Excessive underlip temperature (heat beyond normal conditions) can cause blisters (such as those shown in Figure 231) to form on the air side of a shaft seal’s elastomeric lip. Blisters can also form if the lip material is simply not suited to handle the normal operating temperature. Either way, blisters are problematic because they deform the sealing lip, causing it to lift away from the shaft, thus forming a gap for leakage.

Because blisters can dissipate in the absence of heat, blistering can be tough to diagnose once the assembly has cooled. If blistering is a possible cause of leakage, you should heat the seal and check to see if blisters reappear.

Reducing seal load and thus reducing the buildup of underlip heat can typically eliminate blistering. Alternatively, selecting a different, more suitable elastomer capable of handling higher temperatures may be the answer. See High Temperature Effects for more information.

“Blisters are problematic because they deform the sealing lip.”

Figure 231