Can sleeve be polished by oil ring? 1

[Wkcoo]

Hi all, recently I saw a case of pump thrust bearing high temperature and lube oil turning black in a between bearing pump. This uses oil ring lubrication (bronze oil ring placed on a carbon steel shaft sleeve) and the lube oil analysis shows presence of copper alloy particles, so it is evident the bronze oil ring has contaminated the oil. I try to swapped the oil rings between DE and NDE, it did not solve the high thrust bearing temperature issue. DE side radial bearing temperature has been good even before and after swapping the oil rings. The problem stopped recurring after we replace the thrust side oil ring sleeve and the oil ring with newly purchased parts from OEM.

Given that the oil ring is swapped but did not solve the problem (and no issue with DE side) but after replacing with new sleeve and ring the problem stops, i am thinking the problem lies in the oil ring sleeve itself. A visual comparison of the old vs new sleeve shows that the new sleeve has quite fresh machining marks, but the old one appears slightly polished. Now the leading hypothesis is that the long term slipping motion (~4 years in service) has caused surface finish of the sleeve to decrease, thus resulting lower frictional force between the sleeve and the ring, thus the ring is spinning slower and splash less oil towards thrust bearing.

Has anyone experienced bronze oil ring is able to polish the carbon steel sleeve or pump shaft? Typical understanding is bronze oil ring should be the one that wears and not the steel shaft or sleeve. Any similar experience is appreciated.

 

[lucky-guesser]

The harder material should wear slower, but that doesn't mean it won't wear at all. Especially a few thou over 4 years.

 

[electricpete]

fwiw it certainly seems plausible to me that shaft finish can change over time (it wouldn't have to be wear a few thousandths to change the surface roughness in the range of microinches). And it seems plausible that shaft surface finish could be an important variable affecting friction between shaft and ring that affects the dynamic behavior of the oil ring (we already know that surface roughness being too low can adversely affect sliding bearings but I've never heard it discussed in the context of oil rings)

In general I think the more challenging oil ring applications tend to be high speed, higher shaft diameter. These are often fixed by an application. When troubleshooting the darkened oil problems we often focus on validating oil level (static distance from bottom of oil ring to oil), and oil ring condition, but shaft roughness is something I'd never thought of... will have to start paying more attention to that possibility.

 

[Wkcoo]

Thanks @lucky-guesser, I need to recalibrate my understanding that the harder material will not wear. I manage to find out a tribology study where a steel ball is rubbed against a bronze piece, some material transfer from the ball to the bronze piece is observed.

I actually have another question on the oil ring splashing action, it splashes oil radially but the thrust bearings are not at the same radial plane in the bearing housing. How is it expected for the oil to reach the thrust bearings, hoping the splash is random enough to reach it? I have checked the bearing housing internal wall does not have oil grooves to direct the splashed oil to the bearings.

 

[Wkcoo]

To electricpete, we are still looking for any other potential mechanism causing the thrust bearing high temperature and darkened oil. If the shaft finish is trusted to be the only cause, then it's probably a very unique case that the oil ring is so sensitive with the surface finish. Otherwise we would have seen multiple pumps with issues.

 

[Artisi]

Take out the loose oil flinger and put in a fixed flinger, that will sort out the problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Wkcoo]

To Artisi, to use a fixed flinger in the bearing housing, does the bearing housing design needs to be changed as well? Current bearing housing internal does not have any feature to direct the oil.

 

[electricpete]

You may find responses and links regarding alternatives/replacements for brass oil rings in the following: thread407-374535

At the time of these threads we were looking at an upgrade on one family of overhung pumps with chronic darkening oil problems and oil ring wear. But we never proceeded with the upgrade. As I recall we discussed a lot of options with the pump OEM and some of them were constrained by how far things would have to be disassembled to complete the installation which was influenced by how big were the openings in the casing, but those details are foggy for me.

 

[Artisi]

there are many links on the net - type something like -"bearing oil flinger" gives a good start.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)