J Head pulley bearing replacement

[titchener]

I'm replacing the bearings in the v-belt pulley located on the top of the spindle on a J head clone machine.

There's two threaded collars that hold these bearings in place, one clamps the outer races and one retains the shaft against the inner races.

Is there a torque spec or tightening procedure for these collars?

Thanks,

Paul T.

[HuFlungDung]

It would be poor design to preload ball bearings against one another. I don't know the details of your assembly, but the safe method is to use a spacer between the races. Then, you can safely snug up the nuts, creating a sandwich consisting of shoulder/race/spacer/race/nut.

[titchener]

Sorry I didn't make it clear, this is the stock J head type pulley design, there are "factory" spacer rings between both the outer races and the inner races. An outer collar snugs up the "spaced out" outer races against the housing and the inner collar snugs the spaced out inner races up to the shaft.

I don't have a service manual so I don't know how much torque to put on these collars. I've seen some assemblies where you first tighten the outer collar then tighten the inner one to me a preload drag spec on the bearings.

Paul T.

[machintek]

Bridgeport does not supply a torque for these. Usually I just snug them up. Reasonable judgement must be used. But per Bridgeport, I was told these were sold as a (somehow) matched set. In theory, if you go out and buy 2 bearings, they will fight each other and fail prematurely. They are relatively close to each other. Again, I am only repeating what the factory told me.

George

[Prboz]

Be a bit careful here. Are you sure they are just straight roller bearings and not angular contact? look closely at the size of the race walls if the inner race is small and the outer race is large and on the other side you have the opposite them they could possibly be angular contact. These need to be fitted the correct way around and preloaded against each other. What are the bearing numbers?

By preloading the bearings against one another it gives you greater precision.

just my 2c worth.

[titchener]

These are a pretty common 6207Z bearing, a standard ball bearing often used in electric motors. I would guess Bridgeport wants the race widths to be very close so the spacers work correctly to put the right preload on the bearings.

This is a Birmingham clone BP, but I was pretty surprised to see good quality Japanese NSK bearings in it. This is a Taiwanese Birmingham and the overall quality of the machine is better than I expected, even after seeing it in full detail by taking it apart.

The 6207 is normally an Abec-1 rated bearing, but McMaster sells some that are built to Abec-3 standards so I got those.

I'll check that the race widths of the new bearings are as closely matched as the original ones.

Paul T.

[mxtras]

And how is your repair working?

Scott

[NC Cams]

Did someone say bearings???

Contrary to a prior post, I can assure you that it is NOT uncommon to axially preload ball bearings. It tends to make them run truer by loading ALL the balls evenly and keeping them at the same radius of travel depth in the ball path.

Result is that load is shared by all the balls and you don't get balls skidding as they travel into and out of the load zone. This is what the wave washer does in a lot of electric motors. Makes them run quiter too.

In a situation like this, you'd preload them to make the pulley track truer and run quieter than simply by letting it wander where it wants to go.

How much axial preload??? A no brainer value is 1% to 2% of the radial capacity.

The trick here is to determine a quantity associated with "snug" if you are preloading by mounting bolt titeness. What is "snug" to one fellow is "loose to another.

I'd suggest that you snug up the retaining nut until there is just 0.000 bilateral endplay as measure with a dial indicator, and not by "feel". THis should get you within the 1% factor or close enough

IF spacers are involved, measure the offset of the inner ring after you shove it axially the 1%-2% preload amount. Be sure to do both bearings and add the offset. You then adjust the spacer widths by grinding to maintain the inner ring raceway offset with respect to the outers as measured previously.

Re: abec 1 versus 3. There is less inner ring radial runout/eccentricity of an abec3 than there is in an abec1.

What you do to "match" them is to first find the high point of the eccentricity of both the inner and outer rings - tenths matters as the spec for runout is in microns.

You then match them (low to low and high to high).

This way, they are closer to running/establishing the same axis of rotation and they won't fight each other. You'll find it is often easier to match abec1's than 3's as you theoretically have more tolerance potential to work with and choose from.

That's what the oem's did with their so-called "matched" bearings - match sloppy ones so that they run truer to each other than more accurate ones could/would.

It is NOT the widths you're matching, it is the net axis of rotation of the preloaded raceways that you're trying to match. Raceway centering affects this as does internal clearance and raceway curvature.

You'd never think you'd want looser tolearance bearings, would you, but, alas sometimes you just may want it that way???

One more thing: if these are outer ring rotation bearings, you may want to seal the OD where the shield meets the ring with some RTV sealant.

Why? sometimes with outer ring rotation, the grease tends to centrifuge out quite readily, especially if the grease used is Chevron SRI ( a common bearing grease). If this occurs, you can wash out the grease with Brakleen and regrease with Ronex MP # 2 or #3. This tends to have much less of a tendency to centrifuge out.

Sorry you asked I bet....

[mxtras]

Another good bearing post. I love it!

Keep 'em coming - it's a lot to digest but it's fun!

Scott