552 



THE STEAM-ENGINE. 



of the crank-pins B is likewise turned by suspending the axle on centres cor- 

 responding with the centres of the crank-pins, and made on strong cast iron 

 arms, which are firmly fixed on the ends of the axle, and project beyond the 

 cranks so as to balance the axle, and enable it to turn round on the centre of 

 the crank-pin. The axle is by such means made perfectly true, and the cranks 

 are made of exactly the proper length, and precisely at right angles to each | 

 other. The corners of the cranks are champered off, as shown in the figure, 

 and the ends of the cylindrical parts well rounded out. 



The 'strength and accuracy of construction indispensable in these cranked 

 axles, in order to make them execute their work, render them very expensive. 

 When properly constructed, however, they are seldom broken, but are some- 

 times bent when the engine escapes from the rails. 



The proper motion to admit and withdraw the steam from either end of the 

 cylinder is imparted to the slide-valves by eccentrics, in a manner and on a 

 principle so similar to that already described in large stationery engines, that 

 it will not be necessary here to enter into any detailed explanation of the ap- 

 paratus for communicating this motion, which is exhibited in plan and section 

 in figs. 67, 69. The eccentrics are attached to the cranked axles at E' E". 

 The eccentric E 7 imparts motion by a rod e" to a lever h", formed on an axle 

 extending across the frame of the engine. This conveys- motion to another 

 lever I", projecting from the same axle. This lever I" is jointed to horizontal 

 links m", which at the foremost ends are attached to the spindle /', by 

 which the slide is driven. By these means the motion received by the 

 eccentric from the great working axle conveys to the spindle V an alternate 

 movement backward and forward, and the points at which it is reversed will 

 be regulated by the position given to the eccentric upon the great axle. The 

 eccentric is formed in two separate semicircles, and is keyed on to the great 

 axle, and consequently any position may be given to it which may be required. 

 The position to be given to the eccentrics should be such that they shall be at 

 right angles to their respective cranks, and they should be fixed a quarter of 

 a revolution behind the cranks so as to move the slides to that extent in ad- 

 vance of the piston, since by the position of the levers h" and I", the motion 

 of the eccentric becomes reversed before it reaches the valve spindle. 



The performance of the engine is materially affected by the position of the 

 eccentrics on the working axle. The slide should begin to uncover the steam- 

 port a little before the commencement of the stroke of the piston, in order that 

 the steam impelling the piston should be shut off, and the steam about to impel 

 it in the contrary direction admitted before the termination of the stroke. 

 Through this small space the steam, therefore, must act in opposition to the 

 motion of the piston. This is called the lead of the slide, and the extent 

 generally given to it is about a quarter of an inch. This is accomplished by 

 fixing the eccentrics not precisely at right angles to the respective cranks, but 

 a little in advance of that position. The introduction of the steam to the 

 piston before the termination of the stroke has the effect of bringing it gradual- 

 ly to rest at the end of the stroke, and thereby diminishing the jerk or shock 

 produced by the rapid change of motion. In stationary engines, where the 

 reciprocations of the engine are slow, the necessity for this provision does not 

 arise ; but in locomotive engines in which the motion of the piston is changed 

 from four to six times in a second, it becomes necessary. The steam admitted 

 to the piston before the termination of the stroke acts as a spring-cushion to, 

 assist in changing its motion, and if it were not applied, the piston could not 

 be kept tight upon the piston-rod. Another advantage which is produced by 

 allowing some lead to the slide is that the waste steam which has just impelled 

 the piston begins to make its escape through the waste-port before the com- 



