Aipril15, 1930
I.\-DUSTRIAL
A Y D ESGI,\-EERISG
Determination of Chromium a n d Vanadium in Steel The results of the analvses of different steels given in Table 1-11were obtained by th'e fOlloTYing recommench Procedure: For steels containing chromium to the extent of 1 per cent a 2- to 3-gram sample is taken and for higher contents correspondingly less. The weighed sample is transferred to a 250-cc. Erlenmeyer flask; 25 cc. water are added and then 1.5 cc. of concentrated sulfuric acid for each giam of steel, allowing 1 t o 2 cc. in excess, and 3 or 4 cc. of 85 per cent phosphoric acid. T o hasten solution of the steel, the liquid is boiled. JT7hen solution is complete, sufficient concentrated nitric acid is added dropwse to oxidize iron to the ferric condition; about 1 cc. per gram of steel is sufficient. The solution I \ now boiled to expel the oxides of nitrogen and sufficient nater added to make the total volume approxirnatelv 100 cc. One or two grams of pure potassium bromate are then added and the solution is boiled for 5 minutes. Five grams of animonium sulfate are cautiously added in small portions to the solution (which should again hare been brought t o alqiroximately 100 cc.) ; the solution 1s hoiled until most of the bromine has been expelled. Then 10 c c . of 1 hydrochloric acid are added and boiling is continued until starchiodide paper is no longer colored. After cooling to room temperature, 5 to 8 cc. of 85 per cent phosphoric acid and 1 to 2 drops of 1/400 S potassium iodide are added: 0.1 9 arYenious acid is run in slowly from a buret until the color of the solution has become bluish green and finally 2 to 3 cc. in e w e s . The analysis is then continued as described above for the combined determination of chromate and vanadate. r \ L
CHEMISTRY
145
Table S'II shows that the proposed method gives satisfactory T a b l e VII-Analyses
of B u r e a u of s t a n d a r d s Steels a c c o r d i n g to t h e
Banmu STEEL
Proposed Method
O F STAKDARDS
VALL-ES
Chromium
Vanadium
Pev cent 1.3;
Pev cent 0.21
1 fl4n 1010
0.21
30 b
1 03
0 20s
32
0.89
30
30"
a
1
VALCES
1
Chromium
Vanadium
Per cenl 1.343 1.333
Per cenl 0 220 ri 214
1 005 0.998
0 2007 0.212
0.99
0.208
0.930 0.941
0.018
B Y PROPOSED ~~EX'HOD
II 1
Persulfate oxidation,
b Permanganate oxidation.
Literature Cited (1) Hnmner, J f e t . Chem. Enq., 17, 206 (1917). ( 2 ) Kelley and Conant, J. I N D . ENG.CHEII. 8, 719 (1916). (3) Knop, Z . anal. Chem., 77, 125 11929). (4) Lang, R., Z . a i f o r y . allgem. CAem.. 162, 205 (1926). ( 5 ) Lang, R . , and Zwgrina, Z . EleMrochem., 34, 364 (1928). (6) Lundell, Hoffman, and Bright, IXD. ENG. CHEX., 16, 1 0 6 4 (1923) (7) Spitalsky, Z . Q N O ~ R .Chem., 69, 179 (1911). (8) D;illard and Young, I N D . ESG. CHEII., 20, i 6 4 (1928). (9) IVillard and Young, I b i d . , 2 0 , 7 6 9 (1928). (10) Zintl and Zaimis, Z . a i ~ p e : ~Chem., '. 40, 1286 (1827). (11) Zintl and Zaimis, I b i d , 41, 543 (1928).
The Testing of Automotive Rubber Parts Assembled under Compression' . Part I-Deflection under Compression Franz D. Abbott FIRESTOSE TIRE A S D R C B B E RCO>IPASY,A K R O N , OHIO
Present-day test specifications on automotive rubber parts are not only incomplete and misleading, b u t entirely lacking i n uniformity. This is particularly t r u e of compression and permanent-set tests on automotive mechanical rubber parts assembled under compression. Consequently, a n effort has been made to show t h e urgent need for unification of such methods. Data are presented in Part I to show t h a t high tensile is no criterion of relative resistance to compression set and t h a t hardness is not a measure of deflectability. In addition, deflection a n d permanent set under compression are greatly influenced by t h e size and shape of t h e . . .
