Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/847153
16 | July - August 2017 | www.machinerylubrication.com HYDRAULICS flows through the flow control and back to the tank through the pilot valve. The setting of the flow control determines the rate that the main spool shifts. By allowing the spool to gradually shift, the pump volume is gradually ported through the valve and to the system. Several years ago, I was asked to consult with an oriented strand board plant in Minnesota about reducing shock on its hot press. The lines had been welded many times over due to the leakage that occurred because of pressure spikes. The press used eight 109-gallon-per-minute vane pumps to supply a high volume of oil for closing the press. Directional valves, like the one shown in Figure 2, were used to port the pumps' volume back to the tank when in idle mode and when no longer needed in the rams. When the command was given to close the press, it sounded like eight sledgehammers banging away on the reservoir. Once the press was closed and the solenoids were de-energized, a tremendous amount of vibra - tion and shock occurred in the lines. This was due to the rapid change of flow direction from the pumps. Instead of going to the press, the pumps' volume rapidly changed direction and returned to the tank through the dump valves. It took an entire day to adjust the pilot chokes on all eight pumps. At the end of the day, the pumps were coming in and unloading smoothly. Pilot chokes are considered optional equipment on directional valves. On valves that do not have them, once the pilot valve solenoid is energized, pilot pressure will be ported to shift the main spool at a very fast rate. This allows the pump volume to imme - diately flow through the valve, which generates a shock spike. Pilot chokes can easily be added to existing valves by using longer bolts to mount the pilot valve and block to the main spool housing. Use Crossport Relief Valves Crossport relief valves are commonly used with hydraulic motors when it is necessary to stop a load relatively quickly. The main issues with crossport relief valves are that they are usually omitted from the system, are set too high or are mounted too far from the motor. In Figure 3, a typical circuit is shown with a closed center directional valve, two crossport relief valves and a hydraulic motor. Crossport relief valves perform two functions in the hydraulic system: They absorb the initial shock spike that occurs when oil is first ported to drive the motor and bring the motor to a stop when the directional valve is de-energized. Crossport relief valves should be set 200-400 psi above the maximum pressure required to drive the motor. In Figure 4, the "A" solenoid of the directional valve has been energized to direct the pump volume to the motor. Once the pressure Figure 3. A circuit with a closed center directional valve (1), two crossport relief valves (2A and 2B) and a hydraulic motor (3) Figure 4. A circuit with the "A" solenoid of the directional valve energized to direct the pump volume to the motor Figure 5. A circuit with the directional valve solenoid de-energized to stop the motor and the valve spool shifted to the closed center position 2B 2B 2B Oil Flow Oil Flow A A A 1 1 1 3 3 3 B B B 2A 2A 2A Motor From Pump From Pump From Pump Motor Motor