Common abnormal phenomena in the hydraulic system of construction machinery?
Pressure loss
Because the liquid is viscous, there is inevitably friction when flowing in the pipeline, so the liquid will inevitably lose some energy during the flow. This part of energy loss is mainly manifested as pressure loss.
There are two types of pressure loss: along-path loss and partial loss. Loss along the way is the pressure loss caused by friction when the liquid flows over a distance in a straight pipe with a constant diameter. The local loss is the pressure loss caused by the sudden change of the cross-sectional shape of the pipeline, the change of the liquid flow direction or other forms of liquid 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 should be slightly larger than the maximum working pressure required for system operation. Generally, the maximum working pressure required for system operation can be estimated by multiplying the maximum working pressure required by the system by a coefficient of 1.3~1.5.
flow loss
In the hydraulic system, each pressed component has relative moving surfaces, 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 and cause leakage. At the same time, due to imperfect sealing of hydraulic components, part of the oil will leak to the outside. The actual flow caused by this kind of 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 when the system is working. Usually it can also be estimated by multiplying the maximum flow required by the system by a coefficient of 1.1 to 1.3.
hydraulic shock
Reason: The commutation of the actuator and the closing of the valve cause the flowing liquid to produce instantaneous pressure peaks due to inertia and insufficient response of certain hydraulic components, which is called hydraulic shock. Its peak value can exceed several times the working pressure.
Hazard: cause vibration and noise; make relays, sequence valves and other pressure components produce wrong actions, and even cause damage to certain components, sealing devices and pipelines.
Measures: find out the cause of the impact to avoid abrupt changes in liquid flow velocity. Delay the time of speed change, estimate the pressure peak, and adopt corresponding measures. If the flow reversing valve and electromagnetic reversing valve are combined, it can effectively prevent hydraulic shock.
Cavitation phenomenon
Phenomenon: If air penetrates into the hydraulic system, when the bubbles in the liquid move to the higher pressure area with the liquid flow, the bubbles will burst quickly under the action of the higher pressure, which will cause local hydraulic shocks, causing noise and vibration. In addition, because the air bubbles destroy the continuity of the liquid flow, reduce the oil-passing capacity of the oil pipe, cause flow and pressure fluctuations, make the hydraulic components bear impact load, and affect their service life.
Reason: Hydraulic oil always contains a certain amount of water, which can usually be dissolved in the oil, or mixed in the oil in the form of bubbles. When the pressure is lower than the air separation pressure, the air dissolved in the oil separates and forms bubbles; when the pressure drops below the saturated vapor pressure of the oil, the oil will boil and produce a lot of bubbles. These bubbles are mixed in the oil to form a discontinuous state. This phenomenon is called cavitation.
Location: In the suction port and the suction pipe where the pressure is lower than the atmospheric pressure, cavitation is easy to occur; when the oil flows through the narrow gap such as the orifice, the pressure drops due to the increase in speed, and cavitation is also generated.
Hazard: Bubbles move with the oil to the high-pressure area, and quickly rupture under the action of high pressure, causing a sudden decrease in volume and high-pressure surrounding high-pressure oil to replenish at a high speed, causing a local instantaneous shock, a sharp increase in pressure and temperature, and strong noise and vibration .
Measures: The structural parameters of the hydraulic pump and the suction pipeline of the pump should be correctly designed, and try to avoid narrow and sharp bends in the oil passage to prevent low pressure areas; reasonable selection of mechanical materials, increase mechanical strength, improve surface quality, and improve corrosion resistance.
cavitation phenomenon
Reason: Cavitation occurs with cavitation. The oxygen in the bubbles generated in the cavity will also corrode the surface of the metal element. We call this corrosion caused by the occurrence of cavitation 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.
The hazards and measures are the same as those of cavitation.