An arrangement of discs in a viscous fluid that allows two shafts to rotate at the same or only slightly different speeds. It can be incorporated in a differential to allow a slow, but not a high speed, differential action, ie. preventing wheel-spin without transmission wind-up. ebroadcast.com A particular kind of fluid coupling inwhich the input and output shafts mate with thin, alternately spaced discs in a cylindrical chamber. The chamber is filled with a viscous fluid that tends to cling to the discs, thereby resisting speed differences between the two shafts. Viscous couplings are used to limit the speed difference betweeen the two outputs of a differential, or between the two axles of a car. clublexus.com This is a viscous coupling as found attached to the centre diff' in the transfer case of a Range Rover. It has been cut away to reveal the internal plates. These are alternately keyed to the casing or the centre shaft. Normally a viscous fluid fills the casing and this resists strongly large differences in rotational speed between the plates, ie. it resists the front and rear transfer-case output shafts rotating at very different rates. In this role it limits wheel spin if a front (or a rear) wheel suddenly looses traction. The action is smooth and permits normal differential action when cornering. SEE PHOTO http://www.ebroadcast.com.au/ecars/LandRover/Mech/VC.html The viscous coupling does not help if both a front and a rear wheel loose traction as the Range Rover does not have axle diff' locks (as standard).

Viscous or fluid couplings are used to transfer power. They are widely used in industry and can be found in motor vehicle automatic transmissions. A fluid coupling has two main parts: the input member (impeller) and the output member (runner). Both the impeller and runner have vanes, which are used to direct the fluid flow. There is no mechanical connection between the two shafts, since all power is transmitted by an operating fluid, usually oil. As the impeller rotates, its vanes direct the fluid towards the runner, which is caused to rotate. A major advantage of fluid couplings is that the input shaft cannot be stalled if the output shaft is stopped. In addition, the motor driving the impeller can be operated at its most efficient operating point, regardless of the amount of torque delivered to the runner shaft. Oil is widely used in industry for power transmission applications, not only with the fluid couplings described here, but also with actuators and motors. Oil has several advantages over other fluids used in industrial power applications: it can be used over a fairly wide temperature range, is available in a wide range of viscosities, and is self-lubricating. Oil is also essentially incompressible, unlike compressed, which air used in pneumatic systems. (Another fluid, water, has been used for thousands of years in fluid power applications, in applications ranging from simply water-wheels to hydro-electric power generation.) The ease with which a fluid pours is an indication of its viscosity. Shear stresses are developed in moving fluids and are directly proportional to the fluid's dynamic viscosity. This shearing action imparts motion to the fluid: zero where the fluid contacts a non-moving boundary and at its maximum at the point of contact between the fluid and the moving surface. This mechanism is responsible for the transmission of energy from the input member to the output member of a fluid coupling. A hydraulic torque converter is similar to a fluid coupling. In addition to the rotating impeller and runner, it has stationary guide vanes (reactors) interspered between the rotating elements. The blades in a torque converter have compound curvature (i.e., the blade is fabricated with more than one radius of curvature), designed to control the direction of the fluid flow. The blades are designed so that the fluid will be moving in a direction parallel to the blade surface at the entrance to each section. This will only occur at one specific fluid velocity in a fixed-blade converter. The angle of the blades can be made adjustable and the elements can be adjusted independently of one another to allow the converter to accomodate different loads. [With information from "Mark's Standard Handbook for Mechanical Engineers"]

Viscous cupling is also known as hydraulic coupling and sometimes, fluid flywheel.Like other coupling, its function is to transmit drive between two rotatable shafts - one drive and the other driven.The device consists of two vaned impeller one facing another and mounted on the drive and driven shafts.Both are enclosed in casing and filled with viscous fluid like oil.When the drive shaft rotates,the impeller on it throws high energy fluid to the impeller on the driven shaft thus forcing it to rotate along with it.The fluid returns to the drive shaft impeller giving up its energy to the driven impeller and the circulation continues. In this way a fluid linkage between drive and driven shaft is established but mind it that there is no rigid connection between the two.The coupling can thus transmit the same torque it is provided with.It cannot multiply torque as its cousine torque converter can do. Such type of coupling is used in automatic transmission of a car which transmits no shock or vibration as there is no mechanical linkage.It is used replacing the conventional clutch in the car.