Transmission Gears

The purpose of transmission is to reduce the engine speed and increase the torque available at the rear wheels of the tractor because

 HP =  where T is torque (kg.m) and N is rev/min.

If the engine hp is constant, it is obvious that for higher torque at wheels, low speed is required and vise versa. So the gear box is fitted between engine and rear wheel for variable torque and speed. This is done by suitable design of gear and shafts.

The transmission uses gears to make more effective use of the engine and keeps the engine operating at an appropriate speed. Transmission's primary job is to allow engine to operate in four or five different speeds. Today's four- and five-speed automatic transmissions need torque converters with coolant, radiators and hoses — all of which cause loss of power and efficiency.

Speed varies according to the field requirements and so a number of gear ratios are provided to suit the varying conditions.

About Gears

A gear is a toothed wheel designed to transmit torque to another gear or toothed component. The teeth (or cogs) of a gear are shaped to minimize wear, vibration and noise, and to maximize the efficiency of power transmission.

Different-sized gears are often used in pairs for a mechanical advantage, allowing the torque of the driving gear to produce a larger torque in the driven gear at lower speed, or a smaller torque at higher speed. The larger gear is known as a wheel and the smaller as a pinion. This is the principle of the automobile transmission, allowing selection between various mechanical advantages. A gearbox is not an amplifier or a servomechanism. Conservation of energy requires that the amount of power delivered by the output gear or shaft will never exceed the power applied to the input gear, regardless of the gear ratio. There is actually some loss of output power due to friction.

Types of gear:

Spur gears:

These are Flat with teeth projecting radially and in the plane of the wheel, "straight-cut gears". These gears can be fitted only to parallel axles.

Helical gears:

A refinement over spur gears. The teeth are cut at an angle, allowing for more gradual, hence smoother meshing between gear wheels, eliminating the whine characteristic of straight-cut gears. - Double helical gears - Also known as herringbone gears. These gears have teeth that are 'V' shaped. Each gear in a double helical gear can be thought of as two standard, but mirror image, helical gears stacked. This cancels out the thrust since each half of the gear thrusts in the opposite direction. They can be directly interchanged with spur gears without any need for different bearings.

Bevel gears:

These gears allow to change the operating angle. However one wheel of such gear is designed to work with it's complementary wheel and no other. Four bevel gears in a square make a differential gear, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile.

Worm gears:

This is gear that resembles a screw, with parallel helical teeth, and mates with a normal spur gear. The worm gear can achieve a higher gear ratio than spur gears of a comparable size.

Sector gear:

This is merely a segment of a spur gear, such as one half or one quarter of the circumference, but still attached to the axle in the normal fashion.

Rack and pinion:

This allows torque to be converted to linear force. The pinion is a spur gear, and mates with a toothed bar or rod that can be thought of as a spur gear with an infinitely large radius of curvature. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s).

Crown gear:

This is a special form of bevel gear which has teeth at right angles to the plane of the wheel; it meshes with a straight cut spur gear or pinion on a right-angled axis to its own, or with an escapement such as found in mechanical clocks

Gears used on tractors

1) Selective sliding type:

The gear box consists of:

• Gear housing

• Gear shifting lever

• Main shaft or input shaft

• Output shaft and

• Layshaft or countershaft.

A number of gears are mounted on these shafts. The mainshaft is directly connected to the clutch and carries gears. The gears are liable to slide. The gears are shifted with the help of shifting lever and shifting fork.

The gears are shifted along the shaft, to which they are splined to engage with another gear as and when desired to connect the power train. The gears are of different diameters having different number of teeth. Speed is reduced in proportion to the number of teeth provided on the gears.

2) Constant mesh type:

These gears are always in mesh. Usually the gears are helical in shape.  The transmission is put into operation by engagement of shifting couplings, which slide along the splines on the counter-shaft and the output shaft of the gear box.