[Joe Johnson]

Efficiency of planetary gearboxes are notoriously difficult to understand.

I worked for one of the guys who first put permanent magnet motors in production on a Cadillac door lock system (he has over 100 patents and 4 "Boss Kettering" Awards for valuable intellectual property). Anyway, just to be clear, he was not an idiot. One of his favorite war stories was the time he once put tried to put a printed circuit board motor with a very high ratio planetary gearbox in production for a power window lift system. He used 95% efficiency per stage, thinking, "hey, they are just like spur gears, right?" WRONG! They are not. The are much less efficient and it is not due to the friction at the pivots either because in desperation he put ball bearings on every pivot -- with no noticeable improvement in efficiency.

With low gear ratios you can get quite high efficiencies. After 6 you are asking for trouble.

As to the 85%, I have gotten into trouble over estimating efficiencies. I can think of no cases where underestimating them did so.

I think 85% is a good number to use when you are designing your robot. If you get 90% per stage, life is good, your motor is not working as hard as you had planned for, but if you plan on 95% and get only 85% your motor is a smoking heap of slag...

Joe J.