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There are various factors that have a significant impact on the performance and behavior of a rotary shaft seal, including operating conditions (speed, pressure, temperature, environment), the mating shaft (hardness, surface finish, lead), and the choice of material for the seal itself.

In this blog post, we are going to review the operating conditions that must be considered for a successful rotary shaft seal.

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Pressure

The lip force is the perpendicular force that is holding the lip of the seal against the shaft.

Lip force can be approximated by measuring the differential pressure across the seal (in psi). This gives the pressure value that is needed in evaluating a seal design. Pressure has a major impact on the ability of the seal to do its job. The higher the lip force, and thus the pressure, the more factors such as friction and surface finish have on overall seal performance and expected life.

Velocity

Velocity refers to the speed of the rotating shaft.

Velocity, or shaft speed, is usually measured in sfpm, surface feet per minute. The shaft speed can be found using the equation below, where V will be in sfpm, Dshaft is the diameter of the shaft measured in inches, and ? is the angular speed of the shaft in RPM.  

V = 0.262 · Dshaft · ?

Obviously, high velocity shafts will generate more heat as they interact with the seal, making friction, lubrication (or lack thereof), surface finish, dimensional stability, and thermal conductivity even more important to the design.

PV Value

Pressure and velocity individually do not provide enough information to really evaluate a seal design.

Engineers use a combination of pressure and velocity referred to as the PV value. It is simply the product of the pressure and velocity, PV = P · V with units of psi – sfpm. This is a key parameter in selecting an appropriate material for a seal. When selecting a material for the seal lip, the choices can be quickly narrowed down by checking the PV limit for the materials in question. Material information usually includes recommended maximum values for both pressure P and velocity V.

Temperature

Another major consideration in rotary shaft seal performance is the expected operating temperature.

Seal materials have a range of temperatures in which they perform best, and some are better adapted to extreme heat or cryogenic conditions. Temperature and temperature fluctuations can affect seal performance through dimensional changes in the shaft and/or sealing lip. High temperatures are also associated with high levels of friction.

Conclusion

There are various considerations to take into account when designing a rotary shaft seal, including operating conditions, environmental conditions, and the mating shaft. The key factors associated with operating conditions are pressure (associated with the lip force), shaft speed, PV value, and temperature. Taking the time to evaluate these values early on in the design will save time and frustration later and can aid in quickly narrowing down possible seal materials.

You may also like these article about PTFE Rotary Shaft Seals:

 

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