Bearing Questions....

[tstalcup]

Hi guys - I'm new around here, and I've really enjoyed reading through all the great info here!

I'm looking for some information about ball bearings. I'm cleaning up a woodworking table saw I got at a yard sale, and I've decided to replace the arbor bearings while I'm at it. I took one of the old ones to the local bearing supply house, and got some replacements. I asked about higher quality bearings, and basically got a grunt about them being more expensive.

I'm concerned about the amount of axial play in the inner race of the new bearings. Its not a lot, but its easily noticable when trying to move it back and forth by hand and is more than I want the blade to move axially.

Here's the setup (dimensions approximate):
0.5" shaft, 1" bore through casting for shaft. Each end of the bore in the casting is enlarged to ~1.25" to fit the bearing OD. Bearings are pressed onto the shaft, but will just slip into the casting. So, assembly is:
press one bearing on shaft
put shaft through casting, seating one bearing in casting
press other bearing on opposite end of shaft until seated in casting.

Old bearings: NTN 6202Z. Also marked NTN canada B45-123083-A000
New bearings: FAG 6202.2ZR.C3

Should I worry about this axial play? Is it normal? After a little reading on bearings, I think it may disappear when the shaft stretches the inner race slightly during pressing. Am I way off here? When pressing the second bearing on, do I need to press a little past the fully seated position to give a kind of axial pre-load?

Thanks for your help!

[Dr. Rob]

Not to worry. Axial play in bearings is a normal thing. It varies a little among manufacturers and ABEC rating, but it's always there. Really, it's a good thing, since it provides a willingness to forgive any misalignment in the assembly.

Preload? Sure, that'll work. Don't overdo it, though; it just strains the poor thing. Maybe put a Belleville or wavy washer in there, which will help you apply a reasonable amount of force. A compensator, sorta.

That's what I say, but here's what I'd do: Stuff a coil spring in there, between the bearings. That'll compensate for any bearing slop without overdoing it. No great precision required either.

PS Angular contact bearings would be nice, but perhaps a little pricey?

Edit: Thought just struck me...wouldn't the centrifugal force generated by a blade (Sort of assuming this was a table saw) rotating a a zillion RPM sort of negate the bearing slop?

[This message has been edited by Dr. Rob (edited 03-24-2003).]

[mbensema]

The new bearings are probably a looser clearance then the ones you took out. The C3 is looser then normal and bearings without a designation normally are normal clearance, but for a table saw, that probably will not matter. Once you install the bearings, the play will be taken up and your blade will not have noticable play. Typically the arbor is installed so that the end near the blade is fixed and the bearing in the other end floats for expansion. Adding any shims or anything else could prevent this and create excessive load on your bearings, install it the way it should be and then check for play. If you still have too much, then get normal clearance bearings.

Mike

[lynnl]

Dr Rob, are you thinking about the gyroscopic effect? That would work to counter any tendency of the shaft (arbor) to twist or turn in its orientation. But I don't think that would counter any straight in/out or axial movement. Nevertheless, I'd agree that the spinning blade will tend to remain in the same plane without any external force to move the arbor axially.

[darryl]

Once you press the material into the spinning blade, the radial force will tend to center the bearings and there'll be no problem. Also, the slight, but inevitable off-balance of the blade will induce some radial loading, so play should be very slight even before the material is pushed into the blade. I doubt there will be any problem, anyway, as noted above, the installation of the bearings will likely remove some of the play. In use, I think blade wobble will swamp the effect of bearing play.

[wierdscience]

The c-3 toler.as above metioned is a loser than standard internal clearence,with these bearings you can increase the press fit on both the shaft and the housing bore and still maintain enough clearence.You could close up the housing bore by stipleing it with a small center punch to close it up slightly.

[Thrud]

If this is a low grade contractors saw they might do. But if it is a better saw consider getting the proper bearings. Or at least higher grade bearings. If you do fine wood working a tight saw is manditory and I doubt that you will get consistant cuts with those bearings.

Remember, you have a circular saw spinning at great speed on the end of this shaft. It is not supposed to move around, only turn.