Inversed Timing Belt Transmission

One day, I had an idea for a compact gearing mechanism. It is light, compact and has no backlash (that I’ve noticed). The inner pulley is 20 teeth, the outer ring is 120 teeth and the belt is 150 teeth.

It was a nightmare to compute the dimensions analytically so I finally gave up and made a small python program that solved the problem numerically. This worked out great and I was able to 3d print the prototype shown in the video above.

Behavior under load

I was initially hopeful that the setup would perform well under load, but that turned out to be false. Unfortunately, when a load is applied there is a small inwards force on the belt making it prone to detaching from the outer ring.

Increasing the number of bearings or replacing them with a continous loop of pins/cylinders that rotate as the belt rotates might minimize the issue, but I’ve not gotten around to try it yet.

Prior work

After coming up with the idea, I found a similar mechanism mentioned in the conference paper ePaddle mechanism: Towards the development of a versatile amphibious locomotion mechanism. After publishing the video above I got a comment that directed me to a US patent (US5954611) that seems to describe the same mechanism in detail.