Trouble mounting HTD belts


I’m trying to mount some HTD timing belts that we bought from WCP and I’m having a very hard time getting them on to the pulleys. They should be the appropriate length as I used WCP’s belt calculator. We’re using 18T pulleys and a 5mm pitch on the belts. All but one seem to be just slightly short.

Below is the measurements and the tooth count given by the WCP calculator

3.277 in - 50T
4.357 in - 60T
4.357 in - 60T
3.535 in - 55T
3.340 in - 50T

Are we just not installing them correctly? How hard should this be?


For your belts on 18T to 18T with 5mm pitch I’m getting these numbers which is the same as recalc. Are you using an 18T to some other pulley?

To add onto my comment, are you trying to roll on the belt?


No, all the pulleys in this system are 18T, so 18T to 18T is accurate.

What does it mean to roll on the pulleys?

If all your pulleys are 18T then your numbers above should be the same for the same size belt:

Copied from above:

3.277 in - 50T → Should be same as 50T below
4.357 in - 60T
4.357 in - 60T
3.535 in - 55T
3.340 in - 50T

Rolling on such as you put the belt on pulley A and then put the belt partially on B. Then attempt to spin pulley B so the belt “rolls” on. It seems like you just have the wrong Center to Center and your numbers should line up with the above.

I verified the center to center by measuring with dial calipers. All measurements were within +/-.01 and that was likely due to human error.

Do you by chance have a video demonstration? I’m trying to picture that process on our contraption but I’m having a difficult time.

The best way I found is to put the belt over the two pulleys and then slide onto the shafts at the same time keeping everything parallel. “Rolling on” will work with larger pullies but you are stretching the belt and they are not designed to stretch.

You will struggle with pullies that small trying to roll them on.

Depend of how your bearing are installed, sometime its easier to pop out the bearing, get the belt and pully on and in place and then tension and push bearing back into place.

This way you are trying to put something round into a round hole when under tension compared to trying to push a hex onto a hex. Much harder under tension when they are not parallel.

A lead on the hex shaft will also help. ie file a 1mm bevel on the end of the hex shaft.

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