Damaged SealFigure 2. Arrows show where seal has suffered intrusions from foreign matter.
The majority of standard small-bore (under 8 in. shaft diameter) radial seals are rated up to 3,600 ft/min, while larger diameter seals are rated to approximately 5,000 ft/min. PTFE bearing isolators usually can work at up to 5,000 ft/min, while metallic versions can handle 10,000 ft/min. An application calling for higher speeds requires specialized design considerations.
Options to help mitigate the negative effects of higher shaft speeds include reducing the radial load on the seal lip, switching to a sealing material that can handle higher temperatures, changing lubricant type or viscosity, optimizing the shaft sealing surface or using a non-contacting labyrinth seal design.
Pressure. Standard radial seals are designed for only about 7 psi. System conditions or a fault such as a plugged vent can mechanically load and distort a seal's lip profile, resulting in rapid wear and failure. Solutions to compensate for the effects of pressure include pressurizing the seal cavity to allow the seal to serve as a main pressure retention seal, and redesigning lip profiles to resist deformation under pressure loading and moderate surface speeds.
In situations where higher shaft speeds will be encountered, the permissible pressure differential across the seal becomes smaller. As pressure is applied to the seal, more lip surface is forced against the shaft, which produces greater friction (as does increased shaft speed). Too much friction leads to faster wear and shorter life of seal and shaft -- so, pressure and surface speed must be balanced against each other for maximized seal performance.
Temperature. Operating a seal material beyond its recommended temperature range can cause thermal stress that will harden the compound; the hardening often appears as a series of radial cracks on the seal. (Historically, such heat aging of nitrile rubber seals has represented a more common cause of failure than wear.) Changing the seal material from rubber to PTFE or fluoropolymer can raise a seal's thermal limit.
Surface finish. Shaft surface roughness and directionality rank second only to heat damage as culprits for leakage. Under a microscope, a shaft's surface can be mapped as a series of peaks and valleys. Too smooth a surface may not support an oil film, which can result in a higher-than-desired under-lip temperature. If a surface is too rough, peaks can project through the lubricating film and abrade the lip. The best practice is to consult the roughness and texture specifications developed by manufacturers and based on industry standards. Additionally, consider using electronic tracing instruments to assess surface finishes accurately.
A shaft also may exhibit directional lead (a spiral or screw pattern) from the initial turning or grinding method. While an inward lead might prove beneficial in some respects, an outward pattern can result in more oil under the lip than its pumping action can handle. Keep the potential consequences in mind and inspect shafts accordingly.
Media. Nitrile rubber performs well with a wide range of mineral-based oils as lubricants. However, polar solvents such as acetone can lead to catastrophic swell (observed as a softening) and physical destruction of the seal. Similarly, compounds such as ethylene propylene will swell rapidly from contact with aromatic hydrocarbons and mineral oils. And some lubricants based on synthetics, while resisting oxidation, can attack rubber compounds. The appropriate marriage of seal and lubricant can help avoid seal degradation and contribute to improved performance.