The Locomotive Of Today

134 The Framing, Wheels, etc. : Crank Axles, etc. It is difficult, especially when the steam chest is between the cylinders, to get the webs sufficiently strong, as the cylinders must be some distance apart, and the outer web is liable to be too thin, the crank having not only to turn the wheels but also support part of the weight of the engine. The inside web being further from the wheel is found to be more likely to break, and therefore it is often made thicker than the outer one, as the breakage of a crank axle is one of the most serious mishaps that can occur to a locomotive, especially if broken through tlie web. It is now customary to strengthen the webs with weldless bands of Steel rolled and blocked to the required shape, and slirunk on after heating when the webs have been machined. A common shape for these is an ellipse as shown, but they are to be seen oval, almost rectangular and circular. In the latter case bands are not used, as the webs are of ampie strength at the most usual place of break- 4ge—midway between the axle and crank-pin. To support the pin, which is the next most vulnerable part, some designers have a hole drilled through it and both webs, and a bolt put through with its ends either rivetted over, held by a head on one side and a nut the other, or as shown at G, screwed into a tapped hole in one web by a head outside the other. Crank axles have been built up of separate pieces which are screwed together instead of being in one forging. In order to relieve the cranks of racking stram four bearings are often provided, one on each side of each wheel. The weight is then transmitted directly, and the bending of the axle by it avoided. Straight axles are simple in shape, being round pieces ot metal of suitable diameters. A common form is shown at H, the wheel seat in this case is plain having no collar upon the inside; then the journal being smaller, with a collar at its other end, the central portion is again reduced in diameter. It is most important that all the various diameters should not abruptly change one to another, as by cloing this fractures are very liable to occur at these points; raclii of ampie sizes are therefore strongly recommended. Carrying this idea still further, some designers take one long sweep or double cone having no portion parallel. They have the disadvantage, however, of being somewhat more difficult to accurately fit the bearings to. Axles for American engines are usually even simpler in shape, being either parallel throughout or as shown at I, in which the wheel seat and journal are of the same diameter, being separated on]y by a small fillet, against which the