ANALYTICAL EDITION
MAY 15, 1940
stem did not have to be aligned exactly. If the valve sticks, the cotter pins will shear off and prevent damage to the motor. The valve base was mounted rigidly, so that it would not turn with the stem. I n other designs, it should be possible to couple the motor directly to the rheostat and thus get current, rather than airflow regulation.
Operation I n operation the desired flow is set by the control valve and the by-pass valve is set approximately halfway open. The by-pass will then take care of either high or low deviations from the desired rate.
293
B y using the variable resistance, the correct motor speed can readily be determined. Too high a motor speed will cause continued oscillation of t h e manometer; too low a speed will cause sluggish response. The relays used were obtained from a n old piece of electrical equipment and operate directly from a 110-volt source. Hence, condensers were placed across the contacts to reduce arcing; 0.25-mfd., 110-volt, is adequate capacity. The relay switch should be opened when the apparatus is not in use. A small bottle can be placed in the line after t)he flowmeter to trap the mercury if any relay contacts fail to open.
Laboratory Electric Stirring Motor ( oiiverbe
A
E. B. HERSHBERG 3Ieniorial Laboratory, Harvard University, Cambridge. Mass.
W I I E variety of stirring motors is on the market, and
yet few possess the characteristics desired in the organic laboratory. Small electric motors develop but little power a t low speeds and direct drive is usually unsatisfact'ory for a n y but t h e lightest and most constant loads. If t'he character of the mix changes rapidly, or if the load is re- . leased suddenly, the motor may speed up to a point where breakage occws. For this reason laboratory stirring motors should incorporat,e some type of speed-reduction mechanism which allows increased torque a t low speed. The method of mechanical speed reduction is usually the determining fact'or in the cost, marketability, and life of a stirring motor. The simple belt and pulley drive device has been neglected in favor of more exclusive and complicated designs with possible patent features. T h e mediocre results experienced with a number of these over a period of several years have led t'o the development of t h e motor and pulley assembly with electrical speed control described below. This unit has been found to be very satisfactory for driving wire stirrers (1). T h e series-wound motor is particularly suited for this purpose because of its favorable speed-torque characteristics. Sparking a t the brushes has not been found to be a serious fire hazard and the ordinary organic l a b o r a t o r y p r e c a 11 t i o 11 i
suffice. Shaded-pole induction motors are admittedly safer, but their essentially constant speed characteristics make them useful only in connection with a step pulley where a
FIGURE1 . BODY A.
B.
Push fit bearing seat, 22-mm. (0.8661-inch, -0.0000-inah, 0.0008-inch) bore Set screw, brass, %/a inch, 24 threads, or pipe tap
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
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out a tangle of wire. The locking feature then prevents accidental disengagement. A heavy-duty clamp, F , necessary to support the weight at the furthest extension of the rod, gives adjustment in t r o planes. As a rule the shaft is connected to the stirrer with a piece of heavy-walled rubber tubing, but provision has been made for the use of a chuck which may be purchased standard for a 0.61-cm. (0.25-inch) shaft. In the author's experience the firstnamed method is preferable with ball-bearing glass stirrers (I). Both rheostat assemblies shown in Figure 6 have been used with equal success. That on the left has a switch permitting stoppage without disturbing the rheostat setting, which is an advantage in inspecting the progress of a reaction. The rheostat on the right incorporates an off position, so that the circuit is broken when the dial is turned counterclockwise to the stop. Each is mounted between two Dieces of hard asbestos board and forms a unit detachable from the motor, a feature which is helpful when adjusting either assembly. Other electrical control apparatus, such as autotransformers, are convenient but relatively too expensive at the present time. The following specifications provide for the construction
-3-
of a unit of t h e highest quality material, and in some cases alternate sources of equal reliability have been given. An exhaustive investigation has not been made and other reliable makes should prove satisfactory. T h e construction of these units has been made possible b y the close cooperation of A. H. Gedies, machinist of the Mallinckrodt Chemical Laboratory a t Harvard.
