Machinery Lubrication


Machinery Lubrication magazine published by Noria Corporation

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Page 31 of 76 | September - October 2017 | 27 Pressure Settings Pressures are often randomly adjusted by the local plant knob- turner in an attempt to increase the speed of the machine. When a cylinder or hydraulic motor operates, the pressure will only build up high enough to move the load. Therefore, the maximum pressure limiter in the system should be set 200 psi higher than the pressure required to move the load. If the pressure is set higher than that, exces- sive shock will be generated when starting and stopping the actuator. In a system with a pressure-compensating type pump, the compensator setting determines the maximum system pressure. When this pressure is reached, the compensator spool will shift open and de-stroke the pump. The pump will then only deliver enough oil to maintain the compensator setting. In a fixed-displacement pump system, the relief valve determines the maximum system pressure. Once the relief valve's spring setting is reached, the spool will shift open and port the pump volume back to the tank. I was recently asked to consult with a sawmill that was experi- encing leakage and shock on its lumber stacker. The stacker used a 10-inch-diameter cylinder to raise and lower the stack. The hydraulic schematic recommended that the relief valve be set to 1,200 psi. However, someone had turned the valve up to 1,800 psi. When the cylinder fully bottomed out, the pressure went up to 1,800 psi. A loud bang was then heard, and all lines in the system vibrated. You can calculate how much force was generated when the pres- sure built up to 1,800 psi using the following formula: Force = Pressure x the number of square inches on the full piston side of the cylinder Force = 1,800 psi x 78.54 square inches Force = 141,372 pounds Once the relief valve setting was lowered to the recommended 1,200 psi, the noise and shock severely decreased when the cylinder bottomed out. With the pressure at 1,200 psi, you can calculate the amount of force that was reduced on the cylinder using the same formula: Force = 1,200 psi x 78.54 square inches Force = 94,248 pounds The difference in force between 1,800 psi and 1,200 psi is 47,124 pounds or almost 24 tons. All plant personnel should be made aware of the negative effects caused by pressures being out of adjustment. In addition to reducing shock and leakage, turning down pressures decreases electrical energy consumption of the electric motor and heat generation in the system. If you have shock and leakage in your systems, there is something you can do about it. By properly setting the pressures, installing accumulators and shock suppressors where needed, verifying that the system is piped and hosed correctly, and adding or adjusting pilot chokes, you should be able to reduce or even eliminate shock and leakage in your hydraulic systems. About the Author Al Smiley is the president of GPM Hydraulic Consulting Inc., located in Monroe, Georgia. Since 1994, GPM has provided hydraulic training, consulting and reliability assessments to companies in the United States, Canada, the United Kingdom and South America. Contact Al at gpm@

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