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What Is a Lip Seal?

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  • Written By: Paul Scott
  • Edited By: E. E. Hubbard
  • Last Modified Date: 29 September 2016
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A lip seal is a passive mechanical device used to seal the shaft exit points on electric motors and other rotary and reciprocating output machinery. The seals are designed to exclude dust and moisture from the inner mechanisms of the machinery while having minimal or no negative effect on their performance. Most lip seal designs consist of a rigid outer casing that mates statically with the machine-end cover, and a flexible inner element that forms a dynamic seal on the moving shaft. The rigid portion of the seal is typically made of light-gauge metal or tough plastic, and the flexible part from various grades of rubber. Some lip seals have an additional spring wound into the flexible rubber element which affords additional pressure around the shaft, ensuring the integrity of the seal.

Most rotary or reciprocating machines feature an enclosed internal mechanism that drives a shaft used to transfer the work. The shaft will, in most cases, pass through the casing of the machine on one or both ends. Although essential, these exit points also allow moisture and dust to enter the casing if not sealed. As most of these machines are sensitive by nature, this situation is not desirable. Sealing a rapidly-rotating or reciprocating shaft without adding undue friction is no mean feat, though, especially when shaft wear and the resulting rough, uneven surfaces are taken into account.

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This engineering quandary is where the lip seal comes into its own. These seals are circular in shape, with an opening in the center slightly smaller than the shaft. They are designed to press fit into the end shields of the machine, creating a static dust and moisture barrier at the junction of the seal and shield. The shaft opening is flexible and, being smaller than the shaft, 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 it to be of two-part construction, namely a rigid outer portion which presses into the end shield, and an inner portion flexible enough to accommodate the moving shaft. This is generally achieved by forming a cup out of light-gauge steel or rigid plastic, and then chemically or mechanically bonding a soft, flexible rubber skirt to it along the shaft opening. This skirt is strong enough to exert sufficient pressure on the shaft to exclude dust and moisture, but flexible enough to accommodate wear on the shaft surface and not to degrade machine performance. In some cases, the flexible skirt of the lip seal will feature an integral compartment into which a circular helical spring is inserted, further aiding in sealing the shaft.

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