Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The primary functions of a bearing seal are to keep lubricant in the bearing and bearing chamber, and to exclude contaminants from that area. The dis­cussion in this article relates to seals that are external to the bearing and therefore not a part of the bearing itself.

Key factors in bearing seal selection include:  

• The type of lubricant (oil or grease)
• The type of bearing (rolling or sleeve)
• Shaft surface speed, connection to load (e.g., direct-coupled or belted)
• Seal friction and consequent heat­ing
• Physical space available

The types of seals most com­monly used with rolling (ball and roller) bearings are contact or lip seals (see Figure 1), non-contact seals (see Figures 2 and 6), and to a much lesser degree bearing isolators (see Figure 3). 

Contact seals 
Contact seals utilize a resilient ma­terial that applies continuous pressure to the shaft surface to create an effective sealed interface. It is difficult for fluids or solids particles to penetrate the seal area. The direct contact of seal to shaft results in friction and heat, which can lead to degradation of the seal and the shaft surface finish. If a less effective sealing method can be accepted, an alternative is a non-contact seal. 

Non-contact seals 
The non-contact seal will have much less friction (if any) and heating com­pared to a contact seal. However, the non-contact seal will allow lubricant to leak out of the bearing chamber and liq­uid or physically small contaminants to enter the bearing chamber.     

The bearing isolator is a combi­nation of contact and non-contact features in a single seal. The contact features, however, are used to “drive” part of the seal at shaft rotating speed. Such a seal provides more protection than either contact or non-contact type used individually.

If the bearing is oil lubricated, a con­tact type seal or bearing isolator seal should be used. (An exception to this is the labyrinth seal commonly used for sleeve bearings which is a type of non-contact seal that will be discussed later.) A non-contact seal will allow oil leakage and thus is not acceptable in most applications. Note that virtually all sleeve bearings are oil lubricated, and most rolling element bearings are grease lubricated.  

Bearing isolator seals 
The bearing isolator seal can be used with either grease or oil lubrica­tion, and with either sleeve or rolling bearings. Bearing isolators are more costly than contact or non-contact seals and require more physical space. However, they provide more effective sealing. The first bearing isolators were non-contact labyrinth types that greatly reduced contamination ingress but did not stop moisture or other vapors. A newer type is a contacting type isolator (See Figure 4) that uses rare-earth magnets to apply tension to lapped contacting faces, just like a mechanical pump seal. The contact­ing type stops all solid and vapor contamination but has surface speed limitations; maximum about 4" (100 mm) shaft at 3600 rpm.  

Another variation of bearing isola­tor is a labyrinth design that has an O-ring or other elastomer element which closes the labyrinth channel when the shaft is stopped; centrifu­gal force expands the element and opens the channel when running. This prevents vapor ingress when stopped and eliminates the friction/heat when running. When used with sleeve bearings, a special long-relief bearing isolator is required due to axial end float associated with sleeve bearings. 

A combination of contact and non-contact seal types is a “shaft slinger” as illustrated in Figure 5. It contacts the end bracket when idle and moves away from the end bracket due to centrifugal force when rotating.  

The other type of non-contact seal that is commonly used is the labyrinth seal as depicted in Figure 6. It can be used with rolling or sleeve bearings and with oil or grease lubrication. Suggested oil-lubricated sleeve bearing labyrinth clearances are given in Table 2-7 of EASA’s Recommended Practice for the Repair of Rotating Electrical Ap­paratus (ANSI/EASA AR100-2010). Regarding diametral clearances for grease lubricated rolling bearings, one source suggests about 4-8 mils per inch (0.04-0.08 mm/cm)  for shaft di­ameters below 2” (50 mm), and about 5 to 10 mils per inch (0.05- 0.10 mm/cm) for shafts 2” (50 mm) and larger.

Shaft surface speed must be taken into consideration for contact seals. If the speed is excessive the seal mate­rial will degrade due to overheating from friction, and the shaft surface may be damaged. Table 1 provides limiting speeds for some common contact seal materials. Contact seal friction and wear are also affected by shaft surface finish.  Suggested shaft surface finish tolerances are given in Table 2.

Categories: Bearings, Lubrication

Tags: Bearings Bearing seals

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