1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



9 



IMPROVEMENT ON ECCENTRIC RODS. 



SiK— A plan has long been desired for working the sliding valves of 

 a locomotive engine with two tixed eccentrics, (that is one to each cylin- 

 der) so as to give the lead correctly when the motion of the engine is re- 

 versed, that is to sav, when the engine is working either way. There 

 have long since been locomotive engines constructed with only two 

 eccentrics, and so as to give the required lead to the valves, when 

 working in either direction: but these eccentrics used to work loose 

 upon the shaft, and when the motion of the engine was required to be 

 chano-ed, their situations were altered, by means of levers and catches. 

 But before these catches could get to their proper places, the shaft 

 was obliged to be turned, nearly half way round at least, therefore, 

 each engine was furnished with a set of rods and levers to enable the 

 engine man to work each valve by hand, until the shaft came to the 

 proper place for the catches to go together. This plan, in consequence 

 of the tediousness in reversing the motion, its being so very liable to 

 get out of repair, and other objections, has nearly fallen into disuse. 



The plan now almost universally adopted, consists of four, all of 

 ■which are tirmly fixed to the shaft. These eccentrics are so arranged 

 that two of them work the valves when the engine is going in the 

 forward direction, and the other two work the valves v/hen the 

 engine is going in the backward direction. The four eccentric rods 

 are all connected to one main lever, namely, the reversing lever, and 

 by this lever two of the eccentric rod-ends may be attached to, at the 

 same time the other two will be detached from, the levers which work 

 the valves. With this arrangement the starting, and the reversing, of 

 the engine are so simple as to be performed by the greatest novice ; 

 ■while with the former, the engine man requires considerable practice 

 before he can get properly into the way of starting and reversing. 



A plan for reversing the motion of the engine with greater ease, 

 and for giving the lead to the valves with greater accuracy than that 

 with four eccentrics, can hardly be desired; but it has long been the 

 study of many ingenious persons to contrive a method from which 

 they may obtain "exactly the same result with two fixtd eccentrics. 

 This subject has, to my knowledge, been the cause of many experi- 

 ments, some of which have by accident arrived pretty near to the 

 point of correctness; but on their being performed upon a larger scale, 

 in consequence of the persons engaged in them not being thoroughly 

 acquainted with their ruling principles, they were deemed incorrect. 

 There are those who have studied this subject so minutely, and made 

 so many unsuccessful experiments, as to at last conclude it impossible 

 to obtain this result in the manner alluded to. I have seen several 

 ingenious diagrams intended to prove the impossibility, and I have 

 even known attempts made to prove it impossible by geometrical de- 

 monstration. 



I think it needless for me to enter into the details of the valve work, 

 but, however, I will give you a short description of the method of 

 setting the four eccentrics, which will refresh your memory with their 

 principles, and at the same time perhaps, serve for as good proof of 

 the plan I am about to describe, as can readily be given. 



As the eccentrics, and all the other parts of the valve work, belong- 

 ing to the one cylinder, are generalh' the same as, but quite indepen- 

 dant of, those belonging to the other cylinder. And as each pair of 

 eccentrics require to be set at exactly the same angle with their 

 respective cranks, I think it will render the explanation much plainer, 

 to only take into consideration the two eccentrics belonging to one 

 cyhnder, namely, one for the forward, and the other for the backward 

 motion. 



Suppose A B C D, fig. 1, to be a circle described by the crank, 

 o, the lever to which the eccentric rods are to be attached, EC, a line 

 drawn through the centres of the cylinder, end of the lever, and the 

 crank axle, and B D another line also drawn through the centre of the 

 crank axle, but perpendicular to E C. Suppose it to be at C. Now, 

 when the crank is in this situation, the piston will, of course, be at the 

 end of the cylinder ; and the lead is generally considered as the dis- 

 tance the valve has moved from the middle of its stroke, or as the 

 distance it is open, when the piston is in this situation. To give this 

 lead, when the engine is working in the direction shown by the arrow 

 F, the eccentric must be set about c ; and the perpendicular distance 

 from the line B D to c, is the quantity of lead in the eccentric. Now, 

 when the rod belonging to c, namely, the eccentric rod, is attached to 

 a, tne valve will have tlie lead for working the engine in the direction 

 shown by F, and it will continue to open until the crank arrives at G. 

