E . B . H E R S I I B E R G
C o n v e r s e M e m o r i a l L a b o r a t o r y , H a r v a r d U n i v e r s i t y , C a m b r i d g e , M a s s .
A
W ID E v arie ty of stirrin g m otors is on th e m a rk e t, and y e t few possess th e ch aracteristics desired in th e organic la b o ra to ry . Sm all electric m otors develop b u t little power a t low speeds an d d irect driveis usually u n satisfacto ry for a n y b u t th e lig h test and m ost co n stan t loads. If the ch aracter of th e mix changes rapidly, or if th e load is re
leased suddenly, th e m otor m ay speed u p to a poin t w here breakage occurs. F or th is reason lab o ra to ry stirring m otors s h o u ld in c o r p o r a te some ty p e of speed-reduc- tion m echanism w hich al
lows increased to rq u e a t low speed.
T h e m ethod of m echanical speed reduction is usually th e determ ining fac to r in th e cost, m a rk e tab ility , and life of a stirrin g m otor. T he sim ple b e lt and pulley drive device has been neglected in favor of m ore exclusive and com plicated designs w ith pos
sible p a te n t features. T he m ediocre results experienced w ith a nu m b er of these over a period of several years hav e led to th e developm ent of th e m otor an d pulley as
sem bly w ith electrical speed control described below. T his u n it h as been found to be v ery satisfacto ry for d riv ing w ire stirrers (1).
T h e series-wound m otor is p articu la rly suited for this purpose because of its favor
able speed-torque ch aracter
i s t i c s . S p a r k i n g a t t h e brushes h as n o t been found to be a serious fire hazard an d th e ord in ary organic l a b o r a t o r y p r e c a u t i o n s
suffice. Shaded-pole induction m otors are a d m itte d ly safer, b u t th e ir essentially co n stan t speed characteristics m ake them useful only in connection w ith a step pulley w here a
Fi g u r e 1. Bo d y
A . P u sh fit b e arin g s e a t, 22-m m . (0.8661-inch, —0.0000-inch, + 0.0008-inch) bo re
B . S et screw , b ra ss, */• in ch , 24 th re a d s, or p ip e ta p
IN D U STR IA L AND E N G IN E E R IN G C H EM ISTR Y VOL. 12, NO. 5
P r i l l
F e l t k i n g
B o r e
Fi g u r e 2 . Ca p s, Al u m i n u m Al l o t Ca s t i n g
few fixed speeds are sufficient. Since th e m o to r o perates in v ertical position, ball bearings are th e m o st sa tis
fac to ry in sustaining th e a rm a tu re th ru s t. A n
o th e r ad v a n ta g e of th e ball-bearing m o to r is th a t i t requires grease only a fte r a y ea r or m ore of norm al d u ty an d th e entire assem bly is free from lubrication difficulties. I f a sleeve-bearing m otor is used, as h as been done previously in th is lab o ra
to ry , a single ball betw een th e lower end of th e s h a ft and a se at serves to ta k e u p th e th ru st. B y th is m eans it h as been possible to utilize th e m otors salvaged from o th e r ty p es of stirrers in conjunc
tion w ith th e sam e pulley assem bly, although m ore fre q u en t m otor lubrication is required.
A 4 -to -l pulley ratio w ith a 0.02-horsepower m otor developing its ra tin g a t 5000 r. p. m. has been found th e b est com bination for average use. If desired, a set of step pulleys could easily be su b stitu te d for g reater speed flexibility. T h e b elt is of th e end
less V -type, of ru b b er an d fabric construction.
R o u n d ru b b er belts te n d to v ib ra te excessively an d joined le ath e r belting ca n n o t be used on so sh o rt a radius as th e m otor pulley. I n previous designs using le ath e r belting th e m o to r w as placed on one end of th e rod and th e driven pulley assem bly on th e other. T h is rem oved th e m otor to a certain exten t, b u t th e u n it v ib rated and i t was discarded in favor of th e presen t com pact arrangem ent.
T h e construction of th e ball-bearing pulley as
sem bly is given in F igures 1 to 4, while th e com plete m otor assem bly is shown in F igure 5, an d two a lte rn a te rh e o sta t assem blies in F igure 6.
The ball-bearing pulley, C, Figure 5, is clamped onto the stainless-steel support rod, D. The same screw holds th e 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 to the motor, making it possible to adjust the unit over the work w ith
out a tangle of wire. The locking feature then prevents accidental disengagement. A heavy-duty clamp, F, necessary to support the weight a t the furthest extension of the rod, gives adjustm ent in two planes. As a rule the shaft is connected to the stirrer with a piece of heavy-walled rubber tubing, b u t provision has been made for the use of a chuck which m ay be purchased standard for a 0.61-cm. (0.25-inch) shaft. In the author’s experience the first- named method is preferable with ball-bearing glass stirrers (1).
