A water brake is a type of

fluid coupling A fluid coupling or hydraulic coupling is a hydrodynamic or 'hydrokinetic' device used to transmit rotating mechanical power.

used to absorb mechanical energy and usually consists of a

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating e ...

propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...

mounted in an enclosure filled with water. As the turbine or propeller turns, mechanical energy is transferred to the water due to

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

and

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...

. The shock caused by the acceleration of the water as it passes from pockets in the stator to the pockets in the spinning rotor requires energy. That energy heats the water due to the friction as the water moves through the water brake. Almost all of the horsepower of the system turning the rotor (usually an internal combustion engine) is converted into a temperature change of the water. A very small amount of energy is taken by the bearings and seals within the unit. Therefore, water must constantly move through the device at a rate proportional to the horse power that is being absorbed. Water temperature exiting the unit must be kept under 120–160 °F (50–70 °C) to prevent scale formation and cavitation. The water enters in the center of the device and after passing through the pockets in the stator and rotor exits the outside of the housing through a controlled orifice. The amount of loading is dependent on the level of water inside the housing. Some water brakes vary the load by controlling the inlet water volume only and have a set outlet orifice size depending on the desired hp to be absorbed and some control both input and output orifices at the same time which allows greater control over outlet water temperatures. The housing is vented to the outside to allow air to displace the water as the water level in the unit rises and falls. The amount of torque that can be absorbed is defined by the equation T=kN²D⁵ where T = torque, N = RPM, D = the diameter of the rotor and k = a constant dependent on the size and shape and angle of the rotor/stator pockets. Systems that require the torque of the system under test to be measured typically use a strain gauge mounted on a torque arm that is attached to the housing perpendicular to the input shaft. The housing/stator is mounted on roller bearings and the rotor is mounted on roller bearings within the housing/stator so that it can turn independently of the rotor and frame. The strain gauge connects the torque arm to the frame assembly and keeps the housing from spinning as housing tries to turn in the same direction of the turbine. (Newton's third law). The amount of resistance can be varied by changing the amount of water in the enclosure at any one time. This is accomplished through manual or electronically controlled water valves. The higher the water levels within the brake the greater the loading. Water brakes are commonly used on some forms of

dynamometer A dynamometer or "dyno" for short, is a device for simultaneously measuring the torque and rotational speed (RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the ...

but have also been used on railways vehicles such as the British

Advanced Passenger Train The Advanced Passenger Train (APT) was a tilting high speed train developed by British Rail during the 1970s and early 1980s, for use on the West Coast Main Line (WCML). The WCML contained many curves, and the APT pioneered the concept of activ ...

Hydrokinetic construction (torque absorption)

The Froude waterbrake is based on hydrokinetic construction or (torque absorption). The machine consists of an impeller (rotor) which accelerates water outwards by its rotation. The water has its velocity changed by a stator which causes the water to be returned to the inner diameter of the rotor. For a given mass of water, this velocity change yields a corresponding momentum change – and the rate of change of momentum is proportional to a force. This force acting at some point within the rotor and stator is a distance from the shaft centerline, and a force multiplied by a distance produces torque.

References

{{DEFAULTSORT:Water Brake Dynamometers

Hydrokinetic construction (torque absorption)

See also

References