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).
Valve spring design, surge analysis, stress calculations, and wire material sourcing.
| Identifier | ExFiles\Box 126\4\ scan0146 | |
| Date | 18th May 1936 | |
| #2 - Rolls Royce Ltd. 5-18-36 which is going forward to you under separate cover, also an additional copy of the stress range which we use in designing valve springs. All the stresses are calculated by the Wahl formula which you will find in our catalog. After we have made the springs as free from surge as possible, we measure the amount of surge by noting the variation in coil movement under the stroboscope, or recording it by having a brass needle lightly touch against a zinc oxide coated paper. The additional motion over that which is calculated for the valve movement is thus ascertained and this total movement is used to calculate the stresses through which the spring must operate. We are not able in any instance to absolutely remove all surge of the springs, but we are able to keep it under a very close control so that it never becomes dangerous nor does it result in noisy engine performance. We have tried to calculate the periods at which surge would occur and in this we have absolutely no success. This is because despite all the calculations which often give the exact period of the spring, nevertheless when the spring is put in the motor the torsional whip in the rotating parts throws in uncalculable influences which change the periods and the harmonics so that the only good of the calculation is to indicate which spring would normally give the best performance in a series of springs before they had been fabricated. The best work showing methods of calculation on the surge of valve springs is that of Swan & Savage of the Department of Scientific and Industrial Research of Great Britain. On the back of this article you will note they suggest the use of dampeners. We believe where space limitations prohibit the design of the spring which will in itself be non-surging, that these dampeners be employed. They seem to us to be a way around the problem and not a solution of it. When we cannot get a motor to work on, we often are given a camshaft and the various parts. These we can put in a dummy set up and work along similar lines. The material we use is described in the booklet enclosed with the catalog, Appendix E, Page 79. In this plant we use wire which is drawn from Swedish rods rather than imported Swedish wire. We have found in our researches that while the imported Swedish wire is a good wire, the american manufacturers are producing wire which has slightly better physical properties. In your case it would probably be best to import the Swedish wire direct, and we believe one of the best houses to obtain it from would be Eckstrand and Tholand, unless it would be as cheap to import it from this country, when either the Washburn Wire Co., 118th St.{Capt. P. R. Strong} & Harlem River, New York City, or the American Steel & Wire Co., Worcester, Mass., would be the preferable producers. | ||