Page 224 - Root Cause Failure Analysis
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212      Root Cause Failure Analysis

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                     Figure 17-10  Two-way, fluid-power valve (Nelson 1986).


                      A  number  of  features  common  to  most  sliding-spool  valves  are  shown  in
                      Figure 17-10.  The small ports at either end of the valve housing provide a path for
                      fluid that leaks past the spool to flow to the reservoir. This prevents pressure from
                     building up against the ends of the pistons, which would hinder the movement of the
                      spool. When these valves become worn,  they may lose balance because of  greater
                     leakage on one side of the spool than on the other. This can cause the spool to stick as
                      it attempts to move back and forth. Therefore, small grooves are machined around the
                      sliding surface of the piston. In hydraulic valves, leaking liquid encircles the piston,
                     keeping the contacting surfaces lubricated and centered.


                      Three Way
                     Three-way valves contain a pressure port, cylinder port, and return or exhaust port
                      (see Figure 17-1 1). The three-way directional control valve is designed to operate an



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                      Figure 17-11  Three-way, fluid-power valve (Nelson 1986).
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