JACK, is a word used by practical mechanics to de nete any simple or trifling piece of mechanism.
Jacks and Jack Sinkers, are parts of a stocking-frame, see CHA1N'WORK ; and lace machines are also provided with parts called jacks. A harpsichord has a jack at eve ry key. Brewers have a large vat which they call jack back. Masons employ a hand jack, screw jack, or jack in a box. Carpenters use a jack plane. Jack towels and roasting-jacks are to be found in every kitchen.
We shall here give a description of the roasting-jack and the hand-jack, both of which are complete machines in themselves ; whereas most of the other kinds of jacks are only small parts of other machines.
Hand Jack, or Jack in a Box, is a simple and portable mechanical power for assisting the hand in heaving up or moving heavy bodies to a small distance, as for instance, to raise the end of a large block of stone or piece of tim ber sufficiently to put rollers beneath to remove it, to turn a stone or tree over, or to put a chain round it, to repair buildings, and to lift up an angle of a wall to undermine the foundation. Many other purposes to which a machine of this description is applicable are too obvious. The sim ple lever, or wedge, will, in many instances, perform all that is desired from a hand jack ; but the lever will not retain the load at the height to which it is raised, and the drawing of wedges is a tedious operation. The power of the common or simple hand jack, is obtained by a rack and pinion. See Plate CCXCIX. Fig. 6. A block of wood about 2 feet 6 inches long, 10 inches broad, and 6 inches -wide, is perforated with a square hole or mortice through it lengthwise, for the reception of an iron rack B. This rack is formed with a double claw or horn at its upper end. A small pinion C is made to engage in the teeth of the rack. The axis of the pinion is supported in iron plates bolted to each side of the block, and one end of the axis projects through the side plate, with a square to receive a winch or handle, which, being turned round, the pinion elevates the rack B in the mortice, and raises the claw or horn up to the load to which it is applied. To prevent the weight of the load running the pinion back, the handle is detained by a hook or link a fastened to the outside of the block. When a greater power is required than the simple rack and pinion are capable of exerting, a combina tion of wheel-work is used, as shewn in the same Figure. where AA is the block of wood, which in this case is made sufficiently wide to contain the cog-wheel F, fixed to the pinion C, which acts in the teeth of the rack B. G is a
second pinion of four leaves, working in the wheel F; and the axis of this pinion projects through the side of the block for the winch I-I to be fixed on it. The block AA is made in two halved, and the recess for the wheel F and pinion G is cut out in one of the halves; the other, being laid flat against it, supports the front pivots of the wheel and pinions. The two halves are bound together by strong iron hoops b, b, driven over the outside. The rack has a claw N at its lower end, projecting out sidewise through an opening or slit cut through, in the front half of the block. This claw can be introduced beneath a stone which lies nearly flat upon the ground, and which consequently could not be acted upon by the claw on the top of the rack. To prevent the rack descending when it has a load upon'it, the small click a drops into its teeth, but clears it in going up ; when it is not required to detain the rack, this click can be turned out of the way sidewise.
Fig. 7. is a screw-jack. The block of wood AA is perforated nearly its whole length with a hole sufficiently large to allow the screw B to move up and down without touching. The screw passes through a nut n, fixed firm into the top of the block A ; and if the screw is turned round, it must rise up through the nut, and elevate the claw F. This claw is fitted on the top of the screw with a round collar, which allows the screw to turn round without turn ing the claw ; and the claw N, which projects through a groove or opening made in the side of the block, is fitted to the screw with a smaller collar. The bottom of the block has four short points to prevent the machine slip ping when used upon hard ground. To give motion to the screw, the lower half of it is formed into a square, and a worm-wheel C is fitted upon the square. The teeth of of this wheel are engaged by a worm on the axis of the winch H, and plates of iron a, b, are bolted on each side ri the block, near the middle of its height, to carry the ends of the axis of the winch, and of the worm which is conceal ed behind the worm-wheel C. «'hen the winch is turned round, it causes the wheel C to revolve by the action of the worm in its teeth ; and as the wheel is fitted on the square part of the screw, it compels it to turn with it, but at the same time allows the screw to move up and down.