Mechanics and Construction > Mechanics and Construction

micro arm joints

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johan:
Hello,

I wonder if I could just pick people's brains for some reference websites or books that I can buy to research the different type of arm joints. In particular working with small motors and the pros and cons of various types of linkage (bevel gearbox, worm gear etc)

I'm building a photography device that actually functions very much like a robot arm. Its purpose is to orient objects so I can take images - but it's basically an arm on which is mounted another (rotating) arm on which is mounted another (rotating) arm on which is mounted some clasping devices.

This device is small (15 cm arms) and motion repeatability and rotation speed are not critical. Torque, temperature and vibration are, so I'm tending towards using small worm geared DC motors on each arm joint to rotate the subsequent segment. These seem to have the advantage that being worm geared it means I don't need power for holding torque, no power means no heat, and simpler all round. Something like this 1000:1 motor and a worm gear (yet to be found).

Not having any great experience with these can anyone comment whether that would seem to be a rational choice or is there something better? I have a small nema8 stepper on the way to try out as an alternative but as they require power for holding torque I suspect that's not the answer.

More importantly though, does anyone have any experience or insight or URLs to point me towards which might show the best way to rotate a subsequent rotating arm using a worm gear. I'm a little unlear on the mechanical aspects, ie do I need a sleeve, how best to attach a subsequent arm so that the weight isn't being carried in the wrong way.

Secondly, small gears and linkiages like small worm gears - is there a recommended website for these that people know of?

jwatte:
You don't want the worm gear to have any lateral force -- it's not a structural, holding, kind of assembly.
Thus, you need a bearing of some sort for the rotating/pivoting action of the arm.
Once you have that, attach the worm gear to the rotating arm, and mount the worm leadscrew on the holding arm so that it properly contacts the gear, and then mount the motor so it drives the worm screw.
I've seen a number of worm screw/gears online at sdp-si.com; you may want to try that too.
Anti-backlash worm gears: https://sdp-si.com/eStore/Catalog/Group/210
Mating worms (screws) to those gears: https://sdp-si.com/eStore/Catalog/Group/211

Two other options I would look at:
1) Existing robot arms. For example, Trossen Robotics have a number of arms of different capacity. They will need some power for holding objects, but they are a ready-made solution.
2) Orbiting the camera around the object instead of the object in front of the camera.

johan:
Thank you very much, appreciate the response. Your methodology for making this, with motor last, makes a lot of sense. I'll go away and study the links etc - I have to say it's surprising that t'interwebs doesn't have some sort of wizard or findable explanatory site about combining motors, gearings and bearings!

johan:
D'oh. It's actually been a while since I've been to this site and I'd forgotten about all the reference material here. That gearology PDF is pretty awesome.