New Machine for Laboratory Evaluation of Fatigue of Rubber Compounds Flexed under Compression LESLIEV. COOPER,The Firestone Tire a n d Rubber Company, Akron, Oh19
T
HE failure of rubber compounds to resist fatigue when subjected to flexing under compression, or their tendency to "blow-out," has always been a serious problem, and is becoming an even greater limiting factor in all phases of rubber technology. A new machine has been developed recently in the laboratories of The Firestone Tire and Rubber Company for the measurement of fatigue resistance of rubber compounds. Previous machines of this type have usually been large wheels or drums on which whole tires have been run to destruction. The laboratory machines have not been very numerous or very successful. The Schopper detrition tester
(2) is a laboratory machine for testing the fatigue and abrasioii resistance of rubber stocks in the shape of small balls. Ab. bott (1) obtained blow-outs in his laboratory work on testing rubber used to absorb vibration. Many suggestions have been advanced as to how correct laboratory evaluation of fatigue resistance might be accomplished. The design finally adopted (Figure 1)was made up by Raymond W. Allen, of the Firestone engineering staff, in whose name the patenb application on this Flexometer has beee made. A photograph of the instrument is shown in Figure 2, The Flexometer is definitely a laboratory testing device, but is sufficiently rugged to withstand hard and continu-. ous service. The o p e r q t i o n of the m a c h i n e simply consists of compressing, under definite load, a block of rpbber b e t w e e n two plates, one of which is stationary and the other travels in a circular motion of definite magnitude. The plate o s c i l l a t e s 800 t i m e s p e r minute, this speed being constant u n d e r a l l c o n d i t i o n s . Both the loading and the m a g n i t u d e of the circular motion may be varied widely. The machine has almost unlimited possibilities iq evaluating stocks, since the working condition8 in any compression service may be simulated. The laboratory test block used is in the shape of a frustum of a rectangular pyramid, of which the base is 2.125 X 1.125 inches (5.40 X 2.86, em.); the top, 2 X 1 inches (5.08 X 2.54 cm.); a n d t h e a l t i t u d e , 1.50 i n c h e s (3.81 cm.). This tapered shape was selected so that perfect blocks c o u l d b e p r e p a r e d from any type of stock. T h o s e w i t h h i g h z i n c oxide content, for example, are very brittle while hot. If the test blocks of such stocks have any tendency to adhere, they may be ruined in stripping from the mold. Test blocks of cured articles may be cut into any size and shape, provided their height is not more than 2 inches, as this is the maximum distance between plates. The tapered block is used only wheE the test sample must be cured in the laboratory.
PROCEDURE
FIGURE1. DIAGRAM OF FLEXOMETER
After the test blocks have been made, the procedure fop use of the machine is as follows: (1) The oscillating plate is set on dead center and broughb to a definite starting temperature. (2) The test block is placed an the oscillating plate, di, rectly under the loading plate. Each of these plates has a center inset of wood, 3 inches (7.62 cm.) in diameter and 0.50 inch (1.27 cm.) thick, and the rubber test block is placed between the wooden blocks. The latter act as heat insulators, and tend to hold all generated heat in the tesk block. (3) The load is applied to the block by turning the hand wheel on top until the load is carried by the block and not! by the thrust bearing on the wheel. 350
September15,1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
(4) The height of the block after applying the load is then measured, and the deflection is calculated. (5) The oscillating plate is set the desired amount off center, distorting the test block so that it resembles the frustum of a sloping rectangular pyramid. The diameter of the c i r c l e described by the l o w e r p l a t e w h i l e in motion is designated as the “throw.” (6) The e l e c t r i c a l bell-ringing contacts are adjusted so that they are a definite distance apart. This initial contact opening is called “signal distance.” As the block is deflected under the testing conditions, t h e upper contact is carried d o w n w a r d toward the lower contact, b e c a u s e t h e l o a d , which is a dead weight, continues to rest on the yielding block. This downward m o v e m e n t has been found by m a n y t e s t s t o be a d e f i n i t e c r i terion of the condition of t h e c e n t e r of t h e FIGURE 2. PHOTOGRAPH SHOWINGblock. In other words, FLEXOMETER all b l o c k s of s i m i l a r composition have yielded the same distance, or signal distance, a t the time porosity began. Should the test be continued sufficiently long, the block will actually blow-out, or shatter to pieces, and it is to prevent this actual destruction that the yield distance of slight porosity is used. (7) The duration Of the test, which is now ready to be started, is measured from the time the circular motion is started until the bell rings. The bell does not ring until the test block has been deformed sufficiently to allow the electrical contacts to close.
