Construction and construction machinery_ Hydraulic system failure of concrete pump
Construction and construction machinery_ Hydraulic system failure of concrete pump
The IPF-85B concrete pump truck of a company suddenly broke down when pumping large amount of concrete in road and Bridge Construction: the engine speed of the pump truck decreased from 1800r / min to l500r / min when pumping without load; When the main hydraulic cylinder is reversed, the engine speed drops to 1100r / min. at this time, even if the boom pipe is horizontal, the concrete cannot be pumped out.
The principle of the main hydraulic system of the machine is shown in Figure a. Some people believe that the fault is caused by insufficient power of the engine and internal leakage of the main pump. Therefore, the road test of the pump truck is carried out first. The results show that the pump truck can still run continuously at 60km / h, indicating that the engine power is good; The main oil circuit of the machine is forcibly boosted, that is, the manual reverse valve 4 is closed, and the pressure of the main hydraulic system can reach 28Mpa, which is consistent with the specified pressure value, so the possibility of failure of the main pump is denied; The final confirmed fault shall be on the hydraulic element of the main oil circuit.
The main reasons for the decrease of engine speed during pumping and reversing of main hydraulic cylinder are as follows.
① The output pressure is too low. The standard output pressure of pressure reducing valve 9 is 21MPa. After passing through the pressure reducing valve, the pressure oil of the main pump controls the reversing of booster valve 15 through pilot directional valve 16, so as to reverse gate valve 14 and main directional valve 5. When the output pressure of the pressure reducing valve decreases, it not only affects the charging capacity and speed of the hydraulic system to the accumulator 12, but also slows the reversing of the booster valve, resulting in the untimely reversing of the piston of the main hydraulic cylinder 6 after it is in place, which increases the load of the main pump, which is manifested in the decrease of engine speed and the increase of oil pressure during reversing.
② The nitrogen pressure is insufficient, and the standard air pressure of the accumulator is 7MPa. Its main function is to establish a preload to ensure that the gate valve hydraulic cylinder 13 acts before the main hydraulic cylinder, and to ensure the rapid reversing of the gate valve reversing valve and the main reversing valve. After the nitrogen pressure decreases (generally lower than 5MPa), the pre pressure of the hydraulic system decreases. Therefore, the pressure oil from the main pump must first pressurize the accumulator to make up for the pressure loss after each step-up valve is replaced. During the pressurization of the oil supply circuit, the main hydraulic cylinder and the gate valve hydraulic cylinder cannot be reversed in time due to the failure of the gate valve reversing valve and the main reversing valve to act in time, As a result, the load of the main pump increases, so the engine speed decreases during commutation.
③ The function of piston pull-out valve 10 is to pull out the piston rod of the concrete cylinder. When there is slight internal leakage, the pressure difference between the rod free chamber and the rod chamber of the main hydraulic cylinder decreases, and the resistance increases during the action of the main hydraulic cylinder, increasing the load of the main pump, resulting in the decrease of engine speed during pumping.
④ The throttle hole of the main directional valve is not unblocked. The function of the throttle hole is to balance the pressure difference on both sides of the main directional valve 5 to buffer the impact force when the main directional valve is reversing. The unobstructed orifice can make the pressure difference on both sides become very small, so as to slow the reversing of the main reversing valve, increase the load of the main pump, and reduce the engine speed when the main hydraulic cylinder is reversed.
⑤ The pressure oil from the main pump is stuck in the valve core of the booster valve, the valve core of the gate valve directional valve or the valve core of the main directional valve. After passing through the pressure reducing valve, one way causes the booster valve to act, and the other way pressurizes the accumulator. After the booster valve acts, it causes the gate valve directional valve and the main directional valve to act. Once the valve core is stuck, it will cause slow commutation and reduce the engine speed.
According to the above reasons, it was found that the fault was caused by the stuck valve core of the main directional valve. The valve core is composed of a large valve core and an inlaid small valve core, and its structure is shown in Figure B.
In the process of disassembly and inspection, it was first found that the left and right push of the valve core was not flexible. After gently vibrating out with a copper rod, it was found that the surface of the valve core was worn normally, but one half of the elastic pin 3 was higher than the other half and flush with the surface of the valve core, with obvious friction marks, and the other end of the pin was trapped in the valve core. There is also an obvious bright sliding track on the surface of the valve body and the corresponding area of the column pin. After gently vibrating the valve core, half of the elastic column pin falls out (as shown in 4 in Figure b). Therefore, it is concluded that the fault is caused by the broken piece of the pin, because the broken piece of the pin always tends to fall off in the left and right movement of the valve core, resulting in friction with the inner wall, resulting in two consequences: the valve core is stuck during commutation, resulting in slow commutation and the decrease of engine speed; The spool does not move in place due to the stuck fragment, resulting in poor oil supply in the main oil passage and abnormal increase of oil pressure, that is, the engine speed decreases during the commutation of the main hydraulic cylinder. These two inferred consequences are consistent with the fault symptoms. After removing half of the fragments, the valve core and inner wall are polished, so that it is easier to push the valve core by hand after it is installed. After assembly, the test run shall be carried out. During no-load pumping, the above faults disappear, the engine speed difference is always within ± 50R / min, and the working frequency of the main hydraulic cylinder returns to normal; During the concrete pumping test, when the hydraulic oil temperature rises to 50 ℃ after pumping 500m3 concrete, the operation of each system is still normal. All these show that the root cause of the fault is the falling off of the fragments of the elastic pin.