I
S KEEPISG with the rapid stiides in the automobile
industry, an almost endless variety of rubber mechanical goods is being used in motor-car manufacture. As a result there has arisen a great variety of tests and test specifications, many of which are worthless for evaluating the parts under consideration. In fact, some of these specifications, particularly those requiring compression- and permanent-set tests, are often defeating the purpose for which they were 1 Receiked July 3, 1929, reiised paper receibed December 2, 1929 Presented before t h e Division of Rubber Chemistry a t t h e 77th Meeting of the hmerican Chemical Society, Columbus, Ohio, April 29 t o M a y 3, 1929.
test piece, t h e method of cutting t h e sample, t h e gage a n d methods of measuring it, and condition a n d kinds of surfaces in contact with t h e test piece. Compression and compression-set testing equipment are discussed, and some newly designed laboratory equipm e n t is described. One piece of apparatus, t h e compressetometer, has been found extremely useful i n making deflection, hysteresis, and set tests. These tests may be performed under constant load or constant distortion. The constant-load (spring) compression-set clamp is suggested as a suitable laboratory standard for permanentset testing under compressive loads.
.. . written. The authors of these specifications have either presupposed a uniformity in testing equipment and details of tests, and hence have very briefly stated the tests, or else, through a lack of appreciation of the many factcm influencing the results, have failed to include such important details as size of test piece, gage, etc. Consequently many inferior stocks may pass these specifications if the testing conditions are carefully chosen. There are generally other requirements, such as tensile, elongation, etc., to preclude the possibility of passing certain very low-grade stocks, hut even these requirements do not offer an absoluie safeguard against inferior quality, for a very high tensile, for example, does not
AXdL PTICAL EDI T I O S
146:
necessarily mean high quality in a good motor-support or shock-insulator stock. Undoubtedly modulus, stress, and elongation a t break, and also percentage change in physical properties after aging are important in evaluating many rubber parts, and would be to a certain extent for automobile mechanical goods also, were it not practically impossible to perform many of these tests on the rubber parts themselves owing to their peculiar shape and size. I n many cases it may prove necessary to have the producer furnish n i t h each shipment laboratory test slabs of the same stock for tensile, deflection, and set tests. I n general, these test pieces will be given that cure which produces the maximum desired physical property rather than one identical with that of the
5’01. 2. s o . 2
if a t all, affected compression and permanent-set testing methods. Their tests were conducted a t unusually high pressures per unit area. Xriano (2) also has experimented with molded cylinders of vulcanized rubber compressed in the direction of the axis between parallel plates. The apparatus could compress the cylinders to 45 per cent of their original height. His paper consists of a somewhat mathematical discussion of the form of the compression curve and some factors influencing it, but’ does not give details concerning laboratory procedure. Seither of these papers, however, attempts to correlate the factors studied with permanent set under compression. Hippensteel ( 5 ) recently described a rubber compressiontesting machine, which is capable of t,ests a t considerably higher loads than are necessary for ordinary laboratory tests. Early in the study of automotive rubber parts used to absorb vibration, the author was confronted wibh the necessity for a suitable device for conducting laboratory tests under compression, either a t constant load or constant deflection, in order to compare resistance t,o flexure under compression with permanent, set under compressive loads. Such a device (Figure 1) mas then designed, as a co-testing device with the compression flexometer ( 1 ) . The latter may be described as a dynamic flexure-compression testing device, while the new inst)runient, called a “compressetometer” may be considered as a st’aticcompression testing device inasmuch as permanent set and fatigue tests are performed under steady loads. The compressetometer is suitable for tests a t loads up to 228 kg. (500 lbs.), which is a reasonable laboratory range. The capacity may be increased by substituting heavier springs. This device is much less expensive bhan the machine discussed by Hippensteel and furthermore niakes possible many special , particularly tests a t high and low temperatures. Description of Compressetometer
Figure I-Compressetometer
