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View Full Version : Off-the-shelf precision bearing spacers? Where?



Arthur.Marks
12-09-2010, 09:33 PM
I can't seem to find this, which would seem to be something commonly needed. A precision washer that spaces two bearings which are being mounted face to face. These are angular contact bearings and need access to an oil hole in the gap. So really, I need two washers: one for the outer race and one for the inner race which are matched. I figure that these must be ground flat and parallel to retain the bearings' tolerance (ABEC 7).

1) Am I wrong about the flatness and parallelism tolerance for the spacers?

2) Do these exist for larger bearings that are, for example: ∅90 and ∅62mm ?

I'm seeing these available for smaller bearings, but I can't find anything over 3/4" OD. Such as: http://www.mcmaster.com/#catalog/116/3230/=a2vcpk (scan down the page)

:confused: or am I going to have to have these custom ground?

Jim Caudill
12-09-2010, 11:20 PM
Here are arbor shims from "Precision Brand"
http://www.precisionbrand.com/ProductsCatalog/PBPCategoryDetails.aspx#l1=292&l2=296&l3=0&lcd=296&l4=0

And actually, my local TSC store has large "motor shims"

Dr. Rob
12-09-2010, 11:36 PM
See SKF Series AS, hardened ground flat washers for axial needle bearings.

Very large range of sizes and relatively inexpensive. Easily up to your 90 mm. Thickness though, is always 1 mm iirc, or thereabouts.

Of course INA or Torrington probably have similar. Maybe even McMaster.

.

Arthur.Marks
12-10-2010, 12:10 AM
Hmmm... Good suggestions. I was definitely thinking too specific. I do think I'm going to have to have these specifically ground, though. The flatness/parallelism checks out, but the thickness tolerances for both the above suggestions are +/- 0.002" That's gotta be too much. The inner spacer and outer spacer won't be at all matched in height.

What a stupid place to put the oil hole :(

whitis
12-10-2010, 01:37 AM
The inner spacer and outer spacer won't be at all matched in height.

Usually the point of thin spacers is to make things uneven while maintaining parallelism. Getting thing parallel is one task, getting things to dimension is another, and getting them both parallel and to dimension carries a cost higher than the sum of its parts.



What a stupid place to put the oil hole :(
Oil access is one application where longer spacers on both races may be used with a through hole in the spacer (which needs to be aligned to the oil hole). If you ever spin that spacer, you lose oil delivery. Even then, matching size may not be critical because any difference should be taken out by the preload adjustment.

You can get oil access with relatively thin spacers or without matched spacers, like this: Mount two bearings with a spacer in between their inner races, only. No seals on the sides of the bearings facing each other, seals on the other side. Drop the outer races into a bore with a shoulder and chase them with the shoulder on a preload nut or other adjustable clamping/retention mechanism. A hole drilled through the side of the bore feeds into the gap between the bearings that remains after you take up the preload. Make sure the hole doesn't significantly affect the support of the bearing race.

The general idea is that one oil hole can feed the pair of bearings and you force oil outward from the clean area between the bearings and you don't need any friction causing seals (which wear) on the shaft to keep the oil supply clean and cause it to force through the bearings rather than finding another path of less resistance. It also reduces the number of seals. More than two adjacent bearings can also be fed this way. An oil hole through the bearing race can interfere with the ride of the balls, though you can buy some bearings from SKF with very thin (easily blocked) holes through the races.

If you really need matched pairs, then if they were available off the shelf they would be sold as matched pairs, probably ground simultaneously, just as bearings can be sold as matched pairs or sets. This is more feasible to achieve than individual parts with such tight tolerances that randomly selected parts would match precisely.

macona
12-10-2010, 02:02 AM
If you just order a set of preloaded bearings they will be ground so that when they are placed face to face they will have the required preload. No shims or spacers are required. If you do seperate the bearings from each other they just need spacers of equal length, ground flat and parallel of course. Thats how its done in the milling machine spindles I have dealt with.

