Shaft Seal for Vacuum Apparatus. D. J. Trevoy and IT. A. Torpey, Kodak Research Laboratories, Rochester, 5 T. GREAT
many shaft seals have been described in journals and
A textbooks, and some of them are satisfactory, particularly
those which use precisely machined parts. Ready-made seals of this kind, however, are rarely carried in stock in the laboratory, and are often too cumbersome for use. The operator is thus tempted to resort to some simpler or lighter makeshift, which then fail- t o meet the requirementi
The rod and tube pail. can be used as cable releases, stirrers, valve openers, etc. I t is convenient to provide a cup a t the top of a vertical assembly or a bulb toward the atmospheric end of a horizontal one, t o hold a feR drops of sealing fluid. Also, with the vertical arrangement, it is desirable to have a cover or rotating flange on the shaft t o exclude dust. Tests have shown that shafts will operate continuously a t 0 t o 4000 r.p.m. for months without apparent wear. A typical assembly, 10 inches long, lubricated with di->ethyl hexyl phthalate, w i l l retain vacuum for a month, passing only 1.5 ml. of sealing fluid. The device and a typical application are illustrated in Figure 1. COM\ICSICATIOX 1445 from the Kodak Research Laboratories.
Steel rod
Directional Cold-Finger Condenser. D. J. T~evoy,Kodak Iiesearch Laboratories, Rochester, Ti. Y. COLD
Tc
Pyrex precislor bore
Figure 1. Shaft Seal and Its Application in the Stirred Pot Still
The need for a simple shaft seal is felt particularly acutely in the vacuum laboratory. After much searching, the authors have adopted a scheme which solves the problem quickly, neatly, and effectively, and have standardized exclusively on the procedure described. The seal consists of a drill rod operating within a glass capillary. At first, it was the practice to grind a slightly large rod into a selected capillary, using a fine abrasive. Because the abrasive occupied space between the rod and the capillary, the hole was always too large when grinding was finished. The next step in improving the fit was to taper one end of the drill rod, decreasing the diameter about 0.003 inch in a &inch length. All the grinding was then done with the tapered section, the cylindrical part of the rod being kept completely free of abrasive by frequent washing. With this improvement in the method of grinding, the leakage under vacuum was reduced by a factor of 10. It soon transpired that an even better fit could be secured by matching one of a selection of rods against a precision-bore glass capillary (obtained from the Fischer and Porter Co.) without any grinding whae soever. As now practiced, a few dozen pieces of steel drill rod, with a nominal diameter of 0.0810 inch, are obtained and kept for matching against a few pieces of glass capillary tubing, having an inside diameter of 0.0810 d= 0.0001 inch. When the match is satisfactory, the rod is completely free to move lengthivise or to rotate within the capillary, the clearance being about 0.0001 to 0.0002 inch over the entire length of the bearing. In the authors’ experience, no picces of glass have been rejected; the investment cost is small, and the cost of an individual pair is trifling, considering that no constructional time is lost.
finger to condense vapors has become yenrrally
-4 adopted for small molecular stills (1, 5) of the pot type. Such a condenser trapq not only molecules that arrive from the distilling surface in one strike, but all others that have wandered through longer paths. The problem arises, particularly in research on the mechanism of distillation, of condensing only those molecules that have come directly from the distilland. At other times, a small probing condenser is required that can be placed within the still 80 as to “see” oidy a chosen area of evaporation.
A selective directional condenser that meets these requirements has been constructed (Figures 1 and 2 ) . A tube, A , expanded a t its base to the largest diameter acceptable, is made concave a t the end, B , and is given a retaining lip, C. Glassworking directions for a convenient procedure for construction are shown in the three stages of Figure 2 . OPERATION
A coil of resistance wire (D,Figure 1 ) is connected to stout lead wires, E , and immersed in a filling of heavy oil in the finger. After the probe has been placed in position and the system made vacuum-tight, sufficient current is passed through E t o heat the oil to a temperature well above that of the distilland, a6 shown by the thermometer, F . When distillation conditions are correct, the current is interrupted, the oil is removed by a pipet with a rubber squeeze bulb, and coolant is poured into the finger. The temperature of the finger should be a t least 100” C. below that of the distilland. If the temp e r a t u r e is n o t low enough to freeze the condensate, that part which deposits outside the “seeing” face then drips back into the distilland without contaminating the selected condensate. To terminate an experiment, the coolant is removed, the cold finger is warmed quickly by filling several times with liquid, and air is admitted to the system. When t h e finger is Figure 1. Directional Coldbrought almost to room Finger Condenser Mounted in temperature before Flask breaking the vacuum, 1382
