The bearings of axles of all classes demand much attention from the smith during forging, and the greater the dimensions of the work in progress, the greater is the responsibility of the workman who happens to be managing the particular forging being made. No one but himself knows the quality of the metal employed, the relative position of the components, or the treatment the work receives during the several processes.

Crank-axles involve an additional consideration to that of the bearings. The levers are equally important, many of them being improperly made, either of unsuitable metal or of good metal whose component plates or fibres are at right angles to the proper position. It is an almost unknown occurrence for a smith to receive any instructions concerning these matters; he therefore depends upon what ingenuity or practical knowledge he may possess, and proceeds accordingly. A good smith will therefore be careful to ascertain the quality of the bars he is to use; and, when circumstances permit, he will superintend the shingling, and thus become intimately acquainted with the metal with which he is supplied. The material selected for crankshafts should be of hard, close-grained, tenacious character; and the suitable degree of hardness for the axle portions is attained with hammering, as previously described for other shafts.

A class of simply formed crank-axles is that having one-arm cranks; such axles have their crank-pins extended or produced from the outer sides of the arms or levers. Of this class of cranks, those that are made of two pieces are first described.