Engineering Mechanics

Chapter 30 : Work, Power and Energy „„„„„ 605

30.13. FROUDE AND THORNYCRAFT TRANSMISSION DYNAMOMETER

Fig. 30.5. Froude and Thornycroft dynamometer
It is a commonly used transmission type dynamometer. It consists of a pulley A fixed rigidly to
the shaft of an engine whose power is required to be measured. There is another pulley B, mounted to
another shaft, to which power from shaft A is transmitted. The two pulleys A and B are connected by
means of a continuous belt passing round two other pulleys C and D supported over a T-shaped lever
pivoted at E as shown in Fig. 30·5. A weight is hung at the free end of the lever F.

This dynamometer is based on the principle that when a belt transmits power from one pulley
to another, the tangential effort on the pulley is equal to the difference between the *tensions on the
tight and slack side of the belt.

Let W = Weight hung from the lever,

T 1 = Tension in tight side of the belt

T 2 = Tension in slack side of the belt

L = Horizontal distance between the pivot E, and the centre line of

the action of the weight,

D = Effective diameter of the pulley A,

N = Speed of the engine shaft in r.p.m.

a = Distance between the pivot E and the pulley C or D.

Total force acting on the pulley C

= T 1 + T 1 = 2 T 1

and total force acting on the pulley D

= 2 T 2

Now taking moments about the pivot E,

2 T 1 × a = 2 T 2 × a + W. L

∴ 12 –

2

WL

TT

a

=

We know that the power transmitted by the pulley A

=^12

(– )

60 2 60 120

TT DNWL DN WL DN

aa

π ×π ×π

==

××

* For details, please refer to chapter 33.