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 failures and design history for three types of rear axle road shafts used in old type Bentley cars.
| Identifier | ExFiles\Box 84\4\ scan0106 | |
| Date | 17th September 1936 | |
| K/W. Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} GRY{Shadwell Grylls}- Y202 K/KC/17.9.36. REAR AXLE ON OLD TYPE BENTLEY CARS. -------------------------------- Many thanks for your Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls}8/KW.17.9.36. There are three types of road shafts (or as they are termed in our blue prints "back axle shafts") in use in the old type Bentley cars today. They are:- Piece No. BM.1400. BM.4101 BM.7349. I will endeavour to give you the history of these shafts and their behaviour as briefly as possible, but in order for you to obtain a comprehensive grip of the situation stage by stage, it will be necessary for me to give you the reason for their existence. BM.1400 road shaft was used in all 3-litres, all 4 1/2-litres and Standard 6 1/2-litres up to the year 1929, when owing to the torque obtained through the increased horse power developed by the 6 1/2-litre model, torsional fractures occurred at the splined end of the shaft exactly at the point marked on the blue print 30 m/m diameter where the 5 m/m radius blends into the shaft proper. To overcome this, shaft No.BM.4101 was produced which eliminated the undercut which existed on the original shaft at the 30 m/m diameter. The shaft was actually increased at that point to approximately 35 m/m, since which time there have been only one or two torsional failures to my knowledge. The fractures at the splined or differential end usually occurred according to driver and road conditions etc. at a mileage not lower than 30,000/35,000, but then only on the higher powered cars (6 1/2-litre). Regarding BM.7349, there has been one fracture of this type road shaft at the 44.45 diameter underneath the Timken. This fracture occurred on 8-litre chassis No.YR.5078, the property of Guy Newman Esq., the mileage at the time being 82,736. This is the only known fracture at that end of the shaft. For your further information I am sending the actual fractured end of the shaft in this case for your examination, under cover of our advice note No.4988. -contd- | ||