Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/629709
16 January - February 2016 | www.machinerylubrication.com occurs, usually every knob in the system is turned. The first adjustment made typi- cally is on the pump's compensator. When the compensator's setting is reached and no volume is required in the system, the pump will de-stroke and deliver only enough oil to maintain the compensator setting. If this adjustment is set above the system's relief valve, the pump volume will return to the tank through the relief valve instead of being reduced to a flow rate of almost zero gallons per minute. This causes the temperature to rise above 140 degrees, resulting in O-ring failure as well as pump, motor and cylinder seal leakage. I recently consulted with a paper mill that had heat and leakage problems on its debarker. The unit was running at 205 degrees F. The system was leaking at the directional valve manifold, pump shaft seal and cylinder rod seal. When the pres- sures in the system were set properly, the temperature dropped to 130 degrees F several hours later. Unfortunately, the damage to the O-rings and seals had already been done. The mill had to replace the pump, cylinder and valve O-rings. SHOCK AND PRESSURE SPIKES As mentioned previously, hoses can absorb some of the shock that is generated in a hydraulic system. The other two devices normally used to reduce shock are relief valves and accumulators. The most common problem with relief valves is that the spring pressures are generally set too high. The system relief is usually located near the hydraulic pump and should be set 200 psi above the maximum operating pressure if a fixed displacement pump is employed. If using a relief valve with a pressure-compensating pump, a setting of 250 psi above the compensator setting is appropriate. When properly set, the relief spool will open for a moment, dumping the pressurized fluid back to the tank. Crossport relief valves are commonly found on the hydraulic motor drives of cranes, knucklebooms, and debarker and washer drives. These valves will open momentarily when the load starts moving and then decelerate the load when stop- ping. When oil is initially ported to drive the load, an initial pressure spike will occur. In the case of debarker and planer feedrolls, the spike is generated as the log or board is fed in. The crossport relief should be set to open when the pressure spike rises approximately 400 psi above what is required to drive the largest log or board. Improperly set crossports not only result in leakage at the system fittings but can also cause damage to the motor, machine and other system components. Accumulators are excellent devices for absorbing shock in hydraulic systems. Generally, bladder or diaphragm types are used for absorbing shock. Threaded, in-line shock suppressors that are pre-charged with dr y nitrogen have also become popular within the last few years. When an accumulator is used to absorb shock, there are a few basic guidelines that should be followed. For instance, you should use a small accu- mulator (normally 1 gallon or smaller) and install it as close as possible to where the shock occurs. Also, be sure to pre-charge the accumulator with dr y nitrogen approximately 100 psi below the pressure required to move the maximum load. Remember, any time there is oil leakage in a system, there's a reason for it. The entire system should be analyzed, and the problem for the leaks identified. Compa- nies waste thousands of dollars each year because oil leaks have become accepted. Maintenance personnel must be trained on proper piping, clamping and hose installa- tion procedures. Ever yone in the plant should also be educated on the negative effects that random "knob turning" can have on machine operation. This relief-valve tank line should have been at ambient temperature. Because the com- pensator setting was increased, the excess pump volume dumped over the relief valve when it was not used in the system. An accumulator can be used to absorb shock in a hydraulic system. COVER STORY