Hydrodynamic Transmission

a mechanism for step-less change of torque transmitted from a motor or of the rotational speed (rpm) of the shaft of a machine tool. A hydrodynamic transmission works by the action of a rotary pump and a turbine.

The hydrodynamic transmission was proposed in the beginning of the 20th century. Its design features a centrifugal pump and a turbine, located coaxially in such a way that their wheels form a toroidal cavity partially filled with pressure fluid (low-viscosity oil or water). The fluid is impelled by the pump, which has a wheel connected to a motor. The energy received by the fluid from the pump is transmitted by the turbine to a driven machine.

In a hydrodynamic transmission having only two wheels (a pump impeller and a turbine runner), the torques on both shafts are equal. Such a transmission is called a hydro-dynamic coupling (hydraulic clutch). In rated operation of such a coupling, the rpm of the turbine shaft is lower than the rpm of the pump shaft by 1.5-4 percent. The efficiency of such a hydraulic clutch is 95-98 percent.

Torque converters have three vaned wheels ( a pump impeller, a wheel of the controlling mechanism, and a turbine runner) or more. Such converters may include a single-stage or multistage turbine. The latter design permits an increase in the control range of the rpm of the secondary shaft, as well as a greater ratio between the torque at the turbine runner and the torque at the pump impeller. This greater ratio is used specifically in the “starting-up” mode of operation (that is, with the turbine shaft stationary); the ratio can be as high as 12:1 for three-stage turbines.

Hydrodynamic transmissions make it possible to regulate the torque by varying the amount of pressure fluid in the work cavity. This control scheme is widely used in hydraulic clutches. In torque converters the drop-off in efficiency can be minimized if the control scheme uses variable-pitch rotor vanes. Some torque-converter designs have provisions for disconnecting the control mechanism. Upon disconnecting, such a device becomes a hydraulic coupling (or clutch). This scheme is called a compound transmission. Hydrodynamic transmissions are constructed for a transmission ratio of 0.6 to 6 and an efficiency of 0.86-0.92.

A split hydrodynamic transmission consists of separately located pump and turbine, connected by pipes. This design imposes no limitations on the placement of the turbine in relation to the motor. It permits a division of motor power among several users or, conversely, an addition of power delivered by several motors in order to drive a single machine. Although the efficiency of split hydrodynamic transmissions is no higher than 65-70 percent, this type finds a steadily increasing application in cases where the driven machine must be so located that maintenance becomes impossible or difficult (as in drilling rigs, pumps of aircraft fuel systems, pumps in chemical installations, and the like).

Hydrodynamic transmissions, as a type of automatically operated stepless transmission, are used mostly in automotive transmissions, diesel locomotives, ship power plants, and feed pump drives and as exhaust-fan drives for steam power plants. The power rating of steam power-plant pumps that are driven by a hydraulic clutch can be as high as 25,000 kilowatts.