What types of bearings need preloading during installation and how? I have seen angular contact ball bearings preloaded by locknuts and deepgroove bearings by curved washers or belleville washer or end caps but where and how much? Do any guidelines exist for that and if not properly preloaded , what may be consequences,and how to diagnose?
Regards everybody.

Have you searched internet e.g. SKF website to look for articles on this subject?

Searched internet but with little success. Any consolidation of such information will be helpful.
Regards.

The decision to preload a bearing depends a lot on the application. Not all bearings should be preloaded.

From our company manual it lists the following reasons to preload a bearing:

- enhance stiffness
- reduce running noise
- enhance the accuracy of shaft guidance
- compensate for wear and settling (bedding down) process in operation
- provide long service life.

I'm not sure of the reasons to provide clearance, but I imagine that temperature fluctuations of the bearing or (more importantly) the shaft have something to do with it.

IMHO; none! an A-F brg needs no metal interference or -clearance or whichever term you wish. If You have no clearance in an AF brg then it's a shortened bearing life. How close you can approach zero clearance is dependant on the ABEC#. A #9 can basically approach zero as it is aircraft quality.

35 years ago at Kodak we would preload some bearings that required no spindle movement. Some run-out may have existed but it wasn't compounded. This did however require regular bearing replacement.

Hi guys, I was wondering if you could help me what exactly is preloading?

I should explain that i'm still and apprentice and keen to learn.

Cheers boys

Preloading is the opposite of clearance. Since clearance cannot be a negative number, it is often expressed in lbs. force. During preload, the elements in the bearing are not only in intimate conact, but load is added to the components.

1 - Here is a document related to preload:
http://www.bardenbearings.com/PDF%20CD/PB11.PDF

quote:

Why preload bearings? Preloading is
used to:
• Eliminate radial and axial play.
• Increase system rigidity.
• Reduce nonrepetitive runout.
• Lessen the difference in contact angles
between the balls and both inner and outer
rings at very high speeds.
• Prevent ball skidding under very high
acceleration.

==================================
2 - Attached is another document related to preload. It is an excerpt from the SKF Pump Bearing Handbook. It states

quote:

The purposes of bearing preload are: avoidance of light load skidding, control of contact angles, improvement to internal load distribution, increased bearing stifness, and improved shaft positioning acuracy

The figure shows that the proper amount of preload for angle contact bearings increases fatigue life.
==============================

3 - Here's my take:

We have talked before in other thread mostly about radial bearings. For radial bearings, preload is used to prevent skidding. We said preload is more important for cylindrical roller bearings than for ball bearings.

For radial bearings, at least we have some maximum clearance established by the internal clearance of the bearing. It is not the case for thrust bearings (angle contact bearings and spherical roller bearings). If these thrust bearings are installed without some opposing load, there would be no limit of clearance that can develop under reverse-thrust conditions. So we need some opposing load or very tight endplay limit simply to keep the bearings engaged.

For vertical motor with thrust bearing on top, the weight of the motor rotor may or may not provide sufficient preload to keep the bearing moving in stable condition in presence of factors such as centrifugal and gryoscopic forces on the rolling elements that tend to make the balls want to move irregularly (especially for high D*N bearings). In general if you have a higher thrust bearing like spherical roller bearing, the weight of the rotor is not enough and some designs provide springs underneath the outer race to maintain minimum bearing load during periods of light shaft load (such as uncoupled run). The endplay limit also may not be tight enough to keep the bearing in a stable geoemtery. For a very few vertical motors, the OEM recommends to remove all endplay to prior to uncoupled run, then re-establish a small endplay for normal running conditions. We have one such motor and we found out the hard way what happens if you make an uncoupled run with DOL start without removing the endplay... the noise was tremendous and sent everyone running the other direciton in fear for their lives.

For overhung horizontal centrifugal pumps there is often the back to back angle contact configuration. I think the reason it is used is that precise axial positioning is important for that application especially for mechanical seals as discussed in attachment. As you increase the preload, you increase the axial stiffness... that means changes to the thrust loading have less effect on axial position and better axial position control with higher preload. Of course we also need preload to prevent skidding in these applicatios as well (more important as speed increases).

The installation and adjustment of these back to back beairngs can be a challenge since very small axial movements are involved. The inner rings need to be heated for installation and everything needs to be assembled quickly before the inner rings cool. One trick I have heard is to rotate the two outer rings against each other after installation. You should be able to move them by hand, but they shouldn't spin freely. There is a lot more to the technique than I am familiar with and it has been discussed in some threads here.

Sorry for a lot of rambling. I was addressing the question "why preload" which maybe wasn't the quesiton. Now back to the original question:

quote:

What types of bearings need preloading during installation and how? I have seen angular contact ball bearings preloaded by locknuts and deepgroove bearings by curved washers or belleville washer or end caps but where and how much? Do any guidelines exist for that and if not properly preloaded , what may be consequences,and how to diagnose?

The manufacturers give minimum axial preload values for angle contact bearings and spherical roller bearings.

Also they specify minimum radial preload for ball beairngs and centrifugal roller bearings as we discussed in other thread. These are important but for different reasons (there are not as many problems that can occur from lack of radial preload on radial bearing as from ladck of axial preload on thrust bearing... but still we face the long-term problem of skidding).

The Barden document above gives several examples of method for preload.

If too much preload, bearings may run hot and will have a reduced life due to the overload.

If not enough preload, you can expecte accelerated wear of the bearings. I don't think the skidding will readily show up on vibration until the bearings develop defects over time as a result of the skidding(I am interested to hear if anyone has different experience on that). You may see oil turn dark in the oiler (there are many possible reasons for this). Also I have seen excess wear in the ball pockets of brass cages in back to back bearings of those overhung centrifugal pumps running 3600 rpm... I tried to figure out the cause and as best I can guess it was related to skidding but I'm not sure (does anyone else have thoughts on that?).

Also I would imagine in the case of insufficient preload you may see reduced life of axial-position-sensitive compoents such as mechanical seals.

I would respectfully suggest that Sam Pickens' comments above apply to "internal clearance" rather than "preload". i.e. there are very few applications that demand negative internal clearance in a deep groove bearing....but there are many applications that demand preload (in particular angle contact ball beairngs and spherical roller bearings)

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

SKF_PumpBearingHandbook.ppt (368 KB, 33 downloads)

In my experience with off-highway equipment, I have commonly seen preloading applied to tapered roller bearings. In our application, preload is set via shim packs. These are normally very highly loaded bearings and their life, according to the bearing supplier, is very dependent on getting the preload set correctly.

I'm sure you will find more material on the webistes of the SKFs and Timkens of the world.

Good luck