Chap. XI.

MECHANICS.

965

a-4

Fig 1354.

Fig.1355.

Fig. 1356

placed, serves to communicate to the latter body a recti*linear alternate, but not uniform motion.

a is a wheel partly toothed, which turns in the samedirection on a fixed axis; be is a frame, two of whoseopposite sides are cut into a rack ; to the extremities ofthis frame the bars b a, ct are fixed, which form a singlebody, and which pass through the tenons at the end insuch a manner that the rack and the two bars which maybe regarded as one, st , may require an alternate rectilinearmotion from the circular motion of the wheel a. Wemay also apply the frame which contains the two racksto a bar, by a simple change of construction: thismotion is applied to the alternate motions of pistons ofpumps, sawing machines, &c. By suppressing one ofthe two racks, and furnishing the whole wheel withteeth, we transform alternate circular to alternate rectilinearmotion.

The frames of the preceding machine is enlarged byprolonging the two racks in such a manner that they abuton the two small sides, where they unite in the form of a halfcircle, and the whole wheel is furnished with teeth. It is ne-cessary in this case to employ two cross pieces to communicateat the end of each oscillation a slight lateral movement to theframe, to facilitate the change of gear : in general this motionpresents difficulties in execution, and it does not appearcapable of producing great effects.

This motion is applied to transform the circular continuousmotion of a horizontal axis to vertical alternate rectilinear,which work the piston of a pump, producing a slight balancingin the frame.

Fig. 1357- is a modification ofthe same motion; having re-duced the teeth of the rack toone, the number of that of thewheel is also diminished, and intheir place arms terminated bysmall wheels are substituted, todiminish friction. It will beeasily seen that the rectilinearform of the tooth of the rack

occasions an inequality of the velocity in the alternate motion ;may be varied at pleasure, or the law of velocity rendered uniform,by giving a convenient form to the teeth. This construction is pre-ferable to that of fig. 1354. for machines exposed to very great efforts,which small teeth could not resist.

In fig. 1359. a is a wheel turning on its axis, n,m two cheeks to keepsteady the sliding bar pq rs, which is in the shape of a T; a pivottraverses the groove, and the circular motion of a communicates analternate motion to the bar. This alternate rectilinear motion, veryslow at the end, and accelerated towards the middle of the oscillation,is very simple and of easy execution ; it is a modification of fig. 1354.,and is employed for churns. The circular continuous motion of thewheel can be transformed to rectilinear alternate.

The contrivances that have been devised for converting a cir-cular continuous motion into a rectilinear alternate motion arevery numerous, and many highly ingenious ; the simplest methods ofproducing a rectilinear alternating motion from a circular continuousone are those shown in figs. 1361, 1362, &c., where a beam or stamper is raised verticallyby wheels formed into an irregular outline, or carrying on their circumference wipers orspirals; the stamper is thus raised to a certain height, and descends through the sameheight by its own weight, when the succeeding wiper begins to act upon it, and raise it asecond time. The same is produced by the rack and pinion, provided the pinion has teethonly on a part of its circumference: the toothed wheel elevates the rack, which descendswhen the last tooth has ceased to act, and by the time the last tooth has ceased to act, and bythe time the rack has performed its work, the toothed part of the pinion has come roundagain, for the purpose of raising the rack a second time.

3 q 2

Fig. 1357.

this

Fig. 1353.