BEARING OD AND HOUSING ID -WHAT FIT SHOULD BE THERE? INTERFERENCE OR CLEARANCE. SKF SHOWS BOTH -RANGING FROM INTERFERENCE TO CLEARANCE. BUT AT THE SAME TIME ,IT SAYS THAT FOR PUMPS, MOTORS, THE OUTER RING CAN NOT BE DISPLACED. SO IT MUST BE INTERFERENCE. HOW TO DECIDE. SOME PEOPLE SUGGEST ZERO CLEARANCE OR INTERFERENCE.
PETE, ARE YOU LISTENING?
REGARDS
IRSHAD

It depends on your application. The SKF Bearing Installation and Maintenance Guide provides detailed guidance, but at time does leave you confused.

Typically, the fit between the housing and bearing OD will have slight clearance. The fit between the shaft and bearing ID will be an interference.

If you have a specific application, post it here and we'll try to help or try contacting SKF and see what they recommend.

Steve,
Recently, we stripped one motor to change the bearing. We found interference between vertical motor NDE deep groove ball bearing and housing. We machined the id of the house. Since then, it is running with high vibration , approx. 12-15 mm/sec RMS. It is 6316 deep groove ball bearing at top. The fits recommended by SKF are P7,K7,N7,M7.
If we choose P7 fit, it gives 54 to 14 microns interference. Under such high interference, it will be very difficult to put the bearing in the housing.

I think you are right that it should not be an interference fit at the bearing/housing interface.

In the SKF bearing installation and maintenance handbook, page 35 for radial bearings. For stationary outer ring load
(split or solid housing), the bearing/housing fit should be H6. It is particularly important for non-locating beairng that there not be an interference so the bearing can move within the housing as designed.

Also page 36 for thrust bearings, it is again H something (H7, H8 or H10). Of course for a thrust bearing, there will be some feature allowing the bearing to transmit thrust to the housing other than through the fit.

Also if you google "EASA AR-100", you will get a standard used for motor repair. Table 2-14 of that standard also suggests H6 for a 6316 bearing.

For 170mm od bearing such as 6316, H6 is 0 - 0.0020" loose. H8 is 0 - 0.0035" loose. None of these allow interference even at worst case variation of the housing dimension and bearing dimension.

So if you had an interference to begin with, you did the right thing to enlarge it imo.

If vibration increased after this evolution, some possibile reasons might include:

• the bearing housing was enlarged it too much. (in that case, I expect to see harmonics)
  
• the bearing housing was machined off-center.
  
• a rough edge was created during the machining which prevents bearing from sliding axially
  
• motor was removed and reinstalled and misaligned during the process.

This message has been edited. Last edited by: electricpete,

quote:

Originally posted by electricpete:

the bearing housing was enlarged it too much. (in that case, I expect to see harmonics)

the bearing housing was machined off-center.

a rough edge was created during the machining which prevents bearing from sliding axially

motor was removed and reinstalled and misaligned during the process.[/LIST]

ACCEPTED. THE HOUSING WAS ENLARGED TO 0.060MM MORE THAN BEARING OD. WE CAN HEAR NOISE SUGGESTING MOVEMENT OF OUTER RACE. IS THERE ANY SPECIFIC FREQUENCY ASSOCIATED WITH THIS TYPE OF PROBLEM TO CONFIRM THE PROBLEM.
IRSHAD AKHTAR

Pete has given good advice. One other potential source is that the resulting bearing bore is not perpendicular to the housing, thereby resulting in a cocked bearing.

As Pete mentioned, looseness in the housing can manifest itself by a family of running speed harmonics (1X, 2X, 3X, etc.).

You indicated that the housing is 0.060 mm larger than the bearing OD. This is: (0.060 mm) / (25.4 mm/in) * (1000 mils/in) = 2.4 mils of clearance (0.0024").

For a 170 mm OD, an H6 fit is 20L (0.0020").
For a 170 mm OD, an H7 fit is 26L (0.0026").

So, the bore is a little larger than the recommended H6 fit, but I don't think the H7 fit would result in the vibration that you are experiencing.

What about the shaft to bearing fit?

This message has been edited. Last edited by: Steve Ciesla,

The thermal dynamics are often overlooked I think! Just imagine:
If the outer diameter does not fit into the housing bore and there is some clearance between ring and bore then inevitably there is reduced heat transmission too.
If so then the outerring will increase diameter due to thermal expansion.
If diameter increases then there will become more surface available to exchange the generated friction heat of the bearing. Finally there will be a thermal and dimensional equilibrium with a clearance that is difficult to get under control.
A complicating factor is an bore being not perfectly circular.
A shrink fit bearing should run cooler!
I wonder why these thermal dynamics are not mentioned in the bearing books?
Regards,
Arie Mol

quote:

What about the shaft to bearing fit?

Bearing to shaft fit is interference fit. We had to heat it to 100 deg c for mounting. wHAT IN CASE OF CYLINDRICAL ROLLER BEARING? The fit between housing and bearing should be interference fit as per SKF website information. Am I correct?

Regards
Irshad Akhtar

This message has been edited. Last edited by: AKHTAR,

quote:

Originally posted by AKHTAR:
ACCEPTED. WE CAN HEAR NOISE SUGGESTING MOVEMENT OF OUTER RACE. IS THERE ANY SPECIFIC FREQUENCY ASSOCIATED WITH THIS TYPE OF PROBLEM TO CONFIRM THE PROBLEM.

I don't know that answer. I have seen fretting on a removed outer ring many times, but never seen any vibration that I associated with it. My guess would that if it did show up, it would probably look like random impacting in TWF with raised noise floor in spectrum, or less likely it would look like 1x impacting in TWF with 1x and harmonics in spectrum. Maybe someone else can offer comments on how this might show up in vibration. It would also be a good question to ask on the vib forum.

quote:

Originally posted by AKHTAR:
wHAT IN CASE OF CYLINDRICAL ROLLER BEARING? The fit between housing and bearing should be interference fit as per SKF website information. Am I correct?

Even for cylindrical roller bearing, it should not be an interference fit between bearing and housing. The SKF bearing maintenance and installation handbook makes no distinction between CRB's and deep groove ball bearings in their discussion of fits. Also EASA AR100 specifically addresses CRB's (they have a table) and their recommended fit is H6 for CRB's 10 to 200mm bore.

The function of sliding within the housing may no longer be as important for CRB's since they may allow internal axial movement. However, if you applied interference fit to both the inner and outer ring, you may reduce the bearing internal clearance to an unsafe level. Therefore, we can generally only apply interference fit to one ring or the other. Having an interference fit to prevent relative movement is more important on shaft than on housing (since static loading such as weight continually rotates relative to shaft/inner ring interface). Also interference fit on shaft and clearance fit on housing has advantages in ease of assembly.

The bottom line, most people follow recommendations of bearing OEM's and standards, which (for rotating shaft /stationary housing applications) specify interference on inner ring to shaft and clearance on outer ring to housing.

quote:

Originally posted by Arie
A shrink fit bearing should run cooler!

Arie - I agree the thermal performance of the bearing and ability to successfully slide within the housing is difficult to predict. You have provided some good related discussion before here:
http://maintenanceforums.com/e...751037782#4751037782

Just to clarify, you're not suggesting to intentionally create a static interference fit on both the outer and inner ring are you?

Another problem with interference fits on both shaft and housing would be assembly difficulty.

One cardinal rule of fits with rolling element bearings is to follow the recommendation of the OEM. Just by making measurements one can't alway be certain of what the OEM might have intended. Remember that the OEM may very well deviate from the bearing manufacturer based on his expereince with a particualr type of machine.

John from PA