Concrete Roads
and Their Construction

MECHANICAL DEVICES FOR MAKINC4 ROADS 143 the other, whilst in a gyratory crusher the material is broken by the toothed faces of two cones between which it falls. The crushing effect is obtained by a gyratory motion imparted to the cone by the special gearing. Stamping mills and crushing rolls are less effective, and edge-runner mills are quite unsuit- able. Jaw Crushers.—Among the commonest of this type of crusher are the Blake-Marsden (made by H. R. Marsden, Ltd., Leeds) and that made by Messrs. Winget, Ltd. (Fig. 91). One example of this type of mill is the Heclon crusher made by Hadfield, Ltd. The crushers just described are suitable for coarse aggregate but not for sand, as they are not economical when used to grind a material to a fine powder. Disintegrators or Cage Mills consist essentially of (a) a pair of cages revolving in opposite directions, the material being broken be- tween these cages and passing out between the bars, or (5) a ro- tating shaft carrying loosely pivoted hammer-bars which rotate like the spokes of a wheel and deliver a rapid series of blows on the material contained on a grate until the material is crushed small enough to pass through the grate. These disintegrators (with various minor improvements) are made by several firms. An illustration of the exterior of the “ Devil ” disintegrator made by the Hardy Patent Pick Co., Ltd., Sheffield, is shown in Fig. 93, and an illustration showing an interior view of another disin- tegrator made by C. E. V. Hall, Sheffield, is shown in Fig. 94. Disintegrators are not suitable for grinding to a very fine pow- der, but are very efficient for reducing hai’d lumps in sand, and for producing a proportion of fine flour in. the sand. It is generally preferable to use crushers and disintegrators on the site of the raw material and not in the roadway, though where the latter course is preferred, the crushing machinery may be mounted on a stout truck and driven by a portable engine. Care should be taken, in selecting an engine for this purpose, to choose one which is not readily damaged by the great and sudden varia- tions in the power required. A little consideration will make it clear that each time a hard piece of stone undergoes the crushing process the pressure applied to it increases steadily until it exceeds the maximum resistance of the stone, when the latter yields