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friction forces, temperature-related expansion and contraction, and by forces developed and transmitted by the impact of external loads. Valves often form a part of an actuator assembly and are used for primary movement control of the actuator and also for deceleration of the piston/rod assembly at the ends of their stroke. Failure rate models for valve assemblies are presented in Chapter 6 of this handbook. Chapter 3 includes failure rate models for the actuator rod seals, connector O-rings, gaskets and mechanical seals. The complete failure rate model for the piston/cylinder actuator incorporates modifiers for contamination and temperature effects. The complete model can be expressed as follows: Where: Aac = Aac.B = CCP ~ CT = A-AC ~ ^AC,B ' CCP ' CT (9"1) Failure rate of actuator, failures/million cycles Base failure rate of actuator, failures/million cycles [See Section 9.3.1 and Equations (9-6) and (9-14)] Contaminant multiplying factor (See Section 9.3.2) Temperature multiplying factor (See Section 9.3.3) 9.3.1 Base Failure Rate for Actuator The primary failure effect of internal and external loads on an actuator is wear of the piston and cylinder which results in an increase in leakage past the piston. A criteria of actuator failure would then be a leakage rate resulting from wear which exceeds a maximum allowable leakage rate specified by the user. Wear of the cylinder and piston will occur in two phases according to the Bayer-Ku sliding wear theory (Reference 6). The first or constant wear phase is characterized by the shearing of the surface asperities due to the sliding action of the piston within the cylinder. During this period the wear rate is practically linear as a function of the number of actuator cycles and the wear depth at the end of the constant wear phase is one half the original surface finish. During the second or severe wear phase, wear debris becomes trapped between the two sliding surfaces and gouging of the surfaces takes place. The wear rate begins to increase very rapidly and failure of the actuator is eminent. Therefore, while equations are presented in this chapter for the severe wear phase, for practical purposes the failure rate or life of the actuator can be estimated as that calculated for the constant wear phase. Actuators 9-5 Revision C

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