Polytetrafluoroethylene (PTFE, trade name Teflon®) shaft seals are used in several specific types of applications, such as high pressure hydraulic pump and motor and diesel engine crankshaft applications. Some PTFE shaft seals feature PTFE lips clamped in place using spacers and rubber gaskets inside a metal case. This clamping can be necessary because it’s tough to bond PTFE to rubber or metal. Other designs, however, do bond PTFE to rubber or metal.

Figure 86 shows the “EPT” design, a single lip seal for high pressure applications. The unitized “EHX” design shown in Figure 87 features both a primary lip and a secondary, exclusion lip (both made of PTFE). The EHX design is typically used for diesel engine applications. A unitized seal with PTFE bonded to rubber, which is then bonded to metal, is shown in Figure 88.

PTFE offers advantages over standard shaft seal materials. If your application requires a seal that can withstand high pressures or high temperatures, PTFE shaft seals may be the answer. PTFE seals can withstand temperatures ranging from -65° F to +325° F (-54° C to +163° C). PTFE seals can tolerate shaft speeds of up to 12,000 feet per minute (fpm). Because of PTFE’s unique chemical structure, PTFE seals offer excellent resistance to most chemicals and fluids.

Keep in mind, however, that PTFE shaft seals are not without disadvantages. As with the aforementioned advantages, these disadvantages stem from the nature of the PTFE itself. Because PTFE is stiffer than traditional elastomeric lip materials, PTFE lips will not form the tiny pores (microasperities) that are integral to the pumping action seen in successful shaft seals. To compensate, a spiral groove must be machined or coined into the primary lip surface if it is intended to seal oil under low pressure conditions. This groove can simulate the pumping action by screwing fluid back into the sump, but the spiral groove limits seal usage to applications in which the shaft rotates in only one direction. Examples of PTFE lip seals featuring these spiral grooves are shown in Figures 87, 88, and 89.

PTFE also has much less memory (ability to regain its original shape following deformation) than traditional, elastomeric lip materials. This reduced memory makes the lip less able to maintain consistent contact with the shaft, particularly in the presence of shaft eccentricity. As a result, leakage becomes more likely.

Finally, PTFE lips are more delicate than traditional lips and can therefore be easily damaged during installation. For this reason, a PTFE seal may come pre-assembled onto a wear sleeve. This seal-sleeve combination can be slipped over the shaft with less chance of damaging the seal. Seals may also be shipped with disposable protective sleeves.

For more on PTFE, see the material profiles.

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