Second, the planet gear bearings have to play an active function in torque transfer. Planetary systems split the torque insight from the sun gear amongst the planet gears, which transfer torque to a world carrier linked to the gearbox output. The bearings that support the planets on the carrier need to bear 
the entire brunt of that torque transfer.
Or, in extreme cases, they could select angular contact or tapered roller bearings, both of which are created to withstand axial loads.
In planetary gearboxes, however, it’s much more difficult to design around these axial forces for just two related reasons. 1st, there is typically very little area in a planetary gearbox to include the kind of bulky bearings that may tolerate high axial forces.
The existence of axial forces makes things very 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 additional 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 perform just a supporting function in the functioning of the gearbox. The bearings should just support the rotating gear shafts, however they do not play an active role in torque transfer.
Helical Gears Place Greater Demand on Bearings
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