A hydraulic coupler is a mechanical device that uses a liquid medium to transmit rotational speed. It connects the driving input shaft end to the original driving mechanism and the driven output shaft end to the load shaft end. By adjusting the pressure of the liquid medium, the rotational speed of the output shaft can be changed. In an ideal state, when the pressure approaches infinity, the output speed equals the input speed, akin to a rigid coupling. As the pressure decreases, the output speed correspondingly reduces. By continuously changing the medium pressure, the output speed can be adjusted continuously below the input speed. The principle of power control and speed regulation of the hydraulic coupler, and its efficiency, are based on the aforementioned characteristics. The hydraulic coupler is an energy-consuming mechanical speed regulation device; the deeper the speed regulation (the lower the speed), the greater the loss, especially for constant torque loads. Since the original driving input power remains unchanged, the loss power increases proportionally with the speed loss. For loads like fans and pumps, where the load torque varies with the square of the speed, the original driving input power decreases with the square of the speed, resulting in relatively smaller loss power. However, the output power decreases with the cube of the speed, so the speed regulation efficiency is still very low. The speed regulation efficiency curve of the hydraulic coupler shows an average efficiency of around 50%. A non-rigid coupling that uses a liquid as the working medium, also known as a hydraulic coupling.
The pump wheel and turbine of a hydraulic coupling form a sealed working chamber that allows for the cyclic flow of liquid. The pump wheel is mounted on the input shaft, while the turbine is mounted on the output shaft. As the power source (internal combustion engine, electric motor, etc.) rotates the input shaft, the liquid is ejected by the centrifugal pump wheel. This high-speed liquid enters the turbine, causing it to rotate and transferring the energy obtained from the pump wheel to the output shaft. The liquid then returns to the pump wheel, creating a continuous cycle. The hydraulic coupling transmits torque by changing momentum due to the interaction of the liquid with the blades of the pump wheel and turbine. Its output torque equals the input torque minus the friction torque, hence its output torque is always less than the input torque. The input and output shafts of the hydraulic coupling are connected by the liquid, with no rigid connections between the working components. The features of the hydraulic coupling include: the ability to eliminate shock and vibration; an output speed lower than the input speed, with the speed difference between the two shafts increasing with the load; good overload protection and starting performance, allowing the input shaft to continue rotating even when the load is too great to prevent damage to the power source; as the load decreases, the output shaft speed increases until it approaches the speed of the input shaft. The transmission efficiency of the hydraulic coupling is equal to the ratio of the product of the output shaft speed and output torque (output power) to the product of the input shaft speed and input torque (input power). Generally, a hydraulic coupling can achieve higher efficiency when the normal operating speed ratio is above 0.95. The characteristics of the hydraulic coupling vary depending on the shape of the working chamber and the pump wheel and turbine. If the oil in the hydraulic coupling is drained, the coupling is disengaged, serving as a clutch.
