Contains reports on one research project.National Institutes of Health (Grant 5 P01 GM14940-04)Bell Telephone Laboratories Incorporate

Howland, Bradford H.

Howland, Howard C.

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XIII. NEUROPHYSIOLOGYAcademic and Research StaffProf. J. Y. Lettvin Dr. E. R. Gruberg Janet M. FaulknerProf. J. E. Brown Dr. S. A. Raymond Lynette LevyDr. S-H. Chung B. Howland Diane MajorDr. E. Douglass W. M. SaidelGraduate StudentsR. J. Bobrow J. E. Lisman J. F. NolteR. E. Greenblatt M. Lurie M. SporerP. Haugen K. J. Muller R. S. StephensonC. D. Jones Susan B. UdinA. NEW MECHANICAL DEVICES1. KINEMATIC THRUST BEARINGIn Fig. XIII-1 we show a model of our new kinematic thrust bearing, forthe case N = 17, there being 17 rings and 34 balls. The two end discs havea grooved raceway, the bottom disc is free to move on the 1/4" diameter shaft,and is restrained by a conventional thrust bearing. The configuration as illus-trated is stable, and the device can be rotated at 500 rpm for an indefiniteperiod.The important feature of this bearing is that the forces are self-equalizing; that is,the device is kinematic. We contemplate application as a very high precision thrustbearing; in this case, the averaging or self-equalizing action will enable reduction oferrors in axial positioning caused by oversized balls, irregularities of races, and soforth.An interesting note concerning this geometry is that the configuration is stable onlyfor odd values of N. If N is even, the lattice arrangement immediately collapses withrotation, alternate rings moving toward opposite ends of the shaft. An attempt to illus-trate the kinematic behavior of the device by inserting a 1/32" oversized ball led to asimilar collapse - such a deviation is evidently beyond the limits of accommodation ofthis device.We are, at present, undertaking a computer analysis of these configurations,which will also enable us to explore the axial displacements (axial cam action)caused by radial and angular misalignments of the discs relative to the shafts. Pre-liminary results indicate that it will be possible, by these means, to realize a pre-cision thrust bearing, which, when used in conjunction with a precision worm gear,This work was supported in part by the National Institutes of Health (Grant5 P01 GM14940-04), and by a grant from Bell Telephone Laboratories Incorporated.QPR No. 98 215(XIII. NEUROPHYSIOLOGY)can be adjusted to compensate for the periodic errors that are almost always present inworm gear drives. Such an arrangement would have application to precision pointingof astronomical telescopes.Fig. XIII-1. Kinematic thrust bearings for the case N = 17.2. PRECISION ANGULAR TILT TABLEIn Fig. XIII-2 we show a drawing of an angular tilt table that has features of interestto the precision mechanician. The moving element is supported by a double row of balls,contained by two V grooves, and acted upon by a micrometer screw. The axis of themicrometer screw is laterally offset the amount necessary to provide rolling contactwith the first ball. The arrows in the diagram indicate the approximate relative ratesof motion of the balls. It will be appreciated that the contact between a ball and the sidesof the V groove is rolling motion with a slight spin, and the contact between adjacentballs is rolling motion with a slight slippage resulting from their differing rates of rota-tion. The smooth action of the device depends on the fact that this slippage is absorbedby the concomitant rolling motion.QPR No. 98 21623-32Fig. XIII-2. Diagram of the precision angular tilt table.Photograph of a modified angular tilt table, with rings substituting for the upper set of balls.Fig. XIII-3.(XIII. NEUROPHYSIOLOGY)A model of this device was machined from aluminum alloy and fitted with 29 steelballs, and was then tested with the aid of an electronic gauging system. The reductionratio, that is, motion of the micrometer screw vs the moving end of the platform, wasapproximately 15:1 in this configuration. We found that the action of the device was verysmooth, no mechanical hysteresis being evident. The angular increment of tilt cor-responding to the smallest division of the micrometer, 0. 0001 inch, was approximately0. 2 second of arc. After wide excursions, readings were regularly repeated to approx-imately 1 second of arc, the errors evidently being due to tolerances in the hingebearings. Our next model will use flexural hinges, and will be machined of steel forgreater load bearing capability.Figure XIII-3 is a photograph of a second configuration, in which a set of rings, heredefunct inner races of ball bearings, substitutes for the upper set of balls. The ringsride on a steel shaft, which is cemented into the top V groove. The object of this exer-cise was to reduce the contact stress between rolling elements; the greater conformityof contacting surfaces has this effect. The performance was much the same as before;the reduction ratio was approximately 10:1.The principal disadvantage of the present device is its inherent nonlinearity, whichamounts to several per cent of the total motion. A parabolic fiducial mark on themicrometer scale can be used to correct for this effect; however, more accurate settingswould require the use of a correction table. In certain applications, as, for example,servomechanisms, this nonlinearity is of little importance, and the inherent smoothnessof operation, essentially free from mechanical hysteresis, could be used to advantage.B. Howland, H. C. Howland[Bradford Howland is an M. I. T. Lincoln Laboratory Staff Member. Howard C. Howlandis an Assistant Professor in the Division of Biological Sciences, Cornell University,Ithaca, New York.]QPR No. 98 218

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