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compared to forces binding the piston/cylinder walls, surface fatigue will occur. Hard particles may also cut away surface material. (2) Abrasive Wear - A hard particle entering the gap between the piston and cylinder surfaces can cut away material of the softer surface on a single actuator engagement. The rate of wear will be proportional to the number of particles in contact with the surfaces and the particle hardness. If the hardness of the piston is significantly less than that of the cylinder, a hard particle, absorbed by the softer material causes severe abrasive wear of the harder actuator surface. (3) Surface Fatigue - Particulate contaminants interacting with the piston and cylinder surfaces can dent a surface producing plastic deformation. Large numbers of dislocations will increase the surface roughness and deteriorate the surface material. The result is an accelerated rate of wear and a higher probability of leakage between the surfaces. The deteriorating effects of contaminant particles on the reliability of an actuator must be equated along with the probability of the contaminants entering the gap between the actuator surfaces. The probability of contaminants entering this area will depend on the operating environment, the types and numbers of particles expected to be encountered, and the filtering system to prevent the entrance of particles. The typical actuator contains a bushing to wipe the piston on the return stroke. The life expectancy and reliability of this device must be determined as part of the overall reliability estimate of the actuator. If the piston surface slides over a hard contaminant particle in the lubricant, the surface may be subject to pitting. The abrasive particle has edges with a characteristic radius, denoted by r. When the depth of penetration of the abrasive particle (d) reaches a certain critical value, the scratching produces additional wear particles by pitting. This elastic/plastic deformation process occurs when the maximum shear stress in the complex stress distribution beneath the contact surface exceeds the elastic limit. This maximum shear stress occurs beneath the contact at a depth equal to one half the contact radius. The value of this critical depth is given by (Reference 48). f 2fs, max crit p v sy s,max (9-15) Where: fs.max - Maximum shear stress, lbs/in2 r = Characteristic radius of particle, in FSY = Yield strength of material, lbs/in2 Actuators 9-12 Revision C

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