part in question. This procedure makes it possible for an unethical producer to furnish samples of stock apparently identical in composition, yet of far better physical properties than those possessed by the parts included in the shipment. The question of equivalent cure is also very important and “mechanicals” will often necessarily differ greatly in cure from such test slabs. To date it has been impossible to establish any true mathematical relationship between the results of tensile tests and those of many of the other mechanical tests. Goodwin and Park (5) have shown that the abrasion resistance of a tread compound cannot be judged merely by its tensile data. Likewise the writer ( 1 ) has shown that the data of tensile tests nil1 not serve to evaluate properly the resistance of a stock to flexure under compression. In view of the above facts, i t seems advisable to perform such tests as simulate. partly a t least, service conditions. Probably one of the most important groups of automobile mechanical rubber goods is that including parts assembled under compression, such as bumpers, engine and radiator pads, shims, shock insulators, torque insulators, etc. Sex-ertheless, most investigators appear to have almost n-holly disregarded compression testing, particularly hysteresis and permanent set under compressive loads. Recently, hom-ever, Birkitt and Drakeley ( 3 )have published some very interesting results of work on compression testing. Their data show that gage, area of test piece, arid slippage all influence compression results, but apparently the results reported have only dightly,
The compressetometer, which is mounted in an electric oven, produces compressive loads on test pieces by means of three tension springs. placed outside the oven in order to avoid heat effects as much as possible. By turning the hand wheel, A, in a clockwise direction, the springs are macle to pull a plunger, B , down onto a sample resting on the bottom plate, D . Loads are measured in 5-lb. units on the scale, L . The gage, G, measures deflections in thousandths of an inch, The spindle, S, can be adjusted to various heights to get the correct zero point on the deflection gage for various thicknesses of test pieces. A high degree of accuracy is
‘”
?-
CURE
Figure 2-Hardness
us. Cure
possible. especially when testing sinal1 round disks cut froin slabs, merely by lowering the plunger and noting the position at wliicli it just touches the test, piece. AIore positive accuracy is obtained, however, by first running the plunger down as far as it will go, setting the deflection gage to that reading corresponding to the thickness of the sample to be tested, aiid then raising the plunger and inserting the sample. The latter method is used in all research and special or comparison tests. A fan, F , provides circulation of air. A thermometer is so inserted through the orifice, 0 , that readings can be
April 15, 1930
IND L’STRIAL AND ENGINEERING CHEMISTRY
taken through the glass in the door. As a special precaution in some cases, particularly in getting heat tests started, the thermometer is inserted from the front so that its bulb rests on the plate, D. I n order t o avoid errors due t o temperature effects when standard permanent-set tests are made, i t is necessary to insert the test piece between metal plates (at room temperature) with surfaces similar to those in the permanent-set equipment used. These are then placed in the compressetometer, which is already at the test temperature, and the required load is imposed on the test piece. The majority of compression tests in this laboratory have been performed u i t h the above equipment. Although the compressetometer is relatively easy to operate at, loads u p to 500 lbs , plans have been made to motorize this equipment for tests a t higher loads. The instrument is also t o be autographic so that complete hysteresis curves will be obtainable. . Method of Testing
I n conducting compression and hysteresis tests, the loading and unloading arf’ performed a t the same arbitrarily fixed rates. I n general during the first half of the loading cycle, distortions are read a t every 11.5 kg. (25 lbs.) or less, depending upon the size of the test piece, and finally a t every 23 kg. (50 lbs.) as the slope of the curve becomes steeper (as A y / h increases). For unloading data, readings are taken a t each of the distortions *ecorded during loading. TESTSAT CONSPANT LOAD-In performing permanent-set tests a t constant loads, the test piece is put into the compressetometer under the proper temperature conditions and compressed to the required load. There is a n immediate fatigue effect, as shown by a change in the distortion during the first few minutes after the maximum load is reached. The rate of this change decreases rapidly, and this means a change in load. Consequeni ly, for the first few minutes, and much less frequently for the next 3 or 4 hours, or even during most of the test, the hand wheel is so regulated that the load is kept constant. The drop in load over a 24-hour period due to fatigue of the saniples is very small, ,particularly when the gage is approximately 0.635 em. (0.250 in.) (normal or average gzge for most set tests), so that continued load adjustments are unnecessary except in the most exacting tests. A change of 0.25 em. (0.1 in ) or 40 per cent based on 0.635-cni. (0.250in.) gage, in the position of the plunger causes a variation in the load of only approximately 3.18 kg. ( i lbs.) and no high-grade stock ( i f this gage will show a distortion fatigue of 40 per cent in a 24-hour test a t 70” C. a t ordinary test loadTAX.
HARDNESSD E f L E C T I O N .4T 120 KC.
STOCK CURE
(PENETRATION)
Min. 25
14 5 11
C. 320 320 298
12 60 a
b
T'ol. 2 , s o . 2
0.001 in.
~
cz 54 8 45.4 37.3
100 63 38
~
1
GAGE
PER
Orig.
