I got the chance last night to test what gives first on my Pro… belt slippage or stepper torque holding capacity. I also need to do this on my Original model XXL with 9mm belts…for science!
I used a fish scale that was well rated, had half pound increments, and one person said they calibrated it to be within 4% of a 2kg weight. Not as scientific as metrology equipment like I use at work, but should suffice for this shop experiment. I simply powered the machine on to lock the steppers and pulled using the scale. I marked the back of the X stepper shaft and the Y steppers already had marks from the factory that I used to see if the steppers rotated. I marked the belt’s position in the pulley to see if they had skipped, at least in the Y. The X is covered on the Pro. I repeated each pull 10 times to try to get an average.
The X is the axis most likely to experience issues (on machines equipped with the ZPlus or HDZ, on belt Z machines, that’s the most likely to skip) due to it having one stepper and belt. On my Pro the average skipping point was at around 30 lbs of force. The belt did not skip, but rather the stepper’s ability to hold did. So no teeth skipped on the pulley, rather the stepper rotated.
The Y skipped at an average of 40 lbs of force. And it also caused the steppers to skip, rather than the belt. There are two steppers and belts, but they don’t simply double it seems. That was surprising to me, but makes sense after some thought.
Conclusion: From this I conclude that belts are not a point of weakness when properly tensioned. I fully admit that I have previous experience with belts on 3D printers and have had a Shapeoko for 5 years. In all 20 pulls the steppers skipped and not the belts. I couldn’t come up with a way to test when the belts would skip as there is not an easy way to mechanically lock the steppers that didn’t also lock the belt.
All the machines in this area of the market use the similar torque rated steppers even if they use leadscrews, ballscrews, or rack & pinion. So every machine using similar stepper torque ratings should skip at approximately the same torque range regardless of drive system. Ballscrews do offer some mechanical advantage, but also have more friction. From what is researched, usually ballscrew, leadscrew, and R&P machines in this class have slower acceleration rates/curves because they were skipping steps during acceleration. Most also have similar “gearing” so mechanical advantage is close on all of the machines in this class.