Spin Doctor
12-10-2010, 06:56 AM
In my experience the use of spacers in angular contact bearing set-ups are propriatory pieces manufactured for the particular application*. And yes they have to be ground flat and parallel to under .00005' for ABEC 7. Actually it is not uncommon for spindle manufacturers to use spacers of unequal length in order to incerese the amount of preload in the bearing set-up.

And why Face to Face. DF mounts are unusual in machining spindle set-ups as they are not quite as rigid as a DB or Back to Back mount. And the wide face on the outer race is the "back" side of the bearing hence Back to Back.

* In fact it is not uncommon that spindle manufactures will use spacers that equal either one or mutilple bearing widths of the size bearing being used so that if the original design is not heavy enough the bearing axial load bearing capacity can be raised or the basic spindle design can be used in different situations simply by increasing the number of bearings in the stack up

Arthur.Marks
12-10-2010, 02:48 PM
And yes they have to be ground flat and parallel to under .00005' for ABEC 7.

And why Face to Face. DF mounts are unusual in machining spindle set-ups as they are not quite as rigid as a DB or Back to Back mount. And the wide face on the outer race is the "back" side of the bearing hence Back to Back.

Ouch! :eek: I thought 0.0001" would be more than fine. Half that? Who's gonna grind that for me?! Where did you get that number?
Secondly, here is the diagram I am reading. Did I mistake the bearing arrangement shown? Even if I didn't, I don't see any difference if I chose to mount them DB instead of DF...

http://i771.photobucket.com/albums/xx357/Arrak_Thumrs/Widgets/DB.jpg
Also a handy reference for what we're talking about:
http://i771.photobucket.com/albums/xx357/Arrak_Thumrs/Widgets/GMN.jpg
[EDIT:] Actually, now that I scanned these and they are blown up, I am quite certain I did mistake Face to Face with Back to Back :cool:

Arthur.Marks
12-12-2010, 01:45 AM
Update on this:
We'll see what I can get done on Monday. I took a surplus bearing and separated the races. I then cut them in half on the lathe, which was interesting. The carbide cutoff blade did get through it eventually :)

Monday I'm going to get some proper grinding wheels and see what I can accomplish with my toolpost grinder. I figure I'll end up ahead even if I can't get it as flat as I like. I have a circular magnetic chuck that fits my lathe spindle, so I am going to grind the outer and inner "spacers" at the same time. That should bring them out as matched as I will be able in height. At the worst, it will leave me with a single plane grinding job if I still need to send it out. Inner, outer and one side will be 100% on spec since it came off the same size bearing as the new ones. That's my theory anyway.

Here's the halved inner races:
http://i771.photobucket.com/albums/xx357/Arrak_Thumrs/Widgets/DSCN4046.jpg

Spin Doctor
12-12-2010, 08:31 AM
Arthur, I used to grind bearing spacers quite often in rebuilding spindles is the spacers had become damaged during a bearing failure. And yes .00005" might be a little overboard but I did notice an improvement in service life the spindles. Some of these spindles were being used in a situations that placed a pretty high axial load on the spindles. One instance was the drilling of oil passage holes around 15 inches in deep in a cycle time of about 30 seconds.

In order to grind spacers to this flatness I ground them on top of magnetic parallels that were ground in place and NOT removed from the magnet prior to placing the spacers on top of the parallels. This assured that I was able to grind them to as flat as the grinder itself was able to maintain and not being subject to the potential high or low spots on the magnet. And the parallels would be ground each and every time they were used. Grind wet

Now a lot of these spindles would have spacers of different heights to increase the amount of pre-load built into the spindle. In this case the outer spacer would be popped off of the parallels and the inner one reduced in height the required amount. IIRC .0011" was pretty common for addition pre-load. Also these spindles had a specified dimension from the front face of the spindle either a key on the bottom of box spindles or from a mounting face on cartridegless spindles* These spacers would also need to ground to fit if the bearing spacers them selves were damaged and needed rework or if the bearing widths varied excessively. One manufacturer that was terrible at maintaining widths was RHP before NSK bought them.

