Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/934530
www . machinerylubrication.com | January - February 2018 | 17 suction line. After closer inspection, a crack was found in the suction line pipe. A bad shaft seal can also cause aeration if the system is supplied by one or more fixed displacement pumps. Oil that bypasses inside a fixed displacement pump is ported back to the suction port. If the shaft seal is worn or damaged, air can flow through the seal and into the pump's suction cavity. is recently occurred on a refiner where the hydraulic pump was used to maintain a precise gap between the discs. Several minutes after the system was turned on, foam started coming out of the reservoir. After the pump was changed, a crack was found in the mounting bracket. is led to the shaft being out of alignment, wearing the seal. A misaligned coupling can cause a shaft seal to wear prematurely as well. As mentioned previously, if the oil level is too low, oil can enter the suction line and flow into the pump. erefore, always check the oil level with all cylinders in the retracted position. If a new pump is installed and pressure will not build, the shaft may be rotating in the wrong direction. Some gear pumps can be rotated in either direction, but most have an arrow on the housing indicating the direction of rotation. Pump rotation should always be viewed from the shaft end. If the pump is rotated in the wrong direction, adequate fluid will not fill the suction port due to the pump's internal design. Testing a Fixed Displacement Pump A fixed displacement pump delivers a constant volume of oil for a given shaft speed. A relief valve must be included downstream of the pump to limit the maximum pressure in the system. A fter the visual and sound checks are made, the next step is to determine whether you have a volume or pressure problem. If the pressure will not build to the desired level, isolate the pump and relief valve from the system. is can be done by closing a valve, plugging the line downstream or blocking the relief valve. If pressure builds when this is done, there is a component downstream of the isolation point that is bypassing. If the ML 1. The oil viscosity is too high. Low oil temperature increases the oil viscosity, making it harder for the oil to reach the pump. Most hydraulic systems should not be started with the oil any colder than 40 degrees F and should not be put under load until the oil is at least 70 degrees F. Many reservoirs do not have heaters, particularly in the South. Even when heaters are avail- able, they are often disconnected. While the damage may not be immediate, if a pump is continually started up when the oil is too cold, the pump eventually will fail prematurely. 2 . The suction filter or strainer is contaminated. The strainer typi- cally is 74 or 149 microns and is used to keep "large" particles out of the pump. The strainer may be located inside or outside the reser- voir. Strainers located inside the reservoir are out of sight and out of mind. Many times maintenance personnel are not even aware that there is a strainer in the reservoir. The suction strainer should be removed from the line or reservoir and cleaned a minimum of once a year. I was called in to help a plant troubleshoot a system where they had already changed five pumps in a week's time. I noticed that the breather cap was missing, which was allowing dirty air to flow into the reservoir. A check of the hydraulic schematic showed a strainer in the suction line inside the tank. When the strainer was removed, a shop rag was found wrapped around the screen mesh. Apparently, someone had used the rag to plug the breather cap opening, and it had then fallen into the tank. 3. The electric motor is driving the hydraulic pump at a speed that is higher than the pump's rating. All pumps have a recommended maximum drive speed. If the speed is too high, a higher volume of oil will be needed at the suction port. Due to the size of the suction port, adequate oil cannot fill the suction cavity in the pump, resulting in cavitation. Although this rarely happens, some pumps are rated at a maximum drive speed of 1, 200 revolutions per minute (RPM), while others have a maximum speed of 3,600 RPM. The drive speed should be checked any time a pump is replaced with a different brand or model. 3 Reasons Why a Pump Cavitates A dirty suction strainer is shown.