Common Faults 0f Hydraulic System

Pressure loss

Due to the viscosity of the liquid and the inevitable frictional force when flowing in the pipeline, the liquid must lose some energy during the flow. This part of the energy loss is mainly manifested as pressure loss.There are two types of pressure loss: along the way loss and local loss. The loss along the way is the pressure loss due to friction when the liquid flows a certain distance in a straight pipe of constant diameter. Local losses are pressure losses due to sudden changes in the cross-sectional shape of the pipeline, changes in flow direction, or other forms of flow resistance. The total pressure loss is equal to the sum of the along-path loss and the local loss. Due to the inevitable existence of pressure loss, the rated pressure of the pump is slightly larger than the maximum working pressure required for the system to work. Generally, the maximum working pressure required for the system to work can be multiplied by a coefficient of 1.3~1.5 to estimate.

Flow loss

In the hydraulic system, each pressed element has a surface that moves relatively, such as the inner surface of the hydraulic cylinder and the outer surface of the piston. Because of the relative movement, there is a certain gap between them. If one side of the gap is high-pressure oil and the other side is low-pressure oil, the high-pressure oil will flow to the low-pressure area through the gap, causing leakage. At the same time, due to the imperfect sealing of hydraulic components, part of the oil will leak to the outside. The actual flow caused by this leakage is reduced, which is what we call flow loss.

Flow loss affects the speed of movement, and leakage is difficult to absolutely avoid, so the rated flow of the pump in the hydraulic system is slightly larger than the maximum flow required for the system to work. Usually it can also be estimated by multiplying the maximum flow required for system operation by a factor of 1.1~1.3.

Hydraulic shock

Reason: The reversing of the actuator and the closing of the valve cause the flowing liquid to generate an instantaneous pressure peak due to inertia and the insensitive response of some hydraulic components, which is called hydraulic shock. Its peak value can exceed the working pressure several times.

Hazards: cause vibration and noise; make relays, sequence valves and other pressure components malfunction, and even cause damage to certain components, sealing

Devices and pipelines

Measures: Find out the cause of the shock to avoid a sharp change in the flow velocity. Delay the time of speed change, estimate the pressure peak, and take corresponding measures. If the flow reversing valve and the electromagnetic reversing valve are used together, the hydraulic shock can be effectively prevented.

Cavitation

Phenomenon: If air is infiltrated into the hydraulic system, when the air bubbles in the liquid move to the area with higher pressure with the liquid flow, the air bubbles will rapidly rupture under the action of higher pressure, which will cause local hydraulic shock, resulting in noise and vibration. In addition, because the bubbles destroy the continuity of the liquid flow, reduce the oil-passing ability of the oil pipe, cause fluctuations in flow and pressure, make the hydraulic components bear the impact load, and affect their service life.

Reason: Hydraulic oil always contains a certain amount of air, which can usually be dissolved in the oil or mixed in the oil in the form of air bubbles. When the pressure is lower than the air separation pressure, the air dissolved in the oil is separated to form bubbles; when the pressure drops below the saturated vapor pressure of the oil, the oil will boil and generate a large number of bubbles. These bubbles are mixed in the oil to form a discontinuous state, which is called cavitation.

Location: The oil suction port and the oil suction pipe where the pressure is lower than the atmospheric pressure are prone to cavitation; when the oil flows through the narrow gap such as the throttling port, the pressure drops due to the increase of the speed, and the cavitation will also occur.

Hazard: The bubbles move with the oil to the high-pressure area, and break rapidly under the action of high pressure, resulting in a sudden decrease in volume, high-speed flow of surrounding high-pressure oil to replenish, causing local instantaneous impact, sharp increase in pressure and temperature, and strong noise and vibration .

Measures: Correctly design the structural parameters of the hydraulic pump and the oil suction pipeline of the pump, try to avoid narrow oil passages and sharp bends, and prevent the occurrence of low pressure areas; reasonably select the materials of the parts, increase the mechanical strength, improve the surface quality, and improve the corrosion resistance.

Cavitation phenomenon

Reason: cavitation occurs along with cavitation, and the oxygen in the bubbles generated in the cavitation will also corrode the surface of the metal component. We call this corrosion caused by the cavitation phenomenon as cavitation.

Location: Cavitation may occur in oil pumps, pipelines and other places with throttling devices, especially oil pump devices, this phenomenon is the most common. Cavitation is one of the causes of various failures in hydraulic systems, especially in high-speed, high-pressure hydraulic equipment.

Hazards and measures are the same as for cavitation.

Troubleshooting

Due to its unique advantages, the hydraulic transmission system has been widely used in various fields due to its wide technological adaptability, excellent control performance and relatively low cost. However, due to the unstable quality of components and accessories objectively and improper use and maintenance subjectively, and the components and working fluids in the system all work in a closed oil circuit, they are not as intuitive as mechanical equipment, nor as reliable as electrical equipment. Various parameters are conveniently measured by various testing instruments. In hydraulic equipment, only a limited number of pressure gauges, flow meters, etc. are used to indicate the working parameters of certain parts of the system. Other parameters are difficult to measure, and there are many possible root causes of failures. , which brings certain difficulties to the fault diagnosis of the hydraulic system.

At the production site, due to the constraints of the production plan and technical conditions, the fault diagnosis personnel are required to diagnose the faults of the hydraulic equipment accurately, simply and efficiently; the maintenance personnel are required to use the existing information and the technical conditions of the site to minimize disassembly and assembly. Work load, save maintenance man-hours and costs, use the most convenient technical means, in the shortest possible time, accurately find out the fault location and the cause of the fault and repair it, so that the system can resume normal operation, and strive to no longer be used in the future. The same failure occurs.

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