FACE LUBRICATION. THE THEORIES F004
A seal face lubricant is defined as any fluid that will maintain a film thickness of one micron (.000039″) or more at its operating temperature and load. Seal faces are lapped to a flatness of just under one micron, so a one micron fluid film will keep the lapped faces separated.
As you can guess there are many theories about face lubrication, but few rules. Here are some of the more popular theories:
- Sometimes there is a film of lubricant between the seal faces. This is often refereed to as the “Pressure Wedge Theory”.
- If it is a hydrodynamic film of lubricant there would be no wear or heat generated from friction.
- Elastohydrodynamic lubrication can occur if the fluid you are sealing increases its viscosity with pressure.
- Sometimes there is only vapor. This was observed in the nineteen sixties at Battle Memorial Institute of Columbus, Ohio when they conducted a series of seal face lubrication tests for the aircraft industry. These tests introduced the asperity theory of face lubrication. This idea of creating a vapor between the lapped seal faces was later picked up by some British seal companies and introduced as vapor phase sealing.
- Some people explain this lubrication by stating that the sliding seal face surfaces are protected by the tribology properties of the face material and call it “boundary lubrication”.
- The mixed film theory explains that the seal faces are seeing a combination of hydrodynamic, elastohydrodynamic and boundary lubrications. The face load is supported partially by lubrication and partly by the face asperities. This requires some method of removing the heat that will be generated.
- The dry running theory. This occurs with solvents, cryogenics, dry hot air and most dry gas applications. The seal faces are running on the carbon asperities in the soft face and the graphite film that was deposited on the hard face (the black mark is always visible if moisture is present). If there is no moisture present, an organic has to be impregnated into the carbon/ graphite mixture to release the graphite on to the hard face.
- The three band theory is another popular theory. With this condition you can observe a band of lubricant at the face outside diameter, a band of vapor in the center of the seal faces, and a dry band at the inside diameter of the seal face.
Regardless of what is happening between the seal faces, the rules for operating mechanical seals successfully always remains the same. Keep the two lapped flat faces together and the seal will not leak.