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Adjusting Eccentric Spacers for Resistance

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

  1. Sprags

    Sprags Well-Known
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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
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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 Well-Known
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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
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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 OpenBuilds Team
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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 Well-Known
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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
  7. MikeInMA

    MikeInMA New
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    Sorry to post to an old thread but I am having the same problem. Is the rounded or the flat side of the lock nut supposed to be towards the bearing?

    Also quite a few of the mini v wheels are rough or make a clicking noise.
     
  8. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Flat side should be toward the bearing. This provides more uniform pressure on the inner race of the bearing.
    I'm assuming that you have the spacer properly installed between the bearings so really all you can do is contact the parts store for replacement. Bad bearings happen. Three of the eight mini-V wheel assemblies that came with the Double Wide Gantry Kit that I was assembling yesterday had a bad bearing so I will be contacting the parts store. The best method when the issue is tied to a previous order is use the messages tab under your account. Down at the bottom there is a place to select the specific order and comment.
     
  9. MikeInMA

    MikeInMA New
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    Thanks Rick,

    I was 99.999% sure that they went on with the flat side towards the bearing -- it would be really weird to put a ny-lok on backwards like that, but just wanted to make sure I was not doing something wrong for this application. I did indeed put in the precision shims between the bearings when assembling the wheels -- I also made sure that they were fully seated by just pressing them by hand between a couple of pieces of MDF. The weird thing is that when I take off a "clicker" sometimes I can see that the bearing is not fully seated again. This makes me wonder if the inner race is not proud of the bearing like it should be. I did get eccentrics marked 0.250 instead of 6mm -- not sure if that is right or matters much but I don't think it would cause this sort of issue.

    I really want to get my Sphinx up and running but I have ran into multiple issues and it is getting a little frustrating -- I had a warped C-Beam, and one not cut square. The customer support has been fantastic and replaced those right away last week. I was trying to get the Y axis done today but ended up wasting the whole day taking things apart and putting them back together, swapping mini-V wheels trying to find some good ones, at this point I think all the bearings on this batch of mini-V's are suspect. I am thinking that maybe the parts store just got a bad batch. I am going to contact them tomorrow and see if they will just exchange the whole set of mini-Vs since they are such a pain to get to later if the few remaining "good" ones start clicking or get gritty.

    Thanks for your reply,

    Mike
     
  10. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    As long as the spacers on the opposing wheels are 0.25" long it's fine. If not there will be a slight skew in the way your plate sits.
     
  11. MikeInMA

    MikeInMA New
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    UGH -- the spacers are indeed 6mm, so they sent the wrong eccentrics with this kit.

    Thanks again Rick -- hopefully Openbuilds won't be too annoyed with hearing from me again, but not much else I can do. I bought this kit as part of the black Friday sale and due to being so busy with work I have just had a chance to start putting it together. I will contact them and hopefully they will straighten this out.

    Mike
     
  12. MikeInMA

    MikeInMA New
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    Openbuilds customer support is great -- they shipped out new wheel kits, screws and the correct 6mm eccentrics. I should get a chance to try assembling the Y axis & plates this weekend.

    One other question I have ...

    I made sure that my base frame is as perfectly square as I can with my 12" Starret combination square which I know to be good. My two Y axis C-Beams are just a little longer than the base front-to-back -- (about .5 to .75mm). What would be the best thing to do -- make brass shim at the bottom to take up the gap or take a very light cut with my table saw or chop saw to make the extrusion the right length ? I don't have much experience with cutting extrusion -- I assume I can use a carbide tipped blade for wood with a high tooth count. I just don't want to screw up these beams.

    Thanks,

    Mike
     

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