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).
The demonstration of the Cambridge Portable Vibrograph at the works.
| Identifier | ExFiles\Box 138\2\ scan0167 | |
| Date | 13th September 1938 | |
| REPORT ON THE CAMBRIDGE PORTABLE VIBROGRAPH DEMONSTRATED AT OUR WORKS. 13.9.38. This instrument was demonstrated at our works on the afternoon of 13th Sept.1938 by Mr. Cole of the Cambridge Instrument Co., tests having been arranged upon Merlin G.19 on the Experimental Test-beds and upon a 4 1/4 litre engine provided by the Chassis Dept. It was intended that the work on G.19 should cover some of the ground already performed with the Askania instrument on cylinder block vibration, whilst a transverse vibration of the crankcase walls was to be investigated on the car engine. (1) THEORETICAL CONSIDERATIONS OF VIBROGRAPHS. Vibrographs intended for measuring displacements are of necessity long period instruments, that is the natural frequency has to be lower than any frequency which it is intended to measure and are generally characterised by low damping. The natural period of the Cambridge instrument is assessed at 1/4 second, but the resonance is presumably partially suppressed when hand-held. However, it has been our experience that the act of holding the instrument tends to impose low-frequency vibrations at up to 5-7 cycles per second depending on the method of holding any particular instrument, and this factor appears to set the limit for the low-frequency end of the range. The upper limit of frequency would appear to be set by two considerations, viz. (i) The difficulty of recording the individual cycles upon a time base which is limited in scale. (ii) The failure of the recording mechanism to follow the accelerations involved at the higher frequencies with large amplitudes. It is desirable that the recording linkage be free from all play and be sufficiently rigid to eliminate errors due to flexure in the members. In the latter connection the loading on at the stylus should be as low as possible. At the same time the inertia forces associated with the mechanism should be small so that the reaction upon the vibration system under investigation is insignificant. From the point of view of accuracy in interpreting the diagrams, these should naturally be well-defined, with the width of the trace small compared with the deflections recorded. In order to provide a ready means for estimating the frequency of the vibrations observed, an auxiliary system is required which will record a series of timing marks by the side of the main record. Generally speaking it is desirable to control these impulses from a separate mechanism not associated with the main drive. (2) GENERAL CONSTRUCTION OF THE CAMBRIDGE INSTRUMENT. (a) The vibrograph functions upon a principle similar to our present instrument from Askania Werke A.G., that is it is direct recording and consists of a hand held mass embodying the main mechanism with a stylus rod which is maintained in contact with a point on the vibrating member under test by spring loading. The stylus movement relative to the case is magnified by a linkage 7 times and is used to mark a record of displacement normal to the axis of a strip of celluloid driven at a uniform rate serving as a time base. | ||