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Subject : Centrifugal pump ball bearing seals 13-9

Bearings are normally lubricated with either oil or grease. Oil mist is another alternative,

 but recent fugitive emission regulations are making this form of bearing lubrication lessdesirable. According to the SKF Bearing Comany, both oil and grease have a useful life

of thirty years at thirty degrees Centigrade !"#$F%. &he 'uriron um comany estimatesthat the ()* life of a radial ball bearing in their end suction centrifugal um, oerating at

its B+!best efficiency oint%, to be between one and three hundred years.

So what is going wrong with the lubricant and bearings in our ums- e do not get a

service life any where near those numbers. /t turns out that most bearings fail rematurely

for only two reasons0

• 1igh heat that caused the lubricant to 2varnish2 and then form 2coke2 that will

introduce solids into the lubricant.

• Contamination of the lubricant by water, moisture and solids.

&he water gets into the bearing from three different sources0

• (eakage from acking. As you know, acking leaks.

• 3oisture from a water hose that is being used to wash down the base late and

general area because of acking and other tyes of leakage. /f nothing leaked

there would be no need for a water hose to wash down the area.

• Asiration. A fancy name that means moisture in the air is enetrating into the

 bearing case when the housing temerature reduces.

Original e4uiment manufacturers traditionally use low cost grease and oil seals to

 revent this ingress of moisture and solids. &ake a look at the following diagram0

 

&he sring loaded grease or li seal shown on the left is tyical

of the tye sulied by these manufacturers and has a rated lifeof less than two thousand !5***% hours !"6 days at 57 hours

 er day%.

&he rubber li will olish the corrosion resistant shaft, remove

the rotective o8ide layer that has formed to rotect the shaft

from corrosion, and then roceed to cut a groove beneath therubber li. &his is called 2shaft fretting2 and is e8lained in

detail in aer )*96 of this series.

+ighty three days is not a satisfactory life for anyone, so you are going to have to comeu with an alternative to these short lived destructive seals. At the end of this aer / have

attached a chart that was ublished by the Chicago :awhide Comany showing a life

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comarison between their seal and a cometitor called &he ;ational Seal Comany. At

his writing Chicago :awhide is a division of the SKF Bearing Comany of Sweden.

1ere are a coule of alternatives you should consider to relace the li or grease sealsthat make no sense what so ever0

&he labyrinth seal is a non contacting shaft seal that works

very well as long as the shaft is turning. /t is very effective

against acking leakage and water hoses, but not too effectiveagainst asiration. art 2A2 remains stationary in the bearing

housing while art 2B2 turns with the shaft

&his seal should not damage your shaft and is available in

 both a solid and slit configuration. &here are many materialsand internal configurations available so investigate them

carefully.

3agnetic face seals have been around for many years. &hey

take very little a8ial sace and are very effective against

moisture, water and solids.

/ should mention here that some eole are uncomfortablehaving magnets that close to the bearings.

Sring loaded face seals are the latest and robably the bestalternative available. &he can be urchased in a variety of

materials and siority of remature bearing failures can be revented by following a few simlerules0

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• urchase 4uality bearings. :emember that there are a lot of counterfeit bearings

out there, so be careful.

• /nstall the bearings correctly on a shaft that has been manufactured to the right

finish and tolerance.

• Fill the bearings with the roer amount of grease or oil to the correct level. Be

sure the um is level before you do this.• /nstall face seals to kee moisture and solids out of the bearing case.

• /nstall an e8ansion chamber in the bearing case filling connection.

• /f you use oil, check the oil level on a regular basis

•  ;ever cool a bearing outside diameter. /t will shrink and over heat. /f cooling is

necessary, cool the oil never the bearing.