This may seem counterintuitive, but if you are sealing a rotating shaft under pressure, the answer may well be that you do!
The functioning of traditional non-pressurized radial lip seals is based on the formation of a sealing and lubricating oil film between the seal lip and the shaft surface. The actual sealing mechanism is based on the active pumping action of the seal being counterbalanced by capillary suction forces of the oil-air interface on the air-side of the seal. The effective long-term operation of a radial lip seal is based around the presence of this thin lubricating film.
However, the two basic requirements of a radial lip seal are contradictory, those being minimal leakage and minimal friction. Obviously, a balance is needed as an increase in contact force reduces leakage but increases friction losses and wear. It is safe to say for unpressurized lip seals, most quality manufacturers have found a customer-acceptable balance.
When the pressure needing to be sealed increases, however, the lip contact force increases. The fluid film thickness decreases and the heat generated due to friction increases. More importantly, the higher the lip contact force the greater the possibility of creating mixed lubrication conditions under the seal lip, where the sealing lip does not see constant lubrication. This can lead to changes in the mechanical properties of the rubber in the contact area due to additional vulcanization, physical and chemical aging, and oil absorption, due to the rapid increase in temperature created at the interface.
The engineering staff at American High Performance Seals have taken the decision that maximum service life can be achieved under pressurized conditions by creating a very small uniform leak that ensures the seal will seal the fluid pressure but have a thick enough lubricating film that the seal will always experience full film lubrication. This eliminates the counterbalance of the capillary suction forces and means the seal’s active pumping action will lead to a very small amount of fluid being pumped to the air side of the seal, and not returned.
To enable there to be a stable fluid film, the engineers at AHP Seals have developed pressure-balanced seal designs. Unlike traditional seal designs where the higher the system pressure the higher the lip force (which eliminates the fluid film, creating high friction, heat and wear), the pressure-balanced designs have a very small increase in lip force as the pressure increases and thus maintains the fluid film.
It is recommended that a secondary standard oil seal is fitted outboard of the main pressure seal to capture the small amount of fluid passage past the pressure seal.
Looking for a Unique Seal Design for Your Oil Seals or High Pressure Seals?
Look no further! American High Performance Seals has experienced seal design engineers standing by to answer all of your high pressure oil seal needs. Give us a call at (800) 283-7140 or go to our Contact Us page to complete a simple form describing your specific seal need right away!