There are three different forms of temperature: outside, sump and underlip. The cumulative affect of these temperatures can increase the hardening rate of the elastomeric lip material. This causes the loss of flexibility in the contact area, and ultimately decreases the life of a seal.
Outside temperature can come from any heat source other than the sump or underlip. Long exposure to high outside temperatures can have unexpected affects on the life of a seal. When combining these temperatures with the sump temperatures, an increase in the hardening rate of the elastomer may occur. The other end of the spectrum occurs when outside temperatures reach the lower limit of the lip material temperature range. The affect on the sealing element may result in tearing if there is dynamic runout of the shaft due to a decrease in flexibility and resilience. Unless the seal experiences catastrophic failure, leakage does not normally occur at these low temperatures because the viscosity of the sump media has increased and due to friction the temperatures quickly elevate.
Sump temperature is the most common measure of the three temperatures. There is a direct correlation between seal life and sump temperature. Even if the seal is operating at a sump temperature that is within the given elastomeric temperature range does not mean that the seal life is not being compromised. If long life cycles are an important priority, then a low sump temperature is desired. If long life-cycles are not a priority, then a high sump temperature can have a positive affect on the system. Fluid viscosity, seal torque and power consumption all decrease as sump temperature increases.
The underlip temperature of the contact width is a function of shaft speed, material friction, surface roughness, sump and outside temperature. As these parameters increase, the affect is higher underlip temperature. The material friction is dependant on elastomer properties, radial lip load and lubrication. As was discussed in the Hydrodynamic Effect section, if there is no fluid film available, the seal element would burn up because of extremely high underlip temperature.
The cumulative affect of all these temperatures is that the hardening rate of the lip material is increased and seal longevity is lost.