My company's Chutian IPF-85B concrete pump truck in the road and bridge construction pumping generous volume of concrete suddenly the following failure: pump load in the no-load engine speed from 1800r / min down to 1500r / min; Wang Hydraulic Cylinder commutation engine speed dropped to 1100r / min, this time even if the boom tube in the horizontal state, concrete can not pump. The machine's main hydraulic system principle shown in Figure 1. Maintenance personnel believe that this failure is the lack of engine power and the main pump caused by internal leakage. To this end, we first carried out a road test of the pump truck, the results of the pump truck at 60km / h case can still continue driving, indicating that the engine power is good; and then we forced the main circuit of the machine to step up pressure, Reverse valve 4, the main hydraulic system pressure up to 28MP. In line with the provisions of the pressure value, denied the possibility of failure of the main pump; the final confirmation of the fault should be the hydraulic components of the main oil. The following causes of failure for an analysis, and describes the troubleshooting method. The main reasons that can cause the engine speed to drop during pumping and the engine speed to drop when the master cylinder commutates are: (1) The output pressure of the pressure reducing valve is too low. The standard output pressure of the pressure reducing valve 9 is 21 MPa. , The main pump pressure oil after the pressure reducing valve through the pilot valve 16 to control the reversing valve 15, so that the valve 14 and the main valve reversing valve 5 direction. When the relief valve output pressure is reduced, not only affects the hydraulic system of the accumulator 12 of the pressure capacity and speed, but also makes slow pressure valve slowed, resulting in the main hydraulic cylinder 6 in place after the piston commutation and then increase The main pump load, manifested as reversing the engine speed drop, hydraulic pressure rise. (2) Insufficient nitrogen pressure in accumulator The standard pressure of accumulator is 7MP. , Its main role is to establish a preload to ensure that the valve cylinder 13 prior to the action of the main hydraulic cylinder and valve to ensure that the main reversing valve and between the rapid reversing direction. After the nitrogen pressure is reduced (generally lower than 5Mpa), the hydraulic system pre-pressure is reduced, so after each step-up valve commutation, the pressure oil from the main pump must first charge the accumulator to make up for the pressure loss, Road filling process due to the valve to the valve and the main can not be changed Yan Yan timely action, leading to the main hydraulic cylinder and the valve cylinder can not be reversed in time, resulting in increased load on the main pump, resulting in the reversing of the engine speed decreased phenomenon . (3) Piston draw valve discharge piston pull valve pull-out role is to pull out the check cylinder piston rod, when it appears mild internal leakage, the main cylinder rodless cavity and rod chamber pressure drop, so that the main Hydraulic cylinder resistance increases in the process of action, so that the main pump load increases, resulting in pump down the phenomenon of the engine speed. (4) The main valve orifice is not smooth The role of the orifice is to balance the pressure difference across the main valve 5 to buffer the impact of the main reversing valve commutation, the orifice is not smooth on both sides Of the pressure becomes very small, so that the main valve reversing slow, causing the main pump load increases, causing the main cylinder to reverse the engine speed decreases. (5) the valve poppet, gate valve spool valve or the main valve is stuck by the main pump to the pressure oil through the valve after the pressure relief valve moves the other way to the accumulator Pressure, and after the action of the step-up valve to promote the valve to the valve and the main valve action, once the valve card, it will result in slow to change, causing the engine speed to drop. According to the above reasons - an investigation, and ultimately found fault is the main valve spool is caused by stagnation. The spool is composed of a large spool and a small embedded in the valve spool, the structure shown in Figure 2. In the dismantling process, we first found that the spool around to promote inflexible, with a steel rod gently shock and found that the spool surface are normal wear, but one end of the elastic pin 3 is higher than the other half of the other half and the spool surface Flush, there is a clear friction of the trace, while the other end of the pin is trapped in the spool inside. The inner surface of the valve body also has an obvious bright sliding track in the area corresponding to the pin. After the valve core is gently shaken, the elastic pin is dropped out of the half segment (as shown in Figure 4, 4). We conclude therefrom that the failure of the pins of the pin, because the pin fragments in the spool left and right movement there is always there shedding trend, which caused friction with the wall, resulting in two consequences: when the valve spool commutation, leading to change To slow, causing the engine speed to drop; valve spool due to the block after the reversal of the movement is not in place, resulting in poor lubrication of the main oil gallery, abnormal increase in oil pressure, which is the main cylinder reversing process of engine speed decline. Both of these consequences are consistent with our symptoms of failure. After we cleared the half section of the chip and the valve core and the inner wall was also sharpened, so that the spool fitted with a hand when feeling more relaxed. Assembled after the test machine, no load pump failure disappears above, the engine speed is always at. ± 50r / min, the working frequency of the main hydraulic cylinder also returned to normal. During the gun pumping test, the systems work normally even after the pump pressure is raised to 500ºC and the hydraulic oil temperature rises to 50ºC. All of this shows that the root cause of the failure is the break-off of the elastic pins.