A guide to shaft seals
Machines require rotating shaft seals to retain lubricants and prevent foreign particles from entering sealed cavities, which damage machinery and lead to premature failure.
Machines require rotating shaft seals to retain lubricants and prevent foreign particles from entering sealed cavities, which damage machinery and lead to premature failure. Application conditions can vary considerably, and many seal designs have evolved to satisfy these conditions. Some applications tolerate a small amount of leakage, while others cannot allow any leakage. In general, seal complexity and cost increase as the need for zero leakage increases.
Shaft seals make up two groups: contacting and noncontacting. Contacting seals make direct contact between the sealing component and the rotating shaft. Because there is rubbing contact, leakage is minimal and friction and seal wear occur.
The sealing element in noncontacting seals allows a certain amount of leakage. It is controlled by clearance gaps. Because there is no rubbing contact, leakage is higher, but seal friction and wear are eliminated.
There are three types of contacting seals.
Mechanical face seals prevent leakage in applications that exceed the capabilities of elastomeric radial lip and packing designs. They provide long life without shaft wear. Shaft finish, hardness, and material are not critical. The seal can handle a wide variety of fluids, pressures up to 3000 psi, speeds to 50,000 rpm, and temperatures from -425-1200 F. This type of seal has become popular on pumps where leakage is not allowed.
Radial lip seals are primarily used to retain lubricants and exclude contaminants. The seals function at temperatures from 60-400 F. Advantages include low cost, small space requirements, and simple installation.
Packings are not designed to produce a leak-free seal. They are tightened enough to allow a minimum, but positive, leakage. The leakage is intended to lubricate the packing material. The modest leakage reduces friction and wear. Packing material is supplied in many shapes, including impregnated yarn, continuous strands, square-sections, and interlocking shapes. This seal is generally used on pumps.
Four types of noncontacting seals are available.
Bushing seals are classified as fixed, floating, balanced, and floating ring. The bushing seal is a close-clearance version arranged stationary to the housing or floating with the shaft. The fixed bushing is a sleeve attached to a housing, surrounding a rotating shaft with a relatively close clearance.
Floating bushing seals follow shaft gyrations and have closer tolerances than fixed bushings. Balanced bushing seals eliminate or reduce spring forces and still retain the advantages of a floating bushing seal. Floating ring seals overcome the pressure and alignment problems of single floating bushings by splitting the bushing into several rings. This seal is only suited for sealing liquids.
Labyrinth seals are used mainly in high-speed applications where relatively high leakage rates can be tolerated and simplicity is necessary. A properly designed seal does not require lubrication or maintenance. If wear occurs, the only damage is an increase in leakage.
Visco seals have grooves or screw threads machined on the shaft or in the housing to provide a positive pumping action. Viscosity of the pumped fluid in the clearance gap produces the seal effect. These seals function at certain minimal speeds. For low shaft rotation, a secondary-sealing device, such as a lip seal, must be provided. This type of seal is used where liquids are continuously transferred.
Magnetic seals use a colloidal suspension of magnetic particles, focused by permanent magnets, to create a seal. In addition to providing a nearly perfect seal with negligible wear or friction, they are tolerant of shaft runout. They can be used at speeds up to 120,000 rpm, temperatures up to 400 F, and pressures of 7 psi/stage. Seals are used primarily with gases and exclude moisture, mist, and fine solids.
&HEADLINE>Rotating shaft seals&/HEADLINE>&BYLINE>Joseph L. Foszcz&/BYLINE>
Troubleshooting mechanical face seals
Troubleshooting radial lip seals
Troubleshooting noncontacting seals
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