Defined as the ability to flex or bend when necessary without being damaged, the flexibility of a given rubber material is particularly important in dynamic seals. In combination with resilience, flexibility is what allows a shaft seal’s elastomeric lip to adjust for shaft imperfections. Flexibility determines followability, or the ability of the sealing lip to maintain contact with the shaft despite vibrations or dynamic run-out. In other words, the more flexible the material, the better the lip can adjust to conditions without losing the ability to seal.

Testing a material’s ability to maintain flexibility over time is known as flex fatigue testing, and there are two main tests related to flex fatigue. As described in ASTM D 430, material samples can be subjected to repeated motion (such as bending, see Figure 29) to see how long it takes for surface cracks to appear. Because such cracks would eventually lead to lip rupture in a shaft seal, the length of time it takes for them to appear can be a good indicator of the material’s overall suitability for lip usage. In some instances, it may also help to know how resistant a material is to the growth of a pre-existing crack. ASTM D 813 describes flex fatigue testing for a material sample with a crack already in it.

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