Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/902206
30 | November - December 2017 | www . machinerylubrication.com HYDRAULICS enough to overcome the spring acting on the opposite side, the main spool will shift to the left. Once shifted, the system volume at the "P" port of the main spool will flow through the "P" and "A" ports and then to the actuator (cylinder or hydraulic motor). e oil that exhausts out of the actuator flows into the "B" port, which is connected to the "T" port inside the valve. e oil will then flow out of the "T" port and return to the reservoir. When the main spool shifts (as in Figure 3), the oil on the left side of the spool is ported through the pilot valve spool. Notice in Figure 2 that the circled pilot plug on the right is blocking the flow passage from the tank port of the pilot valve to the tank port of the main spool. When this plug is installed, a line must be connected to the "Y" port on the manifold to externally drain the valve. is provides a flow path for the oil that exhausts out of the main spool's pilot cavity and pilot valve to return to the tank. In some instances, both pilot plugs are removed from the main valve housing. In this case, there are no external lines connected to the "X" and "Y" ports on the valve manifold. In Figure 4, the pilot plugs have been removed. is opens the internal pilot passage from the "P" port of the main valve to the "P" port of the pilot valve. Once the pilot valve is shifted, system pressure is directed through the pilot valve to shift the main spool, as previ - ously described. When the main spool shifts, the oil that exhausts out of the opposite pilot passage will flow through the pilot valve and then into the tank line of the main spool. is is what is known as an internally piloted and internally drained valve. Valves can be internally drained as long as there are no flow surges or pressure spikes in the tank line that affect the shifting of the main spool. Replacing Two-stage Valves When replacing a two-stage valve, it is important that the piloting and draining arrangements are the same as the removed valve. is applies to proportional and AC voltage-operated valves. In certain instances, valves are internally piloted and externally drained. In other cases, they may be externally piloted and internally drained. Refer to the valve's part number to ensure the valve you are installing is the same as the one that was removed. Just because it mounts on the same manifold does not mean it is the same valve. For example, a closed-center, externally piloted and drained Vickers valve has the following part number: DG5S8-2C-E-T-30. e "E" designates that the valve is externally piloted, while the "T" indicates that it is internally drained. If a DG5S8-2C-E-30 valve is put in its place, the valve will not work because it is externally drained. is means the internal drain port is plugged and the main spool will not shift, since no line is connected to the "Y" port. I recall an instance at a plywood plant where they had replaced a valve on the lathe with the exact part number as the valve they had taken off. When the new valve was installed, the spool wouldn't shift and allow the spindle cylinder to extend and clamp the log. After changing several other compo - nents, they found that although the part number was the same, the equipment manufacturer had removed the pipe plugs from the original valve but failed to change the part number. Once the pipe plugs were removed, the valve and spindle cylinder operated normally. If you and your vendor do not have a valve in stock with the config- uration you need, a valve usually can be converted by removing or installing pipe plugs. On the proportional valve used in the examples, the pilot plugs are accessed by taking the pilot valve off the main stage and removing them with an Allen wrench (Figure 5). On some valves, the pilot pressure plug is accessed through the "P" port of the main spool. A not her importa nt t hing to remember is that pilot and drain lines and passages are small and can plug. T T T A A A P P P B B B X X X Y Y Y Figure 4. A main valve housing with the pilot plugs removed Figure 2. A typical two-stage proportional valve Figure 3. Current applied to the pilot valve coil