Cheap stepper motor couplings?

Am just about to fit some production quality stepper motors to my Chester 20V X, Y and Z as a proof-of concept before going to a fuller CNC solution. Will initially drive them from a lashup made of an Arduino plus production quality drivers and a power supply...so essentially just "power feeds" to start with (that bit already built and working fine on the bench)

I will need some couplings between the motor shafts and the leadscrews (full CNC solution will be ballscrews) and I seem to have a choice:
- CNC/production quality Oldham couplings - but they seem very expensive for a PoC, circa £15 each from say Arceurotrade (two hubs and one disk per axis, if I read it right)
- the "flexible shaft" aluminium couplings (seem to be just a spiral cut in an aluminium cylinder) from *Bay, either via an English source (still quite expensive) or from China (cheap)
- something else.

Any ideas please?

You could make you own oldham couplings (just saw cut if you dont have a slitting saw) i've seen people use moulded plastic stuff before, like this;

Edited By Michael Walters on 11/02/2016 18:28:29

I think you'll find the 'flexible shaft' couplings are a series of sawcuts to nearly half way from both sides, with alternate cuts rotated 90 degrees. A spiral cut would act like a helical spring and have little torsional stiffness.

I used a tooth belt drive on my Centec as the endfloat adjusting nut is on the end of the feedscrew and I couldn't see an easy way of fitting an Oldham coupling. It's driven by a PIC with commercial driver, similar to your proposal. If you want the software, which would readily convert to Arduino, send me a PM. One of these days I'll get round to putting some intelligence into it, at present end of travel is set by microswitches.

Duncan,

They are spiral cut and as you say they lack in stiffness. Problem is they are cheap and therefore popular.

Changed loads of them when they have ripped apart. the 3 jaw Lovejoy type are also cheap but these cannot control backlash and not that great at handling miss-alignment.

The Oldham coupling is the only coupling that can handle miss-alignment in two axis whilst presenting zero backlash if you get the right torque disk [ the black ones ]

History trivia:-

Mr Oldham was a Scottish engineer who came up with this coupling for use on Mississippi Belle type paddle steamers. The original torque disks were carved out of wood and designed to shatter if the paddle wheel hit a sunken log.

Previous to this when the paddle wheels were fastened on the end of the drive shaft if they hit a log it cost them either a bent drive shaft or a new wheel. This way the coupling shattered and it was just a case of sliding one half of the coupling up the shaft, insert a new disk, and Belle was your auntie.

[Edit ] kan't spel

Edited By John Stevenson on 11/02/2016 20:34:27

For an overview of coupling types you might want to look here:

http://www.designatronics.com/resources/couplings/metric-flexible.php

Just use a bit of hosepipe and a couple of hoseclips if you think £15 is too expensive and don't see yourself actually reusing something you buy.

I agree with the bit of hosepipe. I've used clear PVC fish tank air hose before now.

Incidentally the plastic coupling in the picture won't allow for any offset between shafts not in line.

Couplings between the stepper motor and the leadscrew are convenient from a construction and maintenance POV as they allow easy assembly and separation of the two items. However if the motor is accurately aligned the coupling does not need to have any flexibility therefore eliminating backlash totally.

If the leadscrew is well supported at both ends you could make a solid shaft coupling and let the motor hang on the end of the leasdscrew. All that is needed then is a tangential strap from the motor body to restrain it.

Well you did ask for ideas!

There are other factors that might need to be taken into account though.

Stepper motors, whilst simple in principle, have limitations that need to be considered to get the optimum performance. I know that in some applications its essential to have some compliance between the motor and its load. If the rotor is ridgidly coupled to a large mass it might not even make the first step of movement.

Ian P

I think the ARC euro price is fairly decent, i would simply go for that, it offers itself as a permanent solution.

Michael W

Thanks John, at least I've learned something today and won't be wasting my money on that type of coupling

I just buy the central Oldham plastic disc (which is the cheap part ) and make the two mating parts from alloy. They don't take long to make and you end up with a cheap solution.

Doug

John Baron, yes it will allow shaft misalignment, the flexible metal connections deal with it.

What about elastomer/pin drive couplings?

Regards,

Geoff

.

Duncan [and John],

This variation suffers negligible torsional 'wind-up'

**LINK**

... much better [in my opinion] than the helical cut.

MichaelG.

Hi Geoff,

I don't think that I was very clear in my post when I said

[quote] won't allow for any offset between shafts not in line. [/quote]

That type of coupling copes well for shafts that are at a small angle to each other, the shafts still have to be co-axial, ie on the same centers at the ends.

NOTE: How do you put pictures into posts ?

Edited By John Baron on 12/02/2016 17:48:11

You can either embed a URL or you need to make a new "album" and post pictures into it, then you can click the camera icon on the post and insert it from the album, hope that helps.

Michael W

Check out the top thread in the top topic on the forum

**LINK**

Neil

Hi Guys,

This is what I meant. I hope that it makes sense now.

PS:

What a horrible procedure just to insert a picture.

Forgot to add thanks Michael W & Neil for album instructions. Though I can't see me wanting to add any more pictures. Sorry.

John Baron,

It's a rather extravagant solution [and probably best avoided] but:

Two couplings and a short rod should accommodate a modest mis-alignment of the type shown in your first example.

MichaelG.

Michael I agree ! This is where "Oldham" couplings score. They can tolerate quite a large axial displacement but are very limited with angular alignment.

I've actually seen rubber tube couplings get so hot that the rubber melts and fails.