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).
Analysis of Bensport piston failures, discussing material, clearance, and design issues.
| Identifier | ExFiles\Box 86\2\ scan0019 | |
| Date | 25th March 1933 | |
| X236. To R.{Sir Henry Royce} from E.{Mr Elliott - Chief Engineer} c. Sg.{Arthur F. Sidgreaves - MD} Wor.{Arthur Wormald - General Works Manager} Hs.{Lord Ernest Hives - Chair} BX. E.4/HP.25.3.33. Bensport Piston. With reference to the failures which have occurred with Bensport piston tops breaking off above the skirt, we are not convinced that this is due to a design weakness in tension at the point of fracture as these pistons were strengthened up on the lines of the Phantom to meet this particular loading. The weight of the broken off portion is 5 lbs and at 4500 r.p.m. represents a load of 827 lbs. The maximum tensile stress having regard to the eccentricity of the load relative to the section is 1050 lbs sq. in. which is not considered high even for the reciprocating nature of the load and the temperature. The material was our old 8% copper piston alloy which seems to be used for these sand cast experimental pistons. We are replacing with chilled die cast standard pistons in RR.53 alloy which will give double the ultimate strength of the material and about three times the yield and proof stresses. It is understood that a thin metal gasket of about .012 was in use when the pistons failed although it was subsequently reported that the piston tops were .03 below the cylinder. This gives .042 initial clearance which is less than what we consider safe. The Phantoms run with .050 clearance but the tolerances on dimensions could vary this figure a lot. Our next run on Bensport will be with large clearance to eliminate any possibility of trouble from this source. It is not easy to see how we could strengthen the design much more, and we are looking into the possibilities of using forged pistons. E.{Mr Elliott - Chief Engineer} | ||