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 connected to the gearbox result. The bearings that support the planets on the carrier need to bear the full brunt of that torque transfer.
Or, in extreme cases, they may 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 create around these axial forces for two related reasons. 1st, there is typically hardly any area in a planetary gearbox to include the kind 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 is critical 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 Helical Gearbox upsize the bearings to accommodate the additional forces.
Since 
they don’t need to withstand any axial forces, spur gear bearings perform just a supporting part in the functioning of the gearbox. The bearings should just support the rotating equipment shafts, however they do not play an active part in torque transfer.
Helical Gears Place Higher Demand on Bearings