## 3 Principle of the Hydraulic Anti-Swaying System  
A hydraulic principle diagram of the hydraulic anti-swaying system is shown in Figure 2. The hydraulic system mainly includes (1) a motor, (2) a hydraulic pump, (3) a pressure-reducing valve, (4) a check valve group (D1-D8), (5) an anti-swaying hydraulic cylinder group (A, B, C, and D), and (6) an overflow valve. For the sake of illustration, it is assumed that the swaying of the spreader is only caused by the motion of the trolley and that the initial motion of the spreader is to the right. Once the motion begins, the tensions in the anti-swaying wire ropes a and b increase from their tensions at the initial position, so a and b are called tensioning side wire ropes. The tensions in the anti-swaying wire ropes c and d decrease from their tensions at the initial position, so c and d are called slack side wire ropes. The piston rods of cylinders A and B, which are connected to the tensioning side wire ropes a and b, extend to the side with the rod cavity under the increased tension in ropes a and b. The pressures of the rod cavity hydraulic oil in A and B increase. This pressure is greater than the outlet pressure of the pressure-reducing valve (3). Thus, check valves D1 and D2 are closed, and the oil pressures in cylinders A and B open check valves D5 and D6, respectively. When the oil pressure is greater than the set pressure of the overflow valve (6), the high-pressure oil flows back to the tank (7) through the overflow valve. At the same time, as the tensions in the slack side wire ropes c and d decrease and the oil from the pressure-reducing valve (3) opens check valves D3 and D4 and enters the rod cavities of cylinders C and D, the piston rods of cylinders C and D, which are connected to the slack side wire ropes c and d, retract toward their respective rodless cavities. Low-pressure oil flows directly back to the tank so that  
Figure 2: Hydraulic principle diagram of the hydraulic anti-swaying system. the slack side wire ropes c and d can remain tensioned while check valves D7 and D8 are closed. Similarly, when the hoisting load swings to the left, an analysis can be performed in accordance with the analysis method described above. The repeated swaying of the load causes cylinders A, B, C, and D to produce the repeated motion process described above. Under the action of the check valve group (4), oil continuously spills out from the overflow valve (6), transforming the kinetic energy of the swaying load into the internal energy of the hydraulic oil buffer, thereby achieving the anti-swaying or swaying-reduction goal. If the load swaying angle is large, the increases in the tension forces of the tensioning side wire ropes are large, and more oil would spill from the overflow valve (6). In addition, under the action of the check valve group (4), the whole hydraulic system essentially controls two hydraulic circuits with different pressures: a high-pressure circuit and a low-pressure circuit. When the load swings, as mentioned above, the high-pressure and low-pressure hydraulic circuits operate simultaneously. The oil cylinder connected to the tensioning side wire ropes causes the high-pressure circuit to be generated and the high-pressure oil to flow back to the tank through the overflow valve (6), and the oil cylinder connected to the slack side wire ropes causes the low-pressure circuit (whose pressure is determined by the pressure at the outlet of the pressure reducing valve (3) to be generated and the low-pressure oil to flow directly back to the tank. The high-pressure oil circuit in the hydraulic system is involved in the work because of the swaying of the load. If the load does not sway, when the load rises, the anti-swaying wire ropes a, b, c, and d are all in a tensioned state.