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Other factors which need to be considered as a check list for reliability include: Chemical compatibility between fluid and seal material Thermal stability Appropriate thickness and width of the seal material . Initial and final seating (clamping) force 3.3 DYNAMIC SEALS In contrast to gaskets and other static seals, dynamic seals are used to control the leakage of fluid in those applications where there is motion between the mating surfaces being sealed. O-rings used in dynamic applications are subject to a sliding action against the gland. This motion introduces friction creating different designs and failure modes from those of static seals. Refer to Section 3.2 for a discussion of seals in general, the basic failure modes of seals and the parameters used in the equations to estimate the failure rate of a seal. There are several types of dynamic seals including the contacting types such as lip seals and noncontacting types such as labyrinth seals. Assemblies with motion usually require lubrication of the O-ring to reduce wear rate. This is usually accomplished with the fluid being sealed. Dynamic seals are further divided as follows: Reciprocating Seal: A seal where the rod or piston moves back and forth through or with the seal. Piston and rod seals shown in Figure 3.7 are examples of reciprocating seals. Rotary Seal: A seal where a shaft rotates with relation to the seal. Typical rotary seals include motor shafts and wheels on a fixed axle. O-rings are not generally used for conditions involving fluid velocities exceeding 800 rpm and/or surface speeds exceeding 600 feet/minute. See Figure 3.7. Oscillating Seal: A seal where a shaft turns and returns with relation to the seal. In this application the inner and outer member of the gland moves in an arc around the axis of the shaft first in one direction and then in the opposite direction with the movement usually intermittent. An example application is the faucet valve. Seals and Gaskets 3-17 Revision G

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