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Both single- and double-roll crushers have been extensively developed for crushing coal, coke, shale and similar soft and friable materials. These machines resemble in general form the roIl crushers we have described, but they are of course much lighter, and their mechanical details are correspondingly simplified. Instead of the knob-like teeth used on the stone-breaking rolls, these coal crushers are usually fitted with spike shaped teeth. and the action has more of a tearing nature rather than the heavy slugging and sledging that takes place in the Fairmount and Edison types.
To simplify the driver double-roll coal crushers are usually geared together, which works out quite satisfactorily because the shocks in machines of this type are light, as compared to those in stone-breaking rolls. This same practice, incidentally, was followed in the earlier, and lighter, rolls used on stone and ore;
it is not used in any of the present-day heavy-duty double rolls. Single-roll coal crushers are driven by the same gear-and-pinion arrangement used in single-roll stone crushers. Peripheral speeds range from about 400 to 800 ft per min, the double-rolls usually running at higher speeds than the single-roll machine.
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