The weight required to cause the downward motion is obtained either by means of the material which has to be transported to the bottom of the hill or by water ballast, while to aid and regulate the motion generally steam or electric motors are arranged to act on the main drums, round which the cable is passed with a sufficient number of turns to prevent slipping.
These machines weigh from 20 to 22 cwt., and are mostly driven by electric motors of 25 up to 35 h.p. as a maximum.
In some instances electric motors have been substituted for compressed-air engines in such machines.
Notable examples are found in the Rateau, Ser and Capell fans, and where an electric generating station is available electric motors can be advantageously used instead of steam.
Modern screening and washing plants, especially when the small coal forms a considerable proportion of the output, are large and costly, requiring machinery of a capacity of ioo to 150 tons per hour, which absorbs 350 to 400 H.P. In this, as in many other cases, electric motors supplied from a central station are now preferred to separate steam-engines.