a1 behavior with amines (linearly eluted without any liquid modifier of the original material) and with chlorocarbons (having a quite peculiar separation order) makes necessary further and more detailed investigations on the gas chromatographic properties of these compounds. The gas chromatographic behavior of other salt forms of crystalline zirconium phosphate and arsenate are under investigation in our laboratories. LITERATURE CITED A. V. Kiselev and Y. I. Yashln, “Gas Adsorption Chromatography”, Plenum Press, New York, N.Y., 1969, Chap. 11, and references therein. F. Bruner, W. Bocola, and G. P. Cartoni, Nature (London), 209, 200 (1966). M. Novotny, J. M. Haves, F. Bruner, and P. G. Simmonds, Science, 189, 215 (1975). F. Bruner, C. Canulli, A. Di Corcia, and A. Liberti, Nature (London), 231, 175 (1971). F Bruner, P. Ciccioll, G. Crescentini, and M. Pistolesi, Anal. Chem., 45, 1851 (1973), and references therein. F. Bruner, P. Ciccioli, and F. Di Nardo, Anal. Chem., 47, 141 (1975). F. Bruner. P. Ciccioli, and G. Bertoni, J. Chromatogr., 90, 239 (1974).
(8) G. Alberti and U.Costantino, J. Chromatogr., 102, 5 (1974). (9) S. Allulli, A. La Ginestra, and N. Tomasslni, J. lnorg. Nucl. Chem., 36, 3839 (1974). (IO) S. Allulli and N. Tomassini, J. Chromatogr., 62, 168 (1971). (1 1) A. Clearfield and G. D. Smith, lnorg. Chem., 8, 431 (1969). (12) S. Allulli, unpublished results, 1975. (13) G. Alberti and E. Torracca, J. lnorg. Nucl. Chem., 30, 317 (1968).
S. Allulli N. Tomassini C.N.R. Laboratorio Metodologie Avanzate Inorganiche Via Montorio Romano 36 Rome, Italy
G. Bertoni F. Bruner* C.N.R. Laboratorio Inquinamento Atmosferico Via Montorio Romano 36 Rome, Italy RECEIVEDfor review February 17, 1976. Accepted April 2, 1976.
AIDS FOR ANALYTICAL CHEMISTS
Simple Bearing for High-Precision Grating Rotation J. P. Walters,’ B. D. Hollar,’ and D. M. Coleman Department of Chemistry, University of Wisconsin, Madison, Wis. 53706
Spectrometers that use a plane diffraction grating can provide a linear display of wavelength if the grating is rotated with a sine-bar mechanism (1,2).While the sine-bar linkage itself is straightforward to fabricate, the angular motion of the grating is so small for moderate wavelength displays that the bearings used for its support must be of high precision, e.g., A.B.E.C. class 7 or 9 tolerances. Concurrently, mounting these bearings requires sophisticated machining, usually a t jigboring tolerances, to provide mounting holes of suitable concentricity and diametrical tolerance. Such fabrication typically exceeds the capacity of most university student shops, requiring instead tool and die grade service a t high expense. In this note, we communicate the essential details of a very simple bearing that provides smooth, low-speed rotation of heavy objects while still being sufficiently straightforward to allow fabrication in a student shop using only a tool-room lathe and production milling machine.
THE “NEST-OF-BALLS” BEARING The approach used follows from the realization that a plane diffraction grating, when used in a Czerny-Turner or Ebert configuration, rotates only over a limited angle (usually less than 60’) and at low rates. Thus, the features of a conventional high-precision ball bearing are largely wasted in this application, except for the sphericity of the balls and smooth surface finish of the races, which combine to allow rotation without cogging or jerking. It then is reasonable to discard all the parts of the bearing except the balls themselves and to eliminate the actual rotation of the balls, since neither wear nor speed of rotation need be considered. Present address, Union Carbide Corporation, Kenmore, N.Y.
When all rotation of the balls in a bearing is eliminated, then a configuration such as shown in Figure 1is possible. For example, in Inset A, three large balls are set with their centers at 120°, and they remain stationary. A smaller ball is placed in the center of this nest. It becomes the non-rolling, rotating member of the bearing. The stationary balls are held as shown in Figure 1,Inset B. A hole is cut in a base plate of suitable shape and the balls are simply dropped into it. A shaft is fixed to the rotating ball by first turning a smooth, but untoleranced, cone into the end of the shaft stock. Then, a small amount of epoxy glue is placed a t the bottom of the cone. The ball is pushed into the cone until it firmly contacts its machined sides and is held thus while the epoxy sets. If the shaft stock is round, then the center of the ball will coincide with the centerline of the shaft to good tolerances (e.g., f O . O O 1 inch or f0.03 mm) with only routine machining care, i.e., a collet in the spindle of the lathe to hold the shaft and a center-drill in the tail-stock to start the cone.
