Belt-Cable Disengagement Error in Simscape Multibody Crane Pulley System

I am modeling a simple crane pulley system using Simscape Multibody, which includes a winch with a spool, an upper fixed pulley, a lower movable pulley attached to a load, and a belt-cable connecting the entire system. During simulation, I am consistently encountering an error stating that the belt-cable has disengaged from the pulley due to the wrap angle becoming less than zero. Although the rope appears visually to be correctly wrapped around both pulleys, the issue still occurs at the start of the simulation. I have observed that even when I provide motion input to the winch, it does not rotate effectively, and the lower pulley moves downward due to gravity. This movement changes the rope geometry dynamically, reducing the wrap angle on the upper pulley and eventually causing disengagement. I have attempted several fixes such as adjusting pulley positions, increasing wrap angle, adding damping in joints, and modifying the rope path using rigid transforms, but the issue still persists. I suspect that the problem may be related to improper cable constraint or initialization of the system. I am sharing snapshots of my model and simulation setup for better understanding. I would like to understand why this issue occurs even when the geometry looks correct and how to properly constrain the system to prevent belt disengagement and achieve a stable simulation.

Answer 1

[Rajasekhar]

Hi Ashutosh,

In Simscape Multibody, belt‑cables are assumed to be always taut and must maintain a positive wrap angle on every pulley. Even if the geometry appears correct visually, any initial downward motion of the moving pulley (for example, due to gravity at t = 0) can reduce the computed wrap angle below zero, which triggers a belt‑cable disengagement error by design.

This behaviour is documented in the Belt‑Cable Endblock description, which states that the cable must satisfy its geometric constraints at all times and cannot become slack.

You may find this documentation helpful: Pulley

If this does not resolve the issue, I recommend contacting MathWorks Technical Support at [e-mail]. They can help determine the appropriate initialisation or constraint strategy.