Introduction to Control System of Tractors

A) Power transmission:

• The power transmission system incorporates a number of distinct parts for transmitting the torque from the engine crankshaft to the traction members.
• The major two jobs (function) of power transmission system are;

I. Transmitting engine power to wheels and

ii. To provide energy for changing the engine torque and speed within the design limits into the torque and speed required by the wheel.

• Engine power is transmitted to the driving wheels through series of intermediates (complete path of power from the engine to the wheels) called power trains.
• The power train transmits power developed by the engine to the driving wheels, power take-off and belt drive.
• This power train consists of:

I. Clutch

ii. Transmission Gears

iii. Differential

iv. Final Drive

v. Drive axle

• Combination of these entire components is responsible for transmission of power generated by tractor engine to rear wheel and implement attached to the tractor hitch.

i. Clutch:

• Clutch is a device, used to connect and disconnect tractor engine from the transmission gears and drive wheels.
• It is necessary for following reasons:

a) For easy cranking.

b) In order to change the gears.

c) To stop the vehicle without stopping the engine.

Types of clutch

a) Friction clutch:

• produces gripping action, by utilizing the frictional force between two surfaces.
• More popular in four-wheels engine.
• It is further subdivided into three classes.

1. Single plate clutch or single disc clutch
2. Multiple plate clutch or multiple disc clutch and
3. Cone clutch

b) Dog clutch:

• has square jaws which are used to drive a shaft in either direction.
• It is mostly used in power tillers.

c) Fluid Coupling Clutch:

• Such type of clutch absorbs shock and vibration, produce smooth starting and easy operation.
• Coupler is mounted on the engine crankshaft and is 3/4th filled with suitable oil.

ii. Transmission gear:

• The tractor engine operates at a high speed, which must be reduced in order to obtain an increase in torque at the traction members.
• Reduction of Speed to increase torque is calculated by using following;

𝑃𝑜𝑤𝑒𝑟 (ℎ𝑃) =2𝜋𝑁𝑇/4500

Where, N = revolution per minute

T = torque (kg-m)

Types of transmission system based on torque control

a) Mechanical drive:

• Transfer and adopts the engine power to the drive wheels of the machine.
• The transmission has two jobs; Select speed ratios, and Reverse direction of travel of the machine.

Common gears used on tractors as a mechanical drive are of two types:

A. Selective Sliding gear type:

• Such type of gear transmits the engine power by means of change speed gears.
• It has two or more shafts mounted parallel or in a line.
• The gear box consists of: gear housing, gear shifting lever, main shaft or input shaft, output shaft and lay shaft or countershaft.
• Speed is reduced in proportion to the number of teeth provided on the gears.

B. Constant Mesh type:

• has parallel shafts with gears in constant mesh.
• The transmission is put into operation by engagement of shifting couplings which slides along the splines on the counter shaft and the output shaft of the gear box.

b) Hydraulic-assists drive:

• is a train of gears that can be shifted ‘on the go’ without interrupting the flow of power.
• control the flow of power through gears that are kept in constant mesh.
• When the operator moves the shift lever, pressurized hydraulic fluid engages the clutches that route power to the selected gears.
• The two types of hydraulic- assist drive are:

1. Countershaft transmission: – It allows shifting one set of gears without disturbing the other gear ratios in the transmission.
2. Planetary transmission: – It works much like the solar system. The basic part are sun gear (center of system), planet pinions and carrier (rotates around the sun gear), and ring gear (surrounds the other parts). Each of the pinions or planet gears, turns on its own axis and at the same time, rotates around the sun gear.

c) Hydraulic drives:

• use fluids to transmit power to the drive wheels.
• The two types of hydraulic drive are:

I. Hydrodynamic drive: – Hydrodynamic drive (torque converter) use fluids at high speeds, but relatively low pressures. Basically, hydraulic pump (hose) forces fluid against a turbine wheel rotating it.

ii. Hydrostatic drive: – Hydrostatic drives used fluids at high pressure but relatively low speed. Basically, energy is transferred by the fluid itself in a closed circuit between the pump and motor. While the fluid does not move through the lines, it is still considered as being at rest or under static pressure. The rising in pressure of the fluid transfers the energy.

