What are the common faults of Rexroth variable piston pumps?

Thread Content

This post was last edited by tianyewei on 2017-11-13 08:22. Variable displacement piston pumps made by Rexroth often experience various faults during operation, such as oil leakage, vibration, noise, and overheating. What causes these issues? When the swash plate angle of a variable-displacement axial piston pump is zero, it is in the neutral position, and at this time the pump’s output flow rate should be zero. But sometimes, a deviation from the midpoint of the median adjustment mechanism occurs, with flow still being output at the midpoint. The reason is that the position of the controller is off, loose, or damaged, requiring re-zeroing, tightening, or replacement. Insufficient angle holding force of the pump, as well as wear of the tilt angle kingpin, can also cause this phenomenon. Today, Autos brings you an analysis of common faults in Rexroth variable piston pumps. If you encounter such problems, the information provided below can serve as a reference, explaining the causes and solutions. The details are as follows: 1. The hydraulic pump does not deliver sufficient flow rate or fails to supply oil (1) Insufficient suction volume. The reason is excessive resistance in the oil suction pipeline or insufficient oil replenishment. Such as the pump running at too high a speed, the liquid level in the tank being too low, air leakage in the fuel inlet pipe, or the filter becoming clogged. (2) Excessive leakage. The reason is excessive clearance in the pump and poor sealing. If the oil distribution plate is scratched by metal fragments, iron shavings, etc., oil will leak from its end face ； The sealing surfaces of the check valves in the variable mechanism are not properly fitted, and the support surfaces of the pump body and the oil distribution plate have sand holes or wear marks. The damaged part of the pump can be identified by checking for foreign objects mixed in the hydraulic oil inside the pump. (3) If the tilt angle of the swash plate is too small, the pump’s displacement is low; this requires adjusting the variable piston to increase the swash plate’s tilt angle. 2. The neutral position is defined as the state in a variable-displacement axial piston pump where the discharge volume is not zero, and the swash plate angle is zero; at this point, the pump’s output flow rate should be zero. But sometimes, a deviation from the midpoint of the median adjustment mechanism occurs, with flow still being output at the midpoint. The reason is that the position of the controller is off, loose, or damaged, requiring re-zeroing, tightening, or replacement. Insufficient angle holding force of the pump, as well as wear of the tilt angle kingpin, can also cause this phenomenon. 3. Fluctuations in output flow: Fluctuations in output flow are related to many factors. For variable displacement pumps, it can be attributed to poor control of the variable mechanism; for example, the entry of foreign objects into this mechanism can cause scratches, wear marks, and other damage on the control piston, leading to unstable movement of the control piston. Unstable movement of the control piston can be caused by insufficient amplifier power or damaged components, as well as poor damping performance of the damper that contains a spring. Unstable flow is often accompanied by pressure fluctuations. To address such faults, it is generally necessary to disassemble the hydraulic pump, replace the damaged components, increase damping, raise the spring stiffness, and adjust the control pressure. 4. Abnormal output pressure: The output pressure of the pump is determined by the load and is approximately proportional to the input torque. There are two types of faults for abnormal output pressure. (1) Excessively low output pressure: When the pump is operating in self-priming mode, air leaks in the oil inlet pipeline, or significant leaks in components such as hydraulic cylinders, check valves, and direction control valves within the system can all prevent the pressure from rising. This requires identifying the leak, tightening or replacing the seals, which will increase the pressure. If the relief valve is faulty or the set pressure is low, the system pressure will not rise; the pressure should be readjusted or the relief valve repaired. If a deviation between the cylinder block of the hydraulic pump and the distribution plate causes severe leakage, and in serious cases the cylinder block may even rupture, the mating surfaces should be re-ground or the hydraulic pump replaced. (2) Excessively high output pressure: If the load in the circuit continues to increase, the pump pressure will also rise, which is normal. If the load remains constant and the pump pressure exceeds the pressure required by the load, then hydraulic components other than the pump should be checked, such as direction valves, pressure valves, transmission devices, and return pipelines. If the maximum pressure is too high, the relief valve should be adjusted. 5. Vibration and noise: Vibration and noise occur simultaneously. They not only pose a hazard to machine operators but also cause pollution to the environment. (1) Mechanical vibration and noise can be generated by factors such as misalignment or seizure of the pump shaft and motor shaft, damage to the bearings and couplings of the rotating shafts, damaged elastic pads, and loose mounting bolts. For pumps that operate at high speeds or transfer large amounts of energy, regular inspections are necessary to record the amplitude, frequency, and noise of various components. If the rotation frequency of the pump coincides with the natural frequency of the pressure valve, resonance will occur; the pump’s speed can be adjusted to eliminate this resonance. (2) Noise generated by fluid flow in the pipes can be caused by a too narrow oil inlet pipe, an oil filter with insufficient flow capacity or that is clogged, air being drawn into the oil inlet pipe, excessive viscosity of the oil, insufficient oil intake due to an overly low oil level, and water hammer occurring in high-pressure pipes. Therefore, the fuel tank must be properly designed, and the oil filters, oil pipes, and directional valves must be selected correctly. 6. Overheating of the hydraulic pump. There are two reasons for excessive heating of the hydraulic pump: one is heat generated by mechanical friction. Since the moving surface is in a state of dry or semi-dry friction, heat is generated through the friction between the moving parts. The second is heat generation due to liquid friction. High-pressure oil leaks into the low-pressure chamber through various gaps, and a large amount of hydraulic energy is converted into heat energy. Therefore, by properly selecting the clearance between moving parts, the volume of the oil tank, and the cooler, it is possible to prevent excessive heating of the pump as well as excessively high oil temperatures. Additionally, a clogged return oil filter that causes excessive back pressure in the return oil can also lead to high oil temperatures and overheating of the pump. 7. The main reasons for oil leakage in piston pumps are as follows: (1) Damage to the main shaft seal or defects/scratches on the shaft ； (2) Excessive internal leakage causes an increase in pressure at the oil seal, thereby damaging or pushing the oil seal out ； (3) The oil drain pipe is too thin and too long, causing oil leakage at the seal ； (4) The external oil pipe of the pump is loose, the pipe fittings are damaged, and the gaskets are aged or cracked ； (5) Bolts of the variable adjustment mechanism are loose, and the seal is damaged ； (6) The cast iron pump casing has sand holes or poor welding.