Yet Another Prusa Mendel Z Coupling

by nophead, published

Yet Another Prusa Mendel Z Coupling by nophead Jun 25, 2011
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Yet Another Prusa Mendel Z Coupling by nophead is licensed under the GNU - GPL license.

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Couples a 5mm motor shaft to an 8mm threaded rod, ensuring they are coaxial, but allowing some angular movement if the rod is not straight. See http://hydraraptor.blogspot.com/2011/06/yet-another-prusa-z-coupling.html.


Place some plastic tubing over the motor shaft and then clamp it with M3x20 bolts. Make sure there is no grease on the motor shaft by wiping with alcohol.

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Hi, I am currently designing a cam holder for the WanHao Duplicator i3, and I was wondering if I can use this model as part of my model. I will give credits to you and this file.


Yes use them for what ever you want.

Thanks a lot, works like a charm.

Thanks, works very well! I put some tape on the motor shaft so it has a bit more grip when it is being clamped with the screws.

May 30, 2015 - Modified May 30, 2015

thanks! these and tubing work much better than the emergency duct tape or snapped couplings I had earlier!

Thanks nophead!

I used 3 layers of heat-shrink that I had available, to an OD of 7.25mm.

Wobble is gone.

This part was my first 3D print because when building my prusa I put too much presure in the standard coupler. I wish I have seen this part here before because when the part finished printing and tested on the motor shaft it was compleately loose :'(. My solution at 2am was to sand the parts so it will fit but latter I put a little piece of cardboard to hold the motor. It is working but tonight I will try to print it again and use the tubing.

Thank you nophead :-D

my measured diameter with tubing in the motor is 6.5 - 6.6 mm ... how much it should be?

It is set to 7mm in the scad file. You must have thinner tubing than me.

so, should I change the .scad file for a smaller diameter?

Ideally yes, or get thicker tubing. There is some adjustment though so you might get away with it.

what you think of the professional couplings? the aluminum ones like this: http://www.ebay.com/itm/5x8mm-CNC-Motor-Jaw-Shaft-Coupler-5mm-To-8mm-Flexible-Coupling-OD-20x25mm-06-/170827364358?pt=LH_DefaultDomain_0http://www.ebay.com/itm/5x8mm-...

they are kinda cheap... do you like them?

No they are like a spring so will not give a stable Z position. They are designed to got between two bearings and have no axial load.

do you think the fact that I have a smaller tubing makes smaller area between the coupling and the shaft so making it less flexible, maybe not enought as it should be?

I'm using these with a rubber hose intended for using with glasses. I got mine at a rafting trip, it's a very soft, almost foam-like rubber. Even though my Z-Motors are bolted down, i get no wobble at all. :-D

Nice :) An extra advantage compared to the prusa standard is that it's usable even when printed on a poorly-calibrated printer.

This fixed ALL of my z-wobble / see saw. The motors do not move at all now. The only difference from this is that i use rubber tubing and clamped it down hard. It is the very tough "surgical tubing" available from medical, swimming equipment, sling-shot, etc.

The vertical corners on my prints are very nice and even now. No "repeating pattern" of slight differences any more.

8-) 8-) 8-) 8-) 8-)

Any reason for the plastic tubing on the motor shaft? I edited the .scad to make the motor coupling end a simple 5mm hole and just attached it directly to the motor.

Seems like you've missed one of the major improvements of this coupling! If you read the description above or nopheads hydraraptor blog you'll see that the plastic tubing is to allow for angular flexibility and help grip the smooth motor shaft!

For people who don't have rubber tubing lying around, I've used approx. 35mm lengths of elastic band wound round the motor shaft instead, and they seem to be working fine.

I ended up making these to replace the original prusa ones, which broke when I had a z axis crash (thankfully protecting the whole x carriage and frame from damage). This got me thinking that a version with a designed-in torque limit would work well as passive crash protection. It would be quite v
ariable depending on infill/material/calibration but if I get time I reckon I'll have a bash at making some prototypes to test the idea.

Anyway thanks for this major improvement on the original parts! :)

If you only fit the bottom nuts in the x-ends then if you crash into the bed the nuts pop out and prevent any serious damage.

Ah yer, good point. I originally had to fit top nuts too to push the axis down as the bushings wouldn't run smooth enough for it to drop, but thats a good point now I've sorted out all my squeaks I could probably do away with them. cheers.

I used this version for my SAE PRUSA. I drilled out the screws to #6 and the main shaft to just under 5/16". I did this so I could press fit the motor shaft with the plastic tube over it to make it tight.

It actually worked out better than the original SAE part this is derived from. Tighter fit. Working on my first couple prints since the upgrade. So far so good!

Just finshed printing and fitting them to my Prusa. They work great, NO wobble at all.

I used 10mm OD PVC tube.

Thanks 8-)

Great. Thanks for testing them for me!

I designed a bearing assembly that mounts to the motors screws and uses a bearing to support the load of the carriage weight and take that load off the coupler and the motors bearings. As designed, it works for motors on top, but you could flip the bearing caddy and lock a couple of nuts on the rod to sit on the bearing. Might work well in concert with your couplers.


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