Outer race movement in a floating ball bearing

[geesamand]

I have an application in which a horizontal shaft is supported by two sealed-for-life deep groove ball bearings. The bearing on one end is a "fixed" setup with transitional housing fit and end thrust load. The bearing toward the middle of the shaft is "floating" with a small clearance fit.

We have occasional trouble with outer race movement of the floating bearing and housing bore wear when the machine is subjected to large magnitude, low frequency vibration. While I can advise against this environment, it is always best to design out the possibility of failure.

What could I try here to solve the problem, assuming I must stay within the same bearing type?

I do not see measurable temperature differences between the shaft and housing, and the shaft is 304S/S or 316S/S and the housing is grey cast iron. I know we can assemble the bearings so that the axial positioning is perfect - would I be nuts to make both bearings a fixed fit?

Thanks,

David

 

[Compositepro]

Do not fix the second bearing. It is designed to move as the shaft length expands and contracts differently than the housing. I would use a center punch to place three or four dimples in the bearing housing. The dimples will raise a little metal by displacement and create a tighter friction fit to prevent the race from turning.

 

[MikeHalloran]

Or, put a groove in the bore that carries the floating bearing, and install an o-ring in the groove.

Or, coat the floating bearing's OD with non-hardening caulk.

Mike Halloran
Pembroke Pines, FL, USA

 

[electricpete]

We have occasional trouble with outer race movement of the floating bearing and housing bore wear when the machine is subjected to large magnitude, low frequency vibration.

The vibration should assist the axial movement, which is necessary to relieve thermal stresses.

Can you elaborate what kind of "trouble" you are seeing with outer race movement? Are you seeing lack of axial movement? Or are you seeing fretting that troubles you? Or something else? I'm not sure I understand what problem you are trying to solve.


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(2B)+(2B)' ?

 

[geesamand]

The problem, in the infrequent cases where it occurs, is slow precession/ rotation of the outer race within the housing bore. It wears thru the cast iron housing and degenerates into a premature machine failure.

I'm aware that the floating / fixed system is designed to relieve axial loads between the bearings.

We have tried the o-ring thing for a customer who had this problem. It did not stop the race from walking.

A non-hardening sealant (Hylomar blue comes to mind) might help but based on the o-ring experience I don't have high hopes.

It would be great to key or peg the outer race and put a dog point set screw into it but getting the bearing modified could get quite expensive.

 

[MikeHalloran]

One long term fix that should be effective is to support the floating bearing in an axially compliant but radially rigid structure. I.e., press or Loctite the bearing OD into a bore that is built into a blade or diaphragm that can react the bearing's radial loads, but is flexible enough in the axial direction to not cause undue thrust loads on the bearing when shaft and housing shrink differently.

You might have to retool the entire housing, or if you are super lucky, a few well placed saw cuts or cored or drilled holes might suffice.



Mike Halloran
Pembroke Pines, FL, USA

 

[electricpete]

My apologies, re-reading the op it was very clear and everyone understood except me.

As a general idea, my understanding is the rotating forces will tend to increase tendency for "spinning" outer ring in housing. Apparently those rotating forces can't be reduced.

I tend to think stationary radial forces will reduce the tendency for spinning. The reason is that I often see severe fretting of vertical motor lower bearings (which have no significant radial load) but don't see nearly as much on horizontal motors (which have radial load). I don't know if it is practical or advisable in your application to increase radial load.

Another thought - A wavey axial "preload" washer pushing between the outer ring of the floating bearing and the housing face might provide a small degree of friction to resist spinning. This is a common standard configuration for floating bearing of electric motors.

There are a variety of snap ring configurations to prevent axial movement. Seems like it would not be hard to make a snap ring that you put into a housing circumferential groove, with a tab that mates with a slot in the bearing outer ring face to prevent spinning …but I guess that is not standard.


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(2B)+(2B)' ?

 

[Tmoose]

page 21 here -

http://techmar.jaslo.pl/files/SKF_uszkodzenia_lozysk_i_ich_przyczyny.pdf

What is the "small clearance fit" between the free bearing OD and the housing? 0.001" or 0.005" ?

How is the shaft driven?

Have you spent time checking the shaft for runout near the bearings and at several points between the bearings and along the cantilevered section?

Can you provide a sketch of the device, indicating radial loads, and whether they are fixed in direction (like a belt drive or gravity) or rotate , like unbalance? Is there some heavy component stuck on the cantilevered end?
Is there a belt drive outboard of the fixed bearing?

Most important Is the spinning bearing's reaction upward or downward?

Have you calculated or measured the first bending resonance of the assembly?

Could you make the bearing toward the middle the fixed bearing?

 

[geesamand]

Tmoose, good questions

Small clearance fit is on the order of .0000-.0010" over a 3.5" dia. Of course, the cast iron housing will steadily wear if the race is moving and create more clearance over time. We once tried a steel housing for better wear resistance (not my idea) and it didn't help much.

Belt driven, outboard of the fixed bearing.

Runout is held within .001 at/through the bearings. We hold a relatively tight balance spec on the rotating parts.

Outer (fixed, combined load, transitional fit to hsg) has radial load vertically upward and constant due to cantilever reaction and belt tension. Axial load coming from the cantilevered end is constant.

Middle (floating, radial load only, slight clearance fit to hsg) has radial load acting vertically downward. There is slight variation in radial load, about 10% of total, coming from the cantilevered end. It's a 6016-2RS bearing and its loaded such that it achieves 70,000 to 200,000 hr L10 life depending on the application.

First critical speed is over 2x of the shaft speed. No indications that resonances are related to these cases of outer race movement.

Fixing the middle bearing is an interesting option because then it can be made into the press fit. The outer bearing would have the consistent radial load of the belt tension keeping it stable. This would be a major change to the product. I expect the next opportunity for that would be in 5-10 years.

The other thing that came into my mind was that when a customer has a non-conforming environment (a rocking support) and this occurs, that they purchase an alternate arrangement for the middle bearing using a cylindrical roller bearing with press fits on both races and Nilos rings for seals. Or perhaps a sealed-for-life 22xxx spherical roller bearing with same fits. These bearings should have enough axial end float to easily absorb any shaft growth.

 

[Tmoose]

"Middle (floating, radial load only, slight clearance fit to hsg) has radial load acting vertically downward."

Seems like consistent vertical loading would preclude outer race creep.

I'm thinking something is making the shaft "crank."
How are the bearings secured to the shaft? Setscrews or eccentric collars on one side of the bearing?

Have you ever measured vibration on that middle bearing?

 

[geesamand]

Yes, in these few severe environments I believe the movement of the entire machine is enough for the middle bearing radial load to approach zero at times. The rotating shaft wants to remain motionless and the machine is moving around it.

We use setscrews and they are secure. Vibration is limited to 0.3 in/s peak (normally 0.2) / 3 mils pk-pk (normally 2). In the severe cases where the bearings might walk I'm not sure if vibration sensors can pick it up as the frequency of the motion can be below 1hz.

 

[Tmoose]

Hi Geesamand,

I'm concerned if the setscrews are all on one side of the race.

Do you make your own housing?
I was thinking that to provide self aligning and axially expansion capabilities commercial ball bearing pillow blocks and flange bearings have to use a sub housing, whereas the fixed bearing can have a spherical OD to provide seal alignment. Static Self aligning capabilities are necessary with any pillow block or flange deep groove ball bearing.

Do you know what the finished shaft sizes are where the bearings mount? Sounds like they must be nearly line-to-line with the bearing bore if the assembled shaft runout is less than 0.001".

Is there a substantial and reinfoced continuous top plate or structure supporting both bearings in common?