tup and the anvil of a steam hammer, and then by means of hydraulic or other pressure applied at the end of a very large spanner, to provide sufficient leverage, the other arm f₂ is gradually bent round at a right angle. Lastly the ends of the crank webs are rounded at g under the hammer, and the appearance of the axle is then as shown in Fig. 11c. The right angle must be tested with a square, and all the dimensions checked to gauges to see that enough material is left throughout to enable the machining to be done subsequently to proper finished sizes. The whole of the above operations require several heats, and the greatest care must be taken in this respect, so that no re-heating is done until after mechanical work, e.g., hammering, has been done on the axle, otherwise the internal physical structure of the steel will suffer injury. Finally all crank axles are re-heated and generally cooled in oil, the scientific considerations underlying “heat treatment” being, however, beyond the scope of this primer. Test pieces are taken from the parts machined off the webs of each axle, and subjected to rigorous tensile and cold bending tests. An oil treated crank axle should have an ultimate tensile strength of not less than 35 tons per sq. in. with an elongation of not less than 20 per cent, in a length of 2 ins.
There are other methods of making crank axles by building up the second crank web with slabs welded to the bloom in such a way that the twisting of this web through a right angle is
