Lip seals are mechanical devices used to seal shaft output points on rotary and reciprocating machinery. They consist of a rigid outer shell and a flexible inner member made of rubber. Lip seals prevent dust and moisture from entering the internal workings of machinery without affecting performance. They are press-fitted into machine end shields and form a static and dynamic barrier along the shaft surface. Lip seals are constructed with a two-part design, with a rigid outer portion and a flexible inner portion. Some lip seals have an integral pocket for a circular coil spring to aid in sealing the shaft.
A lip seal is a passive mechanical device used to seal shaft output points on electric motors and other rotary and reciprocating output machinery. Seals are designed to keep dust and moisture out of the internal workings of machinery, while having little or no adverse effect on their performance. Most lip seal designs consist of a rigid outer shell that mates statically with the machine end cover and a flexible inner member that forms a dynamic seal on the moving shaft. The rigid part of the gasket is typically made of light metal or hard plastic, and the flexible part of various grades of rubber. Some lip seals have an additional spring wrapped in the flexible rubber element which provides additional pressure around the shaft, ensuring seal integrity.
Most rotary or reciprocating machines have an enclosed internal mechanism that drives a shaft used to transfer work. The shaft will, in most cases, go through the machine casing at one or both ends. While essential, these exit points also allow moisture and dust to enter the enclosure if not sealed. Since most of these machines are sensitive in nature, this situation is not desirable. However, sealing a rapidly rotating or reciprocating shaft without adding excessive friction is no mean feat, especially when you factor in the wear on the shaft and the resulting rough, uneven surfaces.
This engineering dilemma is where the lip seal comes into play. These seals are circular in shape, with an opening in the center that is slightly smaller than the shaft. They are designed to be press-fitted into machine end shields, creating a static barrier against dust and moisture at the seal and shield junction. The temple opening is flexible and, being smaller than the temple, stretches slightly to accommodate it. The pressure exerted by the inner edge of the seal forms a dynamic barrier along the shaft surface. The combined barriers effectively seal the moving shaft and the internal mechanism of the machine.
The construction of a lip seal requires that it be of two-part construction, i.e., a rigid outer portion which presses into the end shield, and an inner portion flexible enough to accommodate the movable shaft. This is generally accomplished by forming a cup out of light steel or rigid plastic, and then chemically or mechanically bonding a soft, pliable rubber skirt across the shaft opening. This skirt is strong enough to exert enough pressure on the shaft to keep out dust and moisture, yet flexible enough to withstand wear on the shaft surface and not degrade machine performance. In some cases, the flexible skirt of the lip seal will have an integral pocket into which a circular coil spring is inserted, which further aids in sealing the shaft.
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