Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1206285
HYDRAULICS might explain why so many European machines have used these valves for decades, as historically energy costs have been higher in Europe than in the United States. As U.S. plants begin to be more conscious of energy costs, more American-made machines are using logic valves. Because the valves are mounted in a manifold, less plumbing is required, so the installation costs are lower. Manifolds can be assembled before a unit is shipped. Installation then becomes more a matter of plumbing large manifolds together than installing individual valves. Less real estate is required inside a plant, and fewer leaks are observed. Obviously, if a hydraulic line no longer exists, it can't leak. Of course, there is a downside to having valves mounted in a manifold. Troubleshooting a system with valves that you cannot see can be confusing. erefore, it's more important than ever to understand how to read and use a hydraulic schematic as a troubleshooting tool. Unless you have X-ray eyes like Superman, you must rely on a schematic to understand the hydraulic flow. Directional, Pressure and Flow Control Logic valves are very versatile. ey can emulate almost any type of hydraulic valve and can be used as directional controls, pressure controls, check valves and flow controls. e valves can also handle large amounts of flow with accuracy. eir design may be simple or complex, although generally they are quite simple. While their schematic symbols may take some getting used to, they represent their function very well. Note the three surface areas in Figure 1 – one on top, one on bottom and a third on the side. As you can see, the side surface area is accessed by holes in the side of the cartridge. ere is also a hole on the top, and another hole on the bottom. e dotted line to the top surface area indicates a pilot line. At 3 square inches, the pilot surface is the largest of the surface areas. is ensures the pilot side always generates the most force whenever the same pressure is applied to either the bottom or side as is applied to the top. is is a pilot-to-close logic valve. A pilot- to-open logic valve is also available, but it is not used as often. e most important thing to understand about pilot-to-close logic valves is that if pilot pressure is applied, the valve will stay closed. When no pilot pressure is applied, the valve can be opened with only the pressure required to compress the spring. is will be a very low pressure. e purpose of the spring is to hold the valve closed whenever there is no pressure in the system. Typically, this pressure will be very low (1-5 bar or 15-75 pounds per square inch), depending on which surface area is used to open the valve. Usually, either a small directional valve or pressure control is employed to pilot the logic valve. A logic valve is often used as a pressure control when it is necessary to control pressure while handling a large amount of flow. is makes sense, as it is easier and less expensive to manufacture a small precision spring than a large one. In Figure 3, a small pilot relief valve is utilized to limit the pressure from a large amount of flow through a logic valve. Figure 2. A three-way directional valve can be utilized to determine whether a logic valve is opened or closed. 18 | January - February 2020 | www . machinerylubrication.com , , , , Logic Valve Closed Logic Valve Open Figure 3. A small pilot relief valve can be used to limit the pressure from a large amount of flow through a logic valve. Figure 4. A logic valve can also be used as a check valve. Figure 5. A logic valve with a mechanical variable actuator symbol Oil Flow Oil Flow