BEARING ASSEMBLY AND ROTATION To assemble the bearing, the shaft and cemented ball are lightly pushed into the center of the nest of stationary balls until they move out and contact the edge of the hole in which they are resting. The stationary balls are then fixed in position. A three-point contact is established between the stationary balls and the rotating ball. This three-point contact behaves as a sliding “surface” for bearing motion when the shaft is rotated. The rotation will be free of cogging, jerking, and wobble if the sphericity of all of the balls is high and if their surfaces are smooth. Balls that are micropolished, surface hardened, and spherical to tolerances of less than 0.001 mm are readily available in a variety of toleranced diameters a t ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1261
A -
5 ROTATING B A L L
CONTAIN ROUND HOLE BALLS TO
STATIONARY B A L L
C -
UNDERSIZED HOLE
&
ROTATING BALL HELD I N S H A F T
D -
O V E R S I Z E D HOLE
Flgure 1. Essential features of a "nest-of-balls'' bearing modified such that no balls undergo tumbling rotation
SIDE PLATE A
l
1
N
G SCREW FAST SCAN H g 5461 A 2 n d ORDER
il 1 CENTERING P I N
ROTATING MEMBER
CENTERED P I N
Flgure 3. Performance of the bearing when used to support and rotate a 102 X 256 mm, 590 lineslmm, plane diffraction grating in a 5.0-m Czerny-Turner monochromator
Flgure 2. Use of two bearings and a centering pin to hold a heavy, rotating member such as a diffraction grating
nominal cost from commercial, machine-shop supply houses. An important feature of this bearing is that the stationary balls are self-centering and the diameter of the hole cut to contain them need not be highly toleranced. As shown in exaggeration in Insets C and D of Figure 1,if the containing hole is undersized or oversized, then the effect is to raise or lower the plane of the three-point contact without shifting the centerline of the rotating shaft and ball. This means that the axis of rotation of a supported diffraction grating can be set without the necessity of holding the diameter of the bearing hole to a high tolerance. The latter operation is required if conventional roller or ball bearings are used to support the grating, and is done by jig-boring the hole. The bearing shown in Figure 1 is basically original with regard to all balls being stationary and three-point sliding 1262
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
contact occurring during the shaft rotation. The nest-of-balls concept was communicated in 1944 by Turner (3), and the self-aligning aspect of the bearing brought out simultaneously by Carlson ( 4 ) .Again, in 1961, Jones ( 5 ) accented the high accuracy possible with the bearing, and Moore (6), in 1969, stressed its simplicity. However, in these patents the balls were allowed to rotate, just as in a conventional ball bearing, to allow high rpm. This complicates the bearing fabrication and assembly. Since a diffraction grating need not rotate at high speed, the balls need not rotate at all, and all of the advantages of simplicity, accuracy, and self-alignment are achieved in an assembly that is almost trivial to fabricate.
DIFFRACTION GRATING MOUNTING A second bearing is required to suspend the grating. The method that we have used is shown schematically in Figure 2. All dimensions are referenced to a side plate, or if desired to an angle plate on the milling machine. The hole for the
lower bearing is cut into the bottom support, and the rotating member (which is suitably machined to hold the diffraction grating) is located by the centered pin. The hole for the second bearing is cut into the top of the rotating member, and the axis of rotation is then set by the centering pin, i.e., by the location of the hole in the top support. Although not sought, it is evident that angular misalignment between the centering and centered pins will not affect the smoothness of rotation of the rotating member, a situation that would cause severe problems with conventional bearings. The two bearings are aligned by pushing the centering pin down into the top nest of stationary balls by turning the loading screw. This is done until the rotating member spins freely with no shake, wobble, or play. A very light film of fine oil on the balls is an aid. In practice, a sine drive arm may be attached to the rotating member or fastened around the centered pin. We have fabricated one holder for a 102 X 256 mm plane grating using a worm wheel and gear for the drive. Here, the center hole of the worm wheel was used in place of the centered pin and held the rotating ball. The grating holder was mounted directly on top of the worm wheel. To test the alignment and smoothness of the bearings, the zero-order reflection from a helium-neon laser was observed visually as the grating was rotated. The grating was firmly mounted on a stable 5.5-m optical bed, and the laser beam multi-passed along the bed seven times to provide an optical level arm in excess of 38 m. The grating could be rotated in steps of less than 0.002O over angles up to 70° with no cogging, jerking, or vertical wobble observed in the spot image of the laser beam. The manner in which the above observations apply when the grating is used spectroscopically is indicated in Figure 3. Here, the grating was rotated with a stepper motor driving the worm gear at 0.000069° per step, with approximately 7% relative repeatability in step sizes. The grating was operating in the second order in a Czerny-Turner spectrometer of 5.0-m focal length arranged such that the distance from the camera mirror to the monochromator exit slit was approximately 5 m. This is the optical lever arm, over which cogging or jerking imperfections in the grating bearing are amplified when the grating is rotated. The equivalent mechanical lever arm in the bearing is the distance from any contact point between the balls and the central axis of rotation, which in this example is approximately 3 mm. Thus, the magnification of bearing imperfections is 1667.
