Created on:2021-06-10 13:55

Main parameters and common problems of hydraulic pump

Main parameters and common problems of hydraulic pump

1.6.5 trapped oil phenomenon and unloading measures

(1) The working process of the positive displacement hydraulic pump is generally divided into three stages: first, the liquid is sucked in by the vacuum generated by the increase of the volume of the oil suction chamber (oil suction stage), then the liquid is discharged to the system by the decrease of the volume of the oil discharge chamber (oil discharge stage). Here is mainly to analyze the phenomenon of trapped oil and its unloading measures.

According to the basic working principle of the hydraulic pump, when the hydraulic pump is in the middle stage, its working cavity is in the transitional sealing area between the oil suction and discharge cavities, which traps part of the oil in the sealing area and forms the trapped oil volume. With the rotation of the hydraulic pump, the movement of the squeezer will cause periodic changes in the trapped oil volume: when the trapped oil volume decreases, the oil pressure increases, causing additional periodic load on the bearing and other components of the pump, resulting in impact and noise, resulting in oil heating; when the trapped oil volume increases, the pressure decreases (local vacuum) due to no oil supplement, Cavitation and cavitation may occur. This is the phenomenon of trapped oil. Trapped oil is a harmful phenomenon, it will reduce the efficiency of the hydraulic pump, shorten the service life of the pump, so we must try to eliminate it.

In order to eliminate the phenomenon of trapped oil, the necessary unloading measures should be taken in the structure. The principle is to make the pressure change in the trapped oil volume as much as possible adapt to the pressure when the oil suction and discharge cavities are connected on the premise of ensuring the volumetric efficiency.

(2) Unloading measures because the working cavity of the hydraulic pump is between the suction and discharge cavities when it is in the middle stage, there are three possible situations: negative cover, zero cover and positive cover.

① Negative covering, also known as positive opening, means that when the working cavity is between the oil suction and discharge cavities, the working cavity will communicate with them. At this time, the working chamber will not produce trapped oil, but it will produce large internal leakage, which will reduce the volumetric efficiency, so the negative covering structure is generally not used.

② Zero cover, also known as zero opening, refers to the situation that when the working cavity is between the oil suction and oil discharge cavities, the working cavity is just sealed and the oil suction and oil discharge cavities are just separated. In this case, the oil pressure in the working chamber rises from the oil suction pressure to the oil discharge pressure or drops from the oil discharge pressure to the oil suction pressure step by step, thus causing pressure shock and noise, which is the phenomenon of trapped oil.

③ Positive covering, also known as negative opening, refers to the situation that the working cavity is sealed up for a period of time, which is bound to produce oil trapping phenomenon. However, as long as the phenomenon of trapped oil is reasonably used, the step phenomenon of pressure can be eliminated. Therefore, this kind of positive covering structure and unloading measures based on this structure are commonly used in hydraulic pumps, and the specific structure varies with the type of pump.

For example, the gear pump is in the front and back of the pump, the end cover of the inner surface of the unloading groove corresponding to the trapped oil area, while the axial piston pump is in the valve plate with triangular groove or oil hole.

1.6.6 flow pulsation

According to the kinematics of hydraulic pump, the instantaneous flow of most pumps is not constant in theory (except screw pump), and there is flow pulsation. Flow pulsation has a direct impact on the performance and life of hydraulic components and systems. The greater the fluctuation amplitude of instantaneous flow, the worse the motion stability of hydraulic actuator. For the multi pump oil supply system, the pulsation synchronization may increase the amplitude and worsen the performance. The instantaneous flow pulsation will also cause pressure pulsation, which will cause fatigue damage to the transmission shaft, bearing, pipe, joint and seal of hydraulic pump and motor. In addition, when the pulsating frequency of the instantaneous flow is close to or consistent with the natural frequency of the relief valve, the resonance phenomenon of the valve may also be caused.

Flow pulsation is generally evaluated by flow non-uniformity coefficient, i.e

(1-16)

Where (qinst) max -- the maximum theoretical instantaneous flow of hydraulic pump;

(qinst) min -- the minimum theoretical instantaneous flow of hydraulic pump.

The smaller the flow non-uniformity coefficient δ is, the smaller the flow pulsation is or the better the theoretical instantaneous flow quality is.

The frequency of flow pulsation is related to the structural parameters such as the speed of the pump and the number of squeezers (such as the number of gear teeth of the gear pump, the number of blades of the vane pump, the number of plunger of the plunger pump, etc.). Different types of pumps or pumps of the same type and different geometric sizes have different flow pulsations.

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