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 shown are special ones which allow several tools to be used at once.

After having been bored the tyres are placed on the floor inside a shallow pit, which has a circular gas ring round it, by which they are heated so that they expand. The wheel centre on the axle is then taken up by a crane and lowered into the tyre, which as it cools, shrinks and grips the wheel.



Figs. 28 and 29 show the most usual methods of securing the tyres to the wheels. T is a section through the tyre, and W one through the rim of the wheel. In both examples, there is a lip A which prevents the tyre from sliding laterally over the wheel rim towards the inside of the rail. The flange F, which runs against the rail, prevents the tyre from sliding towards the outside. In Fig. 28 the tyre is secured by set screws S which pass through the rim of the wheel. Some engineers object to set screws, as the tyres tend to