Subject:
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Re: 12t crown bevel gear
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 31 Aug 2001 14:17:28 GMT
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Original-From:
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Brian B. Alano <alano@IHATESPAMkiva.net>
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Viewed:
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787 times
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Clayton Epps wrote:
> I have designed a very small robot arm that has a shoulder that moves up and
> down and a wrist that moves up/down with a pneumatic gripper at the tip.
> [snip]
> The problem so far has been that as one uses more of these 12t bevel gears
> at right angles, there is more and more "give" (axles turning without motor
> turning) in the axles because the bevel gears do not fit perfectly together
> or "mesh" together.
This is often referred to as backlash.
>
> This makes it extremely difficult for determining an accurate position of
> the wrist at any given time.
>
> The second effect (and more difficult effect to explain) is the wrist tends
> to "fall" a short distance when rotated about the wrist axis from upside
> down position behind the shoulder to right-side-up position in front of the
> shoulder (as in dumping object in basket behind shoulder to moving wrist in
> front of shoulder ready for next object). This occurs just after the wrist
> is pointed vertical in the air and begins to rotate down. [snip]
> Thanks,
> Clayton
I think your second effect is probably also due to backlash. Since the fall occurs just past the wrist's balance point, I infer that the gear lash is changing directions. Reducing it in the worm gear is probably the easiest place to start (put a [non-Lego] washer on the worm gear's axle between the worm gear and gearbox to take up the slack). You could also put a rubber band on the wrist to hold it one direction (probably in the direction of lift). This will force the gears to take up the slack. The tension in the rubber band has to be greater than the heaviest load on the wrist, or the slack will return. Of course, your motor will have to work against this rubber band to lower the hand, but this is the price you pay.
Others know a lot more than me. Try searching www.lugnet.com's technic's or robotics newsgroups for "backlash" or "gear lash".
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Message is in Reply To:
 | | 12t crown bevel gear
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| I have designed a very small robot arm that has a shoulder that moves up and down and a wrist that moves up/down with a pneumatic gripper at the tip. I think this is called 2 DOF (degrees of freedom). The goal of this robot is to pick up light (...) (23 years ago, 31-Aug-01, to lugnet.robotics)
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