This expert advice will help simplify your application-specific decisions about external seal types.
The primary functions of a bearing seal are to keep lubricant in the bearing and bearing chamber contaminants out.
Some seals are integral to the bearing; others aren’t. The focus here is on what to consider when selecting external bearing seals. Key factors in making the right choice for an application typically include:
- Bearing type (rolling or sleeve)
- Lubricant (oil or grease)
- Seal friction and consequent heating
- Shaft surface speed and finish
- Physical space available
To select the appropriate seal for an application, match the relevant factors from the above list with the characteristics of the following external seal types.
Common types of external seals
The types of seals most commonly used with rolling (ball and roller) bearings are contact or lip seals; non-contact seals; and, to a lesser degree, various types of bearing isolators that combine the functions of contact and non-contact seals in different ways. The labyrinth seal is a non-contacting type normally used with sleeve bearings.
Fig. 1. The contact-grease-seal arrangement on the left is better for protecting against dust and liquids entering bearing chamber; the arrangement on the right is better for retaining lubricant in the bearing chamber.
Contact seals: A contact seal (Fig. 1) forms an effective sealed interface by applying continuous pressure to the surface with a resilient material. These seals make it difficult for fluids or solids particles to penetrate the sealed area, but direct contact with the shaft creates friction and heat that can degrade the seal and damage the shaft’s surface finish. If a less effective sealing method is acceptable, an alternative is a non-contact seal.
Fig. 2. Example of a non-contact seal used for a ball bearing
Non-contact seals: A non-contact seal (Fig. 2) produces much less friction (if any) and heating than a contact type. Unfortunately, non-contact seals also allow lubricant to leak out of the bearing chamber and liquid, and permit physically small contaminants to enter.
Fig. 3. Example of a combination contact and non-contact bearing isolator seal
Bearing isolator seals: Bearing isolators combine the characteristics of contact and non-contact seals in a single unit (Fig. 3), but use the contact features to “drive” part of the seal at the shaft’s rotating speed. Such seals afford more protection than individual contact or non-contact seals. They also can be used with either grease or oil lubrication, and with sleeve or rolling bearings. Although bearing isolators are more costly and require more physical space than contact or non-contact seals, they deliver more effective sealing.
Fig. 4. Contacting-type bearing isolator (courtesy of Isomag)
Contacting isolators: The first bearing isolators were non-contact labyrinth seals that greatly reduced contamination ingress but didn’t stop moisture or other vapors. A newer version called a contacting isolator (Fig. 4) uses rare-earth magnets to apply tension to lapped contacting faces, just like a mechanical pump seal. Although contacting isolators stop all solid and vapor contamination, they have surface-speed limitations—a maximum of about a 4″ (100 mm) shaft at 3600 rpm.
Labyrinth-design isolators: Another variation of the bearing isolator has a labyrinth design and an O-ring or other elastomer element that keeps the labyrinth channel closed when the shaft is stopped and expands by centrifugal force to open the channel when the shaft is rotating. This prevents vapor ingress while the machine is off and eliminates friction/heat when it’s running. Special long-relief isolators are used in sleeve-bearing applications to accommodate the bearing’s axial end float.
Fig. 5. Example of a shaft-slinger seal that combines contact (1) and non-contact seal types
Shaft slingers. These seals combine elements of contact and non-contact seals (see Fig. 5). Shaft slingers make contact with the end bracket while the machine is idle and move away from it (by centrifugal force) when the shaft is rotating.
Fig. 6. Non-contact labyrinth seal examples arranged in order of sealing effectiveness. (Clearance [C] and length [L] values are the same for each example).
Labyrinth seals: Another commonly used non-contact seal is the labyrinth type (Fig. 6), which can be used with rolling or sleeve bearings, and with oil or grease lubrication. Suggested clearances for labyrinth seals with oil-lubricated sleeve bearings are provided in Table I:
Speeds given are synchronous speeds corresponding to the applicable line frequency and winding poles. Dimensions shown in millimeters are rounded off. The table at left is to be used for horizontal machines with bronze/brass labyrinth seals, absent specific clearance recommendations from the manufacturer. Galling materials, such as cast iron, may require greater clearance. Vertical machines may require less clearance. Labyrinth seal clearance must always be greater than the bearing clearance. A general rule of thumb suggests that labyrinth seal clearance should be 0.002” – 0.004” (.050 – .100 mm) greater than the sleeve bearing clearance.
* The shaft diameter is the diameter at the seal fit; and “up to” means “up to but not including.”
** The diametral clearance is the clearance for the applicable range of shaft diameter.
Reference: ANSI/EASA Standard AR100-2010: Recommended Practice for the Repair of Rotating Electrical Apparatus, Table 2-7.
Suggested diametral clearances for labyrinth seals with grease-lubricated rolling bearings are 4-8 mils per inch (0.04-0.08 mm/cm) for shaft diameters below 2” (50 mm), and 5-10 mils per inch (0.05- 0.10 mm/ cm) for shafts 2” (50 mm) and larger.
Contact seals or bearing isolators are good choices for most oil-lubricated bearings—with the major exception of sleeve bearings, for which labyrinth seals are commonly used. Non-contact seals aren’t acceptable in most oil-lubricated applications because they allow leakage.
The options for grease-lubricated bearings run the gamut, from non-contact and contact seals to various kinds of bearing isolators and labyrinth seals. (Note that virtually all sleeve bearings are oil-lubricated, whereas most rolling element bearings are grease-lubricated.)
Table II. Limiting Surface Contact Speeds for Seal Materials
Shaft surface speed and finish
Shaft surface speed is always a consideration for contact seals. If it’s excessive, overheating from friction will degrade the seal material and possibly damage the shaft surface. Table II provides limiting speeds for some common contact seal materials.
Table III. Shaft Surface Finish Tolerances
Contact seal friction and wear are also affected by shaft’s surface finish. Suggested shaft surface finish tolerances are given in Table III. LM&T
Thomas Bishop, P.E., is a Senior Technical Support Specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO. EASA is an international trade association of more than 1900 firms in 62 countries that sell and service electrical, electronic and mechanical apparatus. Telephone: (314) 993-2220; email: firstname.lastname@example.org; or visit www.easa.com.