I have no idea if I am doing this right, there seems to be little to no documentation that describes the steps involved in properly preloading angular contact bearings. I understand I should use a small spacer between the inner races. Can somebody confirm that I am doing this right?
Firstly, these are going on a pretty nice ground ballscrew for my CNC mill. So there be bidirectional thrust loads for sure. Max RPM will be ~700.
Oh, the bearings are 7200's. That is, 10mm ID, 30OD, and 9mm thick.
They have a 30mm diameter bore that is 19mm deep, the fit is perfect. I made them deeper than need be just in case, since I can always remove material later.
****The important part here****
So, first I measured the axial "slop" of the inner races. I supported each bearing only by the outer race, and applied force to the inner race. The inner race of course is pushed downward until the balls hit the retainer and it can no longer moved. Using a .0001" drop indicator, I measured just how much movement there was. I tested four bearings, my findings were as follows:
I was warned that this dimension will not always be consistent, so I am not startled by the range.
Now, I believe I need to insert a spacer between the inner races, and then clamp on the outer races. I already have an outer race clamp all set up and ready to go.
Do I pick two bearings that go together and add the two dimensions together? So one spacer would be .009" thick exactly, eliminating all room for axial play (assuming my measurements are correct!). But then I would need to add some more thickness to actually preload it, correct? How much extra thickness are we talking about here? Given that steel in this situation won't want to compress much, I would imagine that .001" would be more than adequate, is this correct?
Is my method correct? Any verification is well appreciated! I can make the spacer to an accuracy of .0001", but I don't want to deal with this until I KNOW this is the right way, because making things that thin on a grinder is a PITA since there is not a lot of magnetic force holding it down.