Interesting Reduction Gear Box

[SteveW]

Guys,

I got hold of a small precision electric motor and gear-box assembly the other day and just had to have a look. I was expecting a set of compound gears but on first look could find nothing.

The input of the gear box turned easily following the output shaft. Adding a battery across the motor and the end assembly turned at speed. There was also no obvious means of coupling between the two. On reassembly and powered the high reduction action of the assembly was evident.

After a bit of head scratching I cracked it. The end of the motor shaft had a very subtle cam action. The input end of the gear box was two concentric rings of gear teeth, one male and attached to the output shaft and one female fixed to the body. There was barely any clearance between the two but the inner could easily be deflected to fully engage the outer ring by the cam on the motor output shaft.

When the motor turned the cam walked the inner male gear teeth around the outer female gear sort-of peristaltic pump fashion. The result was a very high reduction ratio with little or no backlash consistent with having just one set of engaging gears.

I just had to share this one.

[davidimurray]

I think it's called a harmonic drive.

Cheers

Dave

[baggo]

Have a look at:

www.harmonicdrive.net/

John

[havoc]

Looks like a planetary gearbox to me. Or do I miss something suble as usual?

[chris vine]

Planetary gearboxes ususally have a sun gear, a planet, or two or three or more and an annulus gear round the outside.

Steve's one is different. What a clever bloke worked it out eh?!

Chris.

[GeorgeRay]

Otherwise known as epicyclic boxes and as a reult have three different ratios depending which gear is held stationary. The harmonic drive has only one ratio depending on the number of teeth in the outer ring and the difference in number of teeth between the inner and outer members.