Solutions to the failure of safe unloading of crawler cranes

Solutions to the failure of safe unloading of crawler cranes

Crawler cranes often adopt pilot control hydraulic systems, which are comfortable to operate and have good micro-movement performance. This system adopts a pilot control oil circuit safety unloading method to prevent possible dangerous working conditions such as overloading of the main and auxiliary hoisting winches or over-luffing of the truss boom. The principle is shown in Figure 1. That is, through the combined action of the torque limiter, overload unloading solenoid valve and logic valve or limit switch, overtilt unloading solenoid valve and logic valve, it is ensured that safe unloading can be reliably achieved when overload or overtilt misoperation occurs, and accidents can be completely prevented.

1. Oil pump 2. Main valve 3. Luffing hoist motor

4. Auxiliary hoisting winch motor 5. Main hoisting winch motor

6. Overload unloading solenoid valve 7. Overtilt unloading solenoid valve

8. Logic valve 9. Pilot handle 10. Oil tank

During the safety test of a crawler crane in the trial production process, it was found that although the indicator lights on the control panel in the control room had already given an alarm when performing overload or overtilt operations, all operations could still continue, indicating that the safety unloading of its safety protection device had failed.

An analysis of this issue shows that faults in either the electrical system or the hydraulic system can lead to the failure of safe unloading. So, the inspection was carried out from both the circuit and the liquid circuit aspects. The purpose of circuit detection is to check whether the torque limiter and limit switch, while providing alarm signals to the panel indicator light, can also transmit electrical signals to the unloading solenoid valve. If they can, the unloading circuit will be connected to achieve safe unloading; otherwise, the safe unloading will fail. According to the schematic diagram, before the torque limiter provides an electrical signal to the unloading solenoid valve, the unloading solenoid valve is in a constant power-off state, that is, the unloading circuit is disconnected. However, once the torque limiter provides an electrical signal to it, the unloading solenoid valve is energized, reversing, and the unloading circuit is connected. For this reason, the pipeline on one side of the logic valve (T1 port) was disconnected. When the operation was carried out, it was found that when the torque limiter did not issue a command, the overload unloading solenoid valve Y1 had no power, the solenoid valve did not reverse direction, and no pressure oil flowed out. However, when the machine entered the overload condition, the valve was powered on and hydraulic oil immediately flowed out of the pipeline. This indicates that the program for the torque limiter to issue logical instructions is normal, and the unloading solenoid valve is also working properly. Similarly, when the limit switch for controlling overtilt unloading and the overtilt unloading solenoid valve Y2 were inspected, they also worked normally. Therefore, the possibility of a circuit fault was ruled out.

After checking the circuit, the liquid circuit was inspected. First, the pipe layout was checked and no errors were found. Finally, the attention was focused on the logic valve. It was suspected that the spring of the check valve inside the logic valve was too hard or the diameter inside the valve was too small, which caused a large back pressure in the unloading oil circuit, resulting in an unsmooth unloading oil circuit and difficult unloading. So, under the alarm state, the pressure at the an or bn port corresponding to the action was measured. It was found that the pressure was about 1MPa, while the main valve only needed a pilot pressure of approximately 0.6 MPa to open. This proved that the suspicion was correct. It was precisely because the back pressure inside the logic valve was relatively large that unloading was difficult.

After identifying the problems existing in the logic valve, it was considered that if the valve was redesigned, not only would the cycle be long, but the cost of the product would also increase significantly. To shorten the product development time and save costs, it was ultimately decided to adopt an "unexpected complication" approach to solve this problem (as shown in Figure 2). That is, without changing the structure of the logic valve, three-way joints are respectively installed on the six control oil ports of the valve. Then, six pipelines are led out and respectively connected to two sets of three-way check valves. Finally, the two pipelines passing through the three-way check valves are respectively connected to the two unloading solenoid valves. These two unloading solenoid valves have the same function as the unloading solenoid valves on the logic valve, namely the overload unloading solenoid valve and the overtilt unloading solenoid valve. Moreover, these two unloading solenoid valves and the two unloading solenoid valves on the logic valve form a parallel relationship in both the circuit and the hydraulic circuit, constituting two groups of solenoid valve groups with the same protection function. During operation, once the torque limiter or limit switch sends an electrical signal, the parallel unloading solenoid valve group with the same function will simultaneously reverse direction to ensure normal unloading, quickly cut off the main oil circuit, and prevent the crane from continuing to work under dangerous conditions such as overload or overtilt.

The improved test results were very good. While the panel indicator light is sounding the alarm, overloading or overtilt operations cannot continue, achieving the purpose of safety protection.