Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1061099
18 | November - December 2018 | www . machinerylubrication.com HYDRAULICS pump and observe the oil as it fl ows into the container. Gradually rotate the relief valve clockwise. If the fl ow rate drops as the pressure is increased, the pump is bad. If the fl ow rate does not change, the pump is good. Extreme caution should be used when conducting these tests so as not to expose yourself to high-pressure fl uid. Accumulators are commonly used to supply additional volume, absorb shock and maintain pressure in the system. They typically are pre-charged with dry nitrogen. For the accumulator to perform appropri- ately, the nitrogen pre-charge must be correct. ere are three easy methods to verify whether an accumulator is operating properly. e fi rst method is to turn off the hydraulic pump and allow the pressure to bleed down to 0 pounds per square inch (psi). Install a charging rig with a gauge on the accumulator's Schrader valve. e nitrogen pre-charge will be indicated on the gauge. Another method is to watch the pressure gauge as nitrogen forces oil out of the accumulator shell when the pump is turned off . e pressure will gradually fall and then rapidly drop to 0 psi. e pressure at which the needle quickly drops to 0 is the pre-charge pressure. e fi nal method is to shoot the sides of the accumulator shell with a temperature gun or infrared camera. e shell should be warmer in the lower half or two-thirds when the pre-charge is correct. I recently consulted with a plant where several accumulators were found to be undercharged. When an accumulator is undercharged, less volume is delivered to the system. Once properly charged, the machine's line speed was increased to reach a higher level of production. ese pump and accumulator examples are indicative of the quick tests that can be made to determine whether a component is operating correctly. 3. How to Adjust the System One of the main problems, if not the biggest problem, is that random adjustments are made on a system in an attempt to speed up the machine or fi x a problem. Often, an adjust- ment is made, but if no change in machine operation is seen, it is assumed that nothing has changed in the system. At one plant, the local "knob turner" thought a hydraulic pump was too noisy, so he turned the compensator adjustment clockwise and cracked open a manual valve in the line immediately downstream. The pump's noise level dropped somewhat, so he left, thinking he had solved the problem. Several hours later, the machine shut down because the oil temperature had risen to 160 degrees F. An electrician was called, who promptly jumpered out the high-temperature shutdown switch. Some 24 hours later, the oil temperature rose to 300 degrees F. e machine then had to be shut down and fl ushed with an expensive solvent to remove varnish and sludge from the system. By rotating the compensator clockwise, the knob turner set the compensator above the relief valve's setting. This enabled the pump volume to return to the tank at a high pressure, generating heat when not required in the system. Opening the manual valve allowed some of the pump's volume to return to the tank at all times under high pressure, generating even more heat. All maintenance personnel must be trained to precisely set pump compensators, relief valves, f low controls, pressure-reducing valves and pre-charging accumulators. If this is not done, shock, leakage, overheating, component failure and damage to the machine can result. 4. How to Read Hydraulic Symbols A schematic should be used to eff ectively troubleshoot a hydraulic problem. erefore, maintenance personnel must know how to read hydraulic symbols in order to trou- bleshoot from the schematic. A commonly used directional valve is shown above along with the symbol for the valve. is symbol indicates fi ve things about the valve: it is Check the oil fl ow out of the relief valve's tank line to assess whether a fi xed-displacement pump is worn. A charging rig with a gauge can be installed on an accumulator's Schrader valve.