Final Drive

Final drive gears are incorporated in  vehicle driving axles and transaxles  for  the following reasons: 

1) to provide a right-angled drive  from either the propeller shaft, or  the   gearboxmainshaft  , to the driven wheels;

2)alternatively, to provide a parallel drive from the gearbox mainshaft to  the  driven wheels;

3)to permit an additional and constant gear reduction in the transmission  system.

The hypoid gear is in common use today due mainly to the fact that the offset pinion allows the propeller shaft to be set below, (for cars) or above, (for commercial vehicles) the crown wheel center. This gives either a reduction in the propeller shaft tunnel, which causes a bump in the vehicle floor, or in the case of a commercial vehicle a reduction in the angle through which the universal joint has to operate.

Large crown wheels reduce the ground clearance under the differential. In the case of pinion axles and planetaryhub reduction axles, the differential and the half-shafts can be made smaller by splitting the reduction. This makes it possible, even in the case of high power outputs, to achieve sufficient ground clearance of the vehicle.

Planetary gears are used mainly in final drives, rear-mounted splitter boxes and, above all, in automatic transmissions. The simplest form of planetary gear (the planetary-gear train) consists of sun gear, ring gear and arm with planet gears. Each of the three elements may act as drive, output or may be at rest. Therefore, great variation is possible as regards the transmission ratio, ranging as far as a reversal of the direction of rotation. The parallel meshing of several gears under load permits a compact construction.