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 crankcase vibration readings and harmonic effects at different engine speeds.
| Identifier | ExFiles\Box 138\2\ scan0164 | |
| Date | 13th September 1938 guessed | |
| (2) It is not easy to deduce any important results from the readings taken at two speeds only, viz:- 2500 and 3000 r.p.m. as tests at closer intervals over a wide range of speeds are required to show the effects of any resonances in the structure. We are satisfied, however, that the readings from the centre and rear of the crankcase are substantially correct but are not completely convinced that the readings taken from the flange of the timing-case are typical of the vibrations at the front of the crankcase, especially as a mounting-foot, which may induce a node, intervenes. We presume that the transverse vibration is occasioned by the bearing loads transmitted through the crankcase webs, this supposition being supported by finding the main vibration at crankshaft frequency. The presence of the marked 2nd harmonic at 2500 r.p.m. in record (1) from the centre of the crankcase is not easy to account for except by supposition, especially as a simple diagram was obtained from the same point at 3000 r.p.m. in record (2). It may be that the second harmonic in the exciting forces fell close to a natural period of the structure at 2500 r.p.m. with consequent magnification of this harmonic. A similar effect is noted from diagrams (5) and (3) respectively from the rear of the crankcase where the pronounced 2nd harmonic at 2500 rpm. has vanished at 3000 r.p.m. We cannot explain the frequency doubling effect at the forward end of the crankcase at 2500 r.p.m., unless this is also bound up with a structural resonance in the region of 5000 p.p.m. ------------------- Lr{Mr Ellor}/HWS. | ||