In most lip seals, increasing fluid pressure increases lip-contact pressure. In a properly designed and installed seal, the lip rides on a thin film of lubricant. Because the film does the sealing, its thickness (usually about 0.0001 in.) must be controlled precisely by the mechanical pressure of the seal element and the shaft finish. If the film gets too thick, fluid leaks. If too thin, the lip wears, friction increases, and stick-slip oscillations can result. Stick-slip oscillations cause a surface wave in the seal, which permits leakage.
Aside from mechanical pressure, the main factors affecting film thickness are sealed pressure and temperature. In most seals, as sealed pressure increases, lip contact pressure increases, reducing film thickness. Increasing temperature, which may stem from increasing shaft speed, reduces viscosity and, hence, film thickness. As the film gets thinner, friction increases, temperature increases, and film thickness continues to decrease. If this cycle continues, it can lead to seal failure. Maximum permissible pressure for radial lip seals range from 7 psi for shaft speeds below 1,000 fpm to 3 psi for shaft speeds above 2,000 fpm.
>Although elastomeric lips can provide a constant shaft pressure under a given set of operating conditions, they are sensitive to pressure and temperature changes in the sealed fluid and tend to become harder with age. Therefore, a garter or finger spring is usually included in the seal assembly to maintain the desired shaft-contact pressure.
Shaft condition also has a profound effect on sealing. Shafts should generally be hardened to at least Rc 30 and have a surface finish of 10 to 20 ∝in. Finishes finer than 10 ∝in. show no improvement in seal life and some laboratory evidence indicates that finishes of 2 ∝in. or finer can shorten seal life because they cannot support an oil film. Finishes without spiral lead are preferred, but if lead is present it should tend to guide the sealed fluid inward.
Correct shaft diameter is also important. An oversize shaft causes the seal lip to exert too much pressure, undersize shafts result in too little pressure. Overpressure leads to early failure and underpressure promotes leakage. Recommended tolerances range from ±0.003 in. for 4-in.-diameter shafts to ±0.01 in. for 10-in.-diameter shafts and larger.