The main working conditions of hydraulic excavator are excavation and loading. For these two working conditions, the lifting speed of the boom is the main factor determining the working cycle time. The faster the lifting speed of the boom, the shorter the cycle time and the higher the working efficiency of the machine. In order to improve the lifting speed of the boom, the excavator not only designs the boom lifting, stick extension and stick retraction actions to be completed by the confluence of two pumps, but also cancels the confluence function of stick retraction when the combined action of the machine requires the rapid lifting of the boom, so as to further accelerate the lifting speed of the boom. This control of the boom is called boom priority mode.
When the excavator is loading or trenching, the boom lifting and stick retraction are generally carried out at the same time. Since the boom and stick have the function of double pump confluence, the two pumps will supply oil to the boom cylinder and stick cylinder at the same time. Under the condition that the engine power allows, whichever action needs more oil, give it more oil. When the boom is lifted quickly, the engine is close to full load. At this time, if you want to lift the boom faster, you have to reduce the oil supply to the stick cylinder, that is, the "boom priority" working mode. However, the "boom priority" working mode does not mean that the boom is always in the "priority state", but only when the boom needs to be lifted quickly, the boom is in the "priority state".
The symptom of "boom priority" function failure is generally that the boom lifting speed becomes slow, and other actions of the machine are still normal at this time. This shows that there is no problem with the common part of the hydraulic system, and the fault causes can be reduced to the main hydraulic system, pilot control part of the boom cylinder and the electronic control part of "boom priority". This article will explain the reasons for the failure of "boom priority" function caused by the main hydraulic system and pilot control part of boom cylinder for reference only.
1. The boom lifting pilot control valve is not working properly
Pilot oil cannot be sent accurately to the boom control valve and boom combiner valve. It can be determined by measuring the oil pressure of the pilot oil circuit. When the pilot handle is operated slowly, the output oil pressure of the pilot digging valve shall be able to continuously change between 294-3727kpa. If not, dismantle and inspect the boom lifting pilot digging valve, check whether its components are worn, damaged or stuck, and check whether the metering spring meets the requirements.
2. Boom control valve and its confluence valve work poorly
Check the movement of the valve core for card jamming, and check the valve core, centering spring and fixing parts for problems. Repair and replace them if necessary.
3. Serious internal leakage of boom cylinder
Accurate diagnosis can be made through flow test, or the boom cylinder can be directly disassembled for inspection, and relevant parts can be replaced if necessary.
4. There is a problem with the pressure relief valve of boom cylinder oil circuit
Measure the set pressure value of the safety valve with a pressure gauge. If it is not within the range of 36800 ± 1470kpa, it shall be adjusted or replaced.
5. There is a problem with the boom anti lowering valve
Check whether the reading core is stuck or damaged, and repair or replace it if necessary.
6. Improper operation
If the boom is quickly lifted and the stick is quickly retracted at the same time, the stick control pilot oil will push the boom combiner valve spool to the neutral position, so that the boom cylinder is in the single pump oil supply state.
