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Machine Design

Face Seals

Mechanical face seals are a good choice when minimum leakage of the sealed fluid is the most important criterion. The primary sealing interface is between rotating and stationary members that form a plane perpendicular to the shaft. The sealing area is a narrow ring where the two faces contact. One of the sealing faces is usually metal or ceramic and the other is usually graphite or plastic.

To keep precision seal faces closed in the absence of hydraulic pressure, some form of loading device, usually a spring, is needed. Loading should be high enough to overcome friction and keep the faces closed under all operating conditions. Unnecessarily high loading will tend to shorten the useful life of the seal. The most common device for supplying a loading force to the seal face is a helical spring. Multiple helical springs, wave springs, bellows, and rubber elements are also used.

Standard face seals have been used for pressures up to 3,000 psi, rotating speeds up to 50,000 rpm, and temperatures from -425 to 1,200°F. Special face seals have been developed for pressures up to 10,000 psi. For extremely high pressures, two or more face seals can be lined up in tandem, splitting the pressure differential equally.

Mechanical face seals usually cost more than radial oil seals or compression packings. However, the elimination of shaft wear may in some applications justify the increased cost.

Face seals usually take about the same space as packings, but substantially more room than radial seals. However, special face seals that take less space than conventional face seals are available.

All face seals, because of their design configurations, tolerate only limited axial motion between shaft and housing. Thus, they cannot be used for long stroke, reciprocating-shaft applications.

Another disadvantage of face seals is their precision. Face flatnesses of 11.6 ∝in. and surface finishes of 2 ∝in. are not uncommon, and the seals must be manufactured to cleanliness standards similar to those in the precision-bearing industry. As with most precision devices, rough or careless handling must be avoided.

Compared with other types of dynamic seals, face seals have longer life and can reduce warranty and liability cost, downtime, and leakage. In addition, face seals are easy to sterilize and eliminate system contamination by packing fragments.

Unbalanced face seals act like pistons. Fluid pressure from one direction loads the primary seal ring and mating ring against each other. Pressure from the opposite direction unloads -- and may even separate -- the rings. The result is either unnecessarily high friction, wear, heat generation, and power waste or else high leakage when the faces separate.

Pressure balancing reduces the piston action, keeping contact load closer to the minimum for effective sealing and enabling the seal to stand larger pressure reversals.

Unfortunately, there are also disadvantages to balanced seals. They typically cost 10 to 50% more than unbalanced seals for the same application because of closer tolerances and more complex seal shapes. They are subject to catastrophic failure if operating conditions do not closely match design conditions. And they usually require more space than unbalanced seals. Therefore, unbalanced seals are used whenever their frictional and pressure-reversal characteristics are acceptable.

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