Page:Steamlocomotivec00ahrorich.djvu/74

 piece, two pieces for the crank pins, all the above being of circular form, and four flat slabs for the webs. The slabs are milled round in a vertical milling machine, the holes for the pins and parts of the axle bored. They are then heated and shrunk on to the circular parts and crank pins of the axle, and finally secured by round plugs screwed into the joints in a direction parallel to the length of the axle.

Wheel Centres. The term “wheel centre” refers to that portion of the wheel which includes the central boss, and the spokes and rim, but does not include the axle or tyre. Wheel centres are now generally made of cast steel. They are turned outside, and also bored for the axles either in a wheel lathe, or—more conveniently—on the revolving table of a horizontal boring mill of the type shown in Fig. 26.

The wheel centres are forced on to the axles in a hydraulic press, the pressure required being from 60 to 120 tons, according to the size of wheel and axle. Small bogie and tender wheels are pressed on by 60 tons or so, but large driving wheels require from 90 to 120 tons. To obtain the necessary grip, the part of the axle or “wheel seat,” on which the wheel fits, is turned slightly larger than the bore of the wheel centre. The amount of this difference or “allowance” for an axle 8¾ in. diameter is about ²¹⁄₁₀₀₀ in., the hydraulic pressure then required being 10 to 12 tons per inch diameter of the axle. If the pressure