FAQ

Cavitation can cause serious damage to the hydraulic system: it can produce knocking noise in the axial piston pump, damage the valve plate and cylinder surface. It can reduce the volumetric efficiency of the pump, resulting in flow and pressure fluctuations. It can accelerate the oxidation of the oil and produce acidic substances that corrode metal parts. Measures to prevent cavitation include: ensuring that the suction pipeline is unobstructed, the pipe diameter is large enough, and reducing elbows and valves. Keep the oil level in the oil tank high enough, and the suction port immersion depth is not less than 200mm. Control the oil temperature within an appropriate range to avoid gas precipitation in the oil. Select oil with good anti-cavitation performance and add anti-foaming agent when necessary. For high-speed pumps, consider using a booster tank or an auxiliary oil supply pump. Check the condition of the suction filter regularly to avoid blockage and increase resistance. When designing the system, ensure that the pump suction port pressure is not less than 0.8bar (absolute pressure).

A reasonable filtering scheme should consider the following factors: A coarse filter (100-180μm) is set at the suction port of the axial piston pump to protect the pump without increasing the oil suction resistance. A fine filter (β₃≥200, corresponding to a filtration accuracy of about 10μm) is set on the high-pressure pipeline to protect precision valves. A medium-precision filter (25-40μm) is set on the return oil pipeline to capture pollutants generated by the system. For servo systems, an offline circulation filtration system can be added to keep the oil cleanliness below NAS level 6. The filter element selection should consider the pressure difference characteristics, dirt holding capacity and material compatibility, and should be replaced regularly (usually when the pressure difference exceeds 0.3bar). A temporary high-precision filter device (3-5μm) should be used when flushing the system, and the normal operating filter element should be replaced after the target cleanliness is reached. Note that the pressure rating of the filter housing should be higher than the maximum pressure of the system to avoid rupture.

Excessive oil temperature is a common problem in hydraulic systems. The main reasons include: the system pressure is set too high or it is overloaded for a long time, and energy loss is converted into heat energy. Excessive internal leakage in the axial piston pump or motor reduces the volumetric efficiency. Insufficient cooling system capacity, radiator blockage or fan failure. Unreasonable oil tank design, insufficient heat dissipation area or too fast oil circulation. Improper oil viscosity selection, too high increases flow resistance, too low increases internal leakage. Effective measures to control oil temperature include: reasonably setting working pressure to avoid unnecessary energy loss; ensuring the normal operation of the cooling system and regularly cleaning the radiator; optimizing the oil tank design and adding auxiliary cooling when necessary; selecting oil with appropriate viscosity; repairing or replacing severely worn pumps, valves and other components. When the oil temperature continues to exceed 70°C, it should be considered abnormal and the root cause should be found.

Common methods for determining the amount of internal leakage include: load settlement method - extend the cylinder and load it, measure the amount of retraction of the piston rod within the set time, and calculate the leakage. Flow measurement method - pressurize the rodless chamber of the cylinder, measure the amount of oil flowing out of the rod chamber, and directly obtain the leakage value. Pressure drop method - close one chamber of the cylinder and pressurize it, record the pressure drop value per unit time, and estimate the leakage. In practical applications, when the cylinder shows obvious self-settlement (speed>50mm/min) or abnormal temperature rise when maintaining pressure, it usually indicates that the internal leakage is too large. Different applications have different requirements for leakage. For example, the allowable leakage of engineering machinery cylinders is usually 1-2 orders of magnitude larger than that of machine tool cylinders. If the leakage of precision equipment cylinders exceeds 5mL/min, repairs should be considered.

Damage to the piston rod surface will directly damage the seal and must be handled in time: Minor scratches (depth <0.05mm) can be polished along the axial direction with fine sandpaper (600# or above). Moderate wear or rust (depth 0.05-0.1mm) needs to be repaired by grinding and re-plated with hard chrome, and the thickness of the chrome layer is usually 0.02-0.05mm. Severe damage (depth >0.1mm or large area peeling) should consider replacing the piston rod. The surface roughness after repair should reach Ra0.2-0.4μm, and the straightness error should not exceed 0.1mm/m. Temporary emergency treatment can be wrapped with polytetrafluoroethylene raw tape on the damaged part, but formal repairs must be arranged as soon as possible. During the first operation after repair, the seal should be reciprocated several times at low speed to adapt the sealing lip to the repaired surface.

Correct installation and maintenance measures include: Ensure the parallelism of the cylinder and the guide rail or load during installation to avoid lateral forces. Before first use, run the cylinder at full stroke and no-load for more than 5 times to exhaust the air in the system and form an oil film. Check the piston rod surface regularly to remove adhering contaminants to prevent damage to the seal. Control the oil temperature within the range of 30-60°C. High temperature will accelerate the aging of the seal. Regularly replace the hydraulic oil and filter element to maintain the cleanliness of the oil (NAS level 8 or below is recommended). When out of use for a long time, the piston rod should be retracted and run for short periods of time regularly to prevent the seal from sticking. Establish a regular inspection system, including the status of fasteners, sealing performance, buffer function, etc.

