Transfer Box and Centre Differential
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.
A description of differentials on both part-time and full-time 4 wheel drive vehicles and a look in to how wind-up occurs in a 4x4 vehicle transmission and how it's avoided.
A basic view and explanation of what transfer boxes/cases are for, ad how they achieve the split of drive and accommodate the different gear ratios required by the driver.