Created on:2021-09-28 14:21

Lifting and transportation equipment_ Fault of power steering mechanism of truck crane

Lifting and transportation equipment_ Fault of power steering mechanism of truck crane

A Puyuan qy50h truck crane of a company had the phenomenon of steering sinking of the steering gear. It was judged that the steering power hydraulic system was faulty. After replacement, the steering performance of one side of the steering gear was improved, but the steering of the other side was not improved. The directional valve was disassembled. After disassembly, it was found that the valve body was scratched and the valve core and valve surface were damaged. After replacing the directional valve, check the hydraulic oil and find that there are metal chips in the hydraulic oil and there are many impurities on the filter element. Replace the hydraulic oil and filter element, clean the hydraulic system, and the steering system returns to normal.

(1) System principle

See Fig. B for the hydraulic schematic diagram of the power steering mechanism of the truck crane. The hydraulic oil tank of the system is installed on the right front of the engine, and the hydraulic pump is directly installed on the engine. The oil inlet filter element is located at the bottom of the oil tank, the check valve is located on the gear pump, and the change-over valve is located in the integral steering gear. The steering gear is composed of worm gear and linkage H-function change-over valve.

The hydraulic oil through the valve core and check valve enters the gear pump, which supplies oil to the steering gear. The distribution valve inside the steering gear is a three position four-way valve with H function. When it is not operated, the valve core will automatically return to the middle position and the hydraulic oil will return to the oil tank; During operation, the valve core displacement is controlled by the steering wheel to change the system flow and direction, and the pressure oil enters from the left or right side of the cylinder to realize the expansion and contraction of the cylinder piston rod, so as to push or pull the steering linkage backward to realize power steering. When the steering wheel is turned to the bottom or positioned at a certain angle, the overflow valve inside the steering gear overflows in order to limit the working pressure of the system.

(2) Fault analysis

Carefully check each component of the system in combination with the manifestation of the fault of the steering power system and the system schematic diagram. First, check the output pressure of the gear pump and find that the output pressure decreases. After a long time of use and wear, the internal leakage of the gear pump increases, and the pressure transmitted to the power assisted hydraulic cylinder decreases, resulting in the gradual sinking of the crane steering. The metal chips generated by the wear of the gear pump enter the hydraulic oil circulation system. With the passage of time, the wear is more and more serious and more debris is generated. When flowing through the directional valve, the joint surface of the valve core and valve body is scratched, the seal is damaged and the directional valve is damaged. When the hydraulic oil returned to the oil tank flows through the filter element, some impurities adhere to the filter element, resulting in damage to the filter element.

When the gear pump fails, the following phenomena generally occur: ① the pressure of the pump outlet pipe is lower than that under normal conditions; ② Turn and sink gradually; ③ The hydraulic oil in the oil tank is overturned; ④ The inspection found that the metal particles in the hydraulic oil tank increased.

When the filter element is blocked, the following phenomena generally occur: ① the hydraulic pump makes empty suction sound when sucking oil; ② Through inspection at the oil pipe at the oil inlet of the steering gear, it can be found that the oil flow pulsation in the oil pipe is strong (stable and powerful under normal conditions).

When the directional valve of the steering machine fails, the following phenomena generally occur: ① oil leakage of the hydraulic valve, easy on one side of the steering wheel and heavy on the other side; ② When normal, the steering is weak, and it is obvious that the steering wheel becomes lighter after the crane supports the vehicle.

Judgment in case of failure of booster hydraulic cylinder: ① external leakage or internal leakage, which is mainly caused by seal wear and oil channeling; ② After troubleshooting the pump and steering gear, remove the ball joint of the crane steering gear and use the power assisted hydraulic cylinder for steering. If the hydraulic cylinder does not act or acts slowly, it can be judged as oil channeling of the hydraulic cylinder; ③ Remove the fixing device on one side of the hydraulic cylinder, the steering wheel is in the middle position, and manually push and pull the hydraulic cylinder. If the hydraulic cylinder can be easily pushed, it is proved that the internal oil channeling (the oil channeling of the valve core should be eliminated first).

(3) Improvement

In order to prevent similar failures of this type of truck crane in the future, the following work should be done: ① regularly check and replace the filter element; ② Check the hydraulic oil regularly and replace it in time if there are many debris and serious pollution of the hydraulic oil; ③ The truck crane with difficult steering shall be repaired in time to prevent debris after the gear pump is damaged and further expand the fault range; ④ Consider placing a magnet device in the steering system oil tank to absorb metal chips.

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