Over the past few weeks, we’ve been discussing the basics of PTFE rotary seals. In today’s entry, we’ll take a look at PTFE radial lip seal design principles.

PTFE radial lip seals generally incorporate a uniformly thin element cross section, made to compensate for the high flexural modulus of PTFE, especially in cases of severe shaft run-out. The thin sections also minimize thermal expansion and compressive “creep” and their effects on maintaining a controlled contact pattern on the shaft surface.

Most PTFE seal constructions have the “body” portion of the element clamped between the two metal cases. To maintain proper retention pressure on the element, a thin element keeps compression set and “creep” at a minimum.

From a cost standpoint, it’s important to keep material to a minimum and the element design as basic as possible. The only exception to thin element design are applications without lubrication and where high wear rates are likely.

The contact pattern in many PTFE seals is wider than related elastomer seals. Sometimes the wide contact area was unintentional, but due to heavy interference on some non-spring activated elements to achieve the desired radial load, the element produced this effect when positioned on the shaft.

Designing a wide contact into a sealing element

There are reasons to intentionally design a wide contact into a sealing element.

PTFE experienced a significant reduction in radial load when exposed to elevated temperatures. To maintain the fluid film between the shaft and the element, a wide contact area is needed to prevent leakage.

Because PTFE is prone to damage from installation or handling, minor nicks in the extreme ID surface are offset with a wide contact area.

In applications where fluid pressure is evident, the wide contact area allows the load to be distributed over a greater area, thus reducing element and shaft wear. The wide contact area on the shaft surface also allows high spring loads to be applied to the elements without increasing wear and/or shaft scoring.

When the wide contact principal; is applied to PTFE lip seal designs – especially spring-activated elements – the design parameters of the seal must prevent “bell mouthing.” This effect will create an oil wedge and produce high leakage rates.

Levels of interference between the element ID and shaft OD may vary. Heavy interference is normally associated with non-spring loaded element designs, and provides greater radial load and enables the element to follow shaft run-out. By incorporating a spring, interference can be reduced.

Designs that depend on large running interferences – this can apply to spring and non-spring design – installation damage becomes a critical problem. Installation tools can work, but it’s preferable to take advantage of the “heat-set” characteristics of PTFE material.

Once the desired sealing interface has been determined and the element is molded to this diameter, the element ID is then stretched over a predetermined diameter and exposed to a heat setting operation.

“Heat set” is simply applying a specific amount of heat to an element and allow it to cool on a mandrel. After heat-set, the element ID is greater than the original fabricated dimension, and installation becomes less troublesome. Once heat-set design is installed, the initial heat generated from the friction will cause the element to attempt to return to its original ID.

Use of a spring with a PTFE element is normally determined by the seal engineer and their design practices. However, there are situations that predict use of a spring:

  • Excessive shaft run-out and/or shaft and bore misalignment.
  • Pressure application where the element takes a compression set and a spring is required to provide immediate response to a sudden pressure reduction.
  • Loss of radial load caused by high temperatures threatens to lead to leakage.
  • To offset the high flexural modulus in low temperature environments.

To learn more about PTFE radial lip seals, be sure to download our rotary seal guide.