Saturday, 25 June 2011

Yet another Prusa Z-coupling

I finally got around to building the Holiday Prusa Mendel I printed over Christmas. I had a few problems with some of the comedy parts and had to revert to using some of the more up to date ones that I sell.

I didn't find the Z couplings worked very well. The requirements are to couple the M8 threaded rod to the 5mm motor shaft exactly coaxially and with no vertical play, but with some angular flexibility to cater for slightly bent threaded rods or any slight angular misalignment.

The rods are not held very coaxially because the clamp is not symmetrical. The alignment depends on how much the two independent clamps are squeezed, which depends on the exact diameter of the shafts relative to the printed diameter of the part.

They are not very flexible either because they have to be strong enough to support half the weight of the X-axis and the extruder. The direction of pull is in the weak direction of the part that tends to de-laminate it, consequently I print them 100% fill to make them strong enough. I would imagine that if there is any wobble in them the constant flexing would eventually fatigue the part and cause it to break.

I looked around at the various attempts to improve these, but I wasn't happy that any satisfied all the requirements above. I did find two sources of inspiration though:

This one by keegi uses a piece of tubing to provide the angular flexibility and it also helps to grip the smooth motor shaft.

This one by Griffin_Nicoll has the strong direction of the part in the right direction, but suffers the same problem as the original because it has two independent clamps. That is easily solved by removing the split in the top section, but then it would be difficult to grip the smooth motor shaft without the clamp halves being exactly parallel, which would depend on the exact shaft and part sizes. It also has no obvious flexibility. Putting the tubing on the motor shaft solves both these problems.

I hacked Griffin's script to make this version: -

I removed the split, changed the holes and the nut traps to fit M3 and changed the motor shaft diameter to 7mm, which is for a 5mm shaft with tubing on it.

Here it is mounted: -

Both halves are identical inside so not matter what the shaft size is they will always centre and align the shafts automatically. The sleeving allows the shaft to flex angularly and also makes a very firm grip on the motor shaft. The part bears weight along its strong direction and is not required to flex at all, so should last forever. Another possible benefit is if the part is made from PLA it is somewhat insulated from the motor shaft by the tubing, so there is less chance it will melt.

I haven't run the axis yet, but it turns very easily manually and there is no wobble at all. I will include these in my kits from now on and I will include the short piece of tubing as it would be annoying to have to buy just 30mm. Note it does require four extra M3x20 bolts, nuts and associated washers.

The files are here on Thingiverse.


  1. I had couplings in both PLA and ABS. I definitely think the PLA worked better.

  2. I was thinking this would be the way to do it. I'll probably need one of these soon for a makerbot upgrade, so I'm glad to see it works.

  3. Looks sturdier, simpler and easier to print, which is always good.

    I assume you might be able to use ducktape (or an equivalent) to simulate the tube and pad the motor shaft? Or would it lack the needed flexibility?

  4. I think it would be flexible enough if you made it thick enough. I would worry it might grip the clamp less because the outside is quite slippery. Also when you roll it round you get a spiral, so you would have some slight eccentricity.

    The tube I used is readily available in the UK from B&Q.

  5. Looks good :-) I'll probably pull it into repo, thanks :-)

  6. Looks great! I did a similar thing but instead of tubing, I used heatshrink!


  7. I would just like to thank nophead for the quick delivery and the well produced product he has sent me.

    Also I think it would be a good idea to rough up the tubing with a little sandpaper, up and down the shaft, just to make sure it has that little more grip.

  8. I'm using the same stepper as shown in the post. You mention some risk of melting on PLA. My motor z-axis steppers do not get warm at at all.

    What about yours?

  9. Yes my motors run cool with the current set to 1A. They get hot on my Mendel because I run it in a heated chamber and I use a bit more current.

  10. Hi, where did you get the plastic tubing from?

  11. I got mine at PetSmart (it's fish aquarium tubing)