Standard Lip Seals 

Lip Seals Lip seals retain the oil inside the air end while keeping air, dust and dirt from entering along the rotating shaft when the machine is off load. Remember, when the inlet valve closes and the shaft continues to rotate, a vacuum may be pulled on the inlet end. Some lip seals are made of a high Nitrile Buna N or Viton rubber encased in steel and were designed to withstand only a few pounds of back pressure.

Compressors with these lip seals rely on discharge valves, oil stop valves and a fast acting sump blow down valve to prevent the system pressure from slipping back up through stopped rotors and pressurizing the drive side.  Rubber lip seals can not withstand high pressures. A leaky lip seal of this type may be an indicator of a bad check valve or oil stop valve which will allow full discharge pressure to reach the seal.

The newer style of lip seals use Teflon ®. These are much stiffer and can withstand high pressure as well as synthetic compressor oils. In general, lip seals are more sensitive, than mechanical seals  to shaft run-out and surface irregularities. A lip seal will often leak oil if the shaft has a total indicated run-out of more than only a few thousands of an inch. You can not assume that because the shaft runs true when spun by hand that it will do so at speed. Therefore, it is even more important that you check the radial or side play in the shaft before replacing a leaking lip seal.

 Lip Seals