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Troubleshooting Concrete Machinery: Unstable or Suddenly Zero Reversing Pressure During Pumping
Release time:
2026-08-14
Data source:
Malfunctions are inevitable during the daily operation of concrete pump trucks. Naturally, these are the moments that cause the most headaches for pump operators.
Malfunctions are inevitable during the daily operation of concrete pump trucks. Naturally, these are the moments that cause the most headaches for pump operators.
Keywords: Concrete pump parts, constant-pressure pump, accumulator, swing cylinder, main hydraulic pump
Applicable equipment types: Trailer concrete pumps, concrete pump trucks, concrete line pumps.
Fault symptoms:
1. Reversing pressure suddenly drops to zero;
2. Reversing pressure drops in stages, eventually falling to zero;
3. Pressure is unstable, fluctuating between high and low levels;
4. After reversing, the pressure fails to reach the set value and stalls at a certain level; upon the second reversal, the pressure either continues to drop (sometimes reaching zero) or continues to rise (sometimes reaching 19 MPa).
The distribution system consists of a constant-pressure pump (power element), two oscillating cylinders (actuators), an intermediate control valve assembly (control elements), and other accessories.
The constant-pressure pump supplies pressurized oil to the system to actuate the swing cylinders, while an intermediate control valve assembly governs the operating sequence of the distribution cylinders. Because the S-valve requires rapid switching, the flow rate from the constant-pressure pump is insufficient during the instant of switching; therefore, an accumulator is incorporated into the system to supplement the flow at that moment. During switching, the constant-pressure pump operates at full displacement and the accumulator releases energy, causing a drop in system pressure. Once the switching action is complete, the constant-pressure pump recharges the accumulator; upon reaching the set pressure, the pump's variable displacement mechanism adjusts the swash-plate to a zero-angle position.
When troubleshooting the aforementioned symptoms, first check the system for relief valve operation (overflow). This can be determined by the following:
1. Check for a rise in system oil temperature; typically, the temperature of the relief component—or even the entire system—will increase.
2. Listen for the sound of relief flow (which usually produces audible noise) and monitor the pressure gauge; upon immediate shutdown, observe whether the switching pressure drops excessively fast.
3. If none of the above symptoms are present, the issue likely lies with the constant-pressure pump. This is a variable-displacement pump: it delivers maximum flow during S-valve switching (with the swash-plate tilted to its maximum angle) and reduces output to near-zero once the set pressure is reached (returning the swash-plate to the neutral position). The pump repeats this cycle with every switch—operating at twice the frequency of the main pump in a closed-loop hydraulic system—making components such as the servo mechanism, valve plate, and pressure cut-off valve prone to wear.
If the swash-plate fails to reach the maximum position during switching, or if pressure cut-off occurs prematurely, pressure instability results. Replacing the pump restores normal operation.
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