What do you really get for a $350 desk top engraver ?

andypugh wrote:

I think we would need a better macro photo to tel if that was a fatigue failure or not, but it leads me to be rather suspicious of the motor-to-shaft alignment and mounting. Is there any type of flex-coupling there?

I couldn't make out any beach marks on the surface. A macro would be interesting, as would pulling the bearing shield/seal and checking for signs of heat or abnormal conditions.

Wouldn't misalignment between motor and driven component tend to break outboard of the bearing?  That's assuming the bearing didn't come with lots of negative preload from the factory, of course.

andypugh wrote:

The moment is greater further from the shaft end, and a ball bearing isn't particulary stiff against torsion. Also the step that the bearing butts against, if machined poorly, might be a stress raiser. 

Why do I get the feeling this is not the first time you've looked at failed (or suspect) industrial components?  Spend any time involved with, or doing, NDT/NDE?

I think Andy is on the right path. I can't think of anything other than shaft misalignment that would cause a stepper motor shaft to snap like that.

the only other clue i can offer up at this point would be the fact that the bearing was not pressed in the motor housing. it came right out so maybe a poor press fit added to this?

machinedude wrote:

the only other clue i can offer up at this point would be the fact that the bearing was not pressed in the motor housing. it came right out so maybe a poor press fit added to this?

Bearings are quite often an interference fit on the shaft but close sliding (or transitional) fit in housing.  Makes assembly easier, especially in production environments.  Interference fits on both the shaft and housing make assembly a nightmare.

Frequently for smaller motors a spring/wave/Belleville washer is used inside one housing end to permit the bearing to move axially as the rotor shaft grows (from heat).  That spring preload also tends to keep the bearing outer races from spinning in the housing bore.

If the bearing was a fairly sloppy fit in the housing there was very little resistance to shaft bending and all the movement would have been concentrated at the shaft bearing seat shoulder.

Andy's suspicion about a fatigue crack from shaft misalignment is looking pretty darn persuasive to me.

i just measured the bearing OD and the housing bore diameter, and the housing is a good .002 larger than the bearing OD. the shaft is either pressed into the ID or it got that way when it broke? the bearing feels like it is spinning just fine so i don't think the bearing was the root cause. the break was right at the shoulder where the bearing would ride and seat on the shaft. no wave washer on the front side and it does not feel like it has one on the other end either.

What kind of shaft couplers are you using, and how is (was) the motor connected/aligned to the ballscrew?