Second, the earth gear bearings need to play an active role in torque transfer. Planetary systems split the torque input from sunlight gear amongst the earth gears, which transfer torque to a planet carrier connected to the gearbox result. The bearings that support the planets on the carrier need to bear the full brunt of this torque transfer.
Or, in extreme cases, they could select angular contact or tapered roller bearings, both 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 space in a planetary gearbox to include the kind of bulky bearings that may tolerate high axial forces.
The existence 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 additional axial forces amount to little more than a hassle. Gearbox designers will most likely upsize the bearings to accommodate 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 really play an active function in torque transfer.
Helical Gears Place Better Demand on Bearings
View our on the internet Helical Gearbox gallery.