APPLIED MECHANICS. 



[REVERSING ORAR. 



the outer and inner circles ; and from O to B, the re- 

 maining Jtli of a revolution, Irt it follow tho inner circle. 

 It in nl. MOIII that, as the shaft rotates and brings the dif- 

 ferent portions of the cam's circumference successively 

 under the roller, tho centre of which we suppose to bo 

 capable of verticil movement only, it successively lifts it 

 one inch, retains it, lifts it again one inch, retains it, 

 and permits it to drop, and remain down according to 

 the conditions of tho problem. The principal point to 

 be attended to in the construction of a cam is, that tho 

 spiral portions leading from the one part of the circum- 

 frrruoe to the next be not too abrupt, and that they bo 

 carefully graduated from the one curvature to the other, 

 so that tho roller be not subjected to sudden jerks, but 

 be made to rise from or fall towards the centre as gently 

 and easily as possible. When the rotation of tho cam 

 is rapid, this is of the greatest importance : the amount 

 of eccentric movement given to the roller should be as 

 small as possible ; or it should be spread over as large a 

 part of the circumference as possible, so that the inclina- 

 tion of the revolving slope may be gentle. 



In some kinds of printing machinery a peculiar form 

 of screw-cam is applied, for the conversion of a continuous 

 rotation into a reciprocating rectilineal motion. 



A cylinder A (Fig. 266), rotating in bearings B B, has 



Fig. 256. 



a double groove of a screw form, cut in its surface, in 

 which a pin C, projecting downwards from the table of 

 tin- printing machine, is free to slide. The table being 

 guided to slide backwards and forwards in a straight 

 line, is caused to move by the revolution of tho cylinder 

 presenting always tho inclined face of its screw-groove 

 to the pin. The screw in double, of opposite direction, 

 with a portion at each end not oblique to the axis of the 

 cylinder. One half-revolution of the cylinder causes tho 

 pin to travel throughout the length of its screw ; another 

 half-revolution retains it at the end of its stroke ; the 

 next half-revolution carries it back to tho opposite end ; 

 tho next retains it ; and so on successively to suit tho 

 alternate rest and motion of tho printing-table to the 

 successive impressions and intervals between them. 



M \ N i : I . K MOTION. Racks and partly--. -are. 1 

 pinions are sometimes used for a similar purpose, thus : 

 A double rack a a (Fig. 257), guided at the ends li and 



Fig. 147. 



f to move only longitudinally, is acted on by a partly- 

 geared pinion e continuously rotating. When the teeth 

 pinion gear with those of the upper limb of the. 

 rack, it is caused to move from b towards c, until tiie 

 pinion, having left that limit, enters into gear with 

 tin- other, giving it the contrary movement, and soon, 

 successively. 



-'.VI represents an arrangement of a similar cha- 

 racter, applied to produce alternating rotary motion. A 

 pinion c roiitimiotmly routes on an axis fitted with a 

 unirnnal-ioint, such as may permit the pinion to gear 

 either with the exterior or interior cogs of the d.ml.V 

 circular rack fc, ti\,-.| .01 the face of a drum or pulley a, 

 and thus to ivr it rot.-.i -.iimd its centre e, in 



direction! alti-rimMy opposite. Thin apparatus having 



been applied to ilriving mangles, has received the name 

 of the Mangle Mutton. 



Fit. MS. 



Fig. 269. 



REVERSING GEAR. On referring to our remark* 

 respecting the communication of motion by pulleys :m<l 

 straps, it will be seen, that while a pulley, driven from 

 another by a direct strap, revolves in the same direction, 

 one driven by a crossed strap revolves in the opposite 

 direction. This principle is frequently applied in ma- 

 chinery where it is desired to reverse the direction of 

 motion. Two pairs of fast and loose pulleys are ar- 

 ranged on a shaft, so that one pair may be connected by 

 a direct, and the other pair by a crossed, strap, with 

 pulleys on the prime mover ; when the direct strap is 

 on its fast pulley, while the crossed strap is on its loose 

 pulley, tho machine is driven in tho direction of the 

 prime mover ; but when the direct strap is thrown on 

 its loose pulley, while the crossed strap is brought on its 

 fast pulley, the contrary motion is produced. In ap- 

 paratus where on arrangement of this kind is applied, as 

 in machines for planing iron, the movement of tho table 

 of the machine to each extreme of its stroke is made to 

 shift the straps by very simple mechanism, which in 

 capable of lieing adjusted so as to vary the amount of 

 stroke at pleasure. 



Fig. 259 represents another method of reversing rotary 

 motion, frequently employed : a and & are two bevil 

 pinions revolving loosely on a shaft d e, and gearing with 

 a bevil wheel c. Between tho pinions is fitted a clutch, 

 sliding on, but revolving with, 

 the shaft d e. The pinions 

 being geared into opposite sides 

 of the wheel, rotate in opposite 

 directions ; and as the clutch 

 is thrown into gear with the 

 one or tho other, tho shaft tl e 

 is caused to rotate in tho one 

 direction or the other accord- 

 ingly. Instead of a toothed clutch, a conical friction 

 clutch is occasionally employed with good effect, because, 

 in tho first place, a very slight movement of tho lever 

 pressing the cone into iti seat on either side, is sufficient 

 to couplo the shaft with either wheel ; and, in tho next 

 place, tho shaft being driven solely by tho friction of tho 

 conical surfaces, cannot be subjected to any strain ex- 

 ceeding tho friction in amount, the cones slipping when 

 subjected to extreme strain. In powerful rolling ma- 

 chinery, this plan prevents a sudden shock, always in- 

 separable from tho use of the clutch, and which may 

 cause tho breaking down of some part of the ma- 

 chinery. 



Fig. 200 indicates a modo of reversing the direction 

 af rotation by means of toothed gearing. A being a 

 toothed wheel on the driving-shaft, and I! one on the 

 shaft to which it is required to convey motion in either 

 direction, C, D, ami K aro intermediate pinions mounted 

 on a frame capable of vibrating on A as an axis. When 

 C is geared with A and I!, as in I., both revolve in tho 

 direction ; but when C is thrown out of gear with 

 K. Mini K brought into gear with it., as in II., the di- 

 rection of its rotation is reversed, m in liratod by the 

 arrows. By some such arrangements, or modifioationi 

 of them, suited to tho circun. any particular 



case, the direction of rotation may be readily vari'iL 



