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I. Functions of the hydraulic coupler 1. It has the capability to reduce shocks and isolate torsional vibrations； It allows for a delay in motor startup, enabling gradual acceleration and reducing the impact between components that can occur due to sudden starting. 2. It possesses the capability to enable the motor to start under light load: Since the torque of the pump impeller in a coupling is proportional to the square of its speed, at the moment of startup the torque generated by the impeller is very low due to the low speed; as a result, the motor starts almost as if it were running without the impeller, leading to a short startup time, low startup current, and smooth starting. This makes it particularly suitable for starting heavy loads with high inertia. 3. Overload protection capability: Since there is no direct mechanical connection in the coupling, when the external load exceeds a certain limit, the torque on the pump wheel stops increasing. The motor continues to rotate as usual, but its output slows down until it stops. The power drawn from the power source is converted into heat, which raises the temperature of the coupling; once the fuse melts and fluid is released, the input and output are disconnected, thereby protecting the motor and the machine from damage. This reduces the incidence of machine failures, maintenance costs, and downtime, and extends the service life of both the motor and the machine. 4. Energy-saving performance: Since the coupler effectively addresses the issues of motor startup and the situation of using a powerful motor to drive a light load, it allows for the use of a motor with a lower horsepower rating compared to rigid transmission systems. Additionally, it reduces the starting current and its duration, thereby minimizing the strain on the power grid. The energy savings amount to 10–20%, especially when starting heavy loads with high inertia. Features of the torque-limiting hydraulic coupling: apart from the bearings and oil seals, there is no mechanical friction; it has a long service life, a low failure rate, and requires no special maintenance. II. Use and Maintenance of Variable-Speed Hydraulic Couplings 1. Pre-startup Checks (1) Check the oil level indicator to confirm that the oil level is appropriate ； (2) Check whether the coupler and cooler pipelines are connected correctly ； (3) Check whether the electrical circuits of each instrument are properly connected ； (4) Check whether the coupling and protective cover are installed correctly ； (5) Check whether the oil level in the coupler’s oil tank is appropriate; when the oil temperature is below 5°C, an electric heater should be used to heat the working oil ； (6) Check whether the coupler conduit is set to the low-speed mode. 2. Operation: (1) The coupling is equipped with an electric actuator for manual operation. Manual or automatically controlled electric actuators are used to adjust the position of the conduit and change the level of fluid in the coupling chamber, thereby altering the coupling’s output speed and output torque. (2) When the coupler’s conduit is fully inserted (0% position), the speed is low; when the conduit is fully withdrawn (100% position), the speed is high (reaching the rated speed and rated power). When adjusting the valve opening from 0% to 100%, the speed should not be too fast, usually within 25 seconds. (3) The speed control range of the coupler varies depending on the working machine. When matched with centrifugal machinery, the speed control range is 1-1/5; when matched with constant-torque machinery, the speed control range is 1-1/3. (4) When a variable-speed hydraulic coupling is paired with a centrifugal machine, the condition of heat generation occurs at a speed ratio of i=0.66, and the power loss due to heat generation is approximately 15% of the motor’s rated power. Therefore, in use, long-term operation near heat generation points should be avoided as much as possible. (5) Constant-torque machinery is equipped with a variable-speed coupling, where the speed ratio i is equal to the efficiency; that is, the greater the speed regulation, the greater the power loss and the more heat is generated. Therefore, it is not advisable to use a speed-regulating hydraulic coupling with constant-torque machinery at high speeds; in particular, it should not operate at high speed ratios for extended periods of time. (6) When the output speed of the coupler is very low, that is, when the position of the conduit is close to 0%, noise that does not occur during normal operation may appear. This is caused by the \"siren effect\" resulting from the meeting of the conduit opening with the oil leakage holes on the outer edge of the impeller. In such a case, simply raising the position of the conduit slightly will resolve the issue; it is not a coupling failure. (7) During operation, it is necessary to check regularly whether the oil temperature and pressure of the coupling are normal. If any abnormalities are detected, the cause should be identified and resolved promptly. 