Coupling Stepper With Rigid Material - Any Problems?

[Brenck]

Hi,

I'm looking for a reaaaaly cheap way to couple steppers to threaded rod. I've used hoses before but they kinda flex a little bit even the big ones or the ones with steel fibers. I was wondering if is there a problem to use a round aluminium bar with a hole passing trough.

Motor shaft is 1/4 and the rod will be turn to 1/4 to, my plan was to make a passing center hole in a 5/8'' alum round bar ( basically like a thick wall tube but perfectly centered ) and the fix the motor and the rod axle with thoose screws used for pulleys. That way everything would be centerd so I would first insert the coupling in the rod, them inser the motor shaft in the coupling and tighten them up, just then i would tighten the 4 screws of the motor to the structure.

I wonder even if perfect alignment occur if I would have problems with vibrations...

Thank You

[High Seas]

I'd say build -- or buy?
And, the shaft might be to rigid = the opposite of the tube approach.
These might work:
http://homanndesigns.com/store/index...index&cPath=21
Quoted as zero backlash - and still will tolerate some misalignment in motor and shaft.
Not cheap as chips but reasonable I'd say.
Cheers - Jim

[LeeWay]

I do use several home made solid couplings on some of my machines. This is only possible if you are able to get them perfectly centered. Otherwise you will have undue wear and vibrations.
I generally design with the intent of making my own couplings. Then if I can't get the axis aligned perfectly to the motor, use something else.

I like the type couplers shown in the link. I only have one of these on my mill and it is stainless steel instead of aluminum.

Thee are the ones I use elsewhere when I have alignment woes.
I get these from Mcmaster Carr, but they are available elsewhere.

[Brenck]

Thanks for the posts,

Thoose flexible couplings are the worst thing I ever seen... Dont last 2 months. I used thoose ( in my 2nd machine ) put them in place looked they looked perfectly alligned, motor torque ratings where more then I had here, in 2 moths, all snapped, oldham its allright but still expansive...

I'm not buyng, as I said realy cheap, getting a round stock of aluminium and putting in a lathe drill the center then reamer the hole to 1/4 would cost $0,50 per coupling, or less...

It would be perfect centered because it would be one single hole passing trough, made on a lathe, I wouldnt have to turn the piece to drill the other side.

I dont know I was conserned if rigid coupling would transfer more of the motor vibration to the rod and then to the machine, maybe aluminium is not the best choice, I dunno...

[LeeWay]

My solid couplings are steel.
You can torque the set screws more in steel.
I wasn't referring to the coupling being true, but it should be as well. I was talking about the motor in relation to the lead screw. It can be difficult to get that perfect, however if I have done it, it can be done.

[Brenck]

Do you think set screws are better? Might be even better if its clamp style, of course is harder to make.

[LeeWay]

Clamp style is best of course, but two set screws each 90 degrees from each other work well. Especially so if you ping the spot they will be sinking into and use the pointed set screws or grind a point on standard ones.

[handlewanker]

Hi Brenck, the problem has been hi lited as an allignment one first and a vibration damper second.

On the mines we had couplings between motor and gearbox that consisted of a plain pulley on the gearbox with four holes in the face, and a second pulley on the motor with four pins sticking out of the pulley face.

The pins fitted into the holes with a rubber bush and allowed quick allignment and resilience for vibration.

A second design we used had each of the pulleys with two pins sticking out of the faces, but spaced radially at 90 degrees to each other.

There was a gap between the pulleys that allowed a hard rubber disc with four holes to take the pins.
Very quick and easy to allign an mount.
Ian.

[Nzoldun]

If you are going the rigid way, alignment is absolutely critical. Drill your hole undersize, BORE with a boring bar and then ream. Machine O.D. after, then part-off.
Only way to ensure bore and od are EXACTLY true .

[Mike Everman]

Originally Posted by Brenck Motor shaft is 1/4 and the rod will be turn to 1/4 to, my plan was to make a passing center hole in a 5/8'' alum round bar ( basically like a thick wall tube but perfectly centered ) and the fix the motor and the rod axle with thoose screws used for pulleys.

Agreed that alignment is the main thing that kills rigid couplings, but of course it's not the coupling that suffers. Machining the end of the screw is one concentricity that will be hard to hit, another is that motor shafts don't always run true either. plus or minus two thousandths of total misalignment, or a tiny, tiny bit of angular misalignment and this won't last long. Setscrews are definitely out, unless you achieve a nice transition fit (kind of a hand tight press fit). Point being that the setscrews are not pushing the shaft across a clearance hole, making it eccentric.

Oldham couplings are incredibly easy to make. You could make one in a drill press in short order. Just for fun, here's an oldham animation:

YouTube - Oldham coupling animation

YouTube - Oldham coupling animation