Lubrication Intervals for Bearings 2

[petronila]

Dear All,

The attached file is a graph used for lubrication intervals for bearings of electric motors, did someone recognize the source of the graph? If so can share the original document, if possible.

Thanks in advance

Petronila

 

[electricpete]

Attached is a figure from SKF Bearing Installation and Maintenance Handbook 140-710 (2001) which looks pretty much the same.... this one adds some additional dashed curves on the left, and clarifies the vertical axis as "relubrication interval" (on the left side) and "L01 grease life" (on the top).... I gather SKF is using those two phrases interchangeably since to my eye they both define what the vertical axis represents.

The graph disappeared in the 2007 version of the same publication. I don't think that means anything became obsolete, only they started to look for new ways to describe it, including their traffic light concept.


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(2B)+(2B)' ?

 

[edison123]

petronila/pete

Is that interval for make up grease (a small amount) or replace the entire grease?

Muthu

http://www.edison.co.in

 

[TugboatEng]

Make-up wouldn't have a clearly defined interval unless the quantity of leakage could be determined first.

 

[petronila]

Great Pete,

It is possible for you to share the whole catalog (link) or at least the pages that include the explanation of how to use the graph?

Petronila

 

[electricpete]

Petronilla - Attached are pages 75/76/77 from the above reference. Page 75 (under relubrication intervals) refers to the graph.

Is that interval for make up grease (a small amount) or replace the entire grease?

edison - great question. It's a little bit of both (could be grease replacement interval, or could be the interval that you divide by two to find the grease replenishment interval). On the first page attached under relubrication procedures, it says "If the relubrication interval is shorter than 6 months, then it is recommended that the grease fill in the bearing arrangement be replenished (topped up) at intervals corresponding to 0.5 tf. The complete grease fill should be replaced after three replenishments. When the lubrication intervals are longer than 6 months it is recommended as a rough guideline that all used grease be removed from the bearing arrangement and replaced by fresh grease"

Make-up wouldn't have a clearly defined interval unless the quantity of leakage could be determined first.

TugBoat - it is pretty common (at least in my industry) to add grease on a time schedule to electric motors (especially the larger ones without shields). The intervals are not based on any observed leakage, they are sort-of empirical based on various recommendations from bearing manufacturers and other experts. The theory of regreasing (what are you really trying to accomplish) is also subject to debate. But as indicated by the terminology on the chart L01 grease life, it suggests SKF believes the grease degrades and an addition of new grease (within reason, not overfilling the cavity) moves the conditions back closer to ideal. To my thinking, there is a lot of oil locked in the grease within the housing cavity that easily migrates into the beairng when the grease is fresh. When the grease ages/hardens it is no longer good at providing a reservoir of oil to migrate into the bearing so replenishing the cavity grease gives a supply of fresh grease that will readily give up its oil again. Some people talk about the ability of new grease to enter the swept area of the bearing or displace old grease in that area (no doubt it depends on he design of the bearing/housing/ports arrangement but I don't think that's typically the objective of regreasing)

What is listed above are just the SKF recommendations. You can find plenty of different recommendations (and in fact I'll be you can find different recommendations among SKF literature). The bearing manufacturers tend to recommend conservative practices with less regard for labor and more regard for keeping pristine lubrication conditions. EPRI has published some guidelines on the other end of the spectrum that push the lubrication intervals longer (and increase the amount of grease added each time to "compensate" although of course 2 ounces of grease added every 2 years does not provide as good lubrication as 1 ounce of grease added every 1 year)


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[electricpete]

I am curious how many of you would proactively change the grease in a motor on a time schedule (like SKF recommends) and how do you do that.

We wouldn't do it unless we saw unusual grease coming out, maybe hot bearing housing, or we were replacing the bearings. And if we did it for any of those reasons, we would replace the bearings in the process because I wouldn't want our guys to try to replace the housing grease without replacing the bearings (especially for open bearings). It seems there would be too much risk for to contaminate the grease sitting in the bearing itself. And I don't really trust that we can remove/replace the grease in the bearing itself, or that we could detect when that grease has dirt in it.

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(2B)+(2B)' ?

 

[TugboatEng]

Pete, we to add grease on a time schedule and it's exactly as you say, based on empirical in the field experience. The experience will be unique for every user which is why I would not expect a component level manufacturer such as SKF to have a recommendation. A machine or system manufacturer is much more likely.

As for the need to supplement the initial grease fill, we can all see it. The lubricating oil does separate from the thickener over time. The mechanical action the bearing works in back in. I think over time, the geese establishes a sort of equilibrium in the housing where the bulk of the grease remains stationary in the housing and only a small fraction of the grease is doing the lubricating. Adding a shot of new grease either breaks that equilibrium and gets things mixing around or it creates additional heat which melts that static grease a bit so it can start working again.

That hardening of grease is just the thickener being concentrated. Anybody seeing this should seriously consider using polyurea thickened grease as it has a much lower thickener to oil ratio and is much more shear stable. Dirt or wear particles can also thicken grease.

 

[Gr8blu]

Pete - having been a motor / generator guy for quite a while, my personal (and professional) response is "grease on a set time interval, modified by application considerations". That means plan on regular re-grease (enough to REPLACE the grease, not just top it up), but adjust the interval based on how the application works. Areas with a lot of airborne contamination or extreme temperatures will require more frequent intervals; those in the "middle of the road" less so.

As a side note: am not a fan of pressure additives, as they tend to decompose into conductive particles - which means bad things for bearings, in my experience.

Converting energy to motion for more than half a century