Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/825468
26 | May - June 2017 | www.machinerylubrication.com Solenoid Enable 6 GPM +6v 24v -6v LVDT S U Power Supply AMP 2.16 Amps HYDRAULICS Figure 5. The linear positioner moved 12 inches Figure 6. An externally mounted amplifier Figure 7. A good power supply is indicated by 24 volts at the amplifier. connecting cables. The light will glow yellow when any of these elements fails. The easiest way to determine where the failure is occur- ring is to remove the LVDT cable from the existing valve and plug it into a new valve. It is not necessary to install the new valve for this test. If the yellow light goes out, the LVDT on the old valve has failed and a new valve should be installed on the machine. If the light stays on when plugged into the new valve, the problem is with the cable or connections. The continuity of the cable should be checked. If the light flickers as the machine operates, this normally indicates loose connections. The zero adjustment is located on the amplifier's front face. This adjustment should be made in the event the cylinder is moving with a zero command signal coming into the valve amplifier. If the load is moving, the spool is not in the closed posi- tion. This is usually caused by the LVDT being out of position. Rotate the zero adjustment until the linear positioner stops drifting or oscillating. If speed or positioning problems are occurring, the command and LVDT signals should be checked at the appro- priate connections on the amplif ier. If these are reading correctly, the problem is most likely in the hydraulic system or the linear positioner. Valves with Onboard Electronics A recent trend has been to mount the amplifier on the proportional valve. This is commonly referred to as onboard elec- tronics (OBE). The valve operates the same as described with the external amplifier. The most common type of OBE valve uses a seven-pin connector. The power supply is input in the "A" and "B" pins on the valve. The command voltage comes into the amplifier via the "D" and "E" pins. To check these voltages, a multi-tester can be used by inserting the red and black leads into the appropriate connectors on the cable. To verif y the power supply, insert the red lead into "A" and the black lead into "B." Twenty-four volts should be indicated. To check the command voltage from the PLC, insert the red lead into "D" and the black lead into "E." A 0-10 volt signal should be shown depending on the command signal from the PLC. A test box (Figure 8) can also be used to verify that the valve is operating properly. The cable from the PLC should be plugged into the box, and the cable on the box into the proportional valve. When the system is turned on and operating, the power supply, command and LVDT voltages will be indi- cated. The valve can also be driven with the test box by moving the "command select" switch to internal. The valve can then be driven with the box's "drive" adjustment. If the linear positioner is drifting, the LVDT may not be in the proper position. To null the valve, the LVDT access cover should be removed. The LVDT centering adjustment should then be slowly rotated until the drifting stops. Oil Cleanliness Proportional valves have extremely tight tolerances between the spool and housing. These tolerances are typically between 0.0001 and 0.0003 inch. It is essential that the oil entering the valve meet the standard set by the manufacturer. The cleanliness level is determined by the ISO 4406 code for the specific valve. For example, the ISO code for a particular valve may be 17/15/12. The three numbers correspond to the 4-, 6- and 14-micron particles in a 1-milliliter sample taken from the system. The "17" represents that the system has 640 to 1,300 particles that are 4 microns and larger. The "15" means there are between 160 and 320 parti- cles that are 6 microns and larger. The "12" indicates that the sample contains 20 to 40