Making custom length timing belt loops

There are a lot suggestions for making timing belt closed loops that aren’t very strong. For example, cutting teeth in half and sewing the two teeth together. Cutting a belt on a diagonal and sewing (or pinning!) through the teeth. But those don’t account for the fact that the strength of the belt is in unidirectional cords running through the middle of the belt, and that the teeth themselves aren’t really very strong.

Ingredients

Steps

  1. Cut the desired length of belt, plus a few centimeters of overlap. I suggest 15 to 20 teeth.
  2. Cut a sacrificial piece of belt to help align the teeth. I suggest about three times as long as the overlap.
  3. At end A, use a fresh blade (the typical x-acto #11 is a good choice) to cut away the designated number of teeth. Start by cutting off most of the tooth, then carefully slice off layers of neoprene until you feel and hear the blade touch the cord. You should see the cord on the cut surface.
  4. At end B, mark the back side of the belt to the same length as the teeth you have removed from end A. This should start opposite the gap between two teeth. Cut away the back of the belt to, but not through, the cord.
  5. Test fit the ends together, with the cut faces touching each other, and with the teeth interlocked with another section of the belt (or the sacrificial piece). If they don’t fit well, you may have to trim the back of end B slightly further until they fit well.
  6. Glue the sacrificial piece of belt down to a flat surface, teeth up.
  7. Set one piece of cling wrap over the belt teeth, loosely, at least a few teeth longer than the overlapped section.
  8. Make sure that there are no unintended twists in the belt, and lay ends A and B on top of the cling wrap, teeth down and cut sections overlapped, with the overlapped portion in the middle and entirely over the section with cling wrap.
  9. Tape or clamp ends A and B down, but not where they overlap.
  10. If using CA, lift up the loose end of end A until you can see the end of B, and put a drop of glue on the end of B. Lower A down, carefully aligning the sides of the belt. (Use another piece of cling wrap to keep the glue off your fingers as you align the sides.) Wait for it to set. Don’t use an accelerator; that may reduce flexibility.
  11. Lift up the loose end of A (if using CA, up to the glue from the previous step), spread glue over the remaining cut-away back, and then drop A down, make sure it’s well aligned, put cling-wrap over it, and clamp it well aligned until it sets (how long depends on which glue you use).
  12. After it is set, remove it from the jig. Trim as needed.
  13. Let cure fully before putting significant tension on the belt. (IC-2000 reaches maximum strength in three hours)

The glued section will (at least with CA glue) be stiffer than the rest, but in my experience it runs fine through toothed pulleys.

Credits: I’m sure I got pieces of this from various places on the internet, but I can’t find this particular process. I think this is a mashup of the glue advice in back-to-back bonding ideas from Bonding GT2 belts and some pointer to IC-2000 I found somewhere, combined with the now-anonymous (deleted account) PSA: how to splice belts for custom length closed loops that used hot glue.

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That is a cool idea! I’ll keep that in mind. I have a lot of troubles finding toothed belts in the right lengths because somehow I always design things with inconvenient between-centers.

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I always use the cut-on diagonal, drill the teeth out, and use a paper clip inserted in the holes to hold them together. You use a small piece of the belt opposite the teeth you are splicing to hold it all together while working on it.

Of course you can use a better wire on larger belts.

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I wouldn’t want to put the pinned tooth variant under nearly as much tension since it doesn’t bond to the cord. I did see that technique and rejected it for that reason.