The Mechanical Handling and Storing of Material

DISCHARGING BY MEANS OF SKIPS AND GRABS 397 the greater the power necessary to raise a given quantity, on account of the compression of the material between the two jaws of the grab, as may easily be understood from Fig. 556. No doubt the amount of matter scooped up increases in proportion as the gripping point is raised, but the difficulty of closing the jaws is proportionately increased. Instead of shifting the action of rotation in this way it would in many cases be more advantageous to form the cutting edge of the blade in a straight line instead of a curve, as shown in Fig. 557, as in such a case the blade will penetrate into the material better at the first onset. If the weight of the grab be insufficient, the jaws will have a tendency in closing to lift themselves out of the material after having sunk to some depth, while the cutting edge of the jaws will describe, instead of an arc, a curve as shown in the dotted lines a c, Fig. 555, C, and in consequence the grab will not take its full load. The illustrations give a clear representa- tion of the movement of the laws of ordinary grabs hinged at or near their centre, but what has been said does not refer to grabs which are hinged from their haunches. In that case the action is different altogether, as the filling of the grab does not depend in the first instance upon the jaws entering the material to some depth through the drop of the grab from a certain height, but depends entirely on a scraping action which will be understood from Figs. 558 and 559, the latter showing not only the scraping but also the closing action of the toggle levers. The Efficiency of the Grab.—The usual effect of a grab depends to a great extent not only on the shape of the jaws but also on the nature of the material to be handled. The jaws of any grab will more readily handle a light material ot a uniform size than one which has a greater specific gravity and contains large pieces. There is no difficulty whatever with most kinds of grabs in the handling of grain, sand, small coal, etc., but when dealing with a material like iron ore a grab which would fill to its full capacity on the former materials would barely fill to half its bulk capacity on the latter. The angle of repose of the material is also an important factor in the filling process, as the greater the angle of repose the more difficult it is to gather the material into the grab. It has also been maintained that the handling of coke or coal by giabs was detrimental on account of the breakage which was deemed inevitable; but there is ieally no need for such breakage if a grab of suitable construction and design is used. As a