Fig. 774. Rectilinear motion of slide produced by the rotation of screw.

Fig. 775. Screw stamping-press; rectilinear motion from circular motion.

Fig. 776. In this, rotary motion is imparted to the wheel by the rotation of the screw, or rectilinear motion of the slide by the rotation of the wheel. Used in screw-cutting and slide-lathes.

Pig. 77S. Uniform reciprocating rectilinear motion from uniform rotary motion of a cylinder, in which are cut reverse threads or grooves, which necessarily intersect twice in every revolution. A point inserted in the groove will traverse the cylinder from end to end.

Fig. 779. The rotation of the screw at the left-hand side produces a uniform rectilinear movement of a cutter, which cuts another screw-thread. The pitch of the screw to be cut may be varied by changing the sizes of the wheels at the end of the frame.

Fig. 780. Uniform circular into uniform rectilinear motion; used in spooling frames for leading or guiding the thread on to the spools. The roller is divided into 2 parts, each having a fine screw-thread cut upon it, one a right and the other a left-hand screw. The spindle, parallel with the roller, has arms which carry 2 half-nuts, fitted to the screws, one over and the other under the roller. When one half-nut is in, the other is out of gear. By pressing the lever to the right left, the rod is made to traverse in either direction.

Fig. 7S1. Micrometer screw. Great power can be obtained by this device. The threads are made of different pitch, and run in different directions; consequently a die or nut, fitted to the inner and smaller screw, would traverse only the length of the difference between the pitches for every revolution of the outside hollow screw in a nut.

Fig. 782. Persian drill. The stock of the drill has a very quick thread cut upon it. and revolves freely, supported by the head at the top. which rests against the body. The button or nut, shown on the middle of the screw, is held firm in the hand, and pulled quickly up and down the stock, thus causing it to revolve to the right and left alternately.

Fig. 783. Circular into rectilinear motion, or the reverse, by means of rack and pinion.

Fig. 784. A cam acting between two friction-rollers in a yoke. Has been used to give the movement to the valve of a steam engine.

Fig. 785. Rotary motion of the toothed wheels produces rectilinear motion of the double rack, and gives equal force and velocity to each side, both wheels being of equal size.

Fig. 786. A substitute for the crank. Rceiprocating rectilinear motion of the frame carrying the double rack produces a uniform rotary motion of the pinion-shaft A separate pinion is used for each rack, the two racks being in different planes. Both pinions are loose on the shaft. A ratchet-wheel is fast on the shaft outside each pinion, and a pawl attached to the pinion to engage in it, one ratchet-wheel having its teeth set in one direction, and the other having its teeth set in the opposite direction. When the racks move one way, one pinion turns the shaft by means of its pawl and ratchet; and when the racks move the opposite way, the other pinion acts in the same way, one pinion always turning loosely on the shaft.

Fig. 787. A mode of doubling the length of stroke of a piston-rod, or the throw of a crank. A pinion revolving on a spindle attached to the connecting rod or pitman is in gear with a fixed rack. Another rack carried by a guide-rod above, and in gear with the opposite side of the pinion, is free to traverse backward and forward. Now, as the connecting rod communicates to the pinion the full length of stroke, it would cause the top rack to traverse the same distance, if the bottom rack was alike movable; but as the latter is fixed, the pinion is made to rotate, and consequently the top rack travels double the distance.

Fig. 78S. Reciprocating rectilinear motion of the bar carrying the oblong endless rack, produced by the uniform rotary motion of the pinion working alternately above and below the rack. The shaft of the pinion moves up and down in, and is guided by, the slotted bar.

Fig. 789. Each jaw is attached to one of the two segments, one of which has teeth outside and the other teeth inside. On turning the shaft carrying the two pinions, one of which gears with one and the other with the other segment, the jaws are brought together with great force.

Fig. 790. Alternating rectilinear motion of the rod attached to the disc-wheel produces an intermittent rotary motion of the cog-wheel by means of the click attached to the disc-wheel. This motion, which is reversible by throwing over the click, is used for the feed of planing machines and other tools.

Fig. 791. The rotation of the 2 spur-gears, with crank-wrists attached, produces a variable alternating traverse of the horizontal bar.

Fig. 792. Fiddle drill. Reciprocating rectilinear motion of the bow, the string of which passes around the pulley on the spindle carrying the drill, producing alternating rotary motion of the drill.

Fig. 793. Intended as a substitute for the crank. Reciprocating rectilinear motion of the double rack gives a continuous rotary motion to the centre gear. The teeth on the rack act upon those of the 2 semicircular toothed sectors, and the spur-gears attached to the sectors operate upon the centre gear. The two stops on the rack, shown by dotted lines, are caught by the curved piece on the centre gear, and lead the toothed sectors alternately into gear with the double rack.

Fig. 794. A modification of the motion shown in Fig. 791, but of a more complex character.

Fig. 795. A bell-crank lever, used for changing the direction of any force.

Fig. 796. Motion used in air-pumps. On vibrating the lever fixed on the same shaft with the spur-gear, reciprocating rectilinear motion is imparted to the racks on each side, which are attached to the pistons of 2 pumps, one rack always ascending while the other is descending.