Fluid Coupling Overview
A fluid coupling contains three components, plus the hydraulic fluid:
The casing, also referred to as the shell (which will need to have an oil-restricted seal around the drive shafts), contains the fluid and turbines.
Two turbines (lover like components):
One linked to the insight shaft; known as the pump or impellor, primary steering wheel input turbine
The other linked to the result shaft, known as the turbine, result turbine, secondary steering wheel or runner
The driving turbine, referred to as the ‘pump’, (or driving torus) is definitely rotated by the prime mover, which is typically an internal combustion engine or electrical motor. The impellor’s motion imparts both outwards linear and rotational motion to the fluid.
The hydraulic fluid is certainly directed by the ‘pump’ whose shape forces the flow in the direction of the ‘output turbine’ (or driven torus). Here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ result in a net power on the ‘output turbine’ leading to a torque; therefore leading to it to rotate in the same direction as the pump.
The movement of the fluid is efficiently toroidal – traveling in one path on paths which can be visualised as being on the surface of a torus:
If there is a notable difference between input and result angular velocities the motion has a component which is circular (i.e. round the rings formed by sections of the torus)
If the insight and output phases have identical angular velocities there is no net centripetal power – and the movement of the fluid is circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is absolutely no flow of fluid in one turbine to the other.
Stall speed
An important characteristic of a fluid coupling can be its stall quickness. The stall quickness is defined as the highest speed at which the pump can change when the output turbine is definitely locked and optimum input power is applied. Under stall circumstances all the engine’s power will be dissipated in the fluid coupling as heat, perhaps resulting in damage.
Step-circuit coupling
An adjustment to the easy fluid coupling is the step-circuit coupling that was formerly manufactured as the “STC coupling” by the Fluidrive Engineering Organization.
The STC coupling contains a reservoir to which some, however, not all, of the essential oil gravitates when the result shaft can be stalled. This reduces the “drag” on the input shaft, resulting in reduced fuel intake when idling and a decrease in the vehicle’s inclination to “creep”.
When the output shaft starts to rotate, the essential oil is trashed of the reservoir by centrifugal force, and returns to the primary body of the coupling, to ensure that normal power transmitting is restored.
Slip
A fluid coupling cannot develop output torque when the insight and output angular velocities are identical. Hence a fluid coupling cannot achieve completely power transmission effectiveness. Because of slippage that may occur in virtually any fluid coupling under load, some power will always be dropped in fluid friction and turbulence, and dissipated as high temperature. Like other fluid dynamical products, its efficiency will increase steadily with increasing scale, as measured by the Reynolds amount.
Hydraulic fluid
As a fluid coupling operates kinetically, low viscosity fluids are preferred. Generally speaking, multi-grade motor oils or automatic transmission liquids are used. Raising density of the fluid increases the amount of torque that can be transmitted at confirmed input speed. Nevertheless, hydraulic fluids, much like other fluids, are at the mercy of changes in viscosity with heat change. This prospects to a switch in transmission functionality and so where unwanted performance/efficiency change has to be kept to the very least, a motor essential oil or automated transmission fluid, with a high viscosity index ought to be used.
Hydrodynamic braking
Fluid couplings may also become hydrodynamic brakes, dissipating rotational energy as heat through frictional forces (both viscous and fluid/container). When a fluid coupling can be used for braking it is also referred to as a retarder.
Fluid Coupling Applications
Industrial
Fluid couplings are found in many industrial application regarding rotational power, specifically in machine drives that involve high-inertia starts or continuous cyclic loading.
Rail transportation
Fluid couplings are located in some Diesel locomotives as part of the power transmitting system. Self-Changing Gears made semi-automated transmissions for British Rail, and Voith produce turbo-transmissions for railcars and diesel multiple units which contain numerous combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were found in a variety of early semi-automatic transmissions and automatic transmissions. Because the past due 1940s, the hydrodynamic torque converter offers replaced the fluid coupling in motor vehicle applications.
In automotive applications, the pump typically is linked to the flywheel of the engine-in fact, the coupling’s enclosure could be section of the flywheel proper, and thus is turned by the engine’s crankshaft. The turbine is connected to the insight shaft of the transmitting. While the transmission is in gear, as engine rate increases torque can be transferred from the engine to the insight shaft by the movement of the fluid, propelling the vehicle. In this respect, the behavior of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmitting.
Fluid flywheels, as distinctive from torque converters, are most widely known for their use in Daimler cars in conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their range of luxury vehicles, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for his or her military automobiles and armored vehicles, some of which also used the combination of pre-selector gearbox and fluid flywheel.
Aviation
The many prominent utilization of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 engines where it had been utilized as a barometrically controlled hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, in which three power recovery turbines extracted approximately 20 percent of the energy or about 500 horsepower (370 kW) from the engine’s exhaust gases and then, using three fluid couplings and gearing, converted low-torque high-rate turbine rotation to low-speed, high-torque output to drive the propeller.
