flow of fluid to and from the two chambers ofthe actuating cylinder.Since the two control valves operateindependently of each other as far as hydraulicpressure is concerned, failure of either hydraulicsystem does not render the actuator inoperative.Failure of one system does reduce the output forceby one-half; however, this force is sufficient topermit operation of the actuator.RACK-AND-PINION PISTON-TYPEROTARY ACTUATORSThe rack-and-pinion-type actuators, alsoreferred to as limited rotation cylinders, of thesingle or multiple, bidirectional piston are usedfor turning, positioning, steering, opening andclosing, swinging, or any other mechanicalfunction involving restricted rotation. Figure10-10 shows a typical rack-and-pinion double-piston actuator.The actuator consists of a body and tworeciprocating pistons with an integral rack forrotating the shaft mounted in roller or journalbearings. The shaft and bearings are located ina central position and are enclosed with a bearingcap. The pistons, one on each side of the rack,are enclosed in cylinders machined or sleeved intothe body. The body is enclosed with end caps andstatic seals to prevent external leakage ofpressurized fluid.Only a few of the many applications ofactuating cylinders were discussed in the precedingparagraphs. Figure 10-11 shows additional typesof force and motion applications.In addition to its versatility, the cylinder-typeactuator is probably the most trouble-freecomponent of fluid power systems. However, itis very important that the cylinder, mechanicallinkage, and actuating unit are correctly aligned.Any misalignment will cause excessive wear of thepiston, piston rod, and seals. Also, properadjustment between the piston rod and theactuating unit must be maintained.Figure 10-10.—Rack-and-pinion double-piston rotary actuator.10-6
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