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).
Letter from France investigating recurring valve spring failures on a 4 B IV model, concluding the cause is bottom coil surging.
| Identifier | ExFiles\Box 82\2\ scan0155 | |
| Date | 5th August 1934 | |
| X4553a Hotel de France, Chateauroux. Indre. France. 5th August 1934. To HS{Lord Ernest Hives - Chair}/RM.{William Robotham - Chief Engineer} From G.W.H. RE VALVES ON 4 B IV. We have received your memo of the 2nd. and HS{Lord Ernest Hives - Chair}/Smth/3/8/34. We have, whilst 4 B lV has broken down, carefully gone into the various reasons why the valve springs are breaking. Firstly, I have to apologise for stating the increase of lift of the first two valves was .031. I asked for the lift of the cams to be taken, and quoted the figures given to me as being correct. Grylls and myself have since measured the lift, and we cannot find any appreciable difference in the cam lifts. The valve lift is as near as we can measure .396". The suggestion of FL Smith that the failure of the springs is due to the spring ends seating on the radious, instead of on the flats, does not appear to hold good, as Nos. 3,4,5,6,7,9,11 and 12 springs have not broken, although all the springs are seating on the radius as shown by his sketch. N°1. spring, i.e. inlet, has broken no less than six times, whereas N°12 has not broken at all. The average life of the first spring set at .004 clearance was 2,600 miles. The lives of the next four springs fitted to N°1 valve set with .003" clearance was 361, 1,416, 640 and 193 miles. The life of the next spring set with a clearance of .006 was 1,987 miles. In our opinion, which is also bourne out by the markings on the springs, is that the failure is due to the bottom coils surging. It is a practice on all car and aero engines to have only a few thousandths coil clearance at full lift, as that surging of the spring cannot increase the stress. Those outer springs have at least .125. Owing to lack of double springs, we ran a single valve spring, which is almost identical with the double outer spring, but with its own bottom washer, a mileage of 1,000 without any sign of failure. we are now going to try out a single type spring as an outer, as this type seats correctly on the head, being a slightly smaller diameter. It will then have .125 clearance at full lift. We shall be delighted to have your views. In case you have not received our telegram, our rear axle is broken. G.W.H. G W Hancock. | ||