Working Principle Of Variable Displacement Piston Pump

The pressure oil of the variable piston pump enters the lower chamber of the variable housing through the oil hole in the pump body and the variable housing through the one-way valve. When the pull rod moves downward, it pushes the servo piston to move downward, and the upper valve port of the servo valve opens. The pressure oil in the lower chamber of the variable housing enters the upper chamber of the variable housing through the oil hole in the variable piston. Because the area of the upper chamber is larger than that of the lower chamber, The hydraulic pressure pushes the piston downward, drives the pin shaft to rotate the variable head around the center of the steel ball, changes the tilt angle of the variable head (increases), and the flow of the plunger pump increases accordingly. On the contrary, when the pull rod moves upward, the tilt angle of the variable head changes in the opposite direction, and the flow of the pump also changes. When the tilt angle becomes zero, the variable head changes to the direction of negative deflection angle, the liquid flow changes direction, and the inlet and outlet of the pump change.

1. Vibration and noise

Vibration and noise occur at the same time. They not only do harm to the operator of the machine, but also pollute the environment.

(1) Mechanical vibration and noise

If the pump shaft and motor shaft are not concentric or jacking, the bearing and coupling of the rotating shaft are damaged, the elastic pad is damaged and the assembly bolt is loose, which will produce noise. For the pump running at high speed or transmitting large energy, it is necessary to check regularly and record the amplitude, frequency and noise of each component. If the rotation frequency of the pump is the same as the natural frequency of the pressure valve, resonance will be caused. The rotation speed of the pump can be changed to eliminate the resonance.

(2) Noise generated by liquid flow in pipeline

Noise will be generated when the oil inlet pipe is too thin, the flow capacity of the oil inlet filter is too small or blocked, the air is sucked into the oil inlet pipe, the oil gap is too high, the oil level is too low, the oil suction is insufficient, and liquid hammer occurs in the high-pressure pipe. Therefore, the oil tank must be designed correctly and the oil filter, oil pipe and directional valve must be selected correctly.

2. 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 kinds of faults for abnormal output pressure.

(1) Output pressure too low

When the pump is in the self-priming state, if there is air leakage in the oil inlet pipeline or there is a large leakage in the hydraulic cylinder, one-way valve and directional valve in the system, the pressure will not rise. This requires finding out the air leakage, fastening and replacing the seal to increase the pressure. If the overflow valve is faulty or the adjustment pressure is low, and the system pressure cannot go up, the pressure should be adjusted again or the overflow valve should be overhauled. If the cylinder block of the hydraulic pump deviates from the valve plate, resulting in a large amount of leakage, and in serious cases, the cylinder block may be broken, regrind the mating surface or replace the hydraulic pump.

(2) Output pressure too high

If the circuit load continues to rise, the pump pressure also continues to rise, which is normal. If the load is constant and the pressure of the pump exceeds the pressure value required by the load, check the hydraulic components other than the pump, such as directional valve, pressure valve, transmission device and oil return pipeline. If the maximum pressure is too high, adjust the overflow valve.

3. The oil discharge is not zero at the middle position

When the inclined angle of the swashplate of the variable axial piston pump is zero, it is called the middle position. At this time, the output flow of the pump should be zero. However, sometimes the median deviates from the midpoint of the adjustment mechanism, and there is still flow output at the midpoint. The reason is that the position of the controller is deviated, loose or damaged, which needs to be reset, fastened or replaced. This phenomenon is also caused by insufficient angle maintenance force of the pump and wear of the tilt angle trunnion.

4. Output flow fluctuation

The fluctuation of output flow is related to many factors. The variable displacement pump can be considered to be caused by poor control of the variable mechanism. For example, foreign matters enter the variable mechanism and scratch step marks, wear marks and scars on the control piston, resulting in unstable movement of the control piston. Due to the insufficient energy of the amplifier or the damage of parts, and the poor efficiency of the damper of the control piston containing spring, the movement of the control piston will be unstable. Flow instability is often accompanied by pressure fluctuations. This kind of failure generally requires dismantling the hydraulic pump, replacing damaged parts, increasing damping, improving spring stiffness and control pressure, etc.

5. Hydraulic pump overheating

The hydraulic pump generates heat for two reasons. Because the moving surface is in the state of dry friction or semi dry friction, the moving parts rub against each other to generate heat. The second is the heat generated by liquid friction. The high-pressure oil leaks into the low-pressure chamber through various gaps, and a large amount of hydraulic energy is lost and converted into heat energy. Therefore, the correct selection of the clearance between moving parts, oil tank volume and cooler can eliminate the phenomenon of excessive heating of the pump and excessive oil temperature. In addition, the blockage of the oil return filter will cause the return oil back pressure to be too high, which will also cause the oil temperature to be too high and the pump body to be overheated.

6. Oil leakage

The oil leakage of plunger pump is mainly caused by the following reasons:

(1) The main shaft oil seal is damaged or the shaft is defective or scratched;

(2) The internal leakage is too large, resulting in the pressure increase at the oil seal, which will damage or rush out the oil seal;

(3) The oil drain pipe is too thin and too long, resulting in oil leakage at the seal;

(4) The external oil pipe of the pump is loose, the pipe joint is damaged, and the sealing gasket is aged or cracked;

(5) The bolt of variable adjustment mechanism is loose and the seal is damaged;

(6) Cast iron pump casing has sand holes or poor welding.

7. The output flow of the hydraulic pump is insufficient or does not output oil

(1) Insufficient inhalation. The reason is that the resistance on the oil suction pipeline is too large or the oil replenishment is insufficient.

(2) Excessive leakage. The reason is that the clearance of the pump is too large and the seal is poor

(3) The inclination angle of the swashplate is too small and the displacement of the pump is small, which requires adjusting the variable piston to increase the inclination angle of the swashplate