What is a Hydraulic Coupling?
3 Answers
Hydraulic coupling is used in automatic or semi-automatic transmissions, similar to the torque converter in AT transmissions. The medium inside a hydraulic coupling is oil, and this device connects two shafts together. AT transmissions are equipped with torque converters, which consist of an impeller, turbine, and guide wheel, and are installed between the engine and the transmission. Also known as a fluid coupling, a hydraulic coupling is a type of hydrodynamic drive that connects a power source to a working machine, transmitting torque through changes in the angular momentum of the fluid. It is a non-rigid coupling that uses liquid as the working medium.
A hydraulic coupler is a device that transmits power through fluid. You can think of it as two impellers installed in a sealed oil tank. When the engine drives the active impeller to rotate, the hydraulic oil inside is flung outward, impacting the other driven impeller to rotate, thereby transmitting the engine power to the transmission. Unlike gears, which connect rigidly, the power transmission is exceptionally smooth—those silky smooth starts in old automatic cars mainly rely on it. However, the hydraulic oil slips during transmission, causing some loss in power efficiency. Nowadays, new cars use torque converters with lock-up clutches instead, which can directly engage at high speeds, saving a lot of fuel. Additionally, hydraulic couplers require almost no maintenance, and the oil never needs to be changed, which is a definite advantage.
This thing is essentially the power transmission bridge of an automatic transmission. The engine shaft is connected to the impeller, while the transmission shaft is linked to the turbine, with the entire housing filled with specialized transmission fluid. During operation, it works like two fans blowing against each other—the impeller agitates the fluid to drive the turbine. Its biggest advantage is its ability to cushion vibrations; for example, sudden engine acceleration won’t directly impact the transmission gears, and it also absorbs noise and vibrations from the drivetrain. However, the hydraulic fluid transmission inevitably causes a 10%-15% energy loss, leading to slightly higher fuel consumption. Nowadays, pure hydraulic couplings are only used in heavy machinery and buses, while passenger cars have long since upgraded to torque converters, which include an additional stator to increase torque.
