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).
Conclusions on the elastic limits and stresses of high-grade spring steel and coil springs, with relevant formulas.
| Identifier | ExFiles\Box 170\3\ img007 | |
| Date | 4th December 1935 guessed | |
| (Sheet 5) Figuring in the old way we should say that we had raised the elastic limit from 100,000 to 130,000, whereas actually we have done nothing of the sort. (9) Conclusions. (a) Probably the true elastic limit of high grade spring steel even after prolonged cold-working is somewhere between 100,000 and 115,000 lbs/in² in shear. (b) Maximum working stresses of 145,000 or 150,000 in coil springs figured for the inside of the coil, and taking account of the ratio of wire dia / coil dia by means of what we call over here the "Wahl" equation, are almost certainly a pure figment of the imagination, for any spring which has been "settled". (c) The coil spring, even of thick wire and relatively small coil dia, is probably a much more efficient instrument than we have given it credit for. Its true maximum stress is probably given much more accurately by the old fashioned formula:- f = (2.55 W.D) / d³ - than by the Wahl "correction". (d) If the coil spring is more efficient than we have believed it follows that the torsion rod offers little or no practical improvement over the coil spring as regards work per lb for a given stress. (e) The max torque obtainable from a tube becomes (2π f.m / 3) {r₂³ - r₁³} corresponding to equal stress. throughout the thickness of the tube. We believe these conclusions are pretty close to the truth. Will you look this over, consider it, and let me have your comments. Kindest regards to yourself and all at Derby. Yours ever, MAURICE OLLEY | ||