iii. Differential Locks:

• The differential lock consists of a movable collar which can be shifted along the splines of the two half shafts.
• As soon as the half shafts are locked, they give the effects of a straight through rear axle, resulting in full traction on both the wheels.

iv. Final drive:

• provides the final and permanent reduction in speed and increases in torque to the drive wheels to reduce the stress on the long axle shafts.
• The final drive transmits the power finally to the rear axle and the wheels.

v. Driving axles:

• It is the component of tractor on which wheels are mounted.

B) Brake:

• Brake is the device usually in the axle housing that slows down the motion or stops the vehicle.
• It is operated by two independent pedals.

Classification of brake

1. Mechanical brake:

a) Drum brake:

• Here, a brake drum is attached concentrating to the axle hub whereas the axle casing is mounted on a back plate.

i) Internal expanding Shoe type:

• Two brake shoes made of frictional material fitted on the inside of the brake drum are held away from the drum by means of springs.
• One end of each shoe is fulcrumed whereas the other is free to move by the action of cam which is turn applies force on the shoes.

ii) External contracting band type:

• This type of brake system is normally available on crawler tractors.
• The drum mounted on the drive axle is directly surrounded by the brake band.
• Braking action of band brakes is obtained by pulling band tightly around a rotating brake drum attached to wheel.

b) Disc brake:

• Two actuating discs have holes drilled in each disc in which steel balls are placed.
• When the brake pedal is depressed, the links help to move the two discs in opposite directions.
• This brings the steel balls to shallow part of the holes drilled in the disc.
• As a result, the two discs are expanded and braking discs are pressed in between the discs and the stationary housing.

2. Hydraulic Brake:

• Based on the principle of Pascal’s law.
• The brake fluid which is usually a mixture of glycerin and alcohol is filled in the master cylinder.
• When the pedal is depressed, the piston of the master cylinder is forced into the cylinder and the entire system turns to a pressure system.
• Immediately, the piston of the wheel cylinder slides outward which moves the brake shoes to stop the rotating drum.
• When the pedal is released, the return spring of the master cylinder moves the piston back to its original position, causing a sudden pressure drop in the line.
• The retracting springs of the brake shoe bring them back to their original position. Thus, the piston of the wheel cylinder returns back.

3. Power brake:

• In power brakes, a separate hydraulic pump is provided.
• When brakes are applied, hydraulic power from an engine driven pump slows the wheels.
• Hydraulic force completely controls the braking of the machine.
• Once the operator presses the brakes pedal to actuate the valve.

C) Steering:

• It allows the driver to guide the vehicle along the straight path or turn left/right as desired.
• The system consists of steering wheel, steering shaft, steering gear, pitman arm (drop arm), drag link, steering arm, tie rod and king pin.
• When the operator turns the steering wheel, the motion is transmitted through the steering shaft to the angular motion of the pitman arm, through a set of gears.
• The movement of the steering arm affects the angular movement of the front wheel.

D) Power take off (PTO):

• It consists of a shaft, a shield and a cover.
• The shaft is externally splined to transmit torsional power to another machine.

E) Hydraulic System:

• enable the operator raise, hold or lower the mounted or semi-mounted equipment.
• It can also be used for depth or height control of the implement attached to, or drawn by the tractor automatic implement hitching, weight transfer to improve traction etc.

Basic components of hydraulic system:

I. Hydraulic pump

ii. Hydraulic cylinder and piston

iii. Hydraulic tank (reservoir)

iv. Control valve

v. Safety (relief) valve

vi. Hose pipe and fittings and

vii. Lifting arms

Operation mechanism of hydraulic system

• The hydraulic pump draws up oil from the oil reservoir and sends it to the control valve under high pressure.
• From the control valve, the oil goes to the hydraulic cylinder to operate the piston, which in turn, raises the lifting arms.
• The lifting arms are attached with implements.
• The hydraulic pump is operated by suitable gears, connected with engine.