3. Maintenance (1) Regularly check the oil level in the tank and top it up promptly ； (2) The oil suction pipe filter should be removed and cleaned after 500 hours of operation of the new machine for the first time ； (3) Carry out maintenance in conjunction with the shutdown of the working machine, and regularly clean the oil supply pump and oil filter ； (4) Regularly check the oil quality and replace it with qualified working oil in a timely manner ； (5) To drain the oil inside the coupler housing, the plug at the oil drainage hole located below the input and output ends of the housing can be unscrewed; alternatively, a stop valve can be installed at the oil drainage hole. III. Trial Operation and Maintenance of Torque-Limiting Hydraulic Couplings 1. Rapid and frequent forward and reverse rotations are not allowed. Theoretically, a coupling can rotate in both directions, but sudden changes in rotation direction can cause excessive inertial forces that damage the connecting components of the coupling. Therefore, it is essential to strictly follow the operating procedures and avoid sudden or frequent changes in rotation direction; this is particularly important when using couplings in main machinery such as tower cranes and bridge cranes. 2. The rotation direction of the coupler’s output shaft is the same as that of the motor shaft; during the first test run, it is necessary to check whether the motor’s rotation direction meets the requirements. 3. When the motor reaches its rated speed, the driven device must start moving; if it does not move, the machine should be stopped immediately to check whether an excessive load is causing braking. 4. During continuous operation, the operating temperature of the coupling shall not exceed 90°C, and the operating temperature of the water medium shall not exceed 100°C. 5. The quality of the working oil should be checked regularly (every 3,000 hours); if any deterioration in the oil quality is detected, it should be replaced immediately. 6. Regularly check the positional accuracy of the motor shaft and the shaft of the working machine, and correct it as needed. 7. Regularly inspect the motor and the mounting foundation of the working machine to prevent vibrations caused by poor foundation rigidity from reaching the coupling and leading to premature damage. 8. Regularly check the wear of the elastic blocks or discs in the flexible coupling, and replace them as needed. 9. It is not allowed to disassemble the coupler housing arbitrarily, as this may damage the seals as well as the assembly and balance accuracy, leading to accidental accidents. 10. It is not allowed to change the coupling oil medium to a water medium. 11. Flushing should not be done arbitrarily, and it is even more prohibited to fill the system to capacity; flushing must be carried out strictly in accordance with the instructions. 12. In couplings with bearing chambers, the bearing chambers are filled with lubricant (commonly known as glycerin) at the time of manufacture. During the operation of the coupling, the lubricating grease thins and seeps out due to heat generated by the bearings; this is not a leakage of the coupling’s working medium, and it is important to make this distinction. 13. To aid in the heat dissipation of self-cooling hydraulic couplings, they should be placed in locations with good ventilation to facilitate cooling. 14. To prevent accidents, the coupling should be equipped with a reliable protective cover. IV. Filling and inspection of hydraulic couplings – Amount of fluid in torque-limiting hydraulic couplings: 1) The amount of fluid in a torque-limiting hydraulic coupling is proportional to the power transmitted; that is, within the specified range for fluid volume, the greater the amount of fluid in the working chamber, the greater its capacity to transmit torque (or speed) ； The same is true in reverse. With a constant external load, the greater the amount of fluid filled, the higher the efficiency; simultaneously, the starting torque and overload capacity of the coupling also increase ； The same is true in reverse. 2) Couplings of a certain specification have a specific power range, known as the power band, which corresponds to the amount of fluid filled in the coupling. The fluid filling level of the coupler ranges from 40% to 80% of the total volume ； When the filling rate is less than 40%, the coupling is uneconomical as it fails to exert its power-transmission capability; moreover, the bearings do not receive lubrication, which accelerates their wear, leads to vibrations, and also increases the risk of overheating. When the fluid filling level is greater than 80%, the hydraulic coupling’s performance is affected due to a lack of sufficient space within the working chamber for changes in fluid flow patterns – resulting in reduced overheat protection capabilities. If the cavity is completely filled with liquid, not only do its properties deteriorate, but the heating and expansion of the liquid can also lead to seal failure or even the rupture of the hydraulic coupler housing. For this reason, it is absolutely essential not to make mistakes in the irrigation, and even less so to go overboard.