CYLINDER VALVES SLIDE. ] 



APPLIED MECHANICS. 



855 



to the moving parts of the engine might be so adjusted 

 as to perform this operation ; and accordingly great in- 

 genuity has been exhibited in contrivances for alternating 

 the flow of steam to and from the opposite ends of the 

 cylinder. 



VALVES. Instead of stop-cocks in the pipes, which 

 are subject to considerable and unequal wear from constant 

 working, and thereby become leaky, valves are often em- 

 ployed, similar to the safety-valve, and worked upwards 

 and downwards by means of levers acting on their stems, 

 which project beyond the steam-tight casing in which 

 they are enclosed. Were these valves made in such a 

 way that the steam pressed on their lower surfaces, and 

 tended to raise them from their seats, it would be difficult 

 to keep them tightly down without very considerable 

 force. If, on the other band, the pressure of the steam 

 acted on their upper surfaces so as to keep them tightly 

 down, considerable force would be required to lift them 

 so as to permit the steam to pass at the proper times. 

 Moreover, as, in large engines, these valves must be of 

 considerable size to let sufficient steam pass through the 

 openings they cover, and as, for effective working, they 

 must be raised and lowered very rapidly, it becomes im- 

 portant to reduce, as low as possible, the pressure upon 

 them, and thus diminish the force necessary for their 

 movement. 



The double-beat valve is a contrivance for covering a 

 large area of steam-passage with a valve subjected to 

 moderate pressure. The steam entering at A (Fig. 164), 

 fills the valve-box, in which an annular or ring-shaped 

 valve B is capable of being pressed upwards or down- 

 wards by a rod C passing through the cover of the valve- 

 box. When the valve is down, its upper and lower coni- 

 cal surfaces rest on corresponding seats, to which they 

 are nicely ground ; while, the valve being raised, the 

 steam flows between the surfaces into the pipe D. The 

 force necessary to raise such a valve, is that of the steam- 

 pressure on the excess of the area of the lower seating 

 above that of the upper, and may be made much less 

 than that on an ordinary valve of equivalent size. We 

 hall suppose, for example, that a valve of 10 inches 



Fig. 1M. 



diameter is subjected to a pressure of 20 Ibs. on the 

 square inch. The force to lift it would be 



(circ. area of 10 ins. diam. = ) 78J sq. ins. X 20 = 1570 Ibs. ; 



and the height through which it must be lifted to give 

 the full passage for steam round it, is 2i inches ; for 2i 

 X (circumf. of 10 ins. diam. = 31^) = 78 sq. ins., the 

 area of a circle 10 ins. in diameter. But were we to use 

 a double-beat valve, having its lower seating 10J ins. in 

 diameter, and its upper 9 ins. in diameter, the area of 

 the one being 86 sq. ins. and that of the other 63^, the 



Eressure on the difference, 23 sq. ins., is 23x20=460 Ibs., 

 ttle more than one-fourth of that on the single-seated 

 valve. Again, the height through which the double-beat 

 valve must be raised is only half that required for the 



single valve ; because, when it is raised, passage is pro- 

 vided for the steam both above and below. Thus, to 

 work the double-beat valve, only }-th or Jth pai-t of the 

 force requisite for the single-seated valve is required. 

 Wherever valves are used, especially in large engines, or 

 under great pressures, for alternating the flow of steam, 

 recourse is had to the double-beat valve, or some expe- 

 dient of a similar character, by which considerable size 

 of passage may be secured without having to lift a great 

 weight, or move it through a great distance. 



SLIDE. But the frequent raising and lowering of any 

 set of valves, however well balanced, would, in quickly- 

 moving engines, be accompanied with noise, and would 

 prove very inconvenient, on account of the complication 

 of machinery required for the purpose, and the greater 

 amount of wear and tear resulting from its use. To 

 avoid these evils, the slide has been contrived ; and it is 

 almost universally employed for alternating the flow of 

 steam to or from the ends of the cylinder, except in en- 

 gines moving very slowly. There are various kinds of 

 slides in use, but they are nearly all contrived on similar 

 principles, with such differences in the details of con- 

 struction as the peculiar views of makers, or the circum- 

 stances of their position, suggest. The most simple 

 kind is called the D-slide, from the circumstance of its 

 form resembling that of the letter D. . 



Figs. 165 and 166 represent two longitudinal sections 



Fig. 165. 



of a cylinder fitted with a D-slide. The steam enters by 



Hg. 1(6. 



a pipe B from the boiler into a cavity called the slide 



