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).
Introduction and technical description of recording accelerometers, including a standard pattern model.
| Identifier | ExFiles\Box 53\1\ Scan188 | |
| Date | 12th September 1932 guessed | |
| INTRODUCTION MEASUREMENTS of the accelerations of moving objects are of value, not only in giving information regarding the changes in the rate at which they move and the effect of obstructions, springing devices, etc., but in securing data whereby the forces acting upon the moving body can be calculated. Since every force can be equated to mass multiplied by acceleration, it follows that a record of the accelerations of a body also forms a record of the forces causing such accelerations. A recording accelerometer is, therefore, an instrument of considerable usefulness, and provides a wide variety of data of practical value to engineers. For example, it is frequently of importance to record the vertical accelerations of an aeroplane, not because the actual accelerations are of much value in themselves, but because from such records the forces due to air pressure on the wings, and hence the maximum strains in the struts and tension wires of the aeroplane, can be calculated, and the factors of safety determined. Further, an accelerometer is of value in recording vibrations, particularly when these are of high frequency. Other applications include the physiological and psychological study of the effect of 'vibrations upon the human system, and the characteristics of the movements of the human body in walking. The accelerometer has also many applications in the direct measurement of the accelerations of moving objects such as trains, electric tramcars, motor cars, or lifts, thus determining, for example, the maximum acceleration or deceleration that can be obtained, or is consistent with other conditions, such as the comfort of passengers or the safe limits of the stress—due to acceleration—to which the materials of the moving object or its structural supports can be subjected. It has been found to be of much value in connection with investigations regarding the springing of vehicles and the effect produced upon them by various road surfaces, and in comparing the “comfort” of different types of tyres, springs, shock absorbers, etc., when fitted to cars. The Recording Accelerometers described in this catalogue are light, robust instruments, by means of which permanent records can be obtained of accelerations in either vertical or horizontal directions. The principal novel feature in their design lies in the method of recording, which does not depend upon optical or photographic methods, and yet is capable of great accuracy even when recording rapidly changing accelerations. The records are obtained by the action of a moving stylus upon transparent celluloid film. The pressure upon the stylus is extremely light and the celluloid flows plastically under the rounded point of the stylus, the line produced having such optical characteristics as to render any point on an enlarged image of the diagram readable to a high order of accuracy. A wide range of accelerations can thus be successfully recorded. The moving parts of the instrument are made small and arranged to move through small distances, so that accelerations of considerable magnitude, or vibrations of high frequency, are recorded accurately and undisturbed by instrumental inertia effects. The stylus-on-celluloid method of recording has also the advantage of durability, in that the record cannot fade, and is impervious to oil or water. Other instruments utilising this method of recording include Stress Recorders, Vibro-graphs, Bridge Deflectometers, Chronographs, Draught Recorders and Micro-Indicators for High Speed Engines. Information regarding these instruments will be forwarded on request. RECORDING ACCELEROMETERS. Standard Pattern. THE standard pattern Accelerometer illustrated in Fig. 1 can be used for recording either horizontal or vertical accelerations, depending upon the position in which it is placed relative to the direction of the movement being investigated. Fig. 2 shows the case opened to give access to the mechanism, while a plan of the instrument, as placed for recording vertical accelerations, is shown diagrammatically in Fig. 3. The mass M is supported by two flat steel strips, H and H1, and is connected by means of an additional steel strip N to a member O moving on knife-edges K and K1. This member carries a flat strip of aluminium L, the extremity of which is free to move between the poles of a magnet P. Fixed to the aluminium strip L is the stylus Q. Any movement, therefore, of the mass moves the stylus across the celluloid film A which is caused to pass round the roller T by means of clock-work mechanism not shown in the diagram. The speed of travel of the film may be varied from approximately 3 to 20 mm. per second. The eddy currents set up by the movement of the aluminium strip between the poles of the magnet critically damp the movement. With the standard spring, the natural period of the whole system is approximately 0·025 second. With this period the sensitivity is approximately one millimetre on the actual record for an acceleration equal to “g,” i.e., 32·2 ft. per sec. per sec. (981 cm. per sec. per sec.); the record can be measured to an accuracy of one hundredth of this value. Other sensitivities can be quickly obtained by substituting suitable control springs, the period being correspondingly altered. The pressure of the stylus on the film can be varied by means of the screw C. An electromagnetic marking mechanism, not shown in the diagram, is fitted so that a time scale, generally tenths of a second, can be marked on the film by means of an additional stylus operating on the back of the film. It is thus possible for the stylus making the record of accelerations to pass over the whole of its range without fouling the time-marking stylus. Alternatively, the second stylus may be arranged to give records of the instantaneous positions of the moving object, or other datum marks. Fig. 2. 9 x 5 x 8 inches. 2369 Fig. 3. 2555 The complete mechanism is enclosed within a compact case, fitted with two hinged lids (see Fig. 2). On the front of the case are carried the various switches for controlling the mechanism, together with the terminals for connection to the time-marker and to the 12-volt battery for the damping electromagnet. The clockwork mechanism is started or stopped by rotating the switch D, but it can also be started independently by depressing the push button G when it is only desired to run a short length of film for calibrating purposes. The records may be easily calibrated by running the film with the instrument set first on its long base and then on its short base, or vice versa, pressing the button G for a second in each position. This operation gives a definite “step” in the record, the height of which represents the acceleration “g” due to gravity. The speed of the film is adjusted by means of the regulator lever E.{Mr Elliott - Chief Engineer} Three toes are provided upon each of two adjacent sides of the base, enabling the instrument to be placed on either its long base, for recording vertical accelerations, or its short base, for recording horizontal accelerations. | ||