Guidelines for motor shaft length and pulley engagement

Question

Searched, but didn't find this discussed here previously. I'm upgrading a hobby-level CNC router (a Shapeoko 3) to have wider belts (it came w/ 6 mm GT2 (2 mm)), I'm going up to 9 mm (otherwise same specs).

I ordered new pulleys and belting from SDP/SI, but when I went to install them was concerned that the end of the motor shaft (6.35 mm diameter NEMA23) is inset from the end of the pulley by almost 5 mm (4.88 mm). With the previous pulleys, they were flush with the motor end (but probably should have been slightly off to allow the belt to run true).

Is that too great a distance? What would be the maximum distance that the pulley could be off the shaft? What guidelines are there for this? How is it determined? (I've tried searching but haven't been able to find anything – am I not using the correct terminology?)

What is more important, that the belt run true or the pulley be on the shaft? There's only a couple of inches between the idlers (pairs of flanged bearings on M6 bolts) and a great deal of engagement.

The pulley in question is SDP/SI part # A 6A51-020DF0908.

Answer 1

If I understand you correctly, you're worried about the radial load on the motor shaft due to belt tension when the pulley is located axially 5mm further from the motor.

Whether or not the loading scenario you've described is OK depends on many factors; the shaft dia, material properties of the shaft, the motor bearings, the speed of the shaft, tolerable shaft run-out, the belt tension as well as the axial location of the pulley.

The better motor suppliers will specify an allowable radial load at a specific axial location on the shaft, typically the center halfway between the motor bearing and the end of the shaft. That doesn't mean you have to load the shaft radially through that point, but the further away from the motor the radial load is applied, the less radial load is acceptable. I'm guessing that you don't have this information.

The most significant consequences of applying a radial load further away from the motor will be increased radial load on the motor bearings and increased shaft deflection. The increase in bearing load will reduce the life of the bearings and the deflection will cause greater shaft vibration. How significant these are really depends on your application.

I'm assuming that you're using a typical toothed drive belt, which shouldn't require a lot of tension to function properly, so the effect should be relatively small. If you're concerned, you could measure the effect with a dial indicator mounted to the motor, measuring the radial deflection of the shaft in line with the direction of the belt pull with no tension on the belt and compare that to the same measurement made with the belt properly tensioned.

Without more information, I would say to try it because the alternative of intentionally misaligning the belt in an attempt to keep the radial load closer to the motor is going to result in increased belt wear and unwanted friction.