Guide Bearing Grease Lubrication Temperature

[mls1]

We have a grease lubricated journal bearing on a vertical shaft of an axial flow pump that demonstrates an unusual temperature cycle. The grease is injected every several hours into six equally spaced, helical grease grooves by a centralized system at high pressure. Each time grease is injected, the bearing temperature rises rapidly then slowly decays back towards equilibrium. Often it doesn't get there before the next grease cycle and temperature rises again.

There have been several theories passed around for the cause. One theory is that the double lip seals on the bearing are allowing the pressure to be maintained high while the grease leaks past the seals and this is causing a hydrostatic operation and higher load on the bearing. Other possibilities are whirl or grease churning (similar to in ball bearings). The pump is on a VFD and the phenomenon occurs at all speeds with average temperature proportional to speed. There is no real evidence the pump is anywhere near critical speed or 50% of critical speed (present best analysis is first harmonic is ~4 times maximum operating speed).

Question: is there a documented phenomenon for grease pressure causing temperature transients in a lightly loaded, vertical plain guide bearing? Any thoughts or references on the subject would be helpful. Thanks!

 

[desertfox]

Hi mils

According to this link a temperature rise in a bearing can be expected after grease injection.

http://www.reliableplant.com/Read/25315/Tips-troubleshooting-bearing-temperatures

 

[mls1]

Thanks, desertfox. I do appreciate your information. I have seen this documentation before and I think it is focused on anti-friction bearings. The churning impacts of grease on roller bearings is well documented. What I haven't seen is specific reference to a similar phenomenon in plain bearings. I think, and please correct me if I'm wrong, that most assume the end leakage on plain bearings prevents overgreasing. In our case, we have a tight double seal. My intuition tells me it is very possible that excess pressure can build up and be retained briefly but a corroborating document would really help.

 

[Compositepro]

It is not the grease pressure that causes heating of the bearing. It is the power consumed by shearing of the viscous grease. If you do not continue to inject grease the shaft will ride on a thin film of grease on the loaded side of the bearing. In all other areas there will be an air gap and no direct contact with grease and thus no shearing action (churning). When you inject fresh grease you significantly increase the area of shearing grease. The excess grease that leaks out will also be sheared while forming a seal to prevent air from entering the bearing and allowing an air gap to form on the unloaded side of the bearing, and in the grease grooves. You are teetering between submerged lubrication and film lubrication.

 

[mls1]

Thanks, Compositepro. I have been considering exactly what you describe but I am confused on one point in this application. The bearing is a guide bearing just above the impeller of an axial flow pump and is submerged in water. The seals are intended to keep water out while allowing grease and wear products to escape (it uses a biodegradable grease to reduce environmental impact). In such a case would there not be 360 degrees of sheer always? If the entire circumference is full of grease and the grease pressure is increased would the eccentricity go down (bearing move toward center) and would that create a higher load?

 

[Compositepro]

In this case, it might be that there is a pressure effect, since your bearing appears to be sealed enough to actually maintain a pressure. My speculation is that this pressure could be preventing cavitation in the grease. As the pressure falls, the grease can cavitate (form a gap), which reduces energy lost in shear.

 

[mls1]

Now that adds an interesting wrinkle. One thing that has been found in the grease taken from the bearing has a very high water content. It would seem reasonable that if the grease is entering and leaving cavitation it would be ingesting water into the lubricant that leaked past the seals when the pressure had dropped. Is there a reasonable way to confirm cavitation is occurring?

 

[Tmoose]

do you have a cross section of the arrangement?

Are the seal lips oriented to:
- exlude water
- contain the grease?

Is the grease cavity:
- pressurized
- vented to atmosphere
- sealed to fend for itself

 

[mls1]

Tmoose - I'm not sure if I can provide the drawing (it has the typical proprietary statements) but I think I can describe it:

1. It's a 6.625" shaft with a double lip seal on each end of the bearing. Both are Garlock Model 53 which has a published pressure rating of 7 psi.
2. Shaft RPM is 140-227 (on a VFD) and the grease supply system is an SKF Dual Line system. There are 6 grease injection ports into 6 helical grease grooves.
3. The seals are to exclude water as the bearing is on lower guide bearing of a vertical pump in a silty river environment (a hydroelectric facility to be exact)
4. The grease cavity is not vented to atmosphere. I'd say it is sealed to fend for itself and pressure is controlled by leakage past the seals.
5. The seals are tight enough that they have been observed to grind a fairly deep groove in the shaft.

Now that I see the 7 psi rating on the seals, it might imply a maximum and expected pressure of 14 psi. The L/D is ~2.3 so the bearing is long and the rated load of the bearing is only 29 psi so a 14 psi grease pressure would seem to me to be significant. It is a total loss system so I would expect it to be sensitive to any change in grease shear.

The lip seals appear to also be designed for oil retention and that may be the basis for the 7 psi rating. The lubricant in this case is NLGI 2 grease.

 

[mls1]

I have one quick update to this. I've communicated with the lip seal manufacturer and with the lip seals pointed out the way they are they can retain a differential pressure of as much as 3-5 psi per seal. The total pressure in the bearing can therefore be 6-10 psi immediately after a grease injection. So that leaves me with two questions:

1. The bearing is lightly loaded with a large surface area so that design pressure is 29 psi. Would a static grease pressure of 6-10 psi be enough to cause hydrostatic operation?
2. The bearing is flooded so I think a divergent cavitation region is expected. Does collapse or partial collapse of the vapor region seem likely given the pressure that can be built up?

Bottom line, is there a plausible explanation for the bearing temperature cycles that results from grease pressure transients caused by the lip seals?