Lip seals compete with mechanical face seals in sealing oil or grease in rotary shaft applications. Under certain running conditions both kinds of seals can be used however, the latter ones are generally used for applications where a significant pressure difference needs to be preserved. Rotary lip seals are preferred for applications where a null or low pressure difference is found.
Lip seal most common components.
The first rotary lip seals appeared during the 1930s however the mystery behind their working performance was not spotted until 1957 when Jagger in UK measured the friction force between the seal and the shaft.
Jagger tuned a lip seal so he was able to vary the loading between the seal and the rotating shaft by simply adding weight to it. He measured the frictional torque at a constant angular speed while varying the load.
By trail and error complex geometries of lip seals have been reached and is part of the knowhow of the lip seals producers.
When the loading was low, the seal significantly leaked. This leakage decreased as the load increased and the friction force increased almost linearly with the load. At a certain specific loading point, the leakage ceased setting this way the “sealing point”. Surprisingly, when the contact was furtherly loaded, the friction force kept increasing linearly without a change in slope and no leakage was observed whatsoever. In other words, the fact of sealing or not sealing did not result in any significant change in the frictional behavior. Due to the aforementioned, together with the fact that at a null loading a certain friction force was measured (contrary to dry tests where the friction-load curve always passes through the origin), allowed Jagger to conclude that a continuous film of lubricant was present between the shaft and the seal.
Later on, it has been repeatedly proofed that depending on the running conditions, a rotary lip seals can work under boundary, mixed and even full film lubrication regimes.
Having a dynamic leak-free seal performing within the full film lubrication regime is wear-wise highly convenient. However, two immediate questions arise:
- Which is the principle behind the hydrodynamic pressure build-up leading to the complete separation of the lip seal and the shaft?
- If there is no physical contact between the shaft and the seal, why there is no apparent leakage?
In fact, the working principle behind rotary lip seals is still nowadays not fully-understood. The following facts have been reported repeatedly by several researchers:
- There is a continuous lubricant film between the shaft and the seal.
- A successfully running lip seal does not present any leakage.
- A certain roughness is needed to prevent leakage in a lip seal. Beyond a certain limit of smoothness leakage is observed.
- A speed governed back-pumping mechanism towards the spring-side of the seal is observed in rotary lip seals. This upstream pumping was spotted from the following situations:
- When the air side is filled with oil the liquid is pumped towards the oil side.
- When a seal in mounted conversely it shows a considerable leakage.
- The pumping rate is increased by moving the garter spring towards the tip of the lip.
- The angle between shaft and seal on the air side must be higher than the one on the fluid side for a good seal performance.
- Velocity dependent bubbling (cavitation) was observed next or directly on the running surface.
- A concave meniscus appears at the air-side of the contact zone. The position of the meniscus is variable.
Examples of lip seal texture at the lip after bedding-in, showing an unsuccesful material (a) and a succesful material (b). Source: SKF.
Diagrammatic representation of the fluid film transfer under a seal lip (upstream puming). Source: Freudenberg Sealing Technologies.
Distortion of the contact band creates asperities at an angle to the shaft axis. Source: Kammüller, IMA, University of Stuttgart.
Although several theories explain the possible mechanisms behind lip seals leak-less performance, as far as the author is aware, none of those could fully predict its behavior. The most accepted theory relies on the pressure build-up on the microscopic level due to the shape of the asperities of the seal surface: micro-convergent gap in the direction of motion (depicted above). Some researchers sustain that asperities alone cannot generate enough pressure to completely lift the seal from the shaft.
Nevertheless, it is believed that several mechanisms contribute to the contact-less lip seal performance. Depending on the specific application and running conditions, one mechanism or another plays a dominant role.