TABLEI. SPECIAL STOCKS USED
IN
351
The operation evaluates resistance to blow-out. Temperature rise data or heat generation measurements may also be made by inserting thermocouples and taking temperature measurements after a certain amount of work has been done on the block. One man can operate at least five of these machines efficiently, or if an automatic stop is installed, one machine can be operated without seriously interfering with other work the operator may be doing. Care should be taken that only well-molded blocks are used and that the machine is correctly set each time. If these two precautions are taken, consistent results will be obtained. The machine is not intricate in design or movement, yet it can readily be used for a wide variety of tests. RESULTS As an example of the results obtained on this machine, six special stocks were mixed and tested. Their formulas and the data obtained are given in Tables I and 11. LITERATURE CITED (1) Abbott, IND.ENQ.CHEM.,20, 853-7 (1928). (2) Schopper, Louis, Catalog 415, p. 25, Leipzig, Germany; American Agent, Testing Machines, Inc., New Yfxk, N. Y. RECEIVED May 19, 1933.
A Circulator for Ice Water L. L. ENGLISH Alabama Polytechnic Institute, Field Laboratory, Spring Hill, Ala.
A
SECOND-HAND automobile vacuum tank purchased at a wrecking establishment and operated with a small filter pump has been very satisfactory for circulating ice water through a condenser during the distillation of lowboiling liquids. The tank is mounted vertically on a tory shelf or wall so that it discharges into a suitable vessel containing a piece of ice and a small amountof water. The circulation is intermittent, and while the vacuum tank is
TESTS
STOCK A B C D E F 100 100 100 100 100 100 Smoked sheet Sulfur 3 3 3 3 4 . 5 3 D.0.T.G. 1.5 1.5 1.5 1.5 1.5 1.5 Channel black 8 36 36 36 36 8 Pine tar 1 1 5 1 1 Mineral rubber . . . . . . . . . 5 Zinc oxide No. 1 180’ 90 90 90 90 Zinc oxide No. 2 180 Cure 60 and 80 minutes at 290° F. (143.3O C.) on regular taper blocks. Testyng conditions: load, 550 pounds (249.5 kg.); throw, 0.55 inch (14 mm.); signal, 0.300 inch (7.60 mm.)
... . . . . . . . . . . . . . . . . . . . . . ...
TABLE11. DATAOBTAINED WITH FLEXOMETER STOCK 60HARDNEBS TIMEUNTIL minute %POUND BELL cure PENETROXETER DEFLECTION RANQ 0.001 % Min. .23.3 53 43 A 22.3 16.5 35 B 23.3 10.6 35 C 17.5 22.7 D 35 19.5 18.0 30 E 22.7 31 40 F
CONDITION OF STOCKAFTER TEST Slightly porous Porous Porous
Porous Porous split Slightly poroua
80-
minute cure
A B C D E F
a
46 35 33 35 29 40 Equivalent t o 0.00254 om.
22.3 22.3 23.0 22.7 17.3 22.7
63 16 10.5 17 20 39
Slightly porous Porous Porous Porous Porous split, Slightly porous
discharging some of the water in the condenser will run back into the reservoir, unless a valve is placed in the intake hose. A suitable valve may be obtained from an old atomizer bulb. With this outfit, 10 pounds (4.54 kg.) of ice will operate a condenser for 3 to 4 hours. RECEIVED June 15, 1933.
