Quote:
Originally Posted by Thad House
Even still, that is 4x greater then what the numbers Austin posted say belts can take. If a 15mm 24T pulley can only take 70 in-lb, teams would be snapping them all the time even with 2 CIM drives.
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First of all, exceeding the torques specified in the Gates guidelines will not cause the belt to break, but to jump teeth.
Secondly, the rated stall torque of a CIM is not achievable on an FRC robot. If you were to stall a CIM with 12V applied at the motor, you would draw 133A, which would probably trip the 40A breaker before you jumped two teeth. More realistically, the stall resistance of the CIM is about 12V/133A 91mΩ. The internal resistance of the battery is about 12mΩ, plus about another 2mΩ for the battery leads and power distribution panel, and another 8mΩ per branch for wiring and the motor controller. For two CIMs and a 13.6V battery charge, I calculate that the voltage on the CIMs would be about 9.75V, which would reduce current and torque by nearly 20%, and still trip the breaker quickly. A more reasonable current draw for a few seconds is 60A per CIM, which would give you less than half the stall torque. When you start stacking more CIMs on the drive train, the limit becomes the 120A breaker, or drawing the battery voltage down below roboRIO brownout.
Finally, I believe that the numbers in Gates' document are applicable for exact C-C calculated pulley spacing. I suspect that many teams with adjustable tension in their belt setups over-tension them. This will shorten the lifespan of the belt (from thousands of hours to hundreds or even tens), but increase the torque that a given size sprocket can deliver to (or receive from) the belt by providing a controlling tension on the "return side" of the belt. With enough tension in the belt, the limiting factor really does become the tensile strength of the belt and sheaves.