The Mechanical Handling and Storing of Material

DISCHARGING BY MEANS OF SKIPS AND GRABS 395 lifted and discharged suddenly in its highest position by a catch which releases the grab so that the material by its own weight forces the grab open, and so escapes. The disadvantage of these appliances when handling friable material such as coal is obvious, particularly when used for forming stock piles. When the pile is high the damage to the falling coal is not so great, but as the point of discharge cannot well be altered, the coal has to drop a considerable distance when the pile is low. Another disadvantage is that one-rope grabs are generally heavier, more costly, their centre of gravity is higher up, and they are, therefore, more liable to tilt over when lowered on to the material. The two-rope grabs, which have a separate rope for lifting and for discharging, although they require more expensive winding gear, have the advantage that they can be gently opened and discharged at any height, and they may be lifted or lowered when open. In case of breakage of one of the ropes the grab would still hang by the other. With such grabs the two ropes should be the same size, as during the discharge the full weight rests on the second rope instead of on the hauling rope. Experiments have shown that the greatest resistance to the movements of the jaws is towards the end of the closing motion ; it is therefore advisable in the construction of grabs to choose means for the transmission of the power, to give the second half of the closing motion the greatest power, such as may be produced by the closing of toggle levers. The desire to build grabs which not only open wide but have a scraping and gathering action has led to the introduction of a compound movement for the jaws. If hinged at a stationary point the jaws of a grab are bound to describe a segment of a circle with their cutting edges, whereas the path of a grab with compound movement describes a curve more like an ellipse, which increases its gathering and filling actions. These have generally so powerful a closing action that even large pieces of ore can be crushed in order to accommodate them in their jaws. Grabs of this description are those of Hoover & Mason; Hulett (which is built by the Wellman-Seaver-Morgan Manufacturing Co.); the Temperley grab; and that of the Brown Hoisting Machinery Co. When unloading railway trucks and small barges, grabs should be used only for material that can be easily handled by them, such as grain, coal, coke, etc. With heavy materials which are much more difficult to handle, the weight of a suitable grab would be so great that the receptacle to be unloaded might suffer damage. “ Orange-peel ” grabs are not suitable, as they are liable to cause injury by their sharp points; it might also be mentioned that they are heavier and more expensive than a two-jaw grab of the same capacity. They are most suitable for excavating purposes, such as for sinking wells, etc. Grabs hinged at the haunches, that is from the extreme outside of the frame, get a greater leverage, and therefore a more powerful closing of the jaws, and so are the most suitable for large pieces of hard material. Salomon’s Experiments with Grabs on Dry Sand.1—A series of observations have been made by Mr B. Salomon, the results of which were published in the Zeitschrift des Vereines deutscher Ingenieure in 1886. He found that the resistance of the grab as dug into sand increased with the depth to which the grab penetrated, and he maintained that his experiments taught him that this increased resistance is also essentially dependent upon the shape of the grab jaws, or rather on the position of the fulcrum to the radius of the curve of the jaw. Perhaps the simplest way in which to explain the results of these experiments is to 1 Dingier’s Polytechnisches Journal, 2nd May 1903, pages 283 and 284.