Overview of Lip Seals

Lip seal is a seal with self-sealing effect, it relies on the lip close to the surface of the sealing coupling, blocking the leakage channel and obtain sealing effect. The working pressure of the lip seal is the sum of the pre-pressure and fluid pressure. When the pressure of the sealed medium increases, the lip is propped open and fits more closely to the sealing surface, further enhancing the sealing performance; in addition, the lip edge has the role of scraping oil, which enhances the sealing performance of the seal. There are representatives of the lip seal according to the shape of the section can be divided into Y-shaped ring, V-shaped ring, U-shaped ring, L-shaped ring, J-shaped ring pieces, lei-shaped ring and other types of seals.
Lip seals are mainly used in reciprocating seals, compared with extruded seals such as O-shaped seals, reciprocating seals using lip seals have better overall performance and higher service life.
The sealing pressure of the lip seal is changed with the change of medium pressure, always work when the medium pressure changes, that can ensure sufficient sealing pressure, but not to produce excessive friction. Extruded seal mainly relies on pre-pressure to generate sealing force, seal contact pressure is pre-determined, the work can not be changed according to the change in medium pressure, if required to seal a large pressure then pre-pressure must also be large, and large pre-pressure will make the seal ring and seal surface contact stress and contact area increases, resulting in a large frictional resistance. Excessive friction can cause breakage of the seal, resulting in difficult start-up at low pressure, or crawling. Lip seals can compensate for small amounts of wear through lip propping and deformation to ensure sealing effectiveness and seal life; while O-ring seals directly affect their sealability as the friction margin decreases.
In addition, O-ring seals are susceptible to tumbling and twisting in reciprocating motion sealing, while lip seals are less susceptible to tumbling and twisting.
Compared with O-ring seals, the main disadvantage of lip seals is that they can only be used as one-way seals, if they are to be used as two-way seals then two seals should be used, which will increase the length of the seal; groove design difficulties, and between the two seals will produce trapped oil, causing damage to the reverse pressure; while the pre-pressure seal section shape left and right symmetry, can be used as a two-way seal, smaller volume, groove design is easy.
Based on the above comparison, in the case of allowing a small amount of leakage, the extruded seal can reduce the groove size; while in the case of high sealing performance requirements, the lip seal with better overall performance should be used.
The function of sealing is to stop leaks, and effective stopping of leaks is the primary requirement for sealing devices. Sealability reflects the level of control over leakage. For motion seals, friction is an important factor in relation to the quality of movement. And sealing and friction always govern each other. Generally speaking an increase in sealability brings an increase in friction, which directly leads to a reduction in movement capacity and quality; and friction accelerates the wear of the seal. Friction can also be a major component of the load on low pressure systems. The resistance capability of a seal reflects the high pressure of the working medium that can be sealed, and is an important indicator of a hydraulic-pneumatic seal.
The basic requirements for seals can be summarised as 1) sealing performance. 2) frictional performance. 3) Pressure resistance performance. 4) Life time. 5) Installation performance. 6) Economy.
The above sealing performance, friction performance and pressure resistance are independent properties, the combination of these three properties gives the comprehensive performance of the seal; the comprehensive performance is maintained for the life of the seal; the installation performance and economy should also be used as an important indicator in the actual design.
The comprehensive performance of the seal is not only related to the performance of the seal itself, but also related to a number of conditions of use of the seal, so to examine the performance of the seal, both to see the performance of the seal or the combination of seals itself, called monomer performance, but also to see the actual sealing performance after the seal is installed in the hydraulic and pneumatic components, called the actual sealing performance.
Seal monomer performance of the following items: 1, friction; friction is related to the performance of the characteristics of the movement, the movement of the seal friction is divided into static friction (starting friction) and dynamic friction (sliding friction) two. Friction by the roughness of the sliding surface, sliding speed and working pressure, seal lubrication state, placement time and many other factors, easy to produce a large change. Friction is difficult to calculate and should be obtained experimentally. Static friction is influenced by many factors and the measurement error is relatively large, the measured value can only be used as a reference. In contrast, dynamic friction can be obtained more stable, important measurement values.
2、Abrasion resistance; Abrasion resistance is a characteristic that has a great influence on the seal life, and like friction performance, it is influenced by a variety of factors. Especially for pneumatic seals, where the lubrication state is poor, wear resistance becomes the main indicator of life. For seals made of synthetic rubber or synthetic resin, the wear resistance of the seal can basically be evaluated according to the wear resistance of the material. However, this does not necessarily correspond to the results of the actual use of the seal, as there are many other factors affecting wear in use. Therefore, it is more meaningful to evaluate the wear resistance of the seal products in terms of the actual use of the seal in the component.
3. Eccentricity resistance; In dynamic seals, a gap is created due to the tolerances of the coupling surfaces of the components. This clearance can cause shaft eccentricity under load and vibration, affecting seal performance. For this reason, the seal must also ensure adequate compensation performance for such eccentricity. In addition, at low temperatures the elasticity of the synthetic rubber seals is reduced, resulting in a reduction in the ability to compensate for eccentricity following, the lower the temperature the more serious this phenomenon, so the eccentricity resistance can also reflect the sealing performance at low temperatures.
4, contact stress; seals rely on a certain contact stress to ensure sealing, seals in deformation conditions must ensure sufficient elastic force. Contact stress determines the shape of the oil film and is a reference for sealing performance and friction and movement performance.
Seals real machine performance should be examined: 1, sealing performance; sealing performance is an important requirement for seals and sealing devices. The leakage at the seal is a measure of sealing performance. Affecting the sealing performance is not only the seal, but the whole sealing device after the installation of various seals, and the combination of various elements of the sealing coupling surface.
2、Eccentricity resistance; here is the eccentricity resistance after considering the actual clearance runout and applying a certain eccentric load.
3. Friction (low working pressure); the size of the friction in practice is evaluated using low working pressure. For reciprocating seals, the starting friction is greater than the dynamic friction, which is more complex and affects the starting performance. The use of low starting working pressure to measure seal friction has practical significance. Therefore, for dynamic seals is generally given a low working pressure.
4, comprehensive performance; comprehensive performance requirements of the sealing device in the specified working conditions, to meet the above performance requirements, and work for a certain period of time, so this indicator reflects the seal is generally given a low working pressure.