Design of Ä Multifrequency Reed-type. Shook Gage with an Inertia-operated Trigger

A reed-type shock gage is an instrument designed to measure the motion of a structure subjected to applied shock. It consists of a series of uniform cantilever beams each of which has a concentrated mass at one end and is clamped to the instrument frame at the other end. The natural frequencies of these beams or reeds are chosen so as to span the range of major frequency components of shock motions encountered on ship structures. The reed-type shock gage described consists of 12 reeds, each having a different natural frequency, supported on a common column, with the free ends provided with scribers which make records of the reed motions on a moving sheet of waxed paper. The gage requires no external source of power, is self-contained, entirely mechanical, and records automatically. The plate holding the waxed-paper record is released when the applied shock actuates an inertia-operated trigger. Means are provided for varying the time duration of the record. A mathematical analysis is given of the motion of the inertia-operated trigger in which it is assumed that the trigger is a beam constrained to rotate about a fixed pivot when the pivot is subjected to a constant accelerj ation. The angle through which the trigger will rotate in time t is given by the expression jj~ (1 cos Vlc^ t) for small angles of rotation, where k, and k2 represent the lumped physical constants of the system. Detailed consideration of the frictional, inertial, and mechanical forces on the trigger is given. Several approaches to the trigger-design problem are discussed, using the above derived expression. INTRODUCTION In the modification of the design of an existing multifrequency reed gage, undertaken at the request of the Vibration Section of the David Taylor Model Basin, several features of interest to designers were incorporated. Most interesting among these, perhaps, was the design and analysis of an inertia-operated trigger. The analysis presented is general enough so that it should serve as a useful guide to designers of other inertia-operated devices. The report first presents the requirements to be met by the gage, ö •T " •* "-. ""•""' -• " T ,_L s morJBBL" <SZ--X2Z**~** then a general statement of how these requirements are met, and last a more detailed description and analysis of the inertia-operated trigger. PRELIMINARY CONSIDERATIONS The original multifrequency reed gage consists of several flat steel reeds of different natural frequencies arranged one above the other in a single plane and rigidly supported at one end by a common column designed to accommodate from 1 to 12 reeds. The column is rigidly mounted to a steel housing of rugged construction. At the free end of each reed is a scriber which bears lightly against a sheet of waxed paper held against a rigid steel plate. When the instrument is clamped to a structure which is later subjected to shock as a result of which it vibrates, the reeds execute various motions and their maximum displacements are recorded on the waxed paper. In measuring shock motions, the Vibration Section found that if in addition to obtaining the maximum displacement of these reeds a displacementtime record could be obtained, the shock motion could be more satisfactorily analyzed. By moving the waxed paper during the shock and subsequent vibrations, such displacement-time record could be obtained. It was desired that the instrument be completely self-contained and independent of external sources of power so that it could be installed in any part of a ship structure without the necessity of electrical wiring or operating personnel. It was also felt that it would be desirable to avoid using an electric drive for the waxed-paper holder and any electrical elements in the triggering device because of the time lags due to the electrical impedance generally inherent in such devices. It was therefore found desirable to have the shock motion itself initiate the motion of the waxed-paper holder directly. Triggering of the gage by random motions of the ship had to be avoided. It was arbitrarily decided to design the gage so that it would not be triggered by an acceleration of less than 1 g with a corresponding displacement of less than 0.01 in. However, provision was made for varying the sensitivity of the trigger if such variation was found necessary. SPECIFICATIONS In view of these preliminary considerations it was decided that the modified multifrequency reed gage meet the following requirements: a. Provision must be made for fastening the gage to decks, bulkheads, and. overheads as in the original gage. b. The arrangement of the reeds and the means of recording should be "retained as on the original design.