Hi Felix,
As you mention, try changing the “Belt direction” in one of the pulleys from “Ends move in opposite direction” to “Ends move in same direction”. Then the simulation shows belts with translational motion and little slip.
In the original model, the issue is that even though the springs are initialized in tension, the force in the belt ends of each pulley remains trivial (even at t= 0 during initialization). This issue is unique to having an odd number of pulleys in a loop.
It was tricky to understand the issue. One way to think about it is considering the simplified model below: the Simscape solver requires that the forces sum to 0 at the nodes: i.e. for pulleys 0, 1, and 2, the model has F0A = -F1B, F0B= -F2A, and F2B= -F1A. If all of the Belt pulleys use “Ends move in opposite direction”, then the additional constraining equations provided by the block are F0A= -F0B, F1A= -F1B, and F2A= -F2B. This set of 6 equations only has the trivial solution of all forces = 0. If Belt Pulley2 uses “Ends move in same direction”, then the last equation is replaced by F2A=F2B, and the system has a nontrivial solution for the forces.
Hope this helps,
Jocie
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