Two cases of displacement adjustment faults of Schwing concrete pumps

1. Schwing KVM37/P2023 concrete pump truck

1.1 Fault Phenomenon

The pumping displacement cannot be adjusted and has always been maintained at the maximum displacement (concrete output 96 m ³ /h, stroke frequency per minute: 19 times). When switching the electric control/manual control conversion switch on the displacement control valve block, it was found that neither the electric control nor the manual control method could change the displacement.

1.2 Fault Diagnosis Process

(1) Analysis of the hydraulic system diagram reveals that the displacement control valve block is equipped with two identical pressure reducing valves (both with a pressure output range of 3.5 to 55bar). One of the pressure reducing valves is used to regulate the pressure of the control oil circuit for controlling the displacement of the main oil pump, while the other serves as a buffer. The initial judgment is that the pressure reducing valve used to regulate the main oil pump and control the oil circuit pressure is damaged, resulting in the inability to regulate the control oil circuit pressure and thus the inability to adjust the displacement of the main oil pump.

(2) The pressure reducing valve for regulating the main oil pump and controlling the oil circuit pressure was disassembled and cleaned. No abnormalities were found.

(3) After disassembling both pressure reducing valves for cleaning and reinstalling them in their original positions, both the electrically controlled and manually controlled displacement adjustment returned to normal.

(4) Reinstall the two pressure reducing valves to their original positions. Use a pressure gauge to test the pressure at the MX1 port (the oil circuit for controlling the oil pump displacement) on the displacement buffer valve block. When adjusting the displacement, it was found that the pressure value remained unchanged at 4bar (the normal value should vary within the range of 3.5 to 55bar). Therefore, it was further confirmed that the pressure reducing valve for regulating the pressure of the main oil pump control oil circuit was damaged.

(5) After replacing the pressure reducing valve, the pump truck was tested and everything was normal.

2 Schwing BP2800HDR Concrete Drag Pump

2.1 Fault Phenomena

The pumping displacement cannot be adjusted to the maximum and has remained at 17 to 18 strokes per minute (the standard maximum stroke is 31 to 32 strokes per minute).

2.2 Fault Diagnosis Process

(1) The main system pressure was detected to be 280bar, and the system pressure is normal.

(2) When the normal pumping stopped, it was observed that the piston rod did not retract, ruling out the possibility of a decrease in displacement caused by leakage in the main cylinder.

(3) Use a pressure gauge to test the pressure at port MX1 (the oil circuit that controls the oil pump displacement) on the displacement buffer valve block. When adjusting the displacement, it is found that the pressure variation range is between 4 and 56bar, indicating that the pressure reducing valve for regulating the main oil pump to control the oil circuit pressure is working properly.

(4) Disassemble and clean the valve core of the control slide valve on the main oil pump (remember the number of thread turns of the displacement adjustment range knob before disassembly for restoration). After cleaning the valve core, reinstall it and restore it, but the fault still persists.

(5) After turning the displacement adjustment range knob in two turns and conducting a test run, the displacement of the tow pump increased. Therefore, adjust the displacement adjustment range knob until the maximum stroke of the pump is 32 times per minute. The fault has been resolved.

2.3 Analysis of Fault Causes

(1) The return spring of the valve core becomes fatigued and soft, or for other reasons, the valve core of the control slide valve becomes loose, resulting in no preload force on the spring.

(2) The hydraulic oil was contaminated, and the dirt entered the control slide valve, causing the valve core to get stuck.