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).
Big end bearing shell preloading, nip, and crankcase journal bearings.
| Identifier | ExFiles\Box 114\3\ scan0132 | |
| Date | 30th September 1937 guessed | |
| - 4 - is very closely connected with that of preload or nip, I propose to deal with this question next, giving you the benefit of my experience at the Depot. First and foremost in all cases when I have been present at the dismantling of an engine fitted with Halls metal big end bearing shells, the shells have been loose in the rods, and I am told by the Depot people that this is the case where all Halls metal big ends have been used with no preload or nip, for example 117.JY the small end of one connecting rod could be moved on its big end shell 3/8" before the shell moved on the crankpin. The remaining rods were not quite so bad but varied from 1/8" to 1/4". For your information K have been preloading or nipping main and big end bearings on the old type Bentley since their inception in 1921 and a loose shell in either a connecting rod or main housing has been unknown. I am aware of the difference in the construction of the two engines and also the different characteristics of the metals used. Nevertheless I am of the opinion that a certain amount of preloading is necessary to prevent this fidgetting, but the figure of .0005 given to K in the fitting instructions received from the Works seems to me to be inadequate taking into consideration the physical properties of the bearing metal in use. We have found by experience that the amount of preload or nip necessary to prevent this fidgetting is a total of .004, i.e. .002 on each half of shell. Any distortion of the bearing shell produced by this preloading is eliminated by the final machining and fitting operations. The shells themselves being sufficiently ductile to take up a permanent set. Con rods that have been dismantled after use with this amount of preloading are found with no preloading left, but they are absolutely firm in the housing. It is early days yet to say definitely whether this preloading will overcome for all time the fidgetting of the shells, but at the time of writing we have received no recurrent cases of fidgetting. One marked feature of a bearing under preloading conditions is that the bedding shows a marked difference. to one without preload in that the preloaded bearing shews a full bedding and little sign of distortion as against a narrow strip of an un-preloaded bearing. 4. Crankcase journal bearing. With the standard .002 bearing clearance we have not had any signs of the metal growing, causing a tightening up of the journal bearings, but we are definitely of the opinion that preload or nip to prevent sinkage is essential and that the amount of .004 total or .002 per side is not too great. We have found the chief sufferers are the centre -contd- | ||