Gears down the engine revolution to provide a ‘direct top’ gear ratio. In the case of cars a final drive ratio of approximately 4:1 is used. Bevel or worm gears are employed to achieve the various functions of the final drive.
The Hotchkiss drive is simplest and most popular form of rear axle suspension.
Hotchkiss drive combines the springing and positioning or locating of the rear axle. It uses a rigid axle with leaf spring mounted at its extremities as far apart as possible on the rear axle.
The Hotchkiss drive consists of a leaf spring and a propeller shaft with two universal joints and one sliding joint.
The front end of the leaf spring is pivoted in pin of bracket which is bolted to the vehicle frame.
While rear end of the leaf spring is supported in swinging shackle with antifriction bush material.
The leaf springs are bolted rigidly to the rear axle casing at middle.
The spring takes weight of body, torque reaction and driving thrust.
The driving and braking torques are absorbed through the front half of the rear leaf spring shown by dotted line.
During driving and braking, the bevel pinion changes the position so the length and angle of propeller shaft changes which will be adjusted by universal joint and sliding joint. Therefore if only one universal joint is at the front end, then the propeller shaft may bend or damage.
To avoid this, another universal joint is provided at rear end.
When the vehicle comes across a bump or shocks, the rear axle moves up and down and it has to move in a circle with front spring supported at the frame as centre.
During this movement of rear axle, the length of the propeller shaft changes which will be adjusted by sliding joint.
Torque tube drive:
In torque drive, the propeller shaft is enclosed in a hollow tube.
The tube is rigidly bolted to the differential housing at one end and is fastened at the other end to the transmission through a somewhat flexible joint (universal joint) situated in spherical cup fixed to the frame.
The torque reaction and driving thrust are taken up by torque tube.
When the vehicle comes across a bump or shocks, the centre line of the bevel pinion shaft will not be shift and always passes through the centre of spherical cup.
Hence, only one universal joint is required at front end and no universal joint at the rear end.
The tube incorporates bearing, which support the propeller shaft.
It is usually located between the (transmission) gear box and the propeller shaft.
No sliding joint is required in the propeller shaft.
In many cars a pair of truss rods is attached between the rear axle housing and the transmission end of the torque tube. The torque tube and the truss rod brace the differential housing to prevent excessive differential housing movement.
In this drive, the leaf springs takes only the side thrust besides supporting weight of the body.