Fig. 59 shows a cock of considerable size, which may be used for water or steam under high pressure. The plug in this example is hollow, and is prevented from coming out by a cover which is secured to the casing by four stud bolts. An annular ridge of rectangular section projecting from the under side of the cover, and fitting into a corresponding recess on the top of the casing, serves to ensure that the cover and plug are concentric, and prevents leakage. Leakage at the neck of the plug is prevented by a gland and stuffing-box. The top end of the plug is made square to receive a handle for turning it. The size of a cock is taken from the bore of the pipe in which it is placed; thus fig. 59 shows a 2¼-inch cock.
Exercise 61: 2¼-inch Steam or Water Cock.—First draw the views of this cock shown in fig. 59, then draw a half end elevation and half cross section through the centre of the plug. Scale 6 inches to a foot.
Instead of drawing the parts of the pipe on the two sides of the plug in the same straight line as in fig. 59, one may be shown proceeding from the bottom of the casing, so that the fluid will have to pass through the bottom of the plug and through one side. This is a common arrangement.
All the parts of the valve and casing in this example are made of brass.
XVII. MATERIALS USED IN MACHINE
CONSTRUCTION.
Cast Iron.—The essential constituents of cast iron are iron and carbon, the latter forming from 2 to 5 per cent. of the total weight. Cast iron, however, usually contains varying small amounts of silicon, sulphur, phosphorus, and manganese.
In cast iron the carbon may exist partly in the free state and partly in chemical combination with the iron.
In white cast iron the whole of the carbon is in chemical combination with the iron, while in grey cast iron the carbon is principally in the free state, that is, simply mixed mechanically with the iron. It is the free carbon which gives the grey iron its dark appearance. A mixture of the white and grey varieties of cast iron when melted produces mottled cast iron. The greater the amount of carbon chemically combined with the iron, the whiter, harder, and more brittle does it become.