Curvilinear gear rack and rail

[rogueMech]

I'm working on a prototype that would call for actuated curvilinear motion along an arc. One method I've seen before is to use a curved gear rack and an arc rail (see image). Anyone know any off-the-shelf suppliers of curved gear racks or have a better method in mind?

Cheers,

 

[rogueMech]

Forgot to displace the image:

 

[MikeHalloran]

That's not a rack, it's a sector. ... i.e., just part of a very large gear. Most any gear manufacturer should be able to make it.

You could, of course, just buy stock rack and bend it. ... but the tooth geometry will be suboptimal, the pitch will probably come out wrong, and bending to a precise radius is not a common skill.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

You can produce the same effect with roller chain mounted along the edge of your curve.

 

[Occupant]

Maybe you'll find something here.

 

[rogueMech]

Yeah, I've contacted schaeffler, NSK, THK, etc, and they don't seem to do this concept as off-the-shelf.

I've seen this concept elsewhere, such as in the image below:

(

http://neurowiki2014.wikidot.com/individual:repetitive-transcranial-magnetic-stimulation-rtms)

I like the concept of a compact arc gantry, and the only way I can think of achieving this is through a curved gear rack.

 

[MikeHalloran]

Compositepro is right; roller chain stretched over an edge with a convex profile can work pretty well as a drive means.

Using what appears to be a DualVee four-roller carriage as the sole support for a load that will also exert a substantial moment may be a problem, because the individual ball bearings in the rollers are generally small and have limited capacity. Additionally, the contact surfaces of the rollers and the guide will need to be hardened, and even then have limited load bearing capacity.

The illustration you provided is a concept, only. In practice, especially in a medical environment, gears and rollers and such are typically concealed and sealed, so that airborne dust and debris don't contaminate them, and so that wear products of the mechanism don't fall into the patient.

I don't think you'll ever be able to bend a rack or a guideway accurately enough to allow good tooth contact or good roller contact. Better plan to machine a full circle and cut it into sectors.

Given a sector like that, you could then consider plain bearings of Rulon or similar self-lubricating material, which will be much more compact than any sort of roller of equivalent capacity.



Mike Halloran
Pembroke Pines, FL, USA

 

[imcjoek]

Well the curved rail is available from THK (and probably others)
Link

I was going to suggest that if the torque you need to transmit is low, a friction driven roller could work.
Although, compositepro's roller chain idea is pretty slick. Alternately, timing belt turned inside out?