Bore FIGURE 2. CAPS,ALUMINUMALLOY CASTING
few fixed speeds are sufficient. Since the motor operates in a vertical position, ball bearings are the most sat& factory in sustaining the armature thrust. Another advantage of the ball-bearing motor is t h a t it requires grease only after a year or more of normal d u t y and the entire assembly is free from lubrication difficulties. If a sleeve-bearing motor is used, as has been done previously in this laboratory, a single ball between the lower end of the shaft and a seat serves to take u p the thrust. By this means it has been possible to utilize the motors salvaged from other types of stirrers in conjunction with the same pulley assembly, although more frequent motor lubrication is required. A 440-1 pulley ratio with a 0.OZhorsepower motor developing its rating a t 5000 r. p. m. has been found the best combination for average use. If desired, a set of step pulleys could easily be substituted for greater speed flexibility. T h e belt is of the endless V-type, of rubber and fabric construction. Round rubber belts tend to vibrate excessively and joined leather belting cannot be used on so short a radius as the motor pulley. I n previous designs using leather belting the motor was placed on one end of the rod and the driven pulley assembly on the other. This removed the motor to a certain extent, but the unit vibrated and i t was discarded in favor of the present compact arrangement. T h e construction of the ball-bearing pulley assembly is given in Figures 1 to 4, while the complete motor assembly is shown in Figure 5 , and two alternate rheostat assemblies in Figure 6. The ball-bearing pulley, C, Figure 5, is clamped onto the stainless-steel support rod, D. The same screw holds the clamp for the electrical leads from the motor, relieving it from any strain due to the connecting cord, The author has found it convenient to include a twist-lock electrical connector, E, close t o the motor, making it possible to adjust the unit over the work with-
Assembly and Specifications BODY(Figure 1). The casting was made of KO. 12 aluminum casting alloy (8 per cent copper, Aluminum Co. of America, Pittsburgh, Penna.) by a local foundry. Both bearing recesses must be machined within the tolerances given to permit easy removal of the ball bearings.
/"
9d
. I
c
FIGURE 3. SLEEVEAND SHAFT
MAY 15, 1940
ANA1,YTICAL EDITION
295
FIGURE 4. PULLEYS AND SPRING BRONZE COMPRESSION WASHER
I
Upper pulley driven pulley machined from a1;minum alloy casting Lower, motor pulley machined from round stock
0
h
kQ86.9 l
Access to the setscren- of the sleeve (Figure 3) is through hole B (Figure l ) , and this is closed with a brass screw plug. A tight fit is obtained by threading only a part of the hole or by using a pipe tap. CAPS (Figure 2). Both caps are identical and are machined from castings of the same alloy as the body. The large end is faced and in this case the bearing recess is made slightly larger than that in the body, in order that the caps may be removed by hand. The compression washer shown in Figure 4 fits in this recess above the bearing and below the felt packing. When the cap is screwed down this exerts a positive pressure against 0
FIGURE 5 . COMPLETE MOTORASSEMBLY
the bearing, holding it firmly against the body, and a t the same time compresses the felt packing washers in the upper recess. Each cap is held in place by two 1.27-cm. (0.5inch) screws. BALLBEARINGS. Two singlerow ball bearings, 8-mm. bore, 22-mm. outside diameter, 7 mm. wide (extra-small type, single row, deep groove, bearing KO.38, SKF Industries, Inc., 440 East 34th St., New York, N. Y., obtained from local hardware distributor). SLEEVE (Figure 3). This is machined from round brass stock and must conform to the tolerances given to permit the ready removal of the bearings without damage t o bearings. SHAFT (Figure 3). Cut from 0.61-cm. (0.25-inch) ground polished finish, 18-8 stainless steel alloy rod with three flats filed in it for the large pulley, sleeve, anti chuck. NOTOR(Figure 5). Serieswound, varying speed, universal motor, 0.02 horsepower, 5000 r. p. m.,, 110 volts, ballbearing (General Electric Co., Schenectady, N. Y., Model 5P35CAlA, type P, frame 35 C. If a more poJverful motor is desired, the 0.033horsepower motor, frame 36, may be substituted. This will require a 100-watt rheostat). V-BELT (Figure 5). Inside diameter 8.9 em. (3.5 inches), outside diameter 9.53 cm. ( 3 . i 5 inches), width a t outside diameter 0.396 em. (0.156