* Cartridgeless Spindles are spindle assemblies that consist of the shaft, bearings, spacers, seals, seal plates and related hardware and are then mounted into say a multi-spindle drill head on machining transfer lines

Mcgyver
12-12-2010, 09:49 AM
A And yes .00005" might be a little overboard but I did notice an improvement in service life the spindles.

Using as you say parallels .00005 parallel should not be too difficult if the grinder's good... but what sort of thickness tolerance is there? that's where i'd always thought the rubber meets the road on a bearing spacer.

Its been awhile since i looked at prices, but iirc the ones that were matched for a preload were magnitudes more expensive than the individual bearings. The bearings are otherwise identical aren't they?

If so, making your own spacers might be a good home shop project as lots of guys have grinders these days....provided the thickness tolerance is doable. Lets say that tolerance is a 1/10th, small but doable on a grinder....and if you screw up and go too small its $0.25 worth of metal, so you try again. Meanwhile if by doing this fidgety work yourself you can take the bearing cost from, I don't know, 250 to 50, that's a home shop win. All theory for me, I haven't done this, but I'd be curious get your take on it Spin Doctor, what are the things I'm not thinking of that would cause trouble?

lazlo
12-12-2010, 10:39 AM
Using as you say parallels .00005 parallel should not be too difficult if the grinder's good... but what sort of thickness tolerance is there?

I don't think the thickness tolerance is as critical as the parallelism. The thickness tolerance sets the amount of preload, which doesn't affect the bearing runout.


Its been awhile since i looked at prices, but iirc the ones that were matched for a preload were magnitudes more expensive than the individual bearings. The bearings are otherwise identical aren't they?

Preloaded spindle bearings are match ground together. As you say, they're ~2 times more expensive than unmatched ABEC-7 bearings. But note that a pair of preloaded bearings is ground so the inner races are even with a specific amount of preload. So you still need a way of providing the preload -- either a spacer or a screw.

Spin Doctor
12-12-2010, 10:40 AM
IIRC the tolerances we had on Assembly drawings from Setco and other manufacturers was .0001 to .00005 on flatness and parallel. This is not the kind of stuff you are going to be doing on the lathe. Also a clapped out grinder makes it difficult to do also. I had the benefit of being able to use a close to new Okamoto 16x32. But with care the same results can be had on a 6x12. Just don't take aggressive cuts and keep the work cool. Also part of the reason for buying sets of 2 or 4 is all of the bearings are matched not only for built in pre-load but also for radial run-out. The high point of the run-out is marked on the inner and out races and these points shold be placed opposite the high point on the shaft and inline. The outers also need to be inline. Plus both front and rear bearing sets need to have their match marks aligned for both the inner and outer races. Will a spindle work if you don't. Sure it will. its just that for optimum performance the devil as always is in the details. Especially when you start dealing with tolerances in the +/- 5 to 10 micron range.

Mcgyver
12-12-2010, 11:54 AM
I don't think the thickness tolerance is as critical as the parallelism. The thickness tolerance sets the amount of preload, which doesn't affect the bearing runout.
.

Agreed, but i was coming at it from what has to be done for the project overall to be a success...ie can one hold the parallel tolerances as well as the dimensional. I was thinking the dimensional might be a more difficult than the parallel

Arthur.Marks
12-12-2010, 01:51 PM
I would think the dimensional tolerance may be relative to the type and arrangement of the spindle housing. For example, on this spindle I am working on, it has two nuts on the very rear of the housing. One threads into the rear of the headstock and only contacts the outer race. The other threads on the spindle and only contacts the inner race. For this instance, at least, I am working on the assumption that any thickness variance is able to be adjusted with the inner nut for preload. After the .00005" post by Spin Doctor, in any case, I was admittedly frozen. I didn't know where to proceed on this. Interestingly, I just noticed now that he is very close to me (~65 miles north). I almost drew out what I needed and contacted these guys: http://www.materialfabricators.com/category_49_Precision+Shims.html

I don't like feeling frozen, though :) So I'm going to see what I can accomplish. Even if it fails, I figure I will learn something from the experience. At this point I'll be happy with mere "success", not perfection. ;)