Elif Temiz, 030070195, 9th Week definitions- Part 3

5-Dynamometer (dynomometer) 

New Definition  (measurement device) (better)

Torque measuring devices used in this connection are commonly known as dynamometers. There are basically three types of dynamometers:

(I)                 Absorption dynamometers: They absorb the mechanical energy as torque is measured, and hence are particularly useful for measuring power or torque developed by power sources such as engines and motors.

(II)                Driving dynamometers: These dynamometers measure power or torque and as well provide energy to operate the devices to be tested. They are, therefore, useful for studying performance characteristics of devices such as pumps and compressors.

(III)             Transmission dynamometers: These are passive systems and are placed at an appropriate location within a machine or in between machines to measure torque at that particular location. These dynamometers are sometimes referred to as torque meters. Driving dynamometers are sometimes called as transmission dynamometers. The first two types can be grouped as mechanical and electrical dynamometers. However, the transmission type is treated separately.

MECHANICAL DYNAMOMETERS

These dynamometers are of absorption type. One of the most familiar and simple devices is the Prony brake as shown in Fig. 4.11. The mechanical energy is converted into heat through dry friction between the wooden brake shoes and the fly wheel of the machine. The torque exerted on the Prony brake is

 T= FL,

 where force F is measured by conventional force measuring instruments, say balances or load cells etc. The power dissipated in the brake is calculated from

where force F (measured at arm L) is in Newtons, L is the length of reaction arm in meters, N is the angular speed in revolutions per minute, and P in watts.

 The Prony brake is inexpensive, but it is difficult to adjust and maintain a specific load.

 Various other types of brakes are employed for power measurements on mechanical equipment. The water brake, for example, dissipates the output energy through fluid friction.

ELECTRIC DYNAMOMETERS

Almost any kind of rotating electric machine can be used as a dynamometer but those specifically designed for the purpose arc convenient to use. Electric dynamometers are of both absorption and driving type. The type of dynamometer to be used depends on the assigned task. For example if the machine to be tested is a power generator, the dynamometer must be capable of absorbing its power. On the other hand, if the machine to be tested is a power absorber, it must be capable of driving it. When the machine to be tested is a power transformer or transmitter, the dynamometer must provide both the power source and the load.

 The electric dynamometers can be grouped into the following two classes:

I. d.c. dynamometers or generators, and

 2. Eddy-current or inductor dynamometers.

 In addition to these, ordinary electric motors and generators may also be used in dynamometry. TRANSMISSION DYNAMOMETERS

Torque can be measured conveniently by means of solid or hollow tubes. These elements are twisted due to the application of a torque. There exist both tensile and compressive strains on the surface at 45° to the tube axis when it is twisted by a torque T as shown in Fig. 4.14(a). The relationship between strain and torque is given by 

where G is the shear modulus of elasticity, ro and r1 are the outside and inside radii of the tube. The angle of twist i is given by 

where 1 is the length of the tube.

( R. S. Sirohi,H. C. Radha Krishna, Mechanical Measurements, pp.69,74)

Previous Definition
The function of the dynamometer is to measure the torque and hence the power of the engine by applying the brake. Thus dynamometer measures the resisting force applied by the brake on the moving body.
Absorption dynamometers
In these dynamometers, the entire energy or power produced by the engine is absorbed by the friction resistance of the brake and is transformed into heat, during the process of measurement. Under this heading the two main dynamometers are:
Prony brake dynamometers
Rope brake dynamometers
Transmission dynamometers
In these dynamometers energy is not absorbed or dissipated but is used for doing useful work. In these dynamometers the poewr of the engine is measured and then transmitted through them. Important dynamometers in this category are:
Epicyclic - train dynamometer
Belt transmission dynamometer
Bevis - Gibson or torsion dynamometer
(Vashist D., Mechanical Engineering:Fundamentals, pg.191-193, Kayra Ermutlu)