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 major service failures on the Merlin XX engine, focusing on main bearings and over-revving issues.
| Identifier | ExFiles\Box 143\2\ scan0186 | |
| Date | 24th June 1941 | |
| BY/HB.{C. E. Harcombe}5/EIM.24.6.41. THIRD ANALYSIS OF SERVICE FAILURES. ------------ Further to our BY/HB.{C. E. Harcombe}5/EIM.21.3.41., we give below our analysis of major Service defects dealt with by this Department during the past three months. In this summary it is proposed to deal only with major issues and the main engine under discussion is the Merlin XX although a certain amount of generalisation is necessary on other Marks of the Merlin. The troubles experienced on the Merlin XX have been to a large extent connected with its manufacture but it is now gratifying to note that recent visits to Crewe showed a marked improvement in the finish of such critical features as the reduction gear and moderate speed drive. For this, much credit is due to the Inspection Department for a well sustained individual effort in which they deserve more support. Main Bearings. The use of 1% tin intermediate bearings is still proving satisfactory and very little trouble is being experienced. We are, however, experiencing a certain number of failures of the ½" silver centre main bearings although Mod.194 is often incorporated. An examination of several bearings which have failed shows them to have broken up in a manner characteristic of the ½" silver lead bronze and in this respect it is pointed out that the breakup is not associated with lubrication but with the insufficient mechanical strength of this alloy under present day operating conditions. By the introduction of Mod.194 we made it possible to re-introduce 1% tin alloy on the centre main bearing as the mechanical strength of this material is better suited for this type of bearing, the only feature against it being a tendency to seize under boundary lubrication. This fault has been almost eliminated by the introduction of Mod.194 and it is, therefore, felt that 1% tin lead bronze could be re-introduced on engines incorporating the improved centre main bearing scheme with complete safety. In the past where crankshaft failure has occurred following centre main bearing failure ½" silver alloy lead bronze has been responsible for more than one of these occurrences. Connecting Rod and Big End Failures. Big end failures are still a source of worry and although during the past months over-revving has been the primary cause we still occasionally get failures which are not attributable directly to this. In regard to the question of over-revving the failures have all occurred during long dives from high altitudes during which 4,000 R.P.M. and probably over have been obtained and there is no doubt that in the majority of cases this has been due to the fact that the Rotol constant speed unit when working under an oil pressure of 350 lbs. per sq.in. could not return the airscrew blades to the coarse pitch position, when fine pitch has been assumed due to the engine cutting out on entering a dive. Fine pitch is gained owing to the airscrew attempting to "constant speed" when the revs fall off, confirming our contention that the constant speed unit did not control and limit the revs under high speed diving conditions. Steps, however, have been taken to increase the pressure under which the constant speed unit operates to 500 lbs. per sq.in. and we understand that very much more satisfactory results are now being obtained and action is being taken in the Service to introduce this improvement. - continued - | ||