Research On Closed Hydraulic System Of Crawler Crane In Over-running Load Condition

Hoisting mechanism is such an important part of the crawler crane that it has an effect on the whole crane performance. The quality of the hoisting mechanism decides the service life and reliability of the crawler crane, and even more important for large tonnage crane, so hoisting mechanism often is one of the key points in the crane hydraulic system design. Closed hydraulic system has compact structure and moves smoothly, it is easy to realize the electrical control and convenient to place the components, so it is widely used in foreign large tonnage cranes, but at home the technology is not mature, and there are still many unsolved questions. Therefore, further research is necessary and it is meaningful to promote the development of Chinese hoisting machinery to large size and intelligentized.Based on the closed hydraulic system test platform of DUT, the characters and the control strategy of the closed hydraulic system in over-running load condition are studied in this paper, and the reliability of the control method is proved by simulation. The main content is as follows:(1) The design requirements of the hoisting mechanism hydraulic system is summed up, the schematic diagram and the difference between the open loop hydraulic system and the closed hydraulic system are analyzed.(2) Characters of the closed hydraulic system in over-running load condition are researched:The function of the motor and the pump exchange during lowering the load, and high pressure oil flows from the motor, meanwhile the engine works as a brake, the speed of the engine and the motor increases with an increase of the load. According to these, control strategy is given:when the engine speed is too fast, reduce the displacement of the pump or increase the motor’s displacement to keep the motor speed within the limit.(3) The closed hydraulic system test platform and the parameters are introduced. The simulation model in over-running load condition based on Simulink is built, and the system dynamic character with different load and the factors which affect the system are studied. The conclusion is:the pipe length can change the damping ratio of the system. The longer the pipe is, the smaller the damping ratio is and the bigger the overshoot is. The leak coefficient has significant influence on the steady-state error and the damping ratio. (4) The closed loop and negative feedback control is given based on the test platform, and the simulation results show that it works in heavy load and light load, but oscillation appears in medium load. After adding the phase-advance compensating, the oscillation is eliminated and the control results become better. The control method is improved on the basis of the expert control and the response time of the system declines.