The performance of a dynamic PTFE seal depends greatly on the pressure and shaft velocity involved. In reality, a seal is in contact with only a small area of rotating shaft, which means that the dynamic forces and energy are going to be concentrated there. Much of the energy that comes from the shaft is going to be dissipated in two forms: wear and frictional heat, both of which can reduce the useful life of a seal. An increase in the force that holds the seal lip against the shaft or an increase in the speed of the shaft makes the effects of wear and heat even worse.
Shaft Velocity, V
The shaft speed is typically measured in sfpm, surface feet per minute. The shaft speed can be found using the equation below, where V is in sfpm, Dshaft is in inches, and ? is in RPM.
Pressure, P
The lip force is the perpendicular force that is holding the lip of the seal against the shaft. It can be approximated by measuring the differential pressure across the seal (in psi).
PV, or the Pressure-Velocity Value
The PV value is simply the produce of the pressure P in psi and the shaft speed V in sfpm. It provides a reference value to represent what different lip materials and configurations are capable of withstanding in terms of pressure and velocity.
PTFE and PV
A PTFE rotary seal can usually be used unlubricated in an application with a PV of up to 150,000 and is ideal for applications with high surface speeds. Because PTFE has such an extremely low coefficient of friction and is self-lubricating, PTFE rotary lip seals can handle high PV values without requiring lubrication. However, lubrication increases the maximum recommended PV value because it not only reduces friction but also helps to conduct away heat that builds up where the lip contacts the shaft surface.
Conclusion
Two key factors that influence the useful life of a PTFE rotary shaft seal are the pressure and the shaft velocity. The product of these two values is used to obtain a reference value known as the PV. When selecting a seal lip material, the PV limit of the potential material is a major factor in selection.
Suggested Links:
Friction and Wear in Polymers and Composites, http://ocw.mit.edu/courses/mechanical-engineering/2-800-tribology-fall-2004/lecture-notes/ch12_polymer.pdf
Friction of Polymers Sliding on Smooth Surfaces, http://www.hindawi.com/journals/at/2011/178943/
Missed Part 1 in this series? View it here: Factors Influencing PTFE Seal Behavior: The Mating Surface – Part 1 in a 3 part series
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