Final
Cm. 0 562 0.576 0 68
0 254
cc. ~
.Sq. r t n .
1 6
6 3
1 64
5.2
1 83
4
7 ,I a
Kg. 18 1
23 0 26
TO G I V E DEfLECTIOs OF:
18 71;h
KG.
Cm. 0 315 0 426
s n . cn.
LOADof 120 ,
j i
K ~ . 8.2 18.1
36.4
37.3'4 KP.
37 8 68.2 110 0
1.9-cm. (0.75-in.) disks used. Approximate assembly deflection of a certain car support, but on larger areas
Compression-Set Clamp
The above discussion has disregarded actual changes or fatigue of the test piece during the test. For both constant load by spring clamps and constant distortion the stock which suffers the more rapid fatigue will be benefited to the extent that it will be resisting the smaller final load per unit area. This effect is negligible in a properly designed spring clamp, as has been shown by tests in the compressetometer. Evidently a constant-load device is a necessity. If a spring clamp is used, a suitable means for measuring deflection of the springs (loads) is likewise a necessity. Probably the simplest type of constant-load equipment is based on one of the constant-deflection clamps described above. If the tie bolts are longer, a short compreshion spring can be inserted between the plate. of the clamp. The teqt
I
h\Y
B 8
'
, I
I ,
1 ;
w
,
/ / i 1":
! ! iiul
z
1
I
Figure 1-Constant-Deflection Compression-Set Clamps A-Double zero-error B-Constant zero-setting at Z
piece is then put' on a steel disk placed on top of the spring and loaded b y compression of the spring. Here again there is a fairly large error in measuring by means of a n ordinary loose scale or caliper and scale. One such type of set clamp which has come into use has a short clutch spring about i cm. (2.75 in.) long, which distorts 0.95 ~111. (0.38 in.) under 120 kg. (265 lbs.) load. It has been shown that percentage deflection increases during tests by an amount' equivalent' to the fatigue of the test piece. This may amount to as much as 0.127 cm. (0.05 in.) on a O.635-clll. (0.25-in.) sample? and may cause a n error of approximately 13.0 per cent based on the spring distortion of 0.95 cxn. (0.38 in.). I n order to proyide ail easier operating and more accurate device, the writer designed the small compression-set clanip shown in Figure 2 (also shown in Figure 1, Part I). This clamp is provided wit'h a spring 10.1 cm. (4 in.) long, n-liich deflects 1.9 cm. (0.75 in.) a t 120 kg. (265 lhs.). The error due to the change in distortion caused by the fatigue of the sample has been cut 50 per cent. Likewise the percentage error arising from an error in measuring this spring deflection has been decreased. For convenieiice t,he clamp is pro-
vided with a depth gage n-ith a scale graduated in hundredtlis of an inch. The gage is rigidly attached to the clamp, and hence is much more satisfactory than a loose scale. Loaddeflection data are then obtained for each spring and suitable curves are constructed so that the clamp can be set to a definite scale reading for a given desired lcjad. X magnifying glass is used in reading the scale. A single nut control increases the ease of operation and permit,s uniform deflection throughout the test piece. Comparison of Tests Using Plunger and Rubber Disk
It can readily be seen that both the constant-deflectioii clamp and the constant-load clamp can be used to test either small buttons cut, direct from the samples (constant-volume test,s) or to make constant-area t'ests by means of a small plunger or anvil. The relative merits of the plunger versus the round rubber disk method were discussed in Part I. The same arguments against the plunger are valid here. Some comparative compress i o n - s e t t e s t s w e r e also made. =111 teits were d a r t e d in clamps originally at room temperature and ro ere of 66 hours' duration a t 70" C. All teqt pieces were 1.9 cin. in diameter and were cut from the same slab of ttock S o . 14, cured for 50 minutes a t 148" C. The load was 42 kg. per sq. cin. (600 lbq. per sq. in.). -411 dkks were sponged off with benzol and d r i e d . The original gage was determined by a R. R- S . g a g e w i t h 1 - c m . (0.39-in.) diameter foot, the final gage by a B. R- S.S o . 2 micrometer caliper 0.396cin. (0.16411.) diameter foot after a 30-minute recorery. Clamp In the exceptional case of Figure 2-Compression-Set S-Test disk a n a m e d disk the average of the minimum and maximum gage was measured 0.15 cm (0.08 in.) from the edge of the disk. The results are given in Table 11. The final load per unit volume is 59.4 per cent greater in case of the plunger method with an increase of 40 per cent in permanent set. This would tend to show that percentage increase in cet is not in the same ratio as percentage increase in load per unit volume and hence the error is slight for small differences in the latter. I n addition, "cupping" occurs in the lower side of the test piece in the plunger method a t high loads (over 42 kg. per sq. cm. or 600 lbs. per sq. in.). This should preclude the use of an ordinary thickness gage unless it is provided with a small foot and also a small pedestal to support the test piece.
