In describing the manufacture of machined crankshafts, C.R. spoilt a very interesting article by making one or two ambiguous statements which may have confused very many readers.
The traditional method of making crankshafts is to forge them from a selected steel. The advantage of this method is that in addition to having a choice of steels there is little wastage as the original billet is just slightly larger in volume than the finished forging; that once the dies are made a production run can be set up; that the grain flow follows the form of the crankshaft giving added strength. Among the disadvantages are that the dies are expensive to make and require a fair amount of maintenance.
Although considerably cheaper to produce, it is only in recent years that certain types of cast iron have been used for crankshaft quantity production. Cast iron is normally a very brittle and rigid material that contains a small percentage of “free carbon”. On cooling the grain flow runs from the surface to the centre of the casting which in a crankshaft would set up many weak spots. However, it is now possible to obtam cast irons where this free carbon is controlled, e.g., in SG (spheroidal graphite) irons, and this removes some of the brittleness. This, together with smaller throws and five-bearing support, makes this type of cast iron crank a cheaper alternative and is used by several manufacturers for their mass-produced lowstressed cranks.
The main disadvantage of a machined crank is in the amount of metal that must be removed and the time it takes to do this. As the grain flow runs longitudinally along the billet so inevitably parts of the crank, particularly the webs, have short grain sections. With the introduction of low-carbon nitriding steels it is possible to refine the core and give surface hardness without the need to heat to very high temperatures as in, for example, case hardening.
Your author used a very unfortunate turn of phrase by describing a sand-blasted finish as “natural steel”. Steel is a very unnatural material and comes in a variety of finishes be it black, bright, ground, turned, etc., etc. One can suppose that what was meant was that the sandor shot-blasted crank had a finish similar to one that was either forged or cast.
It is unlikely that I will ever have to call on the services of Mr. Allen to supply me with a crankshaft but it is gratifying to know that there are people who are prepared to supply the unpopular “one-offs” and small batches to the owners of the rarer and older vehicles, for without such spares available, many would become static museum exhibits or, worse, a heap of rusting scrap.
Stevenage F. J. E. Aylott
[Those statements seem to have been so ambiguous that Mr. Aylott has repeated more or less what I said, with, however, further elucidation, for which I thank him. The idea was not to write an engineering thesis, for which I am not qualified, while Mr. Aylott may be.—C.R.]