Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1929 Page 39 more its Telocity characteristics are made constant the more the apparatus costs, it is economical to be stringent in requirements for constancy in the recorder, of which comparatively few are manufactured, and more lenient in those requirements in the more numerous reproducers. An economical division between the two machines of the total allowable error appears to be in the ratio of one to four. The demand for constancy thus placed upon the recorder is far higher than can be met by gears and bearings of even the most careful construction. It is a demand which can be filled only by special means. Mechanical Filter The fact that a continuous, slightly varying, motion of this sort is mechanically analogous to a slightly varying electric current helps to explain what these means are. A pulsating current can be treated as a direct current on which small alternating currents are superposed. The suppression of the alternating currents, to leave the unvarying direct current desired, can be accomplished by an electric-wave filter which attenuates the undesired alternating components. The electric filter consists of suitably interconnected coils, condensers, and resistances. Since mechanical analogs of these circuit elements are respectively to be found in masses, springs, and dissipative plastics, the mechanical-vibration filter can be visualized in terms of electrical principles. Such a filter, designed in these Laboratories, is incorporated (Fig. 2) in the Western Electric Company's commercial disc-recording apparatus. It uses coil-springs as its capacitances, viscous oil for Its resisances, and the masses of its moving parts as its inductances. The great width of the frequency band to be attenuated, and the pluraliy of the sources of the varying-force, considerably complicate the problem of determining what values of stiffness, weight, and dissipating ability should be used. For example, variations due to gear inaccuracies are most readily absorbed by very flexible springs, whereas dia turbances dtle to varying loads are best prevented from affecting turn 1 //. \r. 8 toller, Record, November, 1828 -IF. A. Frith rirl;. Record, Vniimhrr. [928 "■ To be described in the Journal of thi American Society of Mechanical Engineers Hi in tin I nmn. springs. This structure also multiplies by four the frequencies with which these disturbances occur, since each inaccuracy in cutting is made to occur once for every 90 degrees, in Fig. 3, above: The reduction gearing: a worm and a four-layer worm-wheel. Fig. 4, left: The spring connection between the gear and turntable shaft. Fig. 6, right: Linkage mechanism and its braces, through which the gear drives the oil cup. table speed by the use of stiff springs. The filter finally designed embodies a compromise between these conflicting demands. Its general construction is such that the worm-driven gear drives the turntable shaft through the linearly flexible springs, and relative motion of the gear and shaft is damped by the oil. The gear (Fig. 3) is made in four layers. These layers are clamped together when the teeth are cut, and each layer is afterwards rotated ninety degrees relative to the adjacent layer. All are finally mounted, in engagement with the worm wheel, so that each can move independently of its companions. To each layer two cross-braced posts are rigidly attached, from the tops of which (Fig. 4) springs lead to lugs on a plate fastened to the turntable shaft. Thus each layer of the gear independently drives the shaft through two springs. It is apparent that the offset fourlayer structure of the gear divides by four the amplitudes of the disturbances caused by inaccuracies in the teeth, since at any one time, each affects but one of the four sets of stead of once for every 360 degrees, of rotation. This higher frequency is far more readily absorbed by the filter than the lower would be. The oil connection between gear and turntable shaft is effected by permitting the layer-gear to rotate a vanebearing oil-filled cup, into which dip vanes attached to the turntable shaft (Fig. 5). The mechanism (Fig. 6) through which the gear drives the cup is in this case rigid rather than elastic, but is again one whereby the effect of a gear irregularity upon the cup is quartered in amplitude and quadrupled in frequency. This mechanism is a system of links independently driven by each of the layers of the gear so as jointly to rotate the cup with their average velocity. To each layer, again through the perpendicular posts, is attached a link (Fig. 7). The members of one and the other pair of these links are flexibly joined by crosslinks, to the center of each of which is pivoted one end of a bell crank. The other ends of these two bell-cranks are in turn flexibly joined by a third cross-link, to whose center the member which drives the cup is attached. Fig. 8. Steps in the development of the linkage, showing its successively more rugged construction.