From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Piston alloy composition, specifically copper and iron content, and issues related to heat treatment, distortion, and cracking.
| Identifier | ExFiles\Box 142\2\ scan0050 | |
| Date | 4th July 1925 guessed | |
| - 2 - " In view of the softness of the pistons which are low in tin and copper, we are seriously considering the advisability of calling for copper 7.5% to 8.5% . It is the harder pistons of which analysis is given above that are most satisfactory since the softer ones have very little spring in the skirt. As regards the presence of iron, we have not investigated this in piston alloy but have done so in crankcase (zinc) alloy. Aluminium Manufacturers Inc. have shown us within the past year that 1.0 to 1.5% of iron in this alloy is very desirable, giving consistent results of over 30,000 Lbs sq. in. ultimate and 4.5 to 6% elongation. The U.S. Navy has recently adopted a similar specification as our own with 1. to 1.5% iron for their standard "high tensile" alloy. With regard to "growth" and distortion, we have found this to be more a matter of annealing than of material, although the harder pistons generally appear to grow less than the softer ones. We anneal at 500 - 550° F both on the rough castings and after turning before grinding. This appears to have eliminated most of our trouble, although a certain amount of distortion takes place as a rule after first dynamometer test, the piston increasing slightly and showing hard bearing marks at the upper edge of the skirt on the four spots opposite the diagonal webs supporting the piston pin bosses." (Sgd Cy) Judging by our past experience, I certainly favour the pistons being a little on the hard side rather than too soft, so long as we are in a position to produce these so as to obtain the correct clearance and bedding we require without any stretching being required. At a time when we were experiencing trouble in cracking pistons by stretching, we were certainly able to produce engines which gave satisfactory results from a piston point of view, and which results seem to have remained good for a considerable time. I should like some pistons to be produced which are slightly harder in their composition and which can be ground with reduced clearance, as per the American method of taper grinding, which we can try on test, with a view to being able as far as possible, to produce pistons which give satisfactory results. CONT'D.{John DeLooze - Company Secretary} | ||