Hydraulic shock occurs when oil rapidly starts or stops flowing in a hydraulic system. The flow rate of oil in the pressure line of systems below 3000 PSI is usually 15 – 20 feet per second. In systems above 3000 PSI, the flow rate can be 30 feet per second or more. Shock can also occur when an external force acts on a hydraulic cylinder or motor. Unlike air, hydraulic oil is generally considered to be non-compressible. Oil will only compress one half of a percent when pressurized to 1000 PSI. When a pressure spike occurs in the system, the pressure can increase 4 or 5 times above the normal operating pressure. Since the average duration of a shock spike is 25 milliseconds, the pressure gauge cannot respond fast enough to give an accurate indication. Pressure transducers are normally used to record pressure spikes. Shock spikes that are not properly dampened or absorbed can result in leakage and damage to the lines and components in the system. In Part 1 of this article let’s look at 3 things that can be done to reduce hydraulic shock.
Install an Accumulator
A hydraulic accumulator is pre-charged with dry nitrogen. Some type of separating device such as a piston, bladder or diaphragm is used to separate the nitrogen from the hydraulic oil inside the accumulator. A bladder type (figure 1) or a diaphragm type is recommended to absorb shock. Both of these accumulators contain rubber elements that will be compressed when the hydraulic pressure rises above the dry nitrogen pre-charge. Depending on the system, the accumulator should be pre-charged to 100 PSI below to 200 PSI above the maximum operating pressure in the system. Accumulators that are used for shock can be small in size, usually one quart to one gallon. The accumulator should be installed as close as possible to where the shock spike is occurring. For example, if the pressure spike occurs when a cylinder fully extends, the accumulator should be installed near the port connected to the full piston side of the cylinder. Accumulators are many times used to absorb high flow surges in return lines. In this case, the pre-charge should be lower than the maximum pressure rating of any return filters or heat exchangers located downstream. Any time an accumulator is used in the pressure line an automatic and/or manual dump valve should be installed to bleed the hydraulic pressure down to 0 once the system is turned off.
Directional Valve Pilot Chokes
A typical two stage, hydraulic piloted, solenoid controlled directional valve is shown in figure 2. The valve contains pilot chokes which are located in the block between the pilot valve on top and the main spool on the bottom. The block contains two flow controls that are connected in a meter out arrangement and two bypass check valves. When either of the pilot valve solenoids is energized, pilot pressure is ported through one of the internal check valves and to one side of the main spool. As the spool shifts, the oil in the pilot cavity on the opposite side 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 on reducing shock on their hot press. The lines had been welded many times over due to the leakage that occurred due to pressure spikes. The press used eight, 109 GPM 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’ volumes back to tank when in idle and when no longer needed in the rams. When the command was given to close the press, it literally sounded like eight sledge hammers were banging away on the reservoir. Once the press was closed and the solenoids were de-energized, a tremendous amount of vibration 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’ volumes 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 immediately 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.
Crossport Relief Valves
Crossport relief valves are commonly used with hydraulic motors when necessary to stop a load relatively quickly. The three main issues with crossport relief valves is that they are usually omitted from the system, set too high or they are mounted too far away from the motor. In figure 3 a typical circuit is shown with a closed center directional valve (1), two crossport relief valves (2A and 2B) and a hydraulic motor (3). The crossport relief valves perform two functions in the hydraulic system:
- Absorb the initial shock spike that occurs when oil is first ported to drive the motor.
- Brake the motor to a stop when the directional valve is de-energized.
The 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 momentarily increases to the no. 2A valve setting, the spool will shift open and port the pressurized fluid through the directional valve and back to the tank. When the pressure drops below the no. 2A setting, the valve spool will shift closed and the motor will start rotating. When the directional valve solenoid is de-energized to stop the motor, the valve spool will shift to the closed center position (figure 5). The motor will tend to continue rotating due to the inertia of the moving load. The motor will momentarily turn into a hydraulic pump and deliver oil to its outlet port. The pressure will build until the setting of the no. 2B crossport relief valve is reached. The no. 2B valve will then shift open and direct the oil flow back to the inlet port of the motor. The setting of the no. 2B spring determines how fast the motor will brake to a stop.
If you’re experiencing shock and leakage issues with hydraulic motor circuits, first verify that the crossport relief valves are located in the system. I have seen some systems where they have been omitted allowing the shock to be taken out in the lines, hoses and fittings, resulting in leakage. Secondly, make sure that the crossport relief valves are properly set. When there is a problem in a hydraulic system the first thing that is done many times is to turn the pressure up. Thirdly, the crossport relief valves should be located as close as possible to the hydraulic motor.
A plywood plant in North Carolina was having a problem shearing the motor shaft off their rotary log kicker hydraulic motor. As the logs came down the conveyor the hydraulic motor rotated and kicked the log off the conveyor onto the infeed conveyor to the lathe. Upon inspection, the crossport relief valves were located in a block underneath the directional valve which was mounted 30 feet away from the motor. An additional set of crossport relief valves were installed near the hydraulic motor which eliminated the shearing of the motor shafts.
In the next issue, Part II of this article which will include three additional remedies for absorbing hydraulic shock and reducing leakage.