Analysis and Countermeasures for the "Crawling" of QT60 Tower Crane

The QT60 tower crane used at the construction site often experiences a so-called crawling phenomenon: when the tower body is undergoing adjustment work, the hydraulic cylinder and the components it bears alternate between fast and slow speeds, deceleration and stopping, generating vibration. This situation can cause impact fatigue damage to the tower crane, and in severe cases, even lead to breakage accidents.

1. Investigation of the causes of the "crawling" phenomenon

The occurrence of the "crawling" phenomenon is mainly caused by improper design of the hydraulic system. However, it is often difficult to detect this problem at the beginning. Instead, it is discovered through the inspection of a series of hydraulic components and the application of the "gradual approach to failure" principle. The diagnostic results indicate that the intermittent vibration phenomenon generated when the hydraulic cylinder and load of the tower face descend is not due to the reduction of the hydraulic friction force characteristics and the stiffness of the hydraulic system, nor is it related to the air existing in the pipeline during the pressure adjustment of the system. The "crawling" malfunction may be related to a certain circuit in the hydraulic system.

After testing, the rated working pressure of hydraulic pump 3 is 25MPa. The output oil volume and pressure of the pump are both normal. The maximum pressure of the relief valve 5 control system has been tested and the pressure adjustment is normal. No abnormal situation occurred when the hand hollow directional control valve 6 was reversing. The bidirectional hydraulic lock 7 formed the balance locking circuit of the system. From this perspective, the crux of the problem lies in the bidirectional hydraulic lock 7 -.

Analysis of the Balance Circuit of the Bidirectional Hydraulic Lock system

Due to the function of the bidirectional hydraulic lock 7, when the manual directional control valve 6 is in the middle position, the hydraulic lock is in the unloading state, and the hydraulic cylinder is only tightly locked at any position. When the directional control valve 6 is in the right position, the bidirectional hydraulic lock 7 opens and enters the lifting condition. At this time, due to the resistance of the lifting load to the pressure in the rodless oil chamber, there will be no "negative pressure" phenomenon in the left liquid chamber one-way valve B Therefore, no vibration will occur. When the manual directional control valve 6 is in the left position descending condition, the hydraulic oil from the oil tank pushes open the hydraulic lock and enters the rod cavity of the hydraulic cylinder. Under the pressure of the hydraulic pressure and the load weight, the piston of the hydraulic rod drops rapidly, and then a vibration phenomenon occurs.

When the hydraulic cylinder 8 vibrates, the pressure also vibrates. The variation law is very similar to the intermittent vibration law and the opening and closing law of the one-way valve b through which the return oil of the actuator passes. Therefore, it can be determined that the vibration generated when the hydraulic cylinder 8 descends is closely related to the opening and closing pressure of the one-way valve B.

When the piston and load drop rapidly, due to the excessive speed, the hydraulic pump 3 supplies oil too quickly. The oil supply of the hydraulic pump cannot be replenished in time, resulting in a short-term "negative pressure effect" caused by the rod 倥. At this time, the hydraulic pressure relief of the one-way valve B of the bidirectional hydraulic lock 7 also drops, and the hydraulic one-way valve B is controlled to close. As a result, the oil passage is blocked, and the hydraulic cylinder 8 suddenly stops descending. After the hydraulic check valve B is closed, due to the continuous oil supply from the quantitative hydraulic pump, the oil pressure rises again. The hydraulic check valve B opens once more, ensuring smooth oil return. The piston and load drop rapidly. This cycle repeats until the end of the stroke. The opening and closing of the check valve cause the hydraulic cylinder to vibrate, resulting in the occurrence of the "crawling" phenomenon.

Improvement of the bidirectional liquid lock balance locking circuit

The function of the bidirectional hydraulic lock is to lock when no oil is entering and limit the speed when descending. The improved method is: Keep the liquid chamber check valve A in Figure 1 unchanged. It is only used in the descent circuit to control the rise from being too fast. This situation rarely occurs: By changing the hydraulic control check valve B to a relief valve as a back pressure valve, and to prevent flow loss when the hydraulic cylinder rises, a check valve can be connected in parallel at the same time. In fact, this forms a hydraulic balance valve (speed-limiting valve). The hydraulic system balance circuit can effectively prevent "crawling", ensuring the entire mechanism descends smoothly at a uniform speed. At the same time, it can also make the mechanism pause or make slight movements as required by the working conditions.

4 Conclusion

The elimination of the "crawling" phenomenon and the improvement of its structure of the QT60 tower crane have both adopted advanced technologies from the "Structural Design of New Hydraulic Balance Valves for Construction Machinery", significantly enhancing its operational stability and safety under lifting and luffing conditions, and effectively preventing the occurrence of potential accidents.