Now suppose that on the shaft of a there is a pulley 20 inches in diameter, and that on this pulley there is a belt exerting a pull of 5 lbs., while on the shaft of e there is a pulley 16 inches in diameter, and to find how much this latter pulley would pull its belt, we proceed as follows:
| 2 ) | 20 | = | Diameter of pulley on a. | |||
| 10 | = | Radius of pulley on a. | ||||
| 5 | = | Pull on pulley a. | ||||
| Number of teeth in a | = | 30 ) | 50 | = | Pull at centre of shaft of a. | |
| 1.666 | ||||||
| 60 | = | Number of teeth on b. | ||||
| Number of teeth on c | = | 10 ) | 99.960 | = | Pull at axis of shaft of b. | |
| 9.996 | ||||||
| 80 | = | Number of teeth on e. | ||||
| Radius of pulley on shaft of e | 8 ) | 799.680 | Pull at axis of shaft e. | |||
| 99.96 | Pull at perimeter of last pulley. | |||||
We have in this case treated each pulley as a lever whose length equalled the radius of the pulley, while in the case of the wheel we have multiplied by the number of teeth when the power was transmitted from the circumference to the shaft, and divided by the number of teeth (the number of teeth representing the circumference) when the power was transmitted from the shaft to the teeth.
We thus find that power is composed of three things, first, the amount of impelling force; second, the distance that force moves through; and third, the time it takes to move that distance.
If we take a number of pulleys, say four, and arrange them one after another so that they drive by the friction of their circumferences, then the amount of power transmitted by each will be equal and the velocities will be equal, whereas, if we arrange them as in [Fig. 3354], the power will be equal for each, but the velocities or space moved through in a given time will vary.
What is known as the unit of power is the foot lb., being the amount of power exerted in raising or lifting one lb. one foot, and from what has already been said, it will be perceived that this is the same amount of power as 12 lbs. moving a distance of one inch.
Watt determined that the power of a horse was equal to that necessary to raise 33,000 lbs. one foot high in a minute, and this is accepted, in English speaking countries, as being a horse power.
An engine or machine has as much horse power as it has capacity to lift 33,000 lbs. a foot high in a minute.
CALCULATING THE HORSE POWER OF AN ENGINE.
The horse-power of an engine may be calculated as follows: