1911 Encyclopædia Britannica/Power Transmission
POWER TRANSMISSION. The appliances connected with installations for the utilization of natural sources of energy may be classified into three groups:—
1. Prime movers, by means of which the natural form of energy is transformed into mechanical energy. To this group belong all such appliances as water turbines, steam turbines, steam engines and boilers, gas producers, gas engines, oil engines, &c.
2. Machinery of any kind which is driven by energy made available by the prime mover. To this group belong all machine tools, textile machinery, pumping machinery, cranes-in fact every kind of machine which requires any considerable quantity of energy to drive it.
3. The appliances by means of which the energy made available by the prime mover is transmitted to the machine designed to utilise it. The term power is used to denote the rate at which energy is transmitted. The unit of power in common use is the horse power, and one horse power means a rate of transmission of 550 foot-pounds per second.
In many cases the prime mover is combined with the machine in such a way that the transmitting mechanism is not distinctly differentiated from either the prime mover or the machine, as in the case of the locomotive engine. In other cases the energy made available by the prime mover is distributed to a number of separate machines at a distance from the prime mover, as in the case of an engineer's workshop. In this case the transmitting mechanism by means of which the energy is distributed to the several machines has a distinct individuality. In other cases prime movers are located in places where the natural source of energy is abundant, namely, near waterfalls, or in the neighbourhood of coal-fields, and the energy made available is transmitted in bulk to factories, &c., at relatively great distances. In this case the method and mechanism of distribution become of paramount importance, since the distance between the prime mover and the places where the energy is to be utilized by machines is only limited by the efficiency of the mechanism of distribution)
Prime movers are considered in the articles Steam Engine; Gas Engine; Oil Engine, and Hydraulics and machines in various special articles. The methods and mechanisms of distribution or transmission alone form the subjects of the present article, and the different methods in general use readily fall into four divisions:—
| 1. Mechanical. | 3. Pneumatic. |
| 2. Hydraulic. | 4. Electrical. |
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