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).
Comparative analysis of the Askania and Cambridge instruments for engine vibration investigations.
| Identifier | ExFiles\Box 138\2\ scan0169 | |
| Date | 13th September 1938 guessed | |
| - 3 - (a) contd. pressure to the stylus when using the handle, and it becomes necessary to hold the instrument by the main body. In connection with our work the Askania instrument is more convenient in respect of overall dimensions and reduced weight, also the shape of the main body is suited for easy handling under conditions as in flight when the observer may have difficulty in maintaining a steady position. Additionally, it must be stressed that the stylus arrangement projecting from a tube 3/8" diameter and 4" long confers a very considerable advantage in reaching inaccessible positions required in engine investigations. During the tests which had been arranged we found that certain points of interest on the crankcase of the car engine and the cylinder block of the Merlin were quite inaccessible to the Cambridge, although the more convenient arrangement of the Askania permitted its use. It is on these grounds alone that we have to criticise the Cambridge instrument, and of course it has to be conceded that these are significant only in special applications which, however, form the main bulk of our work. (b) CONSIDERATIONS OF FREQUENCY RANGE, MAGNIFICATION ETC. It was found that the low-frequency vibrations superimposed on the records appeared at a figure rather higher than that usually noted on the Askania. This figure was 5-6 cycles per second, and this increase is presumably attributable to the increased difficulty in holding the Cambridge. Accordingly we feel justified in placing the lower frequency limit at our original estimate of ten cycles per second. The upper frequency limit depends upon the film speed employed. At the maximum speed of 2 cms. per second. it would appear to be possible to resolve frequencies at up to 100-120 cycles per second, but with the drum type instrument this has the effect of curtailing the recording-time. We understand that the makers claim that the recording mechanism is capable of sustaining an acceleration of 100 g.{Mr Griffiths - Chief Accountant / Mr Gnapp} and H.{Arthur M. Hanbury - Head Complaints} Constant verifies that the instrument is satisfactory up to the limit of his tests at 20 g.{Mr Griffiths - Chief Accountant / Mr Gnapp} In any case the latter figure is more than satisfactory for the amplitudes normally recorded at 100 cycles per second. Thus the range of the instrument is comparable with that of the Askania with the added advantage that the resolution of the higher frequencies is much aided by the improved definition. Mechanical magnification of Cambridge Instrument 7x+ 1%. " " " Askania " 6.03 x (measured) We have much less confidence in the ability of the Askania instrument to withstand high accelerations, but calculation leads us to expect correct behaviour at amplitudes of .003" at 100 cycles per second. | ||