The Mechanical Handling and Storing of Material

i66 THE MECHANICAL HANDLING OE MATERIAL The stroke given by the motor to the conveyor is from 6 to 16 in., and the number of strokes from 50 to 100 per minute; the speed of travel of the coal in the trough varies from 1 to 5 ft. per second. Single-Acting Motors.—These can only be used for conveyors so situated that the motor has only to lift or perform one half of the stroke, whilst the weight of the conveyor and its load are responsible for the return stroke. The first motor of this type was built by Gebrüder Eickhoff, and consists of a cylinder with piston and piston rod. The latter extends out through one end of the cylinder and is coupled to a crosshead and two rods, the latter guided on both sides of the cylinder, and these two rods are again coupled by a crosshead which is connected up to the conveyor, either directly or indirectly. If the inlet valve is opened the compressed air enters the cylinder and forces out the piston rod, and with it lifts the conveyor and its load a part of its stroke, when the air supply is cut off and the stroke completed by the expansion of the air and the inertia of the moving conveyor. After the conveyor has come gently to the end of its stroke the gravity of the load exerts itself, so that the conveyor makes its backward stroke automatically. At the same time the piston in the cylinder is reversed; before, however, the stroke is quite finished the air is automatically admitted into the cylinder again. It will thus be seen that only one end of the cylinder is connected with the compressed air mains, whilst the other is open to the atmosphere. The valve gear consists of two trippers on the piston rod which manipulate the slide valve, so that either compressed air is admitted or the cylinder put in communication with the outer air. The latest Flottmann engine, which is fixed directly to the conveyor trough, differs materially from the foregoing, and is strictly speaking a single-acting engine. Figs. 218 to 220 give a good idea of the details of the working parts. The cylinder is open at both ends, and in it move two pistons connected by rods to the junction of two pairs of toggle levers. The illustration represents the two pistons at the beginning of their cycle of movements, that is, close together and with the port for the admission of compressed air open; the pistons are now forced apart, and with them the toggle levers. This action forces the two impulse brackets,’to which the other ends of the toggles are joined, and as one of these brackets is anchored by a chain to the ground, and the other is bolted to the conveyor trough, the latter is forced to make its up-stroke. The air in the cylinder is now allowed to escape, and the weight of the trough by the toggles forces the pistons back to the first position. The working of the gear is as follows : As the pistons are forced apart, and when nearly reaching the end of the stroke, one of them uncovers a port leading to a small auxiliary cylinder with piston, rod, and slide valve, the latter at the extreme end. As a portion of the air enters now behind the piston the slide valve is pushed, forward and cuts off the air supply to the main cylinder. The movement of the slide valve at the same time pushes a rod out at the opposite end, and when the weight of the trough completes the return stroke one of the collapsing toggle levers pushes this rod back again, and thereby reopens the inlet for the compressed air, at the same time pushing the small auxiliary piston back to its first position. The impulse bracket, which is anchored by the chain, is guided by a sliding block and groove fixed to the conveyor trough to ensure the movement of the toggle levers being symmetrical. Double-Acting' Motors are all those which admit compressed air alternately on both sides of the piston, and which move the conveyor in both the forward and backward stroke. The valve or slide gears are either inside or outside the cylinder and consist of