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).
Mathematical comparison and specification for a J.3 propeller shaft, referencing 20HP and 25HP models.
| Identifier | ExFiles\Box 179\1\ img068 | |
| Date | 11th November 1931 | |
| FROM DA{Bernard Day - Chassis Design}/EV.{Ivan Evernden - coachwork} Copy to - HS.{Lord Ernest Hives - Chair} ✔ J.{Mr Johnson W.M.} 3. PROPELLER SHAFT. ORIGINAL DA{Bernard Day - Chassis Design}/EV{Ivan Evernden - coachwork}1/W5.11.31. HS{Lord Ernest Hives - Chair} 85340. We thank you for your BY3/G31131. Our suggested 2.5" dia. for J.{Mr Johnson W.M.} 3. is based on the mathematical comparisons with the existing and previous shafts on the 20HP. and 25HP., and not on the calculated whirling speed. (1) The short chassis 20HP. was as follows: Prop: shaft length 55.900". " " dia. 2.250" X .104". At about 3100 RPM. the car was doing its maximum of approx: 65 MPH. This shaft was OK. (2) The long chassis 25HP. has a shaft: Prop: shaft length 58.900". " " dia. 2.250" X .104". Although this was on the verge of trouble it has been used with speeds up to 3400 RPM. on production until recently - 67 MPH. The axle ratio is 4.55. (3) The length of the shaft on J.{Mr Johnson W.M.} 3. is approx: 55.400", a little less than on the original 20HP. If this car does 75 MPH. the shaft will revolve at 3500RPM. (Axle ratio 4.100, 18" rims, and 14.5 rolling radius of tyres.) Therefore if a 2.250 dia. shaft 58.900 long has proved fit for 3400 RPM. on 25HP. the J.{Mr Johnson W.M.} 3. shaft of 55.400 length and 2.5 dia. pro rata should be safe at - w = 3400 X (58.9/55.4)^2 X [ (2.5 + 2.292) / (2.25 + 2.042) ]^1/2 = 3400 X 1.135 X 1.052 = 4050 RPM. (87 MPH.) (w = const. (D^2+d^2)/l^2) (Duncan) The J.{Mr Johnson W.M.} 3. engine is a long stroke engine and may need even a higher ratio axle than 4.1 to 1. We are therefore instructing a 2.500 prop: shaft for J.{Mr Johnson W.M.} 3. to be tried. Should it fail we should then only go to 2.750" dia., not 3". (1) | ||