MOTOR STIRRING APPARATUS G. ROSS ROBERTSON, UNIVERSITYOB CALIFORNIAAT
LOS
ANGELES, CALIFORNIA
The lack of an inexpensive electric stirrer in the chemical apparatus market has led several large laboratories to draft into service the comparatively new "household mixer" of commerce. This device, widely sold a t a very low price, is readily adapted to laboratory use by direct rubber tube connection of its vertical shaft with an ordinary glass stirring paddle. Its universal motor permits rheostat speed control. To he sure, i t becomes warm when operated a t reduced speeds; but no serious damage seems to occur even after weeks of prolonged duty a t moderated speed. Quite naturally, however, the household stirrer is assembled in a manner better adapted to eggs and mayonnaise than to the chemical laboratory. It is both expensive and awkward to provide such a motor with the customary large laboratory rheostat on a student's desk. Furthermore, the open wiring to 110-volt service near wet plumbing is unsafe. These considerations have led us to combine the household stirrer with a very small rheostat and electrician's conduit fittings to give a compact outfit without electrical hazard to inexperienced users. Figure 1 illustrates the arrangement, including the following parts: One Polar Cub Household Mixer complete with switch, cord, plug, and
steel stirring paddle $5.00 One Allen-Bradley Bulletin 410 Size A Controlling Rheostat, switchboard model, range 8 to 600 ohms 3.00 One standard zinc-plated pressed steel conduit switchbox, single, with plain cover .30 One gal". pipe nipple, '/%' I. P.. 8' long .15 One plumbers' waste-nut, I/%" I. P. .10 One pipe bushing, inch-to-half-inch I. P. .10 One laboratory clamp holder, large size .30 .05
Screws, nuts
List retaii $9.00 Deducting customary discounts to educational institutions, total cost of material (approx.) $7.25 A mechanics' helper can assemble the outfit in less than half a day, bringing the total cost to less than $10. The wooden handle furnished with the motor is replaced with the pipe hushing which is turned or ground down slightly to fit the motor ferrule. While the compression rheostat specified is a compact and rugged device, even more economical substitutes are available a t radio supply stores under the name of "power potentiometers." These list a t $1.50 up, and are wire rheostats with knob handle; for example, the De Jur Power Potentiometer and the Truvolt Variable Resistance. Novelties in this line appear in the market frequently. The 0-500 ohm size seems to be the best commercially available a t the present writing. A slightly higher resistance than 500 2264
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ohms is desirable for very slow speed on very light load, but the commercial radio potentiometers of higher resistance have scarcely enough carrying capacity (0.2 ampere) to control the stirrer safely.
The compression rheostat is capable of handling considerably more than 0.2 ampere provided the power consumption in the device does not exceed 20 watts. Tests in this laboratory show that neither the Polar Cub (Figure 1) nor the 1/200 h.p. motor (Figures 2,3) place a burden of more
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than 12 watts on the rheostat a t peak rheostat load. This peak load occurs a t intermediate speed. The rheostat uses as little as 5 watts a t high and low speeds. On the other hand, the effectiveresistance of this type of rheostat changes slightly with temperature, necessitating a small readjustment of speed after the appliance becomes warm. The principal shortcoming of the above equipment is lack of power a t low speeds. In most operations by students this is of little concern, as the experimenter merely turns the knob until he gets the desired effect. For higher power a t low speed, and thus a t constant speed on light load, the assembly shown in Figures 2 and 3 gives good service. While more expensive than the device of Figure 1, it is still far cheaper than the standard laboratory stirring motor. The motor is a stock 1/200 (or 1,450) h.p. universal motor, which is the smallest standard model of the major manufacturers. It is more expensive and more efficient than the Polar Cub, and gives additional power a t nearly the same current consumption. It is designed for horizontal operation. With the belt drive ratio of 4 to 1, the stirrer is very effective on heavy loads. The mqtor carries a V-groove pulley furnished by the manufacturer. A General Electric motor used in this assembly consumes only 0.2 ampere a t common speeds; 0.35 ampere with locked rotor and 112 volts A. c. directly across the terminals. Westinghouse and other manufacturers offer equipment of much the same class and cost. 4 The motor is mounted on the bottom of a conduit outlet box ( ~ J x inches), in which the superAuous circular knockouts have been hammered back into place and soldered over a Bunsen flame in places where they are called upon to support a screw. The box encloses the rectangular Bradley rheostat of the same size used in the apparatus of Figure 1. The pipe in this assembly is made as short as possible to minimize overhang in the I. P. size, and slips loosely through the two a/P" caps back. It is of which are drilled and mounted on a 3/1" close nipple fastened to the box. The right end of the pipe is screwed into a small rectangular piece of flat iron bar which prevents i t from turning in the box, but permits the box to slide back and forth slightly for adjustment of belt tension. The pulley (2" running diameter, four times that of the motor) is mounted on a suitable length of '/4" brass rod which rotates in a reamed bearing made from a piece of '/lNbrass rod. The brass bearing is threaded externally I/%" S.A. E., and screwed down clearthrough the straight run of the tee; or i t may simply be soldered into the tee, as suggested in the figure. A standard to '/4" bull head tee is used. Common stout twine serves as belt material. In either of these pieces of apparatus it is desirable to provide a laboratory clamp holder of a size just large enough to slip readily along the pipe, but not large enough to be taken off. The holder thus becomes a permanent part of the outfit as issued a t a storeroom.
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