Choose From Three Lubricant Seals
Ken Bannister | October 18, 2018
Your application, operating conditions, and lubricant quality determine which seal design will best keep contaminants from damaging rotating equipment.
If your equipment uses parts that rotate, oscillate, or reciprocate, it’s almost a sure bet that same equipment requires lubricants to protect the moving bearing surfaces and sealing devices to contain the lubricant.
Sealing devices perform two functions. They prevent lubricants or gases from leaking out of a reservoir or bearing surface area and keep contaminants (water and dirt) from entering the bearing surface area. In short, seals are designed machine elements, or “gatekeeper” devices, that separate spaces containing different fluids or substances that may or may not be subject to pressure differentiation.
When asked “what is a seal,” a maintainer is most likely to describe one of three types of shaft seals. Shaft seals come in several shapes, sizes, and materials and must be matched to the application, temperature, and lubricant properties.
Choosing a suitable lubricant for your application will require knowledge of the operational conditions (shaft speed, bearing load, hours of operation), the ambient working conditions (hot, cold, dirty, wet), and the type of shaft seals employed.
To meet the shaft-seal requirements and delivery of long-term reliability, the lubricant must meet specific criteria:
• permit damage-free installation of the seal
• dissipate frictional heat
• increase sealing effect
• prevent seal adhesion, even after a long standstill
• permit easy assembly/disassembly
• be compatible with the sealing material and resistant to ambient media.
Labyrinth, radial-lip, and bearing- isolator are the predominant shaft-seal designs in use today. They differ considerably in that the inexpensive radial-lip seal is referred to as a contact seal and is primarily used to isolate oil systems. The more expensive labyrinth and bearing-isolator seals are referred to as non-contact seals that can isolate lubricants and gases.
In their 2009 Journal of Mechanical Science and Technology paper titled “Comparative analysis of the influence of labyrinth-seal configuration on leakage behavior,” authors Tong Seop Kim and Kyu Sang Cha describe a labyrinth seal as “a non-contacting sealing device that consists of a series of cavities connected by small clearances [in which] flow loses its total pressure while it sequentially experiences acceleration into the clearance due to contraction, friction through the clearance, and dissipation of kinetic energy at the cavity.” The cavities and small clearances create a torturous pathway that results in turbulence acting as a curtain or barrier to restrict outward flow (egress) of lubricant or gas, and inward flow (ingress) of contaminants. Because a labyrinth seal is non-contacting, it technically will not wear out and should never need replacing.
A radial-lip seal is a point-contact seal that uses a circular metal outer band that captively fits into the stationary housing bore. A bonded-elastomer sealing lip, contained in the outer band, sits against the rotating shaft and provides a dynamic and static seal. Most radial-lip seals also use a garter spring to ensure that the single point contact is always engaged on the shaft.
When a shaft is rotating at speed, the shaft seal runs on a thin layer of lubricant between the lip and shaft. Lubricant is also
hydrodynamically pumped into the areas by the centrifugal pumping action of the rotating shaft. This lubrication effect is beneficial to the life of the seal but is only as good as the oil condition. If the oil is in poor condition, the sliding contact surface will cause the seal to heat up and prematurely wear, losing its sealing qualities.
Radial-lip seals are classed and purchased according to surface-speed ranges. For example, a natural buna-N rubber elastomer is good from -40 F to 225 F and works well with petroleum- based lubricants. A Viton seal is good from 0 F to 400 F and is excellent for all synthetic fluids that can “soften” other elastomer types. TFE (tetraflouroethylene) is a harder plastic that requires greater installation care, but is good from -100 F to 400 F. TFE is compatible with most fluids. Although inexpensive to purchase, radial-lip seals are full-contact designs and, as such, require the seal to come into contact with the lubricant, making lubricant choice a decisive factor when purchasing the seal.
The bearing isolator is a more-recent seal design that is gaining popularity. This hybrid compact design is a dynamic non-contact, two-piece unit consisting of a fixed piece (stator) that interconnects and marries to a moveable rotor piece attached to the shaft. The bearing isolator is an easy-to-install split-hybrid design that uses o-rings and an elastomer connecting ring, known as the unitizing element.
When specifying seals, first understand your operating conditions. Then consult with your lubrication and bearing seal providers to determine which seal will best protect your assets. EP
Ken Bannister is co-author, with Heinz Bloch, of the book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for Engtech Industries Inc., Innerkip, Ontario, he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact: firstname.lastname@example.org or 519-469-9173.