Pre Loaded Bearing

[sreid]

I guess I have always assumed that one could purchase deep groove radial bearings with an initial preload. And I said so in a design meeting. Then looking at an SKF catalog, I don't see how to specify this. I'll call SKF Monday but I thought I'd ask here this weekend to see if I'm just Blowing Smoke.

The application is intermittant slow rotation (1 rps) with light loads. We can't tolerate any play in the bearings (precision mechanism). We would mount the inner race with Loctite and the outer race with a Tolerance Ring to avoid additional bearing race squeeze and to eliminate race play.

 

[israelkk]

I think you are mixing inner play between the inner race and the outer race (radial free movement of the balls between the races) with the free play of the outer race to the housing and the inner race to the shaft.

To prevent the free play between the races to the housing and the shaft you can use fress fit or glue or tolerance ring. For the inner bearing free play the correct and accurate way is to order a matched pair of bearings designed to limit the free play and to induce a designed preload when assembled correctly.

 

[JJPellin]

I have never heard of a deep groove radial bearing with pre-load. And if they did make them, I would not recommend it. As the bearing heats up, the balls will tend to grow more than the races, shaft and housing. Purchase a bearing with the proper intenal clearance so that when it is at running temperature you do not oveload the balls.

Johnny Pellin

 

[electricpete]

For precision applications, you can specify reduced internal radial clearance (work with the bearing manufacturer to make sure this is suitable for your application).

I have the impression that angle contact bearings are frequently used in precision applications.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[unclesyd]

Isn't 1 rpm a little slow for a ball bearing?

 

[sreid]

unclesid,

It's one rev per second. I can't state the exact application but the mechanism will be used to move a platform +/- 1 mm and to any stopping position in between. A positioning servo drives a gearbox which drives an eccentric (Crankshaft?). We need to achieve, say, 2 micron repeatability (I will probably have to compensate for positioning accuracy in software).

I can probably get away with ABEC 7 C0 bearings but no clearance would be ideal. I could also use angular contact bearings in pairs but that doubles the number of bearings and the inner and outer races have to be clamped (mechanical and assembly complexity).

I'm open to any suggestions.

 

[unclesyd]

I've got to get another excuse as the "Senior Moment" thing is wearing thin.

The only real help I can give is that it can be done.

We manufacture our own spinneretts for synthetic fiber which requires positioning on the order of your OP. I was never directly involved in the positioning aspect of the manufacturing process but do remember a few things about our machines. These machines were capable of making any hole pattern one could draw.

The original machines (1960's) used ball bearings whereas the later models use some high precision needle bearings.
Our positioning was accomplished by using a pantograph that translated the require movement of the work piece from a cam. The ratio of movement imparted by the pantograph was greater than 50::1 and could be stacked or run in parallel to achieve the desired motion of the work piece.
As I recall the early ball bearings were supplied by Torrington. We were fortunate that there was bearing depot at the local Naval Air Station and they would checkout each bearing. Later I know that we gleaned some information from Pope about their spindle bearings that was incorporated in the machines. There was also some work with RBC on their thin section bearings.
For the life of me I can't recall the supplier of the needle bearings and right now can't think of anyone to call.

Here is the type cutting and precision we were doing.
We beat the precision stated here.
Shapes

http://www.nippon-nz.com/english/jigyo/index.html

Precision

http://www.nippon-nz.com/english/jigyo/pdct_212.html

 

[MikeHalloran]

You can preload _pairs_ of bearings, but not a single bearing.

Machine tool spindles use bearings paired at the bearing factory, with the axial faces of the races lapped so that when they are squeezed together, a controlled preload exists. (Effectively, they are used as angular contact bearings, even though they are still called radial bearings.)

Electric motor manufacturers commonly preload generic radial bearings by fixing one and assembling the other with a 'preload spring' between the bearing race and a shoulder.

At the 2 micron level, do not be surprised to see trajectory errors associated with the passage of a single ball past a given spot on a race.


Mike Halloran
Pembroke Pines, FL, USA

 

[sreid]

Unclesyd,

Thanks. My real concern is how much of a numb-nuts (SP) I'm going to appear when the preloaded radial bearings are not available. Probably I'll say that I really meant accurately controlled clearance bearings.

But theoretically, I don't see why you could not make preloaded radial bearings. It's a matter of selective fitting the races and balls. But, as others have said, there are many practical (and possibly historical) reasons this isn't done or is not commonly done.

1) They might brinnel just sitting on the shelf.

2) Every idiot in the world would select them, use press fits on the races, run them at high speed and then blame the bearing manufacturer when they immediately fail.

3) Many other reasons.

MikeHalloran,

You are absolutely correct about bearing running accuracy. The accuracy of the gear box will be at least 10 times worse and I'll probably need to have high accuracy linear encoder feed back to get to the exact position I need.

 

[MikeHalloran]

I just remembered something. You can't get radial ball bearings with a preload, but you _can_ get 'X' bearings with an internal radial preload:

http://www.kaydonbearings.com/catalogs/catalog300.pdf#page=1

Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

MikeHalloran beat me to it as I was about to post about the "X" bearing from RBC.

Also look at the following brochure from RBC.

http://www.rbcbearings.com/literature/pdfs/RBC-specialty.pdf

 

[MikeHalloran]

The other nice feature about X bearings is that if your shaft is short enough, you only need one.



Mike Halloran
Pembroke Pines, FL, USA

 

[sreid]

All,

Thanks for the X Bearing suggestion, I'm looking into them but so far they are larger that I need (I need something in the range of 1/2 in, ID, 1 in. OD).

I'm now certain the bearings I thought were preloaded radial bearings were actually X bearings (SKF calls them 4 point bearings). Their external appearance is the same.

I also think I see why preloaded radial bearings do not exist; any axial load would quickly overload the bearing.

 

[HawksDesign]

Hi,

You could try and get a real accurately machined interference fit, between the shaft, bearing and bearing housing. I would suggest you try this out, as this should take care of fine clearences in the bearing.
Regards

 

[sreid]

We're going to go with duplex angular contact bearings. Thanks for everyone's help.