OF THE PISTOX.] 



APPLIED MECHANICS. 



857 



it and the edge of the disc below, a groove which con- 

 tains the packing ring D. This ring is made to fit the 

 Fig. 168. 



cylinder, and is cat obliquely across at some point of its 



circumference E, a parallelogram-shaped hole being cut 



if the middle, and filled with a piece of metal truly 



fitted to it. By thus cutting the ring across at E, it is 



permitted to expand in diameter ; and the slits made at 



E are covered by a plate F inside the ring so that no 



Fig. 169. 



1 



J 



steam can pass by them from one side of the piston to 

 the other. Several bent pieces of steel-plate G are 

 placed between the ring and the boss of the piston, so 

 as to push the circumference of the ring outwards. 



As the inside of the cylinder and the edges of the 

 piston and its cover become worn by constant rubbing, 

 the packing-ring is made to expand, and still to work 

 tightly in the cylinder, without permitting the flow of 

 steam past the piston. Sometimes, for small pistons, 

 the packing-ring is merely made thicker at the side op- 

 posite its slit ; and being at first slightly larger than the 

 cylinder, so that it must be compressed when pushed 

 into it, its own elasticity makes it expand to fit the 

 cylinder even after considerable wear, without the 

 necessity for steel springs within it (Fig. 170). 



The piston-rod, in passing through the cylinder cover, 

 is surrounded by a cavity called a stuffing-box, and filled 

 with soft twisted hemp and tallow, called pac'.insj, which 

 is compressed in the cavity by means of a g and, forced 

 down upon it by tightening screws. By the use of this 

 packing, while the rod travels upwards and downwards, 



VOL. I. 



steam cannot pass round it ; for even if the rod be worn 

 somewhat irregularly, the elasticity of the packing serves 



Fig. 170. 



to prevent the leakage of steam. Occasionally vulcanised 

 india-rubber is employed for the purpose of packing. 



The slide-rod, and, indeed, all rods about an engine 

 for moving valves or parts within cavities containing 

 steam or water, have to pass through packing of this 

 kind. 



SHUT-OFF AND THROTTLE VALVES. For 

 opening or closing the communication between the boiler 

 and the cylinder, so as to admit steam from the one to the 

 other, to diminish the quantity or entirely arrest it, a 

 stop-cock is generally employed for small engines, and for 

 larger ones a shut-off valve. The stop-cock, being suffi- 

 ciently well-known, we need not describe . The shut-off or 

 stop-valve is generally made as indicated in Fig. 171 . The 

 steam-pipe A communicates with the valve-box, having 



Fig. 171. 



a pipe B proceeding from it. The mouth of this pipe is 

 fitted with a conical edging or seating, to which a conical- 

 edged valve C is nicely, fitted. Through a stuffing-box 

 D in the cover of the valve-box, passes a screwed rod, 

 connected by a free joint to the valve C ; when the rod 

 is turned round by a handle E, so as to screw it inwards 

 through the gland D, the valve is pressed firmly down 

 on its seating, and thus all communication from A to B 

 is cut off. By unscrewing the rod, the valve is raised 

 from its seating as much as may be required for the 

 passage of steam. 



The throttle-valve is for the purpose of choking or 

 throttling the passage of steam in a pipe, so as to regulate 

 the quantity passing through it in a certain time, with- 

 out perfectly arresting it. It consists of a disc A (Fig. 

 172) mounted on a rod passing across the pipe through 

 a stuffing-box at one side. On turning the disc edgeways 

 towards the current, the steam is allowed to pass ; but 

 when it is turned across the pipe, none can pass except 

 such a small quantity as can leak round the edges of the 

 disc. This valve is almost universally applied to engines 



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