Machinery Lubrication magazine published by Noria Corporation
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When it is desired that fluid only travel in one direc- tion, a logic valve can be used as a check valve, as shown in Figure 4. If the flow is moving to the right, the logic valve opens once the spring tension is overcome, but any flow to the left will be blocked as pressure is applied to the pilot surface area. In Figure 5, a logic valve is shown with a mechanical variable actuator symbol. is symbol means there is a screw that can be adjusted to limit how far the logic valve can be opened, thus causing the valve to behave not only as a directional control but also as a flow control. The variations that can be applied are infinite, allowing logic valves to emulate almost any type of direc- tional, pressure or flow control. e important thing to understand is that the operation of the logic valve is solely dependent on its surface areas. Remember the following formula: force = pressure x area. When tracing the flow on a schematic, consider the size of each surface area and the pressure applied. With this in mind, it is very simple to determine in which direction flow will travel. Installation Logic valves are built to exacting tolerances. e internal clearances are rarely more than a few ten-thou- sandths of an inch. It generally is recommended that any type of valve be installed with a torque wrench, but for a logic valve, torque settings are critical. Most of these valves are mounted beneath a cover that is secured by four Allen bolts, as shown in Figure 6. If the bolts are not evenly torqued, the logic valve may not work from the time it is installed. Failure Modes By far the most common failure of a logic valve is due to contamination, either by particles being introduced to the manifold or generated by component wear and ML www . machinerylubrication.com | January - February 2020 | 19 Figure 6. Most logic valves are mounted beneath a cover that's secured by four bolts. START YOUR FREE SUBSCRIPTION www.machinerylubrication.com overheating of the fluid. When several of these valves are mounted in a manifold, they tend to contaminate each other. While the system fluid may be changed and the system flushed, usually the pilot fluid is the same as that which was added at startup. It often never leaves the manifold. To avoid chasing contaminants through a large mani- fold, you should flush the manifold when a logic valve is replaced. Many companies have suffered from contam- inants moving throughout their manifolds, causing one logic valve failure after another. Common sense tells us that all the valves in a manifold are under similar stress at similar pressures, have the same fluid traveling between them and are not mounted very far apart. erefore, if one valve becomes contaminated, the rest of them cannot be far behind. ML About the Author Jack Weeks is a hydraulic instructor and consultant for GPM Hydraulic Consulting. Since 1997 he has trained thousands of electricians and mechanics in hydraulic troubleshooting methods. Jack has also taught radio-wave propagation for the U.S. Air Force and telecommunica- tions equipment operation and repair for the Central Intelligence Agency at American embassies overseas.