ECCENTRIC AND ITS FITTINGS.] 



APPLIED MECHANICS. 



861 



attached the eccentric-rod EP. The eccentric disc, or 

 sheave, being firmly fixed on the shaft, is caused to re- 

 volve with the latter, and its centre is thus made to de- 

 Bcribe a circle round the centre of the shaft. The whole 

 sheave thus becomes a crank-pin of extended diameter ; 

 and as it can slip freely round within the ring, the end 

 F of the eccentric-rod is caused to move upwards and 

 downwards, during every revolution, through a distance 

 equal to the diameter of the circle through which the 

 centre of the sheave revolves. The radius of this circle 

 is called the thnna of the eccentric ; and its diameter, or 



Fig. 183. 



the distance through which F is caused to move during 

 a half-revolution, is called the stroke of the eccentric-rod. 

 The getting of the eccentric upon the shaft, or the fixing 

 of its position with respect to that of the crank, is a 

 matter of nice adjustment for causing an engine to act 

 well, as we shall endeavour to describe. Let A (Fig. 184) 



Fig. 184. 



represent the outline of a cylinder and piston, with the 

 ports at one side, and a long D-slide D fitted to them ; 

 C the circle in which the crank-pin revolves, and E that 

 in which the centre of the eccentric revolves in con- 

 nection with it. We suppose the piston at the top 

 troke, and the crank-pin at C. If it be intended that 



the crank shall revolve in the direction of the arrow in 

 1, the eccentric revolving in the same direction, the slide 

 should be just opening to admit steam above the piston, 

 and to permit its exit from below it. The centre of 

 the eccentric must, therefore, be at some such position 

 as E' so that, as it continues to revolve, it may con- 

 tinue to open the slide to steam above and to eduction 

 below for some time, and then be ready at e' when the 

 crank reaches C' to reverse the movement. Again, if 

 the rotation of the crank be in the opposite direction, aa 

 in 2, the eccentric centre must for the like reason be at 

 E", a point on the side of the vertical central line op- 

 posite to that occupied by E'. In an engine always 

 moving in one direction, the eccentric can be fixed on 

 the shaft in such a manner as to give the proper move- 

 ment to the slide ; but when it is desired that the engine 

 should move in the opposite direction, some expedient 

 must be devised for altering the position of the eccentric 

 on the shaft through an angle measured by twice E' F', 

 or E* F", so as to throw its centre as much to one side 

 as it was to the other side of the crank-pin. The sim- 

 plest mode of effecting this object is to make the eccen- 

 tric sheave loose on the shaft, with a piece A B (Fig. 185) 

 projecting from its face, and a piece, or stop, C D pro- 

 jecting from the shaft. When 

 the shaft moves in one direction, 

 the eccentric remains at rest until 

 the end C of the stop comes 

 against B, when it is caused to 

 revolve with the shaft. But, if 

 the shaft rotate in the opposite 

 direction, it leaves the eccentric 

 behind, until D comes round to 

 A, when it is again caused to 

 revolve, but having its centre 

 changed with respect to any fixed point on the shaft, by 

 a quantity determined by the extent of slip before the 

 opposite edges of the stops come in contact. When 

 this arrangement is adopted, the end of the eccentric- 

 rod in generally made of the form indicated in Fig. 186, 

 Fig. ISO. 



Fig. 185. 



having what is called a gah or round-bottomed notch A 

 fitted to the pin of the slide-rod B. When the eccen- 

 tric-rod is pushed or drawn aside, so as to relieve the 

 slide-rod pin from the gab, the slide can be moved up- 

 wards or downwards by hand, independently of any 

 motion of the eccentric-rod, and the movement of the 

 steam above or below the piston thus changed at pleasure. 

 C is a guard or stop to prevent the withdrawal of the 

 rod too far from the pin. The slide-rod is generally 

 made capable of being worked by a lever conveniently 

 situated for the hand of the attendant. When he wishes 

 to reverse the motion of the engine, he quickly with- 

 draws the gab from the slide-rod pin, and by means of 

 the hand-lever moves the slide in the direction opposite 

 to that in which it was formerly moved by the eccentric. 

 He thus Jhrows the steam-pressure on the opposite side 

 of the piston if it were formerly ascending, it begins to 

 descend, and conversely the movement of the crank 

 and shaft is thus reversed ; the stop that drives the 



