Second, the earth gear Helical Gearbox bearings need to play an active function in torque transfer. Planetary systems split the torque input from sunlight gear amongst the earth gears, which in turn transfer torque to a world carrier connected to the gearbox output. The bearings that support the planets on the carrier have to bear the full brunt of this torque transfer.
Or, in acute cases, they could select angular contact or tapered roller bearings, both of which are made to 
withstand axial loads.
In planetary gearboxes, however, it’s much more difficult to design around these axial forces for just two related reasons. First, there is typically very little area in a planetary gearbox to incorporate the type of bulky bearings that can tolerate high axial forces.
The presence of axial forces makes things completely different for the bearings that support helical gears. But it’s important to make a distinction between fixed-axis and planetary gearboxes. In fixed-axis gearboxes, the excess axial forces amount to little more than a hassle. Gearbox designers will most likely upsize the bearings to support the additional forces.
Since they don’t need to withstand any axial forces, spur gear bearings play just a supporting part in the functioning of the gearbox. The bearings simply need to support the rotating gear shafts, but they do not play an active role in torque transfer.
Helical Gears Place Better Demand on Bearings