Second, the earth gear bearings need to play an active role in torque transfer. Planetary systems split the torque input from the sun gear amongst the planet gears, which in turn transfer torque to a world carrier linked to the gearbox result. The bearings that support the planets on the carrier need to bear the entire brunt of this torque transfer.
Or, in extreme cases, they could select angular contact or tapered roller bearings, both which are Helical Gearbox created to withstand axial loads.
In planetary gearboxes, however, it’s much more difficult to design around these axial forces for two related reasons. First, there is typically very little space in a planetary gearbox to incorporate the kind of bulky bearings that can tolerate high axial forces.
The existence of axial forces makes things completely different for the bearings that support helical gears. But it is important to make a distinction between fixed-axis and planetary gearboxes. In fixed-axis gearboxes, the excess axial forces total little more than an inconvenience. 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 enjoy just a supporting role in the functioning of the gearbox. The bearings should just support the rotating gear shafts, however they do not play an active function in torque transfer.
Helical Gears Place Greater Demand on Bearings