Because the hydraulic system of the molding machine uses many oil cylinders, and the load of each station is not the same, it is very meaningful to apply the variable pump in the system.
Working principle: The axial piston pump with the sliding shoe structure is the most widely used axial piston pump at present. The piston placed in the cylinder body is in contact with the swash plate through the sliding shoe. When rotating, the swash plate pulls or pushes the plunger out of the cylinder to complete the oil suction and discharge process. The oil in the working chamber formed by the plunger and the cylinder hole is communicated with the suction and discharge chambers of the pump respectively through the oil distribution plate. The variable mechanism is used to change the inclination of the swash plate, and the displacement of the pump can be changed by adjusting the inclination of the swash plate.
Maintenance of plunger pump: The swash plate axial plunger pump generally adopts the form of cylinder rotation and end face distribution. The end face of the cylinder body is inlaid with a friction pair composed of a bimetallic plate and a steel oil distribution plate, and most of them adopt the method of plane distribution, so the maintenance is more convenient. The oil distribution plate is one of the key components of the axial piston pump. When the pump is working, on the one hand, the high-pressure oil in the working chamber pushes the cylinder body to the oil distribution plate, and on the other hand, the oil film pressure between the oil distribution plate and the cylinder body forms a pair of pressures. The hydraulic reverse thrust of the cylinder pushes the cylinder away from the oil distribution pan. The designed hydraulic pressing force Fn of the cylinder block to the oil distribution plate is slightly larger than the hydraulic reverse thrust Ff of the oil distribution plate to the cylinder block, that is, Fn/Ff=1.05~1.1, so that the pump works normally and maintains a high volumetric efficiency.
In fact, due to the contamination of the oil, there is often slight wear between the oil distribution plate and the cylinder block. Especially at high pressure, even slight wear can increase the hydraulic reverse thrust Ff, thereby destroying.
Common fault handling
1. The output flow of the hydraulic pump is insufficient or does not output oil
(1) Insufficient suction: The reason is that the resistance on the suction line is too large or the oil replenishment is insufficient. Such as the pump speed is too large, the liquid level in the oil tank is too low, the oil inlet pipe leaks, the oil filter is blocked, etc.
(2) Excessive leakage: The reason is that the gap of the pump is too large and the sealing is poor. For example, the oil distribution plate is scratched by metal fragments, iron filings, etc., and the end face leaks oil; the sealing surface of the one-way valve in the variable mechanism is not well matched, and the bearing surfaces of the pump body and the oil distribution plate have blisters or grinding marks. The damaged part of the pump can be identified by checking the foreign matter mixed in the hydraulic oil in the pump body.
(3) The inclination angle of the swash plate is too small and the displacement of the pump is small, which requires adjusting the variable piston to increase the inclination angle of the swash plate.
2. When the oil displacement is not zero in the neutral position, when the swash plate inclination angle of the variable axial piston pump is zero, it is called the neutral position, and the output flow of the pump should be zero at this time. But sometimes there is a phenomenon that 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, and it needs to be re-zeroed, tightened or replaced. Insufficient angular maintenance of the pump and wear of the tilt trunnion can also cause this phenomenon.
3. Output flow fluctuations Output flow fluctuations are related to many factors. The variable pump can be considered to be caused by the poor control of the variable mechanism, such as foreign matter entering the variable mechanism, and scratches, wear marks, scars, etc. are drawn on the control piston, resulting in unstable movement of the control piston. Instable control piston movement can be caused by insufficient amplifier power or damaged parts, and poor damper performance of the control piston with spring. Unstable flow is often accompanied by pressure fluctuations. Such failures generally require dismantling the hydraulic pump, replacing damaged parts, increasing damping, increasing spring stiffness and control pressure.
4. 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) The output pressure is too low When the pump is in the self-priming state, if the oil inlet pipeline leaks or there is a large leak in the hydraulic cylinder, check valve, reversing valve, etc. in the system, the pressure will not rise. This requires finding the air leak, tightening and replacing the seal, and the pressure can be increased. The overflow valve is faulty or the adjustment pressure is low, and the system pressure cannot go up. The pressure should be re-adjusted or the overflow valve should be repaired. If the cylinder block of the hydraulic pump deviates from the valve plate and causes a large amount of leakage, in severe cases, the cylinder block may be broken, and the mating surface should be reground or the hydraulic pump should be replaced.
(2) The output pressure is too high: if the circuit load continues to rise, the pressure of the pump 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, the hydraulic components other than the pump should be checked, such as directional valve, pressure valve, transmission device and oil return pipeline. If the maximum pressure is too high, the relief valve should be adjusted.
5. Vibration and noise: Vibration and noise occur at the same time. Not only do they pose a hazard to the operator of the machine, but they also pollute the environment.
(1) Mechanical vibration and noise: If the pump shaft and the motor shaft are not concentric or dead, the bearings and couplings of the rotating shaft are damaged, the elastic pad is damaged and the assembly bolts are loose, all of which will generate noise. For pumps that run at high speed or transmit large energy, check regularly and record the amplitude, frequency and noise of each component. If the rotational frequency of the pump is the same as the natural frequency of the pressure valve, resonance will be caused, and the rotational speed of the pump can be changed to eliminate the resonance.
(2) Noise generated by the liquid flow in the pipeline The oil inlet pipeline is too thin, the flow capacity of the oil inlet filter is too small or blocked, the oil inlet pipe sucks air, the oil gap is too high, the oil level is too low, the oil absorption is insufficient, and the high pressure pipeline liquid hammer, etc., will produce noise. Therefore, the fuel tank must be properly designed, and the oil filter, fuel pipe and directional valve must be selected correctly.
6. Hydraulic pump overheating: There are two reasons for the overheating of the hydraulic pump. One is the heat generated by mechanical friction. Since the moving surfaces are in a state of dry or semi-dry friction, the moving parts rub against each other to generate heat. The second is heat generation by liquid friction. The high-pressure oil leaks into the low-pressure chamber through various gaps, and a large amount of hydraulic energy loss is converted into heat energy. Therefore, the correct selection of the gap between the moving parts, the volume of the fuel tank and the cooler can prevent the excessive heating of the pump and the excessively high oil temperature. In addition, the blockage of the oil return filter will cause the oil return back pressure to be too high, which will also cause the oil temperature to be too high and the pump body to overheat.
7. Oil leakage: The main reasons for oil leakage of the plunger pump are as follows:
(1) The main shaft oil seal is damaged or the shaft is defective or scratched;
(2) The internal leakage is too large, causing the pressure at the oil seal to increase, and the oil seal is damaged or punched.
(3) The oil drain pipe is too thin and too long, causing oil leakage at the seal;
(4) The external oil pipe of the pump is loose, the pipe joint is damaged, the gasket is aged or cracked; (5) The bolt of the variable adjustment mechanism is loose and the seal is damaged;
( 6) The cast iron pump casing has blisters or poor welding.
There are many manufacturers producing plunger pumps now, and the structures of imported and domestic parts are different. Each pump should be used in strict accordance with its factory instruction manual. When repairing the pump, you should first check whether the pump is installed and used properly in the system, so as to find out the cause of damage in time, eliminate hidden dangers, and ensure the normal operation of the system. The repaired hydraulic pump should be tested by certain testing equipment before it can be used. If it does not have the detection conditions, it should also be repeatedly debugged in the system to make it work normally.
