Causes and maintenance of the failure of the "boom Priority" function of excavators

The main working conditions of hydraulic excavators are excavation and loading. For both of these 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. To enhance the lifting speed of the boom, the Caterpillar OB series excavators not only design the boom lifting, boom extension and boom retraction actions to be completed by the combined flow of dual pumps, but also cancel the combined flow function of boom retraction when the boom needs to be lifted quickly through the combined action of the machines, further accelerating the lifting speed of the boom. This control method for the boom is called the "boom priority" mode.

The principle and implementation conditions of "boom priority"

When an excavator is performing loading or trenching operations, the lifting of the boom and the inward retraction of the boom are usually carried out simultaneously. As both the boom and the boom have a dual-pump combined flow function, the two pumps will supply oil to the boom cylinder and the boom cylinder at the same time. Under the condition that the engine power allows, more oil will be supplied to the action that requires more oil. When the boom is lifted rapidly, the engine is close to full load. At this time, if you want the boom to lift faster, you have to reduce the fuel supply to the boom 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". It only puts the boom in the "priority state" when it needs to be lifted quickly.

The hydraulic actuator with "boom priority"

When the boom is lifted, the pilot oil of the pilot control valve for lifting the boom causes the boom control valve 2 to act, guiding the oil from the upper main pump to the large chamber of the boom cylinder. When the oil pressure of the pilot oil exceeds 2250kPa, the boom convergence valve operates, guiding the oil from the lower main pump to the large cavity of the boom cylinder. At this time, the boom cylinder is in a state of dual-pump convergence oil supply, causing the boom to rise rapidly. In terms of the control system, when the "boom Priority" button is pressed, when the oil pressure of the boom lifting pilot oil exceeds 2646±196kPa, the boom priority pressure switch installed on the pilot oil pipeline closes. After the computer receives the closing signal, the fine control solenoid valve is energized, and the pilot control oil circuit of the boom manifold is connected to the return oil circuit of the system. At this time, the boom manifold is in the neutral position. In this way, even if the boom is retracted inward during the rapid lifting process of the boom, it can still ensure that the boom rises quickly.

The reasons for the failure of the "Boom Priority" function and the troubleshooting methods

The symptoms of the failure of the "boom priority" function are generally manifested as a slower lifting speed of the boom, while at this time the other movements of the machine are still normal. This indicates that there is no problem with the common parts of the hydraulic system. The cause of the fault can be narrowed down to the hydraulic main system of the boom cylinder, the pilot control part, and the "boom priority" electrical control part.

The main hydraulic system and pilot control section of the boom cylinder

(1) The pilot control valve for the boom lifting is not working properly

The pilot oil cannot be accurately delivered to the boom control valve and the boom convergence valve. It can be determined by measuring the oil pressure of the pilot oil circuit. When operating the pilot handle slowly, the output oil pressure of the pilot excavator valve should be able to vary continuously between 294 and 3727kPa. If not, the pilot excavator valve of the boom lift should be disassembled and inspected to check if its components are worn, damaged or stuck, and also to check if the metering spring meets the requirements

(2) The boom control valve and its combined flow valve are not working properly

Check whether there is any jamming during the movement of the valve core, and whether there are any problems with the valve core, centering spring and fasteners. Repair or replace them if necessary.

(3) There is severe leakage inside the boom cylinder

Accurate diagnosis can be made through flow testing, or the cylinder can be directly disassembled for inspection. If necessary, relevant parts should be replaced.

(4) There is a problem with the pressure safety valve of the 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 should be adjusted or replaced.

(5) There is a problem with the boom anti-drop valve

Check if the reading core has any jamming or damage. Repair or replace it if necessary.

(6) Improper operation

If the boom is lifted rapidly and the boom is retracted quickly at the same time, the pilot oil controlled by the boom will push the valve core of the boom combined valve to the neutral position, thus putting the boom cylinder in a single-pump oil supply state.

2 The electronic control part with "boom priority"

(1) There is a problem with the monitoring board

Press the "Boom Priority" button on the monitoring board and confirm whether the "Boom Priority" indicator light is on. If the indicator light does not light up or is unstable, the monitoring board should be replaced.

(2) The pressure switch fails to send the correct signal to the computer

By taking advantage of the function of the computer to display the working status of some switch components, the switch can be inspected. A multimeter can also be used to check whether there is a short circuit or open circuit in the circuit between the pressure switch and the computer, as well as whether there is any problem with the switch itself. If there is a problem, it should be eliminated. If a multimeter and a pressure gauge are used in combination, the pressure switch can be precisely judged. The closing pressure of the pressure switch should be 2646±196kPa, and the return pressure should be 2156±196kPa.

(3) The fine control solenoid valve 9 does not operate normally

① Check the solenoid valve circuit. Start the engine and select the "boom priority" working mode. When the boom control handle is in the middle position, the solenoid valve loses power. When the handle is pulled to the full-speed lifting position of the boom, the electromagnetic trigger is triggered. If the test results are opposite, it indicates that there is a problem with the computer and it should be replaced. If there is no power all the time, it indicates that the circuit between the electromagnetic alarm and the computer is open or grounded. If there is always electricity, it indicates that the circuit is short-circuited to the power supply. A multimeter can be used to identify the cause of a short circuit or open circuit and eliminate it.

② Check the solenoid valve. Check the output pressure of the solenoid valve with a pressure gauge. That is, keep the bucket cup in the retracted position and lift the boom at full speed. At this time, the output oil pressure of the solenoid valve should be greater than 3727kPa. When the boom handle is in the neutral position, the output oil pressure of the solenoid valve should be less than 294kPa (i.e., the return oil pressure of the hydraulic system). If not, check whether the valve core of the solenoid valve is stuck or worn and whether the electromagnetic coil meets the requirements.

(4) The solenoid valve for the walking speed operates abnormally

The function of this valve is to control the fast and slow states of the walking motor. When power is lost, the walking motor is in a low-speed state. After being powered on, the motor is in a high-speed state. However, when it is powered on, the pressure oil it outputs will simultaneously push the limit valve core of the boom confluence valve, keeping the boom confluence valve in a neutral position, and the function of the boom dual-pump confluence will thus disappear. In the "Compaction" mode, the computer also powers it to slow down the movement of the boom. Therefore, in the "boom priority" state, the walking speed solenoid valve should be out of power. The inspection and troubleshooting of the walking speed solenoid valve can be carried out by referring to the method for checking the fine control solenoid valve.

As there are many factors that can cause the failure of the "boom priority" function, when troubleshooting, it is necessary to carefully inspect the relevant parts. Since circuit inspection is relatively simple and convenient, one should start from the circuit aspect.