OT Eccentric Gear set.

elevmike

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Join Date
Feb 2004
Location
Detroit, MI
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I'm trying to build a model of a machine. What I need is to find a gearset where the speed of the driving shaft would be constant, but the output shaft would speed up for 180 deg then slow down for 180 deg. After one full rotation both shafts must return to their origional positions. This would require an eccentric gearset where the drive shaft and output shaft are both offset from the center of the gears. I tried this with some Lego gears, but the problem is both gears are identical and thus binds. So apparently this type of gear set must be matched, as best as I can discribe it: male & female. I've googled and googled and looked through hundreds of pages but all with no luck & was hoping somebody here might have run accross this before.

Any links or suggestions would be appreciated. Thanks, Mike
 
Simon,

This needs to be a mechanical soulution.

Steve, Thanks. The drag link should work for sync-ing the model but I'm not sure if it would be acceptable for the real thing.
 
You might try looking for elliptical gears. This could accomplish what you want. Another possibility is a Geneva mechanism, although this is more for intermittent motion. Still another device that might possibly work is a differential (I think Cherry is a company I used in the old days for web adjustment in printing) where the position of the ring gear is shifted by a worm gear.
 
Hi Tom,

Elliptical gears would cause the output to speed up and slow down twice per revolution, but eccentric gears would cause the output shaft to change speed once per revolution. (I think??)

The both shafts need to be in constant motion, so the Geneva mech is out. I looked at phase drives, but that would require another mechanism to change the phase. I was hoping to make it more simple.

I'm no gear expert, but when I take two identical gears, and offset the shaft, I ALMOST get the desired result, but the gears bind after about 300deg of rotation. I'm not shure if this is because the gears have the same number of theeth, or if the shaft offests are not just right.
 
Tom Jenkins said:
If they change twice per rev, you could put in a 2:1 final reduction or increase.

I'm not sure I get what you mean? The output shaft needs to increase in speed from 0 to 180 then reduce in speed from 180 to 360. For example in terms of rpm: (RPM / 2) for 1/2 rotation and (RPM x 2) for 1/2 rotation. On a time-velocity graph, the speed variation of the output shaft would be in the form of a sine wave.
 
Oopps!!

Tom,

Your right, I do need an elliptical gearset. After looking at the model again the shaft speed changes TWICE per rotation. Thanks for the nudge...

So where in the world do I find an elliptical gear set??
 

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