Water Vapor in Bearing Cavity

[ejc]

When specifying seals for bearings, I have come across these claims.

When bearings start, they increase temperature, resulting in a build up of pressure in the bearing chamber. This is usually expelled through the vent, or through the seal. When bearings stop, the heat dissipates, resulting in the bearing chamber sucking air through the seal. This air has water vapor in it, which then enters the lubrication, and reduces bearing life.

Has anyone in the bearing industry seen this phenomenon? Is the small amount of vapor really affecting the lubrication system / bearing? How much would a contact seal really increase bearing life opposed to a non-contact seal, provided the only difference is this start-up / stop water vapor issue?

Any help would be appreciated.

Thank you.

 

[electricpete]

To start it is interesting to note that some people claim as little as 100 ppm water is deterimental to the life of rolling bearings. Saturation level of water in oil is typically in the neighborhood of 75 ppm at 60C.

Certainly I have seen levels higher than that in the oil although the cause is not exactly clear.

We have had problems with moisture intrusion causing bearing rust on machines which sit idle for long periods of time. I have not heard of it for machines with frequent start/stop. I personally would be most interested in the ability of the seal to prevent breathing during secured operation.

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[diamondjim]

I think bearing and car engines are much
the same. It pays to run them periodically
to ensure lube is on all surfaces. I think
the water condensates and this is the worry
especially near the top surfaces. Even grease
lubed bearings need to be rotated periodically
to ensure grease is distributed evenly around
the bearing surfaces.

 

[electricpete]

I believe that an even better solution than a good bearing seal is some means to keep the bearing slightly warmer than surroundings. Motor winding space heater seems to do the job well for some vertical motors we have with oil-lubricated top bearing that sit idle for periods of time... have not had moisture problems in the oil.

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[electricpete]

I just came back and saw diamondhim slipped a post in between my two. My comment "even better solution..." had nothing to do with his post.

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[Hippo41]

I've seen this a lot on the big sugar mill drives in South Africa. The big crushers are shut down during 'off-crop' for repair, and can acculmulate a surprising amount of water in the housings in a relatively short time upon cooling and drawingin the very humid tropical air.

It created problems on start-up unless maintenance people were trained to take the caps off the bearings housings and check for water content just before startup.

Corrosion on the raceways of the spherical roller bearings was evident leading to early failure on start-up.

We solved the problem by changing the grease to a calcium-sulphonate based product (made by FINA at the time, now TotalFinaElf). It was far more 'water tolerant' than standard Lithium or Lithium-Calcium EP2 bearing greases.

Rotating the machinery during a long term shut down might not be practical, and if you have a lot of water you'll only emulsify what you have there anyway. But it is still a good practice for motors, pumps etc in storage.

Lester Milton
Telford, Shropshire, UK

 

[sreid]

As a side note, the "Water Pumping" problem is almost impossibe to fix through sealing. The conditions that cause it are high humidity and temperature cycling (hot days, cold nights). As long as there is some leakage through the seal the water vapor will condense out in the closed space. And the rate that water can be pumped in is truly amazing.

 

[ejc]

What I'm hearing is that there is no seal that will solve this problem. What about a closed bearing cavity (non-vented) and a contact-type seal? I'm refering to a lip-type seal or a face type mechanical seal / isolator.
Has anyone tried to use these to seal out this water vapor?

Thanks.

 

[Hippo41]

Labyrinth and Taconite seals are better than felt or rubber as you can maintain a steady flow of grease into the labyrinth to form a seal using some simple gas charged grease dispensers.


Lester Milton
Telford, Shropshire, UK

 

[NaikD]

All,

We have a science instrument that uses two angular contact ball bearings seperated by polymide bearing retainers. The polymide bearing retainers functions as a lubricant reservoir. The lubricant used is Apiezon C oil.

The science instrument will be in storage for about 7 years. Following things come to my mind.

1. Lubricant loss due to vapor pressure in 7 years
2. Instrument may require a drive test once in a year (I do not know the frequency) to make sure the bearings etc. work properly.
3. Degradation of lubricant in 7 years.

Can anyone add to this from your experience? What I have missed to account for? What would be the frequency for item 2?

Thanks in advance for your help.

Regards,

- Dipak

 

[electricpete]

It might help to know what is the environment. Climate controlled is better still not a guarantee.

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[brgguy22]

The environment is the key issue. If the bearing is in machine in a climate controlled factory the humidity effect should be minimal. The type of grease used is the other important factor. The other option is to use open bearings and provide enough lubrication to buffer against the external air infiltrating the bearing itself.