Need Help! Where can I buy some very thin 47mm OD shims for between 7204 bearings outer races?


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Thread: Where can I buy some very thin 47mm OD shims for between 7204 bearings outer races?

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    Default Where can I buy some very thin 47mm OD shims for between 7204 bearings outer races?

    I am looking for some outer race shims for 7204 bearings. They are 47mm OD.

    I cannot find any really thin shims this size anywhere and not sure where would make them for a reasonably price.
    I believe I need some .0020 and lower thickness shims.
    I am not exactly sure what thickness will get me enough preload for near zero deflection.

    I am also not sure that I should even expect better than this on a machine with a heavy bed with metal on metal old school ways. It takes a good amount of force to get it moving.
    The original manuals have no permissible backlash specs to know what the expectation of the mechanics were. The only thing it has is some overall specs of the machine cutting.

    I am getting backlash of .0021" (Y) and .0019" (X) and want to try to shim it out. This is with new bearings at about $350 a set and rebuilt ball screws and this is as good as it gets (bearings are NSK 7204A5TRDUHP4Y). Dissapointed, but I am not sure what I can do. I think I was mislead by the manufacturer and several bearing houses on these being the right the thing to use. Knowing what I know now, why would anyone spec a 25 degree bearing when even the original was 30 (most support bearings are 60 degree now it seems). This thing is a bed mill with metal on metal box ways. Not a huge machine, but still over 4000 lbs and big portion of that is the bed and it takes some grunt to move it around.

    I can see the bearings inner race retaining nut deflecting with my indicator near this amount before you even feel much pressure building. You can basically rotate the screw by hand a little back and forth easily until it deflects the maximum amount and then starts actually putting enough force on the ball screw nut to move the table.

    These double 7204 (25 degree contact angle) assembled in DB configuration have an axial rigidity rating of 140 N/um. I am not exactly sure what this means, but my assumption is that 140 Newtons of force could deflect them 1 uM?
    140 Newtons is only 31.47 pound force. Now 1um is not much, but if multiplying that force to something reasonable to move the bed keeps on deflecting, then that comes up to a significant movement. However, there are no charts showing what this curve would look like that I found so far. Obviously it would not be linear and there is a certain point deflection will stop and then the bed move.

    For reference what seems common now would be a 20TAC47 bearing set (2mm wider though for the set), but they have an axial rigidity rating way higher at 630 N/uM. They also have a much higher preload of 1450 N.
    I think the static load ratings are also 53 kN (vs 7.40 for the ones I have now). I will likely mod my retaining plates and use these bearings if I have to do this again. I think I can just shave the lip and get the 2mm needed for them to fit. These bearings can also be found cheaper.....

    This seems like it does not take much force to have some deflection, but I do not fully understand all the ratings. All I know is I have them installed well, greased properly, and everything was clean as can be. Both axis have near the same deflection and I feel like I should expect better or make it better??

    With that said, I cannot even budge the Y axis by hand with the ball screw out (just pushing on it). I have to use a pry bar to get it to move. It moves seemingly easy with the screw by hand, but it has to be overcoming that weight and stiction to get moving and I think these bearings just can't do any better without shimming for a higher preload than the most they give you an option for (heavy: 649 Newtons (which I think is 145.9 Pounds force). Pushing the bed with 145 lbs isn't going to move it in Y. I know that. I weigh 190 and I can push with both hands and both feet as hard as I want and not budge it. It is even hard to move it with a long pry bar. I can move the X manually (ball screw out), but it takes takes embedding a foot somewhere and pulling real hard. So to me, this all adds up to the bearings are just going to deflect without shimming, but if I could get an NSK person on the phone, maybe I could get some actual answers.....

    The ways are in excellent shape, the gibs are adjusted, and everything moves super smooth now with the rebuilt screws and new bearings, but that deflection of the bearing on direction change is still backlash. I have the backlash compensation set to get it right on the money, but I was expecting better than this after getting back much tighter rebuilt ball screws and new bearings. And everyone says backlash compensation is not the best....

    The X did not improve at all with new bearings and rebuilt screws and I think that is because everyone told me the wrong bearings. These new ones are 25 degree and the original are 30. I bet I could put the 40+ year old bearings back in there and get less backlash. The machine has very low usage even though it is old (was never in production, only used for teaching).

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  2. #2
    Community Moderator Jim Dawson's Avatar
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    Default Re: Where can I buy some very thin 47mm OD shims for between 7204 bearings outer race

    McMaster has shims https://www.mcmaster.com/shims

    You may have to modify to fit, but that's easy. I just use one the right ID and turn the OD to fit on a stub arbor in the lathe.

    or

    Actual bearing shims
    https://us.misumi-ec.com/vona2/detail/221004991334/

    Jim Dawson
    Sandy, Oregon, USA


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Where can I buy some very thin 47mm OD shims for between 7204 bearings outer races?

Where can I buy some very thin 47mm OD shims for between 7204 bearings outer races?