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).
Modifications to engine lubrication and an investigation into connecting rod bolt failures.
| Identifier | ExFiles\Box 143\2\ scan0187 | |
| Date | 24th June 1941 | |
| - 2 - BY/HB.{C. E. Harcombe}5/EIM.24.6.41. Continued : A modification has recently been put through by the Experimental Department to introduce continuous grooves on Nos. 3 & 5 main bearings in order to supply more oil to Nos. 3 & 4 big ends under over-revving conditions and in this respect it is pointed out that only the increased capacity oil pump to Mod. 174 has enabled this change to be made. It is thought that this alteration will have considerable affect on big end reliability and Nos. 3 & 4 big ends should thus give less grounds for complaint. Work is also being continued by this Department on lubrication development and it is understood that a scheme for using two holes in the crankpin is now being considered for standardisation. This is a scheme which has been worked at by us since January 1940 and previous to this was fitted to the Bentley car long before the War where it produced a marked improvement but only when an adequate oil supply was actually reaching the crankpin. The modified blade rod lubrication which has also been running for a similar length of time, incorporating flats in the bearing block and the deletion of the annular groove in the blade rod bearing is also being put forward for standardisation. Merlin XX. 645 on which tests were carried out in an endeavour to produce big end failure following high speed climbing and diving conditions was equipped with the two holes in the crankpin and the improved blade rod lubrication as well as flats on the crankpin. This engine completed 118 hours satisfactorily and the condition of the blade rod bearings was better than normally experienced on the standard type under similar circumstances and the fork rod bearings were very satisfactory. While it is agreed that an engine thus equipped in the Experimental Department went down in 2 hours running it should be noted that the flats of the crankpin were not feathered away on the trailing edges. As long ago as 1939 when Mod. 194 was being developed the non-feathering of the trailing edges of the flats produced a scraper which wrecked a bearing. In the sphere of Service running an almost new type of failure has been experienced in that connecting rod bolt failure has occurred, this has been generally associated with the plain rod but one case of fork rod and bolt failure has also been examined. An investigation of the broken parts in each case shows that one bolt has failed in fatigue, generally across the centre large diameter collar and the opposite bolt has failed as a tensile test piece, considerable necking taking place across the weakest section. This trouble is considered to be associated with unequal tightening of the bolts, one being elastically tighter than the other. As a result of this the bolt which is slacker tends to stretch a little in Service and in consequence allows the cap to move, hingeingeing on the tighter bolt. This places a bending moment on the tighter bolt and a fatigue crack is set up at the inside edge relative to the connecting rod, further running allowing the crack to spread with subsequent bolt failure and complete breakup. It has often been stated that the tightening of the Merlin connecting rod bolt is not critical provided it is - continued - | ||