How to create such belt? I tried to create it by creating two circles, two tangent lines (one 6mm above), and arcs. Joined them, then created one tooth, and by using arraycrv tries to get 128 teeth, but here is the mistakes
ArrayCrv might do the trick.
Rather than create a complete section of the belt, just create the outline of the tooth, then linear array that to make 64 teeth (not 128) and join them. Create a single straight line representing the back of the belt, correctly offset from the teeth.
Create a curve to represent the back of the belt as it runs around the two pulleys. Make sure this is exactly 128 teeth long. Then split it in two along the axis of the two pulleys, so you have two equal sections.
Use FlowAlongCrv to copy the 64 teeth onto one of the half curves, using the straight line as your base curve and the half curve as the target, making sure rigid=No. Repeat for the other half curve or mirror the first flowed teeth and join to complete the loop.
From here you can extrude the solid belt from the back loop and the tooth loop.
The reason for splitting the construction into two parts is that FlowAlongCrv wants ends.