Machinery Lubrication

Machinery Lubrication May-June 2017

Machinery Lubrication magazine published by Noria Corporation

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22 | May - June 2017 | HYDRAULICS Hydraulic By Al Smile y, GPm HydrAulic conSultinG Since the mid-1980s, propor- tional valves have become increasingly popular and have replaced servo valves in many applications. The cost of a proportional valve can be as much as 50 percent of the cost of a servo valve of the same size. However, servo valves continue to be used where precise positioning or speed control is required in aircraft, aero- space and turbine generator applications. Proportional valves can be used as flow and pressure controls, but the most common use is as a directional valve. While the design of proportional directional valves may vary from one manufacturer to another, they all essentially perform the same function: controlling the direction and speed of a hydraulic cylinder or motor. By using feed- back devices like linear-displacement transducers or rotary encoders, the position of the actuator can be precisely controlled. Direct-operated Proportional Valves Direct-operated proportional valves are used when flow through the valve is approx- imately 25 gallons per minute (GPM) or less. Two-stage valves, which incorporate a pilot valve and main spool, are used when higher flow rates are required. In order to troubleshoot the valve and system, you must be able to read hydraulic symbols. In Figure 1, the symbol for a direct-operated proportional valve is shown. Notice the four squares in the symbol. These represent the number of positions in which the valve spool can be shifted. When there is no power to the valve coil, the spring will shift the spool to the position shown on the far left. This is known as the "fail safe" position. In this condition, all flow is blocked through the valve. The symbol for the solenoid indicates that the valve operates off a variable elec- trical signal. This is usually 0-10 volts or in some cases 4-20 milliamps. The "S/U" symbol on the valve represents the linear variable differential transformer (LVDT), which is used to electrically specify the posi- tion of the valve spool. The feedback from the LVDT is normally a direct current (DC) voltage signal. The actual components of the proportional valve can be seen in the valve cutaway in Figure 2. To operate the valve, an amplifier and power supply are required. The power supply is typically 24 volts and is used to power the amplifier. The command voltage is input from the programmable logic controller (PLC) and determines the posi- tion of the valve spool. The "enable" is a relay from the PLC that must be made to send a current signal to the proportional valve coil. In some cases, the enable signal is not used. When the power supply is turned on and the enable relay is not made, the LVDT will send approximately minus 12 volts back to the amplifier, signifying that the spool is in the "fail safe" position (Figure 3). Once the enable relay is closed, a current signal will be sent to the solenoid. The current creates Understanding and Troubleshooting Valves Proportional Solenoid Enable Command 0v 0v LVDT S U Power Supply AMP Figure 1. A direct-operated proportional valve

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