Machinery Lubrication

Machinery Lubrication November-December 2017

Machinery Lubrication magazine published by Noria Corporation

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28 | November - December 2017 | www . Directional valves are some of the most funda- mental components of a hydraulic system. When a directional valve is sized for an application, it must be large enough to handle the volume of oil necessary to operate the cylinder or hydraulic motor. For applications that require less than 10-15 gallons per minute of oil flow, a direct solenoid-operated valve is used (as shown in Figure 1). To shift the valve spool, current is applied to the valve coil. is creates magnetism within the coil, which pulls in the plunger. e plunger then acts on a pushpin, which shifts the valve spool. e solenoid generates approximately 30 pounds of force to shift the spool. Once the spool shifts, oil is ported through the valve and then to the cylinder or hydraulic motor. Two-stage Valves On systems where higher flow rates are required, two-stage valves are normally employed. A typical two-stage proportional valve is shown in Figure 2. e top valve is known as the pilot valve. e purpose of the pilot valve is to direct pilot pressure to shift the main spool. Since these are larger in size than the direct operated valves, more force is needed to shift the main spool. Instead of using very large solenoids that demand high current to operate, hydraulic pressure is utilized to shift the main spool. ere are springs on both sides of the main spool which hold it in the center position when the pilot valve is not shifted. e springs are usually rated between 50-115 pounds per square inch. When the main spool on this particular valve is in the center position, the "P," "A," "B" and "T" ports are blocked. Two-stage valves may be internally or externally piloted. In Figure 2, notice the pilot plug (circled in red) on the left. is plug blocks flow from the "P" port of the main spool to the "P" port of the pilot valve. When this plug is installed in the valve, pilot pressure must come from an external source and be connected to the "X" port on the valve manifold. Many presses and mobile equipment employ a separate pump for supplying the pilot fluid. In some cases, oil is ported downstream of the pump through a pressure-reducing valve and then to the "X" port. By utilizing a separate pump or pressure-reducing valve, a lower pressure is used to shift the main spool. is prevents damage to the spool instead of using the higher system pressure to shift the valve. When current is applied to the pilot valve coil (as shown in Figure 3), magnetism in the coil causes the pilot spool to shift to the right. Pilot fluid then flows from the "X" port through the pilot valve and to the pilot cavity on the right side of the main spool. Once the pilot pressure builds up How Directional Valves Affect Oil Flow in Hydraulic Systems HYDRAULICS Al Smiley | GPM Hydraulic Consulting Coil Plunger Spool Figure 1. A direct solenoid-operated valve

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