This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
[Footnote: Paper read before the Institution of Mechanical Engineers.--Engineering.]
By DANIEL LONGWORTH, of London.
The movable-fulcrum power hammer was designed by the writer about five and a half years ago, to meet a want in the market for a power hammer which, while under the complete control of only one workman, could produce blows of varying forces without alteration in the rapidity with which they were given. It was also necessary that the vibration and shock of the hammer head should not be transmitted to the driving mechanism, and that the latter should be free from noise and liability to derangement. The various uses to which the movable fulcrum hammers have been put, and their success in working--as well as the importance of the general subject which includes them, namely, the substitution of stored power for human effort--form the author's excuse for now occupying the time of the meeting.
[Footnote 1: The hammers have been for some years used by A. Bamlett, of Thirsk; the American Tool Company, of Antwerp; Messrs. W.&T. Avery, of Birmingham; Pullar & Sons, of Perth; Salter & Co., of West Bromwich; Vernon Hope & Co., of Wednesbury, etc.; and also for stamps by Messrs. Collins & Co., of Birmingham, etc.]
Until these hammers were introduced, no satisfactory method had been devised for altering the force of the blow. The plan generally adopted was to have either a tightening pulley acting on the driving belt, a friction driving clutch, or a simple brake on the driving pulley, put in action by the hand or foot of the workman. Heavy blows were produced by simply increasing the number of blows per minute (and therefore the velocity), and light blows by diminishing it--a plan which was quite contrary to the true requirements of the case. To prevent the shock of the hammer head being communicated to the driving gear, an elastic connection was usually formed between them, consisting of a steel spring or a cushion of compressed air. With the steel spring, the variation which could be given in the thickness of the work under the hammer was very limited, owing to the risk of breaking the spring; but with the compressed air or pneumatic connection the work might vary considerably in thickness, say from 0 to 8 in. with a hammer weighing 400lb. The pneumatic hammers had a crank, with a connecting rod or a slotted crossbar on the piston-rod, a piston and a cylinder which formed the hammer-head. The piston-rod was packed with a cup leather, or with ordinary packing, the latter required to be adjusted with the greatest nicety, otherwise the piston struck the hammer before lifting it, or else the force of the blow was considerably diminished. As the piston moved with the same velocity during its upward and downward strokes, and, in the latter, had to overtake and outrun the hammer falling under the action of gravity, the air was not compressed sufficiently to give a sharp blow at ordinary working speeds, and a much heavier hammer was required than if the velocity of the piston had been accelerated to a greater degree.
As it is impossible in the limits of this paper to describe all the forms in which the movable fulcrum hammers have been arranged, two types only will be selected taken from actual work; namely, a small planishing hammer, and a medium-sized forging hammer.
[Footnote 1: To the makers, Messrs. J. Scott Rawlings & Co, of Birmingham, the author is indebted for the working drawings of these hammers.]
The small planishing hammer, Figs. 1 to 3, next page, is used for copper, tin, electro, and iron plate, for scythes, and other thin work, for which it is sufficient to adjust the force of the blow once for all by hand, according to the thickness and quality of the material before commencing to hammer it. The hammer weighs 15 lb., and has a stroke variable from 2½ in. to 9½ in., and makes 250 blows per minute. The driving shaft, A, is fitted with fast and loose belt pulleys, the belt fork being connected to the pedal, P, which when pressed down by the foot of the workman, slides the driving belt on to the fast pulley and starts the hammer; when the foot is taken off the pedal, the weight on the latter moves the belt quickly on to the loose pulley, and the hammer is stopped. The flywheel on the shaft, A, is weighted on one side, so that it causes the hammer to stop at the top of its stroke after working; thus enabling the material to be placed on the anvil before starting the hammer. The movable fulcrum, B, consists of a stud, free to slide in a slot, C, in the framing, and held in position by a nut and toothed washer. On the fulcrum is mounted the socket, D, through which passes freely a round bar or rocking lever, E, attached at one end to the main piston, F, of the hammer, G, and having at the other extremity a long slide, H, mounted upon it. This slide is carried on the crank-pin, I, fastened to the disk, J, attached to the driving shaft, A. The crank-pin, in revolving, reciprocates the rocking lever, E, and main piston, F, and through the medium of the pneumatic connection, the hammer, G. The slide, H, in revolving with the crank-pin, also moves backward and forward along the rocking lever, approaching the fulcrum, B, during the down-stroke of the hammer, and receding from it during the up-stroke. By this means the velocity of the hammer is considerably accelerated in its downward stroke, causing a sharp blow to be given while it is gently raised during its upward stroke.
To alter the force of the blow, the hammer, G, is made to rise and fall through a greater or less distance, as may be required, from the fixed anvil block, K, after the manner of the smith giving heavy or light blows on his anvil. It is evident that this special alteration of the stroke could not be obtained by altering the throw of a simple crank and connecting rod; but by placing the slot, C, parallel with the direction of the rocking lever, E, when the latter is in its lowest position, with the hammer resting on the anvil, and with the crank at the top of its stroke, this lowest position of the rocking lever and hammer is made constant, no matter what position the fulcrum, B, may have in the slot, C. To obtain a short stroke, and consequently a light blow, the fulcrum is moved in the slot toward the hammer, G; and to produce a long stroke and heavy blow the fulcrum is moved in the opposite direction.