Lubricating a chromed bearing surface

[Lovison]

We just experienced a failure of which everyone wanted to blame the lubricant. But I wanted to toss this out to see what feed back I'd get.

We have a micarta CG (plastic graphite impregnated) sleeve bearing that is being grease lubricated. The clearances are 0.010" on a side which is considerable. The lubricant is a Castrol Mol-Lube 860/150-2 ES (lithium complex) which is a great lubricant on bearings but is it ok to use on a sleeve bearing? The lubrication engineer says it should be just fine.

The pump ran about an hour and suddenly the amps went up and the bearing area where the sleeve bearing is installed heated up. We shut down the pump and checked the bearing.

The upper sleeve bearing looks good but the lower end (where the grease comes in) was scoured and looked ragged?
The first time this happen we felt it was mechanical and took it all apart but the second failure was identical.

Did we mentioned that the shaft was chrome plated on the bearing surface. This is a vertical pump with three radial sleeve bearings and the uppermost bearing heated up where the 2nd intermediate didn't and the lower bearing is submerged. Same failure twice and we can really pinpoint the problem. Could it be a lubrication problem or is it still a mechanical influence?

Test was made in a vertical position operating at duty speed 1780 RPM and ran well for about an hour before this sleeve heated up. The shaft is straight and there was no vibration. The bearing sleeve is scoured both vertically and horizontally to allow grease to be distributed troughout the entire bearing length. The test pumpage is only water at ambient temperature.

Any input?


Let me know your thoughts. Wayne E. Lovison
service-parts@naglepumps.com

 

[vanstoja]

Is the lower submerged bearing above or below the pump impeller? Are there impeller wear ring seals above and below the impeller and are they labyrinth teeth or plain annular? How far away from the pumps best efficiency flowrate were you operating? One possibility is tahat if you're operating far off best efficiency pump flowrate, you could be getting high radial hydraulic thrust loading on the impeller. If you bend the lower rotor shaft enough to contact stator surfaces hard at either the lower radial bearing or the impeller seals (less likely with labyrinth teeth which can melt away) the hard rubbing initiates a conical whirl mode with the largest orbital excursion at the top end of the shaft. A really hard rub down below can cause reverse rotor precession (opposite to rotor spin direction) and you're into "full annular rub" (sometimes called "dry friction whirl" where the rotor tries its hardest to get outside the stator. This can cause catastrophic bearing damage in a very short time period. I'm inclined to believe it is possible to have a milder form of this phenomena which can do measurable though not necessarily catastrophic damage to bearings. If your 10 mil bearing clearances are radial you can tilt the rotor 20 mils between upper and mid bearings which bring impeller seals and lower bearing into stator contact. You don't need any grease lubrication screwups to bring about the above scenario.

 

[Lovison]

Vansotja

thanks for your feed back but we think we've discovered the problem. The radial sleeeve cartridge assembly consist of an upper and lower lip seal which encapsulates the sleeve bearing. These lip seals had too much contact load in turn they generated too much friction thereby heat.

We've changed the lip seal design and load characteristics and will open slightly the radial sleeve clearances. It seems grease lubricated bearings require a little more clearance then we normally apply to water flush designs.

Our design has three radial sleeve bearings each positioned to prevent shaft deflection while under load. The shaft surfaces were coated with chrome of which some are saying the surface may be too smooth and the grease can't bind to the shaft? This is still under investigation.

The bearing that failed and we can see that on the surfacing of the sleeve bearing was the upper most bearing that is never submerged. The two lower bearings didn't show any signs of this bearing damage most likely because they are submerged or cooled. Regardless we are changing all of the lip seal designs and bearing tolerances and will rerun new performance test.

We knew we had something wrong because during our testing everything was find then after about an hour the amps started to go up and when we shut the pump down it just seized right up. The flange areas containing the upper radial seal were warm and this pointed directly to the area where the lip seal was installed.

It was a good learning lesson. I'd suggest others to look into this occurrence if after a shaft repair is made or if they buy their bearings from other than an OEM.

We make custom built equipment so we run into these things every now and then so it keeps us on our toes. I appreciate your comments and thanks again for the response.

















Wayne E. Lovison
service-parts@naglepumps.com

 

[rhc]

Hi Lovison
Amen to your observation: ........"It seems grease lubricated bearings require a little more clearance then we normally apply to water flush designs."...........
I am unfamiliar with ......."micarta CG (plastic graphite impregnated) sleeve bearing"........ but from your description suspect it belongs with the re-enforced phenolic plastic group of materials.
As a group they are perfectly happy to operate in water oil or grease, some even dry but like most non metallics they do not conduct heat very well at all.
When lubricated with cold water they require continuous uninterupted flow. Close running tolerances can be achieved.
Lubrication with oil or oily fluid is treated much the same as water but generally with slightly more clearance because oil dissipates heat less efficiently than water.
(all other factors eg swelling being considered)
Fast rotating shafts are the Achilles heel of this group of materials . We sometimes come across marine applications where someone has decided the existing water lubricated bearing shall be lubricated with grease and the bearing has burned out. Accepted thinking is that grease lowers friction therefore the bearing won't get hot. This is only partly true. With grease, total friction may actually increase and yet there is no mechanism to dissipate heatother than via the shaft or through the walls of the bearing. Left unchecked the bearing temperature will steadily rise and the walls of the bearing will expand. At some point full circumferential contact will be made with the shaft. The bearing clamps around the shaft and the clamping spreads rapidly along the shaft. Catasprophic failure will result. These bearings don't melt but instead turn charcoal.
By increasing the clearance to tolerate what might be considered a maximum acceptable bore closure due to temperature rise it may be possible to operate the bearing. Even so the contact surface of the bearing will run hotter than if allowed to run in oil or water and the life of the bearing will be reduced.
Where slow speeds are involved grease is perfect.

 

[rhc]

I need to correct a typo ........."Fast rotating shafts are the Achilles heel of this group of materials . We sometimes come across marine".......
Should read ......" Grease and fast rotating shafts are the Achilles heel of this group of materials . We sometimes come across marine"............
Sorry about that.