External leakage of hydraulic cylinders is common in the following parts: Leakage at the piston rod seal is the most common, mostly due to seal wear or piston rod surface damage. The seal needs to be replaced and the rod surface needs to be checked. Chrome plating repair is required if necessary. Leakage at the joint surface between the cylinder head and the cylinder barrel is usually caused by aging of the seal ring or loose bolts. The seal should be replaced and the bolts should be tightened according to the torque requirements. Leakage at the oil port connection is mostly caused by loose joints or damaged seals. Re-tightening or replacement of the seal is required. Leakage caused by welding parts or casting defects requires professional welding repair or replacement of parts. Leakage at the buffer regulating valve may be caused by valve core wear or seal failure. The valve seat needs to be ground or the seal needs to be replaced. When dealing with leaks, special tools should be used for disassembly to avoid damaging the sealing mating surface.

Slow movement may be caused by many reasons: First, check whether the system pressure has reached the set value and whether the axial piston pump output is normal. Excessive leakage in the oil cylinder is a common cause. Wear of the piston seal will cause the high and low pressure chambers to communicate with each other, and the seals need to be disassembled and replaced. Scratches on the inner wall of the cylinder will also increase leakage. Minor scratches can be repaired by honing, and severe wear requires replacement of the cylinder. Excessive external load or mechanical jamming will increase movement resistance. The guide rails, hinge points and other parts should be checked. Excessive oil viscosity or low oil temperature will increase flow resistance. The appropriate viscosity oil should be selected and preheated. Control valve core jamming or improper flow adjustment will also limit the speed of the oil cylinder. The valve function needs to be checked.

Axial piston motors and pumps have many similarities in maintenance: both require attention to oil cleanliness and viscosity, monitor operating temperature, and regularly check internal wear. However, as an actuator, motors also require special attention: maintenance of load connection parts, including coupling alignment, bearing lubrication, etc., to avoid additional radial forces. Motors usually need to withstand greater external impact loads, and the bearing condition and internal fasteners should be checked more frequently. The variable mechanism of a variable motor may operate more frequently, and special attention should be paid to the wear of the control piston and servo valve. For motors with integrated brakes, the thickness of the brake wear plate and the brake spring force must also be checked regularly. Compared with pumps, the motor housing oil leakage is usually larger, and it is necessary to ensure that the oil leakage line is sufficiently unobstructed.

External leakage is common in the following areas: Leakage at the shaft seal is mostly due to aging of the seal or wear on the shaft surface. The seal needs to be replaced and the roughness of the shaft surface needs to be checked, and repaired if necessary. Leakage at the housing joint surface may be caused by loose bolts or damaged gaskets. The bolts should be tightened or the gaskets should be replaced according to the specified torque. Leakage at the oil port connection is usually caused by loose joints or damaged sealing rings, which need to be re-tightened or replaced. It is worth noting that excessive internal leakage will increase the pressure at the shaft seal and cause seal failure. Therefore, when dealing with external leakage, the internal wear should be checked at the same time. For motors that are activated after long-term disuse, the sealing material may lose its elasticity due to oil loss and hardening. The sealing status should be checked before starting.

Correct maintenance measures include: regularly check and replace the hydraulic oil, and keep the oil clean to at least ISO4406 20/18/15 level, and higher cleanliness is required under high temperature conditions. Monitor the system operating temperature to avoid long-term operation under conditions exceeding 90°C, and increase cooling capacity if necessary. Regularly check the tightness of the motor mounting bolts and the alignment of the shaft to prevent additional radial forces from damaging the bearings. For variable motors, the flexibility of the variable mechanism and the smoothness of the control oil circuit should be checked regularly. Establish a preventive maintenance plan, including regular disassembly and measurement of key matching clearances, such as the plunger and cylinder bore, the distributor and the cylinder body, etc. Note that the motor housing oil drain pressure must not exceed the specified value (usually not more than 2bar), and ensure that the oil drain line is unobstructed.

Starting difficulties may be caused by the following reasons: Insufficient system pressure cannot overcome static friction. Check the pressure setting and the status of the axial piston pump. The internal parts of the motor, such as the plunger or the swash plate, are stuck. This is mostly caused by insufficient lubrication or oil contamination after long-term parking. You can try to manually turn the motor and replace the clean oil. The oil temperature is too low, resulting in too high viscosity, which increases the starting torque requirement. The hydraulic oil should be preheated to the operating temperature range. The variable mechanism in the minimum displacement position will also cause starting difficulties. It needs to be adjusted to the appropriate position. For motors that have been out of service for a long time, the drain port should be filled with clean oil before starting to remove the internal air.