Axles and Differentials
The Best Explanation of Differentials ever, an old one but very good.
The differential is there to allow the individual drive wheels (Left and Right) to turn at different speeds but with equal torque. This allows the vehicle to drive around bends without winding the transmission up, scrubbing the tyres or breaking something.
On rear wheel drive vehicles the differential may connect to half-shafts inside an axle housing, or drive shafts that connect to the rear driving wheels. Front wheel drive vehicles tend to have the pinion on the end of the main-shaft of the gearbox and the differential is enclosed in the same housing as the gearbox. There are individual drive-shafts to each wheel.
Epicyclic centre (3rd) Differential & Viscous coupling
Centre differentials are there to allow the torque to be sent to the front and rear axles with equal torque, while at the same time allowing them to turn at different speeds. Thus preventing wind-up.
An epicyclic centre diff allows more torque to go to the rear axle than the front.
The Epicyclic centre diff has an unequal torque split which is produced at the 2 outputs. Generally, 33% is supplied to the front axle and 67% is supplied to the rear (which has the more heavily loaded rear wheels).
The Torque difference is achieved by the difference in leverage applied to the output shafts by the sun and annulus. Torque is a product of force and distance, and as the force apllied to both sun and annulus is equal it follows that thier diffrent radii from the output centre line will vary the torque.