Machinery Lubrication

Machinery Lubrication May-June 2017

Machinery Lubrication magazine published by Noria Corporation

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Page 28 of 76

24 | May - June 2017 | HYDRAULICS magnetism in the coil, which pulls in a plunger to shift the spool. Since the command voltage is zero volts, the spool will continue to shift until the LVDT shows that zero volts is fed back. The spool will then be shifted into the "electrically closed" position (Figure 4). Approximately 1.35 amps of current are required to shift the spool into the "electrically closed" position. To move the linear positioner 12 inches (Figure 5), a command voltage is input into the PLC. The amplifier converts the command voltage into a current signal, which is applied to the valve coil. A command voltage of 6 volts is input into the amplifier. The amplifier will then send a higher current (2.16 amps) to the valve coil. This shifts the valve spool into the straight arrows position. This position is commonly called the "A" position. The increased current causes the valve spool to shift until the LVDT feeds back minus 6 volts. The spool will then stop shifting and maintain its position. Oil is then directed through the valve spool and into the full piston side of the linear positioner. The speed in which the positioner moves is determined by the amount of spool shift. In this example, if the valve has a maximum flow rating of 10 GPM, then 6 gallons per minute will flow through the spool when shifted with a command voltage of 6 volts. As the cylinder moves, the linear- displacement transducer sends an analog or digital signal back to the PLC. For example, if one digital pulse is sent back to the PLC for every 0.001 inch of movement, the positioner rod will move until 12,000 pulses are fed back, signaling that the posi- tioner has moved 12 inches. The command voltage will then drop to zero, and the proportional valve spool will once again shift into the "electrically closed" position. The cylinder will hold its position until commanded to move to a different stroke. Troubleshooting the System If an externally mounted amplifier is used (Figure 6), lights on the front panel will denote a fault in the system. When the power supply is turned on and the enable signal is received, the "on" light will illumi- nate green. The enable voltage may range from 8.5-40 volts, although 10 volts is common. If the light does not illuminate, the enable and power supply voltages should be checked at the amplifier connec- tions. If no input enable voltage is present, the wiring and output signal should be checked from the PLC. If the power supply drops below 21 volts, the red "UB" light will come on. This generally means the power supply or wiring is bad. When the power supply is good (Figure 7), 24 volts should be indicated at the amplifier. The yellow light on the bottom of the amplifier face is used to show when the LVDT is bad or there is a problem with the Figure 2. Components of a proportional valve Figure 3. The valve spool in the "fail safe" position Figure 4. A valve spool in the "electrically closed" position

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