CNC motor Coupler Death and Repair

[Toymaker]

How may hours running and at what rpm to generate the first failure?
K2

I couldn't give you even a ball-park guess on those questions. I've never installed a Hobbs run-time counter.

 

[Toymaker]

Here in Portugal they call it "Seal and Glue". It's a very high strength acrylic sealing/gluing substitute for silicone and can be painted. It's used in the construction business. The Amazon example is just one of the brands.
https://amzn.eu/d/fQpSj7g

Looks similar to a product found in the US called Liquid Nails. Liquid Nails Which does dry semi-rigid.

Always nice to collect known working fixes.

 

[Toymaker]

Try these .......
mmm......link doesnt upload

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I use this type coupler on all three axis on my CNC Mill, and have never had a failure. But, the Z axis on my lathe uses a Nema 34 stepper with 1100 oz-inch of torque, while the mill uses Nema 23 steppers with just 291 oz-inch.
Given the increased torque of the much larger motor, I opted for the higher torque coupler.

 

[Steamchick]

Hi Toymaker, (NOT wanting to teach those that already know...) the couplings suggested by Bluejets are intended for angular misalignment, not axial displacement. Of course, the best machines have closely aligned shafts, the cheaper machines often need to be set to a better alignment. Couplings of whatever design will have manufacturers' limits on what axial lateral displacement or angular displacement they can accommodate. The failed elastomer-cross design - in my opinion - may have failed in a "normal" life-time for the ageing and number of cycles, but the failure mode I am fairly sure is a fatigue failure from the combination of age, stress, and number of cycles. If you have used the machine for just a few hours a month then age and stress dominate. That is life. But if you have used it for 40 hours a week for 7 years then you have exceeded the guaranteed life so replace like with like.
Your epoxy repair I reckon will be transmitting higher loads (higher than intended!) To the shaft bearings so those will fail rapidly with continued use of your epoxy coupling. So I recommend you replace with the correct coupling ASAP.
Just my ideas....
K2

 

[stanstocker]

couplers like that are designed to accommodate any misalignment between the shafts, perhaps the reason the rubber deteriorated is that it was being continuously flexed by misalignment, perhaps the epoxy replacement will shift the deterioration to a more expensive to repair part (either the lead screw and nut or the stepper motor itself) (and, no, epoxy does not flex like rubber). perhaps you should have filled the coupler with silicone rubber or shoe-goo or ...

According to Lovejoy, this style coupler is only intended for use where alignment is within 0.5 degrees or better. In situations where the builder has decided they look sort of like universal joints and then tried to use them as such they will fail quickly. With any significant misalignment they will tend to bind unless space is left between the spider and the jaws, not a recommended practice.

Funny how such a simple looking component actually has all sorts of things to consider isn't it?

Cheers,
Stan

 

[Toymaker]

The Z axis stepper motor is mounted to an adapter plate, partially shown in this photo, which has over-sized mounting holes which allow adjustments in the up-down and side-to-side alignment. I had planned to use shims to compensate for pitch and yaw miss-alignment, but when all the screws were locked into position, the coupler could be easily turned through 360 deg of rotation with zero detectable binding. Alignment was, and is, pretty close to perfect,...the Z axis can still be spun easily with just two fingers (OK, actually that's one finger and one thumb)

With zero detectable binding, I'm confident that I'm not transmitting unnecessary loads onto either the motor bearings or the lead screw bearings.

 

[Charles Lamont]

One of the things I learned during a 6-month posting to Metalastic in Leicester was that rubber should always be used in compression.

 

[Toymaker]

One of the things I learned during a 6-month posting to Metalastic in Leicester was that rubber should always be used in compression.

Assuming the rubber spider part is not glued to the metal halves, it certainly looks like the metal jaws of a jaw-type coupler always put the lobes (legs) of the rubber spider in compression during normal operation,...yes?

On my broken coupler, when I replaced the rubber spider with epoxy glue, I cleaned only the outer surfaces of the metal jaws to allow the tape-dam to stick, and intentionally left all of the inside surfaces dirty and greasy, as I did not want the epoxy to stick to the aluminum jaws, but instead, simply form a very close fit epoxy spider. Hopefully my plan was successful and the epoxy spider did not adhere to the metal jaws, thereby placing the epoxy in compression only during operation. I guess I wont know how successful I was until, and if, my epoxy spider fails and I remove it and inspect it. Hopefully that day will never come

 

[Jasonb]

You are better off with an Oldham type coupler if there is radial misalignment than a flexible one. As with everything you get what you pay for so the rubber in those inexpensive flexi couplings may not be the best and coupled with a reaction to the WD 40 will have a shortened life.

Huco oldham coupling on my mill, discs are Acetal