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128 much metal has to be turned off to produce the new proper contour C of the tread and shape of flange, and secondly, in the case of engines with four or six wheels coupled, the wheels wear down to unequal diameters, so that when the smallest wheel of the set of say six wheels has been turned down, all the other five wheels must be turned to exactly the same diameter, and more metal has to be removed than would be necessary if each wheel were independent. Coupled wheels of even slightly unequal diameters cause a great strain on the coupling rods, and also produce a “nosing” or side to side action of the engine when running. In Fig. 50 A is the contour of the tread of the tyre when new. This wears down to line B, and the tyre is then re-turned to the new contour C.

Tyres are usually about 3 ins. thick when new but, owing to wear and re-turning, they eventually attain a thickness which will not allow them to be safely re-turned. Wheels up to about 5 ft. 6 ins. may be re-turned so long as the tyres are in sound condition and provided that the tyres will not be less than 1½ in. thick after re-turning but, for wheels larger than 5 ft. 6 ins., the limit of thickness after re-turning is about 1¾ ins.

Axles, especially driving crank axles, are subject to very heavy bending and twisting forces, and are watched very carefully for flaws and cracks. If a minute crack begins to show itself the axle may be allowed to run until the crack develops, when the axle is immediately