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Shaft centering in an end support bearing.

Discussion in 'General Talk' started by TimS, Nov 6, 2019 at 3:28 PM.

  1. TimS

    TimS New
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    Hi everyone. I have a ball spline I'm designing a 3D printer around and ran into an issue once I started measuring the shaft to put it into CAD. The shaft itself is a 3 spline 16mm diameter x 500mm spline section with 15mm diameter stepped end supports and a 3rd step at one end for a motor coupling.

    Problem is that my calipers are showing the end supports at 14.75mm at one end and 14.8mm at the other end. This at best would be a slip fit with a 15mm ID bearing and has me worried about shaft wobble as it turns. So I started thinking of solutions.

    My first and probably most cost effective option would be to drill out the shaft end and tap it. Then use a conical bolt to tighten the bearing against the step while centering the bearing on the shaft. The second option would be to have the shaft machined to a 14mm press fit. This option I can't do myself so it'd arguably be more expensive.

    Am I overthinking this? Am I wrong about the shaft tolerance and a 15mm bearing is okay? Are there other solutions that I may be missing?

    Thanks for any help.

    Tim
     
  2. Christian James

    Christian James Well-Known
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    The shaft needs to be a fraction smaller to allow at least a sliding fit or better a push fit. What you don't want is a sloppy fit because the shaft may turn in the bearing center and wear down making it worse. Also, a micrometer is better than a caliper for accurate measuring. Anyway, I get the impression that you haven't actually tried a bearing on the shaft yet?
     
  3. TimS

    TimS New
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    Correct. I only have the shaft on hand. I picked it up because I needed to take measurements off it to get it into CAD. I'm building the rest of the design around it so I was waiting until I had a BOM before ordering anything else.

    Perhaps I should just order the bearing and see what I can do with it. I believe I should only need one with the stepper and its mount supporting the opposite end.
     
  4. Alex Chambers

    Alex Chambers Master
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    Use a bearing to support both ends - don't load your stepper unnecessarily. Bearings are relatively cheap to replace when they wear.
    Alex.
     
  5. TimS

    TimS New
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    Makes sense. Thank you.
     
  6. TimS

    TimS New
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    Alright. I got my end bearing and it seems to work out well with next to no play on the shaft.

    I needed to come up with a solution for the pulley as well. My nut comes with a 50 tooth 2gt pulley on it and I need to reduce its speed in order for my x-axis resolution to match the other axes. If I use 20 tooth pulleys with 1/16 microstepping on all other axes my resolution works out to 12.5 micron. To match up the x-axis I'd need to have a reduction of 2.5:1. No planetary or worm gear set exists matching that ratio so I'm looking at using a 5:1 planetary gearset and setting the microstepping to 1/8. Doing the math (and using prusa's calculator) this should give me the same 12.5 micron resolution.

    Is my line of think on that correct? Is there an easier way I'm missing? I tend to over think things so I can sometimes miss an easy solution.
     

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