Mechanical Couplings Difference

[kolias]

Don’t know if this is the right place to discuss couplings but I wonder is there a difference in using either of the ones shown in the attached picture and why?

I have always used the type A to connect a stepper motor to a ball screw shaft on a typical 3 axis cnc and never had a problem but perhaps type B would be a better choice?

Thanks

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[Jim Dawson]

I have never liked type A for transmitting much torque. They work great for driving encoders. It's a spring and thus would have some ''windup''. Also I don't like keyways and setscrews for couplings.

Type B will transmit a lot of torque, and also is easy to disassemble when needed. The full around clamping action of the coupling on the shaft is more robust than a keyway and a setscrew.

[kolias]

Many thanks for the education Jim, it makes sense and I will switch to type B.

[joeavaerage]

Hi,
I used A type on my original mini-mill. As Jim mentioned I did note a little 'lost motion' in the coupling as it would twist under load. The lost motion
amounted to 4um and that compares to the natural resolution of 1um. I often considered replacing them, although really 4um is pretty damn good and probably
better than the over-all accuracy of my machine.

THEN....I had a crash. The steppers are coupled to 10:1 low lash planetary gearboxes and have a lot of torque, about 25Nm at a crash stall. I found that the type A coupling just shore
off. I was very pleased, I was going to badly break or damage something and yet the coupling broke, cheaply and easily replaced. I came to realise that its like a mechanical 'fuse'.
I decided to keep the type A couplers so that I did have a 'fuse' and would accept the minor loss of resolution. I used that machine for seven years and produced a mountain of good work
with it.

My new mill is much bigger and more powerful with servos direct coupled to the ballscrews. I could not tolerate the lost motion of the type A couplers but used twin disc couplings,
they were pretty cheap, $60 for three of them. They have a 20mm nominal bore shaft size, and they work well. They will not shear off if I crash though......I haven't yet, 18 months and counting!

Craig

[kolias]

Thanks Craig, very inetersting