July 14, 2022
The first planetary drive I helped take apart had three stages of through hardened spur gears encased in a simple cast iron tube with plates on both ends. Both input and output shafts rotated on taper roller bearings, but everything else had simple bronze bushings. The sun pinions were shrink-fitted to the previous stage’s “carrier plate” and the third stage “carrier plate” was keyed to the output shaft. It was a big pile of parts dripping with foul smelling tar like oil. The sun pinions had some pitting wear, but otherwise only the bronze bits needed replacing. Looking at it from a “conventional” gearbox point of view, it was at least twice the number of revolving elements to get slightly less efficiency. Only if you put yourself in the shoes of the original designer — back in the early 1920s — did it start to make sense. Yes, there were lots of gears, but they were simple to make and did not require large pieces of raw material. Everything required, including the housing, could be machined on an engine lathe, a drill press, a Bridgeport mill, a Barber-Colman hobber, and a Fellows shaper. Anti-friction bearings were not very cheap — or very good — in those days, so the design worked well with bronze bushings. A small shop could build some “high power density” drives without the need for sophisticated castings, heat treat, or grinding capabilities. Labor was cheap compared to high-quality material, and this product did not require highly skilled operators. Modern users might be concerned about low operating efficiency because of all those meshes, but in that time period no one bothered to measure efficiency. Planetary drives are usually “concentric” arrangements and ours looked much like an electric motor. In fact, they were so common that post-World War II efforts to replace them with less complex helical drives were redirected to adopt that “concentric” arrangement to compete. Getting two helical stages to “line up” requires geometry compromises that a designer reluctantly employs. The “cost drivers” in planetary drives tend to be the internal gears and the planet carriers. Some additional thoughts on those next time.