994

THEORY AND PRACTICE OF ENGINEERING.

Book 11.

Fig. 1482.

Fig. 1482. To transform alternate rectilinear to alternate circular motion, abed is arulemade to move in a groove, another rule, tvdkvt, is attached to the first by the hinge r insuch a manner, that the system of two rules openand shut round r, like the two limbs of a pocketrule; a plate of metal Imhn , which moves paral-lel to the line bd, carries two pins p, q, whichlodge in the grooves practised in the rule

tvdkvt. It will be perceived that in virtue of thisdisposition of the piece Im, hn moves from theside of e; the two rules being supposed open, theyought first to close by the action of the pin q onthe curve t' t/, and then to be drawn by the con-tinuation of the motion of the piece Inhm. Theinverse of this takes place when the motion of thispiece is made in the opposite direction: such isthe mechanism employed by M. Droz to put underthe die the piece which is to be struck, and todrive away that already struck.

The alternate rectilinear motion of the alternate a _

hand is very slow towards the extremities of its _ )'

course, and accelerated towards the middle in order ^

to receive the pieces which fall from a hopper, and todeposit them under the die without the least shaking;this motion is communicated to it by means of a pinfixed to the running piece lnhm y which enters anoblong opening practised at the extremity of the lower arm 67 $; the axis y is horizontal andperpendicular to the plane, passing through the axis of the great screw of the die, in such amanner that making this lever move in either direction, we push the pin \ forwards orbackwards, and consequently the piece Inhm; the upper extremity of the lever 878 is sub-jected to move between two curves of metal, constructed according to given rules ; each ofthese curves is attached by its extremity to the axle of the great screw, and they aredisposed in such a manner, that the pin \ retires when the great screw lowers, and re-ciprocally by this we see how the play of the mechanic hand is united with the motion ofthe die, and what is the result of it; in fact, the circular alternate motion of the die beingsupposed constant by means of the two horizontal curves which form a groove, in which theupper extremity of the vertical lever dyS is transformed into circular alternate, very slow atthe extremity of the oscillations, and accelerated towards themiddle. The same result takes place for the lower extremity8 , but then we change the planes, and we preserve the samelength to the oscillations; if the axis 7 is the centre of thelever 87 S, we can render them greater or smaller by bringingthe upper point nearer or farther.

Fig. 1483. a b is the beam of a steam-engine, c the centreof rotation, n a bar of iron which turns freely round an axis r,placed at the extremity a of the beam, and which divides thebar n into two equal parts; the bar of iron n is attached at oneof its extremities to the piston rod, 0 , and at the other extremityto the iron bar de , which turns round a fixed axis.

Fig. 1484. Let c be a drum; a gorge or spiral goes fromone extremity a of this drum, and after a certain number ofrevolutions terminates at the other extremity 6 ; let fh be thetotal course which the carriage must make; take hg equal tothe height at of the drum A; fix a cord at f, and another atg both should be of the length fg; the other extremity of thefirst cord is fixed at a, and that of the second at 6 , after beingrolled in an opposite direction on the spiral gorge of the drumA : these two cords ought always to touch at the same point ofthe surface of the drum A ; according to this arrangement wesee, first, if the drum A turns uniformly round its axis, its motionof translation will vary, as the radii of the spiral; secondly, inproportion as one of these two cords unwinds, the otherwinds, consequently they are equally stretched; thirdly, thedrum A will have no tendency to move in the direction of itsaxis ac.

This latter circular alternate inconstant motion is changedinto alternate rectilinear, following the law which the end ofthe machine requires.

Fie. 1483

Fig. 14M.