The Story So Far
There are many different arrangements for planetary drives, but they all require internal gears. As mentioned previously, we do not know by whom or when the internal gear was invented — just that they have been around for thousands of years. We covered the basics of this unique gear element in previous postings, but now is a good opportunity to highlight some of its quirks with regard to planetary drives.
Very seldom, in my experience, has the internal gear been the limiting component for gearbox rating. Modern planetaries frequently use surface hardened and ground internal gears, but this is not bending strength- or pitting durability-related. Higher precision and resistance to debris-related wear are the primary motivations in most cases.
One of the things to keep in mind for all epicyclic drives is that debris tends to stay in the mesh and cause lots of damage. Currently, our accepted standards for rating gears cover only tooth breakage [strength] or flank pitting [durability]. Wear, and debris damage from wear, are not calculated and require testing or observation of units in service. Planetary drives may need pressure lubrication just to move the debris out of the mesh.
Another potential failure mode for internal gears is cracking from the root fillet to the outer surface — or a keyway or bolt hole. Finite element modeling may be needed at the design stage, backed up by extensive testing. There is so much we do not know about these types of failures — including for example the effect of surface finish inside the holes and minimum wall thickness limits.
It would be nice to reduce the mass of the internal gear, but the thinner the ring gets the harder it is to keep it round. With carburized rings you very quickly move into the “die quench” zone, with added cost and expensive tooling.
A Simple, Elegant Legacy
The “legacy” planetary units I learned on were all spur gears. Later we moved into double-helical designs that required special guides for the shaper, along with provisions to keep the two halves in proper angular alignment. This is how a very simple, low-hardness part gets complicated and expensive.
That perhaps is the planetary story in a nutshell, i.e. — they start off as a simple, elegant concept and end up a complex collection of “improvements.”