INTERMITTENT MOTION.] 



APPLIED MECHANICS. 



895 



through the centre of the crank bisecting the vibration 

 of the lever, or cutting the arc in which the pin of the 

 lever vibrates in such a manner, that its deviations from 

 the straight line at its middle and extreme points are 

 equal. So also the line in which the point G is required 

 to reciprocate, when prolonged, bisects the vibration of 

 the arm E. By this arrangement the deviation of the 

 pins moving in circular arcs from rectilineal motion is 

 rendered as small as possible, and lateral strain from 

 obliquity of connecting-rods is proportionally reduced. 

 By varying the length of the arms C and E, the amount 

 of vibration may be varied at pleasure ; for while the 

 angles in which they vibrate, remain equal, the lengths 

 of the circular arcs in which their pins vibrate, are pro- 

 portional to the lengths of their radii. 



By proper arrangements of cranks, levers, and con- 

 necting-rods, in respect of lengths and relative positions, 

 it is generally possible to convert a given rotary or re- 

 ciprocating movement into one of greater or less extent, 

 in a different plane or direction. When the recipro- 

 cating movement of a lever is applied to produce the 

 rotary motion of a crank, it is necessary either to fit the 

 spindle of the latter with a heavy fly-wheel to bring the 

 crank over its dead-centres, or to adopt some other com- 

 bination of cranks, as in the duplicate marine or loco- 

 motive engines, so that while one is on the dead-centre, 

 the other is receiving motion from its connecting-rod. 

 The crank or eccentric can give reciprocating movement 

 to a lever, only in such a manner, that for each revolu- 

 tion of the crank the lever makes one complete double 

 stroke, or an excursion from one extreme of its vibration 

 to the other and back. Sometimes it is desirable that 

 each revolution of the rotating shaft shall cause a num- 

 ber of reciprocating movements of a lever or rod con- 

 nected with it. There are several methods of effecting 

 this object. For moving a stamper 6 (Fig. 253) so as to 

 crush or pulverise materials subjected 

 to jt, a wheel a is fitted with several 

 curved arms or wipers, which, as the 

 wheel revolves, come successively in 

 contact with a pin or projection c on 

 tin: stamper, lift it, and let it drop. If 

 the wheel have six arms, the stamper ''< 

 makes six strokes during each revolu- 

 tion. Such an apparatus is frequently 

 applied in cases where repeated strokes 

 of falling heavy bodies are required for 

 special operations, as for tilt-hammers 

 used in iron manufacture, seed-crushers, fulling-mills, 

 and washing-machines. For lighter work, such as tho 

 movement of pendulums or balance-wheels of time- 

 keepers, there are numerous ingenious arrangements of 

 escapement-wheels having their circumferences cut into 

 teeth of suitable form, and capable of acting on the pallets 

 presented to them, with the regularity and precision 

 required. 



INTERMITTENT MOTION. Sometimes it is de- 

 sirable to convert continuous rotary movement into one 

 that shall proceed by fits and starts. A crank or eccen- 

 tric A (Fig. 254) on the continuously rotating shaft con- 

 nected by a rod B with a lever C, causes it to vibrate 

 once during each revolution of the crank round a spindle 

 D, on which is mounted a ratchet-wheel E, having teeth 

 like those of a saw. The lever C is fitted witli a pidl F 

 hanging freely from a pin, and formed so as to drop into 

 the space between the ratchet teeth. One half-revolu- 

 tion of the crank A causes the pall to move over one or 

 more teeth without putting them in motion, as it can 

 slide freely along their inclined sides ; but the other 

 half-revolution bringing back the lever and pall which 

 bears against the abrupt face of a tooth, causes the 

 ratchet-wheel to rotate through the same arc with the 

 point of the pall. In many cases where this arrange- 

 ment of ratchet and pall is applied, it is necessary to 

 vary the amount of movement of tho ratchet-wheel at 

 each stroke. This is effected either by varying tho throw 

 of the crank A, or the position of the connecting-rod 

 pin on the lever C, so .as to make the back stroke of the 

 pall pass over two or more teeth, as may be required. 



In machines for boring, turning, planing, and shaping 

 metals, in machinery for sawing timber and such like 

 operations, this arrangement is applied for moving the 

 material operated on, so as to present a new portion to 

 the action of tho tool or saw at each stroke. 

 Pig. 251. 



CAMS. In describing the methods of regulating the 

 flow of steam in engines, we alluded to the cum as an 

 apparatus by which the continuous rotation of a shaft 

 is made to lift a valve, sustain it for a certain period, 

 and then let it drop. Cams are also applicable to many 

 other mechanical movements of a similar character : and 

 as their forms may be varied indefinitely, so almost any 

 recurring movement can be eflected by their use. As 

 an example of what may be effected by a cam, let us 

 suppose that, having a rotating shaft, we desire it, during 

 Jth of a revolution, to raise a weight 1 inch ; during Jtli 

 to retain it at that height ; during the next Jth to raise 

 it 1 inch more ; to retain it there during }th ; during the 

 next Jth to drop it the two inches through which it had 

 been raised ; and during the remaining Xth to retain it at 

 its lowest position until it is again raised. 



Let A (Fig 255) be the shaft revolving in the direction 

 of the arrow, and let three circles be described round its 

 centre, the inner circle being of any convenient radius, 

 and the others having radii respectively 1 inch and 2 

 inches greater. Let the circles be divided into eight 

 equal segments, and let H be a roller connected with the 

 weight to be lifted, bearing on the inner circle at B. 

 Jig. 2:5. 



From B to C, Jth of a revolution, let the circumference 

 be part of a spiral curve touching the inner circle B and 

 the middle circle at C ; from C to D, Jth of a revolution, 

 let it follow the middle circle ; from D to E, I Mi of a 

 revolution, let it bo another spiral curve touching the 

 middle circle at D and the outer at E ; from E to F, Jth 

 of a revolution, let it follow the outer circle ; from F to 

 G, Jth of a revolution, let it again be a spiral touching 