In Figure 3, the entrance and exit slits of the monochromator were set at 0.050 mm, meaning that approximately 52 steps of the stepper motor are required to scan the geometrical image of the monochromator entrance slit across the exit slit. It is expected that during this time, the photoelectric signal observed in scanning up the leading edge of a line (e.g., Hg 5461) would show a regular rise, Le., show no discontinuities, unless the bearing and associated gearing involved in the grating rotation did not rotate, or detented in the reverse direction, etc., because of mechanical imperfection. Two such scans are indicated a t the bottom of Figure 3. The first scan shows one irregularity (A), while the second (repeat) scan shows two (B and C). If we interpret the presence of the second irregularity in the second scan as due to imperfections in the bearing, and note that it consists of three steps, then the irregularity in equivalent linear motion a t the ball contact points inside the bearing would be approximately 2 X mm. While this can be considered only an estimate, we understand that such minute irregularities are possible if the contact points in the bearing occur only through a film of oil. The data in Figure 3 clearly verify the practical utility of this simple bearing.
ACKNOWLEDGMENT The assistance of Russel Riley, Robert Schmelzer, and Robert Lang of the Chemistry Department InstrumentMachine Shops is acknowledged and appreciated. LITERATURE CITED (1) R . M. Badger, L. R. Zumwalt, andP. A. Giguere, Rev. Sci. Instrum., 19,861
(1948). (2)J. P. Walters, Anal. Chem.. 39,770 (1967). (3)E . P.Turner, U S . Patent 2,351,890, June 20, 1944. (4) B. G. Carlson, US. Patent 2,352,469, June 27, 1944. (5)G. L. Jones, U S .Patent 2,990,221, June 27, 1961. (6) R. F. Moore, US. Patent 3,463,564, August 26, 1969.
RECEIVEDfor review December 19,1975. Accepted February 13,1976. Portions of this work were conducted as part of the requirements for the Master of Science degree in chemistry a t Wisconsin (BDH). The financial support of the National Science Foundation under Grant GP 13902-X is acknowledged.
High-Output Potentiostat for Electrosynthesis Studies Rodney L. Hand' and Robert F. Nelson** Department of Chemistry, University of Idaho, Moscow, Idaho 83843 and Department of Chemistry, University of Georgia, Athens, Ga. 3060 1
Recent articles have demonstrated the ability of electrochemistry, in conjunction with a thorough knowledge of the kinetics and mechanisms of chemical reactions, to exert a significant degree of control over product distributions through variation of such parameters as applied potential, current density, concentration of electroactive species, and electrolysis time, thus illuminating exciting long-range possibilities in the field of electrosynthesis. To achieve meaningful results, a potentiostat is required which is capable of performing electrolyses on at least gram quantities of ma-
'
Present address, Allied Chemical Corp., 550 2nd St., Idaho Falls, Idaho 83401. Present address, Department of Chemistry, University of Georgia, Athens, Ga. 30601.
terial in a short period of time. Unfortunately, most currently available commercial instruments are not well suited to this task and/or are quite costly. These commercial instruments generally fall into two broad categories which can be classified as high voltage/low current or low voltage/high current power sources. The former group of instruments, which is noted for low-output and fast-response characteristics, is designed primarily for kinetic studies employing electrochemical relaxation techniques. These instruments are poorly suited for electrosynthesis studies because of the prohibitively long times required for electrolyses of reasonable amounts of material. Those instruments capable of high output currents usually have low applied potential limits and are thus designed primarily for work in media with very low resistances; they ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1263
