Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1189076
operating problems, such as reduced speed and stalls. It may even result in accumulator damage. In general, it is better to under- charge than overcharge. However, a bladder accumulator that has lost all or most of its charge may have its bladder crushed at the top of the shell and become ruptured by the Schrader valve assembly. Adding Volume Volume accumulators have a dump line to release pressure when- ever the system is shut down. e pre-charge can be checked without the charging rig by watching the gauge fall when the system is shut down and the dump line is opened. e gauge will drop slowly because the dump line normally is downsized to avoid turbulence in the reservoir. When the gauge reaches the current pre-charge of the accumulator, it will then drop immediately to 0 psi. is also is a good way to tell if an automatic dump valve has opened as it should. When the system is shut down, if the gauge drops imme- diately to 0 psi without gradually dropping first, it is likely that the gauge is isolated from the accumu- lator, and whether the accumulator has dumped will be unknown. Shock Absorption When an accumulator is used for shock absorption, it is not desir- able that there be much, if any, oil in the accumulator during opera- tion. e accumulator will respond more rapidly to pressure spikes if the compression process has already begun. For this reason, it usually is recommended that shock accumula- tors be pre-charged to nearly 100 psi below the maximum load pressure of the actuator it protects. It is not advisable to use a piston accumulator to absorb shock. Bladder and diaphragm accumu- lators are more responsive because they don't have to overcome the static friction of the piston seal, and the mass of the piston need not be accelerated or decelerated. e piston accumulator is like a hydraulic cylinder with no rod. It is pre-charged with nitrogen and no oil in the bottom. When the system is pressurized, the nitrogen compresses as the bottom of the accumulator fills with oil. e nitrogen pressure www . machinerylubrication.com | November - December 2019 | 19 Over time, oil bypasses the piston seals in a piston accumulator, displacing nitrogen. Mounting a piston accumulator horizontally can cause the piston seals to wear more rapidly. If a bladder accumulator is mounted horizontally, a cavity may form between the bladder and the shell, causing fluid to become trapped. Bypassed Oil Uneven Wear at Piston Contact ML Bladder Cavity