Tuesday, 5 October 2010

Wobbly Ball Bearing

Well, here comes the first design problem: The z-axis ball bearing near the stepper motor does not seem exactly perpendicular to the threaded rod, so when the rod turns, the outer rim of the ball bearing moves back and forward, which in turn puts force on its mounting.


Wobbly Illustration.
All very wobbly, really...

I'm still guessing and searching, but... The surface of the nuts I use to fix the bearing to the threaded rod may not be exactly perpendicular to their thread, and/or the nuts twist slightly when they press against the bearing. Secondly, the ball bearing does not have a tight fit to the threaded rod (spacing is less than the thickness of a sheet of paper). This may cause mis-alignment when mounted, though the "wobble" effect seems more than that.

I haven't been able to fix it yet, so suggestions are welcome...

4 comments:

Andy said...

I'm not sure what is causing that bearing to move as you've described, but perhaps an option would be to seperate out the two functions of the bearing. Use the existing bearing to provide axial alignment for the threaded shaft. Then, add another assembly for use as a thrust bearing.

Or, how about adding a sleeve over the threaded rod on each side of the bearing so the nuts are not directly contacting the bearing assembly?

Thomas said...

Andy, thanks for the suggestion. The "sleeve" would probably the second-best solution (after machining the rod). For now I'm using tape.

The axial bearing issue bothers me a bit, because I'm using a standard ball-bearing to hold the load along the axis. The bearing might not last the distance...

Andy said...

Another thought- you could use a bicycle headset bearing. They are designed for significant thrust loads, and would last forever in this sort of application. You could probably buy a cheap headset at your local bike shop for $10 or less. The only problem would be the diameter would be larger than what you'd need.

Thomas said...

Andy, I had a look. they are indeed a bit large for what I do, but it's a good idea to keep in mind for some other up-and-coming project ideas.