Page 150 - Tribology in Machine Design

136 Tribology in machine design

where T l and T 2 are the effective tensions on the tight and slack sides of the
belt or rope respectively.

4.11. Frictional aspects The brake-horsepower of an engine is the rate of expenditure of energy in
of brake design overcoming external resistance or load carried by the engine. The difference
between the brake-horsepower and the indicated horsepower represents
the rate at which energy is absorbed in overcoming mechanical friction of
the moving parts of the engine
friction horsepower = indicated horsepower —brake-horsepower

and

The operation of braking a machine is a means of controlling the brake-
horsepower and so adjusting the output to correspond with variations of
indicated horsepower and the external load. A brake may be used either to
bring a machine to a state of rest, or to maintain it in a state of uniform
motion while still under the action of driving forces and couples.
In engineering practice, the latter alternative is useful as a means of
measuring the power that can be transmitted by a machine at a given speed.
A brake that is used in this way is termed a dynamometer and is adapted for
the purpose simply by the addition of equipment which will measure the
friction force or couple retarding the motion of the machine. Dynamo-
meters fall into two classes:
(i) absorption dynamometers, or those which absorb completely the
power output of the machine at a given speed;
(ii) transmission dynamometers, which, but for small friction losses in the
measuring device itself, transmit power from one machine to another.
Generally speaking, the power developed by an engine may be absorbed by
either mechanical, electrical or hydraulic means. In friction brake dynamo-
meters all the power of the engine is absorbed by mechanical friction
producing heat.

4.11.1. The band brake
Figure 4.40 shows a mechanical type of friction brake used in a crane. The
brake drum D is keyed to the same shaft as the crane barrel E. The flexible
band which surrounds the drum, and consists of either a leather or narrow
strip of sheet steel with suitable friction material lining is connected to
points A and B on the lever pivoted at F. A load P applied to the lever at the
point C causes the band to tighten on the drum and friction between the
band and drum surface produces the necessary braking torque. Assuming
that the drum tends to rotate anticlockwise, let 7\ and T 2 be the effective
belt tensions and a the radius of the circle tangential to the lines of action of
T] and T 2. Then

Figure 4.40