Page 435 - The Mechatronics Handbook

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have no speed limitations. More generally, and especially for large machinery, dynamometers include a
specialized driving or absorbing machine. Such dynamometers are classified according to their function
as absorbing or driving (sometimes motoring). A universal dynamometer can function as either a driver
or an absorber.
Absorption Dynamometers
Absorption dynamometers, often called brakes because their operation depends on the creation of a
controllable drag torque, convert mechanical work into heat. A drag torque, as distinguished from an
active torque, can act only to restrain and not to initiate rotational motion. Temperature rise within a
dynamometer is controlled by carrying away the heat energy, usually by transfer to a moving fluid,
typically air or water. Drag torque is created by inherently dissipative processes such as: friction between
rubbing surfaces, shear or turbulence of viscous liquids, the flow of electrical current, or magnetic
hysteresis. Gaspard Riche de Prony (1755–1839), in 1821 [22], invented a highly useful form of a friction
brake to meet the needs for testing the steam engines that were then becoming prevalent. Brakes of this
type are often used for instructional purposes, for they embody the general principles and major operating
considerations for all types of absorption dynamometers. Figure 19.50 shows the basic form and con-
structional features of a prony brake. The power that would normally be delivered by the shaft of the
driving engine to the driven load is (for measurement purposes) converted instead into heat via the work
done by the frictional forces between the friction blocks and the flywheel rim. Adjusting the tightness of
the clamping bolts varies the frictional drag torque as required. Heat is removed from the inside surface
of the rim by arrangements (not shown) utilizing either a continuous flow or evaporation of water. There
is no need to know the magnitude of the frictional forces nor even the radius of the flywheel (facts
recognized by Prony), because, while the drag torque tends to rotate the clamped-on apparatus, it is held
stationary by the equal but opposite reaction torque Fr. F at the end of the torque arm of radius r (a fixed
dimension of the apparatus) is monitored by a scale or load cell. The power is found from Eqs. (19.58)
and (19.64) as P = Frω = Fr2πN/60 where N is in rpm.

Threaded handwheel for
adjusting frictional force Torque arm radius r

Beam

ω
F
Shaft
Smooth
surfaced
pulley or Load cell
flywheel or
platform scale

Friction block

FIGURE 19.50 A classical prony brake. This brake embodies the defining features of all absorbing dynamometers:
conversion of mechanical work into heat and determination of power from measured values of reaction torque and
rotational velocity.

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