Ml 



APPLIED MECHANIC& 



[BOCBNTRIO LINK MOTION CAMS. 



eccentric is brought round to the opposite side ; and 

 when, after a few alternations of the slide l.y hand, the 

 reversal of the engine U fairly established, the gab of 

 the eccentric-rod is permit tod again to drop over the 

 alide-rod pin, and the contrary motion continues. 



When there are two engines working together, such 

 as marine or locomotive engines, the number of opera- 

 tions required for reversal makes it a rather ditli.-ult 

 matter. There are two slides, two eccentrics, and two 

 sets of hand-levers ; both gabs have to be thrown out of 

 gear, and both levers worked for a time by hand ; while 

 great care is demanded on the part of the attendant, lest 

 he work them in such a way as to neutralise each other, 

 or oppose the reversing effect which he desires to pro- 

 duce. To overcome these difficulties, an ' ingenious ar- 

 rangement, called the link motion, has been introduced 

 in marine and locomotive engines. For each of the 

 engines there are two eccentrics fixed side by side on the 

 haft, and their rods are jointed to an arc A (Fig. 187) 



with a circular slot in 

 it, in which the pin B of 

 the slide-rod can freely 

 slide. The two eccen- 

 trics are fixed, on the 

 shaft in such positions 

 that one is adapted for 

 the motion of the shaft 

 in one direction, while 

 the other suits its motion 

 in the opposite direction. 

 When the rod of the one 

 is nearly in a direct line 

 with the slide-rod, it 

 gives it its reciprocating 

 motion, while the other 

 merely causes the arc 

 to oscillate without 

 affecting the motion of 

 the slide ; but when the 

 arc and rods are pulled 

 aside, so as to bring the 

 slide-pin under the other i 

 eccentric rod, its motion is given to the slide, and the 

 engine is thus reversed. With such an apparatus, then, 

 one simple movement of a hand-lever, connected with 

 the arcs of both engines, causes their immediate reversal. 

 But this is not the only advantage of the link motion. 

 It will be readily seen that the middle point A of the 

 arc being brought round to the slide-rod pin, the latter 

 will be left nearly at rest ; for the opposite ends of the 

 arc being moved nearly in opposite directions by the 

 eccentrics, will merely oscillate round A, as a fulcrum or , 

 centre. By bringing the arc to this position the engines / 

 are stopped, because the slide being at rest, admits noj 

 alternation of steam above or below the piston. Again, v ^ 

 by shifting the arc so as to bring the pin to any point on X 

 either side of A, more or less movement of the slide in 

 Fig. iss. either direction is produced 



at pleasure, and thus the 

 quantity of steam passing 

 through the ports into the 

 cylinder may DO varied, and 

 consequently the speed of 

 the engine, according as the 

 slide is caused to expose a 

 greater or less amount of 

 opening at the ports for its 

 admission. 



CAMS. When valves are 

 used instead of the slide, 

 for alternating the flow of 

 team to the cylinder, it is 

 not unusual to move them 

 by apparatus called cam*. 

 A (Fig. 188) U a pipe leading 

 from the boiler, B a port to 

 the cylinder, and C a valve, 

 either simple or of the 

 doable-beat kind, closing the passage from A to B. The 



valve-rod CD, passing through a stuffing-box in the 

 cover of the valve-box, terminates in a roller D, which 

 bean upon a cam F fixed on a shaft E, caused to rotate 

 by the engine. This cam is a disc partly circular, and 

 with part of it, F, projecting to a greater distance from 

 the centre. As long as the roller D bears upon the cir- 

 cular portion, the valve O remains down upon its seat ; 

 but as the projecting part of the cam is brought by 

 its revolution under the roller, the valve-rod is pushed 

 up, aud the valve lifted to permit the passage of the 

 steam. By varying the extent of the projecting part, 

 the valve can be Kept open during a greater or less 

 portion of a revolution of E. When it is desirable to 

 vary this period, the cam is sometimes made with 

 numerous steps of various extents, on any of which the 

 roller may be made to boar at pleasure (Fig. 189). 

 Fig. 189. 



For engines revolving slowly, this mode of working 

 valves is very valuable, as it gives the power of admit- 

 ting steam to the cylinder during a less or greater part 

 of a stroke, and thereby of taking advantage of its 

 expansive power within the cylinder. But when the 

 rate of revolution is rather rapid, it is difficult to make 

 the cam and roller capable of working without noisy and 

 injurious blows, resulting from their rapid alternations. 



GOYKUXoK. In stationary engines, which are sub- 

 jected to continual variations of work, and yet are re- 

 quired to move with great regularity, it is essential to 

 provide some means of governing the speed. The most 

 simple and efficacious apparatus for this purpose is the 

 conical pendulum or governor, invented by Watt (Fig. 

 190). We have already described it in general terms as 

 applied to windmills. 



Fig. 190 



For Rteam-enginos it is used in a similar manner. The 

 vertical spindle is put in motion by the engine, and 

 revolves quickly or slowly, according to the velocity of 

 the engine. When it revolves rapidly, the balls fly out- 

 wards, and raise the grooved brass which slides on the 

 spindle A (Fig. 191), and thus moves a forked lever I:, 

 which, by proper rods and levers C, causes the throttle- 

 valve D to turn round in the steam-pipe, and check the 

 passage of the steam to the cylinder. When the engine 

 revolves slowly, on the other hand, the balls fall in, the 

 brass sinks, aud the throttle-valve is presented edgeways 

 to the steam, and permits a more free passage. 



As, in the pendulum of a clock, the length from the 

 point of suspension to the bob must be regulated to beat 