Both rheostat assemblies shown in Figure G have been used with equal success. T h at 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 th a t the circuit is broken when the dial is turned counterclockwise to the stop.
Each is mounted between two pieces 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 autotransform ers, are convenient b u t rela
tively too expensive a t the present time.
T h e following specifications provide for th e construction of a u n it of th e highest q u ality m aterial, and in some cases a lte rn a te sources of equal reliability h av e been given. An exhaustive investigation has n o t been m ade an d o th e r re
liable m akes should prove satisfactory.
T h e construction of these u n its has been m ade possible b y th e close cooperation of A. H . Gedies, m ach in ist of th e M al- lin ck ro d t C hem ical L ab o rato ry a t H arv a rd .
A s s e m b ly a n d S p e c ific a t io n s
Body (Figure 1). The casting was made of No. 12 aluminum casting alloy (8 per cent copper, Aluminum Co. of America, Pittsburgh, Penna.) by a local foundry. Both bearing recesses m ust be machined within the tolerances given to perm it easy re
moval of the ball bearings.
J s F l a t s
R e a m
1 0 - 3 2 l a p
I s / s o "
P r i v e F i t
Fi g u r e 3 . Sl e e v e a n d Sh a f t
MAY 15, 1940 ANALYTICAL E D IT IO N 295 Fig u r e 4 . Pu l l e y s a n d Sp r i n g Br o n z e Co m p r e s
s i o n Wa s h e r U p p e r p u lle y , d riv e n p u lle y m a ch ined from a lu m in u m a llo y c astin g L ow er, m o to r p u lle y m ach in ed fro m
ro u n d sto ck
the bearing, holding it firmly against the body, and a t the same tim e compresses th e felt packing washers in the upper rccess. Each cap is held in place by two 1.27-cm. (0.5- inch) screws.
Ba l l Be a r i n g s. Tw osingle
row ball bearings, 8-mm. bore, 22-mm. outside diameter, 7 mm. wide (extra-small type, single row, deep groove, bear
ing No. 38, SK F Industries, Inc., 440 E ast 34th St., New York, N. Y., obtained from local hardware distributor).
Sl e e v e (Figure 3). This is machined from round brass stock and m ust conform to the tolcrances given to perm it the ready removal of the bear
ings without damage to bear
ings.
Sh a f t (Figure 3 ). C ut 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, and chuck.
Mo t o r (Figure 5). Series- wound, varying speed, uni
versal motor, 0.02 horsepower, 5000 r. p. m., 110 volts, ball
bearing (General Electric Co., Schenectady, N. Y., Model 5P35CA1A, type P, frame 35 C. If a more powerful m otor is desired, the 0.033- horsepower motor, frame 36, may be substituted. This will require a 100-watt rheostat).
V - Be l t (Figure 5). Inside diameter 8.9 cm. (3.5 inches), outside diameter 9.53 cm.
(3.75 inches), width a t outside diam eter 0.396 cm. (0.156 ' / O S 2
T a p
/ 6 ‘
Access to the setscrew of the sleeve (Figure 3) is through hole B (Figure 1), and this is closed with a brass screw plug.
A tight fit is obtained by threading only a p art of the hole or by using a pipe tap.
C a p s (Figure 2). Both caps are iden
tical and are machined from castings of the same alloy as the body. The large end is faced and in this case the bear
ing recess is made slightly larger than th a t in the body, in order th a t the caps m ay be removed by hand. The compres
sion 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
F i g u r e 5. C o m p l e t e M o t o r ' A s s e m b l y
296 IN D U STR IA L AND E N G IN E E R IN G CH EM ISTR Y
Fi g u r e 6 . Rh e o s t a t As s e m b l i e s
inch). Rubber and fabric construction (Eastm an Kodak Co., Rochester, N. Y., No. 57012 mechanism drive belt for Kodascope E or E E ). This belt is a replacement p art on motion picture projectors.
E l e c t r i c a l C o n n e c t o r (Figures 5E, and 6 ) . Midget cord connector w ith 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 body (Figure 5) and the female end to the rheostat with 150 to 180 cm. (5 to 6 feet) of rubber- covered lamp cord.
S u p p o r t R o d (Figure 5£>). The m otor block is drilled and tapped (0.25-inch pipe size) a t the center to accommodate the 18-8
stainless-steel alloy rod, 1.27-cm. (0.5- inch) diameter, 30 cm. (12 inches) long.
C l a m p (Figure 5, F). H eavy-duty right-angle clamp fastener. By using a large swivel clamp fastener, motion in much and has shown no deterioration over a period of 6 months, though used above the m anufacturer’s current rating.
The rheostat is mounted between two pieces of hard asbestos
mometer is brought into perpendicular position, the therm ometric substance—e. g., carbon dioxide—is adm itted up to a pressure of 400 to 500 mm. and condensed, and the therm om eter is sealed off