Have you ever encountered this

[Bart_]

Pulled a small winch apart to check, service, lube. It specced single stage planetary. I find inside 6tooth sun gear, 22 tooth 3 planetaries, stationary ring gear 48 teeth and the planetary cluster floated, no output from its drive plate. Then I see another ring gear "same size" as the other and I figure WTH how can this work at all. And that ring gear is the spline out, it's dish shaped. So the pondering begins as I varsol the grease out, checking gear teeth condition etc. Found a cracked but usable needs-lube bushing on the motor which was neat, it was a 4 brush face type commutator not drum shape.

So pondering the impossibility of it working at all, I start counting gear teeth again and find the spline out dish ring gear has 51 teeth.

So the stupid thing is a harmonic drive 153:1 where they did pitch circle 48 teeth meh close enough to pitch circle 51 teeth, concurrently mesh them both on the same planetary cluster. From a mech eng view that seems like taking liberties with how things are done.

For harmonic drive look up the Canadarm. On there they spring the shape of the ring gear to mesh "wrong" gears for big reduction in 1 step.

Learnt something new. Learnt is a wurd isn't it?

 

[Reach]

Very interesting sounding setup from your description. Pictures would be wonderful if you have them too.

Sure, learnt can be a wurd, just don’t call it a word!

 

[Bart_]

Sorry I reassembled it full of grease already, no pics. Think of a normal ring gear split in two each half takes up 1/2 of the planet gear width. Except one half has 48 teeth, the other half has 51 teeth. So as the tooth engages it shoves its way in and displaces the 51 tooth a hair. Definitely not rolling involute contact.