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INDUSTRIAL .4ND ENGINEERIYG CHEMISTRY
FIGURE 6. R H E O S T .4S3EMBl,IES ~P
inch). Rubber and fabric construction (Eastman Kodak Co., Rochester, N. Y., No. 57012 mechanism drive belt for Kodascope E or EE). This belt is a replacement part on motion picture projectors. ELECTRICAL CONNECTOR (Figures 5E, and 6). Midget cord connector with cord grip (Harvey Hubbell, Inc., 1930 Thomas St., Bridgeport, Conn., catalog No. 7478. Obtained from local electrical supply shop). The male end is connected to the motor leads which are clamped to the bodv (Figure 5 ) and the female end to the rheostat with 150 to 180-cm. ( 5 to 6 feet) of rubbercovered lamp cord. SUPPORTROD (Figure 50). The motor block is drilled and tapped (0.25-inch pipe size) at the center to accommodate the 18-8
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stainless-iteel alloy rod, 1.27-cm. (0.5inch) diameter, 30 cm. (12 inches) long. CLAMP(Figure 5 , F ) . Heavy-duty right-angle clamp fastener. By using a large swivel clamp fastener, motion in the third plane can be obtained if desired (Central ScientificCo., 1700 Irving Park Blvd., Chicago, Ill., Catalog No. 72315). RHEOSTAT(Figure 6). A 5(rwatt, 400-ohm rheostat, with or without an off position in the extreme counterclockwise position according to the mounting shown, has proved satisfactory (Hardwick, Hindle, Inc., Newark, N. J., Type B-50). Previously, lobwatt rheostats (Ohmite Manufacturing Co., 4835 West Flournoy St., Chicago, Ill., Model K, 4O0-ohm1 stock No. 0454) had been used for 3 years. The smaller unit, however, costs only half as much and has shown no deterioration over a period of 6 months, though used above the manufacturer's current rating. The rheostat is mounted between two pieces of hard asbestos sheet 0.3 cm. (0.125 inch) thick, and a 8.9-cm. (3.5-inch) dial plate calibrated 0" to 100' over 325" is used to indicate the setting. This may be purchased from a local radio supply store. It is connected to the power supply through 60 to 90 cm. (2 to 3 feet) of lamp cord and a male electrical plug.
Literature Cited \ I ) Hershberg, IND.ENQ.CHEM.,-4nsl. Ed., 8 , 313 (1936).
Thermometer for Low Temperatures A. FARKAS AND L. FARKAS, Department of Physical Chemistry, The Hebrew University, Jerusalem, Palestine
HILE t h e well-known vapor pressure thermometers of Stock are the most accurate instruments for measuring low temperatures, they appear unhandy if the measurements are to be performed in a limited space. T h e v a p o r pressure thermometer having the form shown in the accompanying figure eliminates this drawback, and has proved very satisfactory in use in this laboratory. The construction of parts A , B , and C is evident from the figure. Coil D has a small hole in its end which prevents the mercury in B from entering tube C, while it ensures communication between Bland C. Tube E is sealed to the short glass rod, F , which in turn is sealed inside tube A . The capillary tip, G, of tube E prevents the escape of mercury from E in case the thermometer is kept in the horizontal position. The instrument is filled as follows: Bfter adding sufficient mer-
cury, t h e thermometer is brought into horizontal position with coil D upward (the mercury should now reach the level indicated by the dotted line), and is evacuated. Then the thermometer is brought into perpendicular position, the thermometric substance-e. g., carbon dioxide-is admitted up to a pressure of 400 to 500 mm. and condensed, and the thermometer is sealed off a t a. The thermometer can be calibrated empirically or by using the known vapor pressures of the thermometric agents. In the latter case the capillary depression of the mercury in tube E has to be allowed for. One thermometer of the dimensions given in the figure will cover about 15" to 2 5 O , depending on the gas with which it is filled. Ten thermometers filled with the substances listed in the table will cover nearly every range of temperature down to -215" C. Once a thermometer has been filled and sealed, i t can be handled just like any ordinary mercury thermometer. Thus it will not be affected if it is kept in the horizontal position. Substance
Temperature Range O
Pentane Butane Propane Carbon dioxide Ethane
c.
+ 5 to - 35 to -
35 73 71 t o -100 - 93 to -112 -112 to -140
-
Substance
Temperature Range
c. Ethylene Nitric oxide Methane Oxygen Nitrogen
-122 -159 -175 -193 -205
t o -150
to -175 t o - 188 to -205 to -215
