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Adjusting Eccentric Spacers for Resistance

Discussion in 'CNC Mills/Routers' started by Sprags, Jan 22, 2018.

  1. Sprags

    Sprags Veteran
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    So I started building my Sphinx C-Beam Machine last night.

    And the first plate I assembled was a Y-axis plate. I found that I had to adjust how much I needed to torque down on the M5 locknuts in order to allow the wheels to spin without too much resistance. I noticed that some bearings feel smoother than others.

    I then assembled the next Y-axis plate and because I did it once the next one went faster. I did not need to loosen the nuts from torquing down too much on them.

    I installed both on the c-beam and can tell that the first one rolls easier than the second one. Which is odd. I would have thought that since I did not put any excessive forces on the bearings that the one i assembled second would have been better. Now..i reality neither one is really bad..they just aren't the same. And since the Sphinx C-Beam uses two stepper motor to move the Y-axis I have to believe that if they both move equally as free then that is better for the stepper motors and the accuracy of the machine.

    So I am thinking I need to get some silicone lubricant to spray onto wheel bearings because I have now loosened the screws and they still feel like there is still some friction. And perhaps I should start all for adjusting the tension by rotating the eccentric spacers so they roll as loosely as possible and then adjusting them going from perhaps the small wheels to the large wheels from inside to outside.

    Any suggestions would be appreciated.

    Thank you,

    Blake
     
  2. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
    Staff Member Moderator Resident Builder Project Maker Contest Winner! Builder

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    Did you by any chance forget the shim inside the wheel? Clamping forces are transferred from inner bearing race, to shim, to adjacent race. Forgetting the shim, leads to what you described above. Check the OpenBuilds youtube channel for videos showing how to assemble wheels, eg from 0m59 on for example
     
  3. Sprags

    Sprags Veteran
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    NO I did not forget to install the shims..one between the bearings inside the wheel and the one that gets installed between the eccentric spacer and the wheel/bearing/shim assembly. But now that you mention it, it seems that a shim should be installed between the wheel assembly and the locknut but the build guide did not call out for one. Would that help?

    The larger wheel assemblies have a shoulder to keep the bearings separated in the assembly process so unless you torque down on the locknut A LOT those bearings should never make contact. I never torqued down on any of the locknuts to the point where the wheels could not spin

    The friction I am feeling is not significant..but it is still noticeable if you know how to properly assembly stuff. I've built lots of things throughout my from motorcycle engines to jet engines ( i worked for GE Aviation). I actually work for an aerospace bearing and seal manufacturer now and i run the machining department for bearings. I know how to machine stuff.
     
    #3 Sprags, Jan 22, 2018
    Last edited: Jan 22, 2018
  4. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
    Staff Member Moderator Resident Builder Project Maker Contest Winner! Builder

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    The shoulder gets clamped by the outside bearing race - thats what "mounts" the wheel to the bearings. But the clamping loads are not transferred through that shoulder - its transferred through the precision shim between the bearings. Without the shim (or if by any chance someone, not you specifically, just saying this for future searches if this thread comes up - got shims from some obscure supply chain in the east and they arent the exact same thickness as the shoulder) the forces actually press the bearing inner and outer races apart, which is why thats the goto reply when someone mentions tightening and loosening the bolt, changes the friction. As per design (and many millions of tested wheels) that doesnt happen, unless the shim isnt doing what its supposed to. That said, its always hard to troubleshoot those sort of things without the parts in hand, so I am leaving the judgement up to you.

    RE: Shim between locknut and bearing: No need, the locknut is smaller than the outer race of the bearing, and shouldnt rub
     
  5. Giarc

    Giarc Master
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    I ordered a few of the wheel kits from the "far east" out of curiosity. The shims were not 1mm. They were all less than that. Also, about half of the wheels cracked during my testing.
     
    Peter Van Der Walt likes this.
  6. Sprags

    Sprags Veteran
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    The wheel kits I used were OpenBuilds wheels. Both sizes of wheels had two spacers that measured 1.01 to 1.03 mm when I used my Mitutoyo digital caliper. I know a micrometer is more accurate but I don't have one.

    I used some silicone lubricant and experimented with adjusting how much to tighten the locknut on the bearings. I found that how much to torque down on each locknut varied greatly in order to allow the wheels to spin without any of the rough feeling. The smaller wheels were more prone to having that rough feeling.

    As I said I made sure to use a shim between the bearings when building the assembly.

    I did notice that even though a member did say the locknut is smaller than the outer race and does not hit the outer race is not true if you install the nut with the hex side towards the bearing....which is what I had done. I did not see anything that explained that the nut should be installed with the round end towards the bearing. Since the round end of the nut is the end that has the locking ring part of the nut that seems like should actually be better since that way ensures that it is making full contact with the screw threads.
     
    #6 Sprags, Jan 23, 2018
    Last edited: Jan 23, 2018

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