0 (2)

The algebraic sum of the moments about any given axis must equal zero. The problem is greatly simplified by summing moments about C, the intersection of the soil reactions R_{1} and F (fig.1). The line of action of force W_{1} also passes through this axis. The moment equation is:

W_{2} x_{1} – Py_{1} – R_{2}x_{1} = 0 (3)

From these three equations the values of the soil reaction may be readily calculated in terms of the tractor’s weight and the drawbar pull.

Solving equation 3 for R_{2 }:

And equation 2 for R_{1}:

R_{1 }= W_{1} + W_{2} – R_{2 }(5)

Substituting the values of R_{2} from equation 4 for R_{2} in equation 5:

The stability of a tractor is, to a great extent, determined by R_{2 }and the tractive capacity by R_{1}.

The term expresses the change in soil reactions R_{1} and R_{2}, resulting from the drawbar pull P. The soil reaction R_{1}, supporting the rear wheels, increases as P increases and the soil reaction R_{2 }decreases. This relationship is true until P becomes large enough to cause to become equal to W_{2}, which in turn causes R_{2} to become zero. Any further increase in P will cause the front wheels to leave the ground.

Whether the tractor will become unstable and tend to turn over backward will depend on a number of factors, such as the location of the center of gravity of the tractor and the location of the hitch points to the tractor and to the implements being pulled. These factors are discussed later.

Although there is no actual shift of weight, this change in soil reactionsR_{1} and R_{2} is commonly known as weight transfer.

If zero is substituted for R_{2 }in equation 4 and the equation is solved for P, an expression is obtained for the value of the drawbar pull P at which the soil reaction against the front wheel becomes zero.