Final Drive Purpose

For turning the power flow at right angle from propeller shaft to the rear axle as well to provide mechanical advantage or leverage from the propeller shaft Final dive consists of a pair of helical or spiral bevel gear, i.e. a pinion connected to the propeller shaft and a ring gear connected to a flange on the differential case. It is designed to provide a reduction ratio of 4:1 and 5:1 I.e. when top or direct drive gear is engaged in the gearbox, rear wheels are driven at 1/4 to 1/5 of the engine speed. When an axle transmits power to the road wheels it is known as a ‘Live Axle’ but when an axle is not capable of transmitting engine power to the road wheels it is known as ‘Dead Axle. The rear transmission includes the final drive, the differential and the rear axle all of which are housed in the rear axle casing.

The purpose of the rear transmission is;-

(a) To transmit the power from the propeller shaft to the rear axle through a right angle.

(b) To enable the driving wheels to be driven at a lesser speed than that of the engine.

(c) To operate as a compensatory gearing by which the inner and outer wheels are made to revolve at different speeds while negotiating a corner.

Categories of Final Drive

There are two main categories of final drive: –

(a) The chain type final drive: The chain type final drive is now only used in case of motorcycle. In this class of final drive, chain and sprockets are used to connect rear wheel with gearbox.

(b) Gear type final drive: The gear type final drive consists of drive pinion and a ring gear. There are three distinct type of gear type final drive namely,

(i) Spiral Bevel or Crown Wheel and Pinion Drive. The bevel pinion and crown wheel was originally made with straight cut teeth. Now days, spiral bevel gearing is used because it operates less noisily. The pinion is carried on a short shaft mounted on ball or roller bearings in the rear axle casing. The pinion shaft is driven by the propeller shaft through either a universal joint or a splined sleeve. The pinion is meshed into and drives the crown wheel and the number of teeth on the pinion relative to those on the crown wheel determines the reduction.

Spiral Bevel Drive
Spiral Bevel Drive

(ii) Worm Driveor Worm and Wheel. The Worm drive comprises a worm shaft and worm wheel assembly in which the worm shaft is connected by a flanged coupling to the propeller shaft. The worm shaft has deeply pitched screw cut on it, which meshes with the teeth on the periphery of the worm wheel. This shaft is mounted along the top or bottom of the rear axle casing. The gear ratio is determined by varying the pitch of the worm and the number of starts. The worm is usually made of hardened alloy steel and the worm wheel is of phosphor bronze or gunmetal. When the worm is arranged on top of the worm wheel it is known as ‘Overhead Worm Drive’ it has the advantage of giving more round clearance to the back axle casing, but it is more difficult to lubricate. When the worm is underneath the worm wheel the arrangement is known as ‘Under slung Worm Drive’ which gives reduced ground clearance and ensures adequate lubrication. Worm shaft mounted in bearings and is free to rotate, and prevented from moving backward or forward. Any oil shortage would have serious results with the bevel gearing because of the greater friction between the teeth of the worm and worm wheel.

Worm Shaft and Worm Wheel
Worm Shaft and Worm Wheel

(iii) Hypoid Drive. This is modified version of the bevel wheel and pinion. The main difference is that the axis of the bevel pinion is below that of the Crown Wheel. Arrangement enables the chassis cross members and the floor of the body to be lowered several inches without touching the propeller shaft. Hypoid gear axles run in a special grade of oil, which must always be used. This type of final drive is often used on heavy vehicles because of its strength. Lubrication is of the greatest importance.

Hypoid Drive
Hypoid Drive

Differential

(a) Purpose of Differential. The purpose of the differential is to enable the drive wheels to revolve at different speed, although both the wheels are driven by the same propeller shaft. When a vehicle is running in a curved path the wheel on the outside of the curve, must travel through a greater distance in a given time that of the inside of the curve.

(b) Construction of Differential. This mechanism comprises of two differential (Planet) pinions and two bevel gear wheels (sun wheel) in mesh, which are mounted in a hollow casting, termed the differential cage and bolted to the crown. The crown wheel and cage are mounted in the rear axle casing on roller bearing. The differential pinions are mounted across the cage on small axle known as the ‘spider’. The bevel gear wheels are mounted in recess in the cage and are capable of rotation in the recesses. The splined ends of the driving axle shaft fits in the bevel gear wheels. The crown wheel is not connected directly to the driving axle shaft but to the differential cage which houses the differential pinions, in constant mesh and the differential pinions are with the bevel gear wheels.

Mesh and Direction of Final Drive
Mesh and Direction of Final Drive

Operation of Differential

(a) When Vehicle Steered on a Straight Course. The power from the engine is transmitted to the differential crown wheel, through the gearbox, propeller shaft and bevel pinion which houses the crown wheel to rotate, the differential cage which is rigidly concerned to the crown wheel when the vehicle is being steered on a straight course, the differential pinions do not rotate on the spider but, revolve integrally with the cage. The pinion thus function as bridge pieces or dogs which interconnect the two bevel gear wheels, driving axle shafts and wheels all revolve together, about the same axis and at the same speed when the vehicle is being driven on a straight course.

(b) While On Circular Path or Course. When a vehicle is being driven in a circular path or course the outer driving wheel must travel a greater distance, than the inner driving wheels. Consequently, if no skidding is to take place, the speed of the outer driving wheel must be relatively faster and the speed of the inner driving wheel relatively slower the crown wheel speed remains constant. Actually the increase in speed of the outer wheels are in direct proportion to the decrease in speed of the inner wheels and the average speed of both wheel is the same as that of the crown wheel.

Final Drive

(a) Continuous Hum

  • Tyre noise on the road. Certain kinds of road pavement surface cause tyre noise.
  • Incorrect adjustment of the wheel bearings, the drive pinion bearing or the differential
  • Drive gear and drive pinion tooth contact incorrect.

(b) Hum while coasting

  • Worn out wheel bearings.
  • Inspect drive gear adjustment and readjust if necessary.
  • Scored ring gear and pinion.
  • Pinion bearing rough.

(c) Hum with Engine pulling

  • Crown wheel and pinion too deep in mesh.
  • Pinion bearing rough.
  • Scored ring gear and pinion.
  • Scored or pitted differential side bearings.

(d) A regular knock

  • Broken or chipped teeth on crown wheel or pinion. Differential trouble (i.e. planet pinions, sun wheels broken or chipped teeth)

(e) Rear wheels does not rotate the engine when drive is passed

  • Pinion shaft key sheared.
  • Axle shaft broken.
  • Ring gear or pinion teeth stripped.
  • Differential gear or differential teeth stripped.
  • Differential cross-broken.
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