Sunday, December 9, 2018

Another Interesting 3D printer Failure- How NOT to Design a Belt Clamp

UMMD has a belt driven Z axis using steel core polyurethane GT2 belts from China.  The belts are held in 3D printed clamps and use short pieces of the same belt to lock the belts into the clamps.

This is a test piece, but the clamps used in UMMD's Z axis use this technique to clamp the belt.
Several months ago I noticed the Z axis belts were flopping around a little, as if they had lost tension.  Initially I thought it was due to the top pulley mounts shifting under the constant pull of the belt tension, and later, after readjusting them, I thought maybe it was caused by broken steel wires inside the belts.

I recently retightened the belts and within a couple days, found they were flopping around again.  I decided it was time to take them off and see what was going on.

Here's what I found:

The polyurethane portion of the belt was sliding and stretching over the steel cables at its core.

The tension on the belt caused the end of the belt to stretch over the steel wires that run through it.  That's not good.  The same thing could happen to any belt held in a clamp the way I have designed the clamps in this machine.  I think it's better to use clamps that fold the belt over on itself to lock it- that way the belt is less likely to slide and stretch on the core.

Here's how the XY stage belts are clamped in UMMD.  The belt folds back over on its own teeth to lock it in place.  This seems to be a better design... it's been working trouble free for >2 years.

I replaced the belts with some glass core, 9mm wide Gates GT2 belts (the same type used in the XY stage), and then redesigned and printed new clamps that fold the ends over on themselves so that this won't become a problem again.

The new design, which is very similar to the design used in SoM's X axis, which has been working for about 5 years:

SoM's X axis belt clamp has been working for >5 years.

I used this printed gauge to check the slot widths for the belt material I had on-hand,

Gauge used to check slot widths needed to clamp different belts.  1.0-3.0 mm in 0.1 mm steps.

then designed the new belt clamp to fit the bed lifting brackets and the belt:

The new belt clamp for UMMD's Z axis.  The space around the posts is just wide enough for the belt to fit, and the entry and exit slots are just wide enough for the folded belt to fit with the teeth interlocked.
Note: the belt clamp file linked above is not the exact dimensions I used in UMMD- I was unable to locate the original file so I recreated an approximation of it that you can easily customize to fit your printer and belt.


  1. Hey Mark

    Do you have the CAD model for that new one you made for the UMMD? the one for the SoM is obviously different but i am unsure what measurements you used for the new one.

    1. The clamp really needs to be custom fit to your printer and belt, but I will add a link to a CAD model of the UMMD clamp in the post, above.

      Belt thickness varies a lot from one manufacturer to another so I suggest you print out a belt gauge (I will post a link to that, too) with slots in 0.1 mm increments so you can check the correct slot size for the belts you are using. Check sizes for both a single pass of the belt and for the belt folded over on itself. The folded belt slot size is most important- you want the belt to be squeezed a bit to ensure that the teeth can't slip over each other and release the belt. You may find the belt gauge useful for sizing slots for things besides belts, too, so keep it around after you print it.

  2. considering you're lying the belt on the toothed section and they're slightly squishy (can squish by 0.3mm on mine) wouldn't the belt go slightly out of alignment depending on the belt tension? how did you calculate where the entrance should be? did you measure the belt thickness?

    1. Even if it squished by 0.3 mm - and I find that hard to believe it could do that under any normal tension- the resulting positional error would be so small it wouldn't matter.

      I start by wrapping the belt around the pulleys and measure the diameter, then in the CAD model, I place a solid block for the belt clamp in its final location. Then I draw a line representing the smooth back side of the belt between the top and bottom pulleys, based on the measured diameter. I make the slot for the belt where the line touches the belt clamp block. The width of the slot is determined by fitting the belt into a printed slot gauge ( I check both a single pass of the belt, and the doubled belt with the teeth engaged.

      Some day we'll be able to model belts that will follow motion of the mechanism...


Leave comments or a questions here and I'll try to post a response as soon as I can.