The Planetary Gears heat

A planetary gear train running at very high speeds and continuous duty is apt to generate enough heat to justify cooling. With ordinary gear-and-pinion systems, the load often requires quite a large chunk of gearing and surface area, which amounts to an appreciable heat sink. The compactness of planetary units can limit the rate of heat dissipation, and therefore additional measures are sometimes taken; the lubricant may be circulated through a heat exchanger, or a cooling fan is employed. If the operation is continuous, the system has less opportunity to cool than if it runs intermittently, and without sufficient cooling, the speed allowance might need to be lowered. Or, as mentioned, other kinds of speed reducers can be attached ahead of the planetary reducer, although this is an added encumbrance.

The range of speeds for planetary gearing varies widely depending on the application. Often, the size of the gear drive largely affects the speed rating, since a higher pitch line velocity can mean a heat increase far beyond any cooling effect from the larger geometry. Indeed, there are small planetary drives that run at tens of thousands of rpm.