_Answer._ They are made of cast iron with wrought-iron or steel tires around the outside. Fig. 161 represents a perspective view of a pair of locomotive wheels and axle. The central portion of the wheel, that is the hub, spokes and rim, are cast in one piece. Usually the hub and the rim, and sometimes the spokes, are cast hollow. The central portion of the wheel, that is the part which is made of cast iron, is called the _wheel-centre_. [Illustration: Fig. 161.] QUESTION 265. _How are the tires fastened on the wheel-centres?_ _Answer._ The insides of the tires are usually turned out somewhat smaller than the outside of the wheel-centre. The tire is then heated so that it will expand enough to go on the centre. It is then cooled off, and the contraction of the metal binds it firmly around the cast iron part of the wheel. As an additional security bolts or set-screws, _a_, _a_, fig. 161, are screwed through the rim and into the tire to prevent it from slipping off in case it becomes loose. In some cases the wheel-centre and the inside of the tire are turned conical, and the tires are then put on cold and held on with hook-headed bolts, _C_, as shown in fig. 162, which is a section of the tire and the rim of the wheel. The wheel-centre is made largest on the inside. As the strain against the flange of the tire is inward, the cone of the wheel-centre resists this strain. If it was curved or tapered the reverse way, the strain would come on the bolts, and it would also be impossible to remove the tires without first taking the wheels off the axles. This method of putting on tires has the advantage that they can be removed quickly and without heating the tires.[70] [70] It is exclusively used on the Baltimore and Ohio Railroad. [Illustration: Fig. 162. Scale 3 in. = 1 foot.] QUESTION 266. _Are there any standard sizes for the inside diameters of tires?_ _Answer._ Yes. To avoid the great inconvenience arising from the diversity in the _inside diameters_ of tires, the American Railway Master Mechanics’ Association has recommended that the inside diameter of tires should be made 36, 40, 44, 50, 56 and 62 inches. The thickness for the first three sizes to be 3 in. and the last three 2¹⁄₂ in. QUESTION 267. _How are the driving-wheels fastened on the axles?_ _Answer._ The hubs are accurately bored out to receive the axles, and the latter are turned off so as to fit the hole bored in the wheel. The axles are then forced into the wheel by a powerful pressure produced either with a hydraulic or screw press, made for the purpose. In order to prevent the strain upon the crank-pins from turning the wheels upon the axle, they are keyed fast with square keys driven into grooves cut in the axle and in the wheel to receive them. The ends of these keys are shown at _b_, fig. 161. QUESTION 268. _How are the crank-pins made?_