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).
Conference held to investigate a broken valve failure on a Wraith III car following its French trial.
| Identifier | ExFiles\Box 108\1\ scan0208 | |
| Date | 17th September 1937 | |
| Rm{William Robotham - Chief Engineer}/Gry.{Shadwell Grylls} Rm{William Robotham - Chief Engineer}/Swdl.{Len H. Swindell} DA.{Bernard Day - Chassis Design} DA{Bernard Day - Chassis Design}/Mc.S. Handwritten: Rm{William Robotham - Chief Engineer}, 600a EY.6/G.17.9.37. WRAITH III - /26-G-VI. RE: BROKEN VALVE. A Conference of the following people was called this morning to fully consider the conditions which resulted in a broken valve on the above car, and in it breaking down on its French trial:- Present:- HY.{Tom Haldenby - Plant Engineer} Rm{William Robotham - Chief Engineer}/Gry.{Shadwell Grylls} Rm{William Robotham - Chief Engineer}/Swdl.{Len H. Swindell} DA.{Bernard Day - Chassis Design} DA{Bernard Day - Chassis Design}/Mc.S. From the Laboratory report, and an examination of the photographs, as well as the other exhaust valves taken from the same engine, it was clear that the cause of the trouble was not due to one of the upper waterways in the head being blocked with core wire and sand. From an examination of a section of the head it was obvious that the slight blockage of the waterway was not in any sense the cause of the failure. All the exhaust valves exhibited circumferential cracks, which, whilst they were of small depth, were obviously the cause of the ultimate failure of the valve under discussion. The valves had apparently not been overheated, and the failure must therefore be ascribed to the valve seating itself from the high acceleration position on the cam instead of from the slow acceleration 'toe' provided for the purpose. The cracks were due to the fact that the material had been sufficiently hot to reduce its tensile strength considerably, but not sufficiently hot to make the material ductile, under which circumstances the clearance would have continued to be taken up without the valve failing. Another fact which pointed strongly to this conclusion was that although a slow acceleration period on the toe of the cam, sufficient to cover .015 lift, was provided, the mechanism was noisy if a tappet clearance of .004 was exceeded. By modifying the toe of the cam to give a slow acceleration up to .020 it was possible to run the engine with .008 to .010 tappet clearance without same becoming noisy. Mr. Swindell had already instructed a reduced lift of the valve, with the object of reducing the [illegible] on [illegible] chanism [illegible] this reduc [illegible] lift has be [illegible] gether. | ||