pic: Another Belt-In-Tube Drivetrain

[Chris is me]

Originally Posted by Cothron Theiss

What are your concerns with using smaller pulleys? I assume that the belts would slip before the pulleys or belt would fail, but is belt slippage your concern with a pulley smaller than 24t?

Belt skipping / ratcheting often damages the belts as the teeth are being loaded in an odd shocking manner, so from a robustness standpoint ratcheting is to be avoided as it is itself a precursor to belt failure.

Belt / pulley strength is a function of two things - the diameter (tooth count) of the pulley, and the width of the belt. A larger diameter pulley decreases the force put on the belt teeth a given output torque. A wider belt decreases the stress placed on the tooth by spreading the force out to a wider area. As belt sizes get thinner and as pulleys get smaller, eventually you start to skip teeth via ratcheting, and eventually you'll fail a belt.

In a drivetrain, with HTD belts, 24T pulleys and 9mm wide belts are riding on the edge of feasibility - these drives tend to fail belts by ratcheting and eventually tensile failure. I haven't experimentally determined how small of a pulley you can go to in order to safely use a 15mm wide belt - I'm sure I could estimate it with some math, I just haven't done it. I seem to recall 18T pulleys would cause 15mm belts to skip in a drivetrain, but I don't remember if that was speculation I heard someone else say or something actually based on real world experience.