 But if the crank be turned in the direction shown by H, the eccentric 

 will cause the valve to move in the wrong direction, and, consequently, 

 allow the steam to act contrary to the motion of the piston ; therefore, 

 another eccentric e, is furnished, which is set at exactly the same angle 

 with the crank as c, but on the opposite side. Both of the eccentric 



rod ends are connected with the reversing lever, as I have before ob- 

 served, by which they may be detached from, and ■attached to the lever 

 a, at pleasure. It will be seen, by a little attention to tlie figure, that 

 the changing of the eccentric rods, when the crank is at C, will pro- 

 duce no alteration in the position of the valve, neither is it necessary 

 it should, because the piston is then at the end of its stroke, and, 

 although the crank be required to turn in the other direction, the 

 steam will still be required to act upon the same side of the piston. ^ 



F;g- I. 



:bi g 



Fig. 2. 



Let us now suppose the crank to be at B, the eccentrics will now be 

 at/, g, and the |)iston about the middle of the cylinder. When the 

 engine is intended to work in the direction of F, the rod belonging to 

 y; must be attached to the lever, which will cause it to stand at //, and 

 consequently the valve will be wide open, with the exception of the 

 little difference caused by the lead. To reverse the motion, that is to 

 say, to set the valve for working the engine in the other direction, 

 the valve must be made to slide so as to open to the same extent, to 

 allow the steam to act upon the contrary side of the piston. This is 

 accomplished by the reversing lever, which detaches the rod belong- 

 ing/, and attaches that belonging to g, which, by means of its forked 

 end, draws the lever from Ti to /, and consequently causes the steam to 

 act on the other side of, and force back, the piston. 



By a little attention it may be seen that, while the crank is in any 

 point of its revolution, the chang:ing of the eccentric rods will produce 

 that alteration, in the position of the valve, required to reverse the 

 motion of the engine ; therefore, I think the two points, in which we 

 have supposed the crank, will be sufficient to explain the manner in 

 which the lead is effected, and the motion reversed by the two fixed 

 eccentrics to each valve. 



I shall now proceed to explain the principles of a plan for giving 

 the lead to the valves, ■and reversing the motion of a locomotive en- 

 gine, with two fixed eccentrics, instead of four. In the following ex- 

 planation, for the same reason as in the foregoing, I shall only speak 

 of the valve, &c., belonging to one cvlinder. 



Suppose (as in fig. 1,) the circle A B C D, fig. 2, to be described by 

 the crank, E C, a line drawn through the centres of the cylinder, and 

 crank axle, and B D to be drawn perpendicular to E C. Suppose the 

 crank to be at C, and the eccentric at f. After having determined 

 the quantity of lead to be given by the eccentric, draw the lines F G, 

 and H I, at the same angles with the crank, as you would set the eccen- 

 trics in fig. 1, to the same quantity of lead. Then draw the line J K, 

 perpendicular to H I, and that end of the lever to which the eccentric 

 rod is attached when the engine is working in the direction of L,must 

 come in this line ; supposing the valve to be worked from the lever M. 

 Bv a little attention it will be perceived that, by setting the end of the 

 lever in this situation, the valve will have the same quantity of lead, 

 ■as it would if the lever and eccentric were set as in fig. 1. To cause 

 the engine to be right for working in the contrary direction, no altera- 

 tion is necessary in the situation of the valve ; still it would not do to 

 let the eccentric rod remain attached to s, therefore, I introduce 

 another lever v, the end of which comes into the line N O, which is 

 drawn perpendicular to F G, and, by means of the reversing lever, I 

 detach the eccentric rod from t, and attach it to r, which will still 



C 



