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).
Technical memorandum discussing engine vibration analysis, focusing on flat 12-cylinder engines and torque components.
| Identifier | ExFiles\Box 132\2\ scan0207 | |
| Date | 16th December 1940 | |
| Wd{Mr Wood/Mr Whitehead}/GSB.11/RS.{Sir Henry Royce's Secretary}16.12.40. - 2 - The best treatment of this is given by Den Hartog in "Mechanical Vibrations". The present writer has evolved an alternative method for one of the less satisfactory steps in the analysis. C. On Page 4 it is stated that a flat 12 2-stroke would have a very bad 3/rev. (3rd order) vibration, and amplitude figures are given. This is wrong. There are no odd orders of vibration in such an engine, nor indeed in any engine (2 or 4 stroke), which is made up of a number of flat twin units, one for each crank, and so has a 180° bank angle. This is because, as regards the torque components, on any one crank, of orders 1, 3, 5, 7 etc., the torque component due to one cylinder is in opposite phase to the component due to its complementary cylinder acting on the same crank. D.{John DeLooze - Company Secretary} The 6th order vibration is a "major" order in the flat 12 just as is the 3rd order in a normal 6-cylinder, but with the important difference that, being a higher order, the torque components will no doubt be considerably smaller than for the major order of the 6 cylinder. Careful numerical examination is necessary. For this and higher orders, 2 node vibrations should also be considered, but these will be minimised if there is simultaneous firing at cranks equidistant from the centre - a principle applied by Halford to coupled engines and introduced to single engines by Rg{Mr Rowledge}/Cbt. E.{Mr Elliott - Chief Engineer} With regard to the general conclusions drawn in the covering memo Rm.{William Robotham - Chief Engineer}16/ML/7.12.40, although in agreement with the desirability of reducing reciprocating masses, it should be pointed out that, owing to the steepening slope of the side of the resonance curve, the permissible increase in R.P.M. with a given reduction in reciprocating mass becomes less and less as the resonance peak is approached. Since, according to Sheet 6, the power is beginning to fall at 4000 r.p.m., there does not seem to be any object in running at a higher speed than this until the m.e.p. curve has been improved. Wd{Mr Wood/Mr Whitehead}/G.S. Bower. | ||