April 15, 1930
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table 11-Comparison-Set
CLAMP
DISK: 1 2
MINIMUM GAGE Orig.
Final
Cm.
Cm.
Cm.
0.658 0.470 0.674 0.508
0.178 0.166
.
0.665
SET
Loss
PLUNGER: 1 0.665 0.420 0.245 2
1 1
0.417
1
T e s t s Using Disk a n d Plunger
0.248
70
VOLUME
LoA:c,PER
Orig. Final
Orig. Final
Cc.
Kg.
Cc.
28.6 24.6 Av. 2 6 . 6
1.87 1.87 1 . 9 1 1.91
37.0 37.4 Av. 3 7 . 2
1.85 1.85
Kg.
1050 1050 1040 1040
1 . 1 8 1045 1 . 1 1 8 1045
1670 1670
Dead-Weight vs. Spring Compression-Set Clamps
Without doubt a properly arranged dead-weight equipment should be more nearly capable of 100 per cent accuracy in load than the spring clamp. However, in order to eliminate frictional "wobble" troubles such equipment should be vertical acting, as in Figure 3, A , instead of "horizontal acting" as shown in B. A dead-load device is unwieldy and liable to error due to a sudden application of the load (slippage errors). For testing a piece 12.9 sq. cm. (2 sq. in.) in area a t 28 kg. per sq. cm. (400 lbs. per sq. in.) using 364 kg. (800 Ibs.), for example, some hydraulic or pneumatic equipment would be much more convenient than the actual weight method. However, for the sake of simplicity and for practical accuracy, the constant-load spring clamp is very satisfactory when properly designed, and the adoption of some fairly compact device, such as shovm in Figure 2, would be an important step forivard in the program of unification of methods of testing.
P1p D
C I T
that the spring clamp gives higher compression-set results than the dead-load method "due to the stiffening action resulting from the heat-expansion of the spring." Others have maintained that heat tends to weaken the spring. The latter view is strengthened by the curve in Figure 5. I n fact, the variation due to temperature in the l o a d -d e f 1e c t i o n c u r v e of t h e type of spring used in the clamps shown is probably f a r w i t h i n t h e other errors inherent in any s m a l l , c o m p a c t non-precision i n s t r u m e n t of t h i s t y p e . Nole-For present purposes a precision instrument is considered only as one which will measure distortions of load or gage of the sample t o within 1 per cent and 0.0025 cm. (0.001 in.), respectively.
It is true that there is an apparent stiffening of the spring d u r i n g a c o m p r e s s ion-set test which may amount to of Constant-Load. as much as 0.127 cm. Figure 4-Calibration Spring '0.05 in.) i n c r e a s e i n length a t 120 kg. (265 lbs.) in tests on 1.9-cm. (0.75411.) disks, cut from slabs of approximately 0.635-cm. (0.25-in.) gage. This is due, however, to fatigue of the stock, and in fact the load is really falling instead of increasing. Some data are given in Table 111. The tests were conducted a t 70" C. a t 136 kg. (300 lbs.) on 2.9-cm. disks of 0.95-cm. stock. T h e error in load is less than 3 per cent, and is less on lower gages and a t higher loads. For two samples tested simultaneously the error is doubled.
6 Figure 3-Dead-Weight 0-Oven T-Table
Table 111-Error Constant-Load C l a m p s D-Test disk C-Telescopic test clamp
The question has been raised as to the effect of repeated use on the calibration curve of the springs used in the constant-load clamps. Figure 4 shows that deflections of a new spring a t 120 and 182 kg. (265 and 400 Ibs.), respectively, were 1.7 and 2.59 cm. (0.67 and 1.03 in.j, whereas after almost 3 months of continual use a t loads between these two values for periods varying from 22 t o i 2 hours at 70" C., the distortions were, respectively: 1.9 and 2.76 cm. (0.7 and 1.09 in.). Of course other calibration curves were necessary in the meantime, but most of the change occurred within the first few tests a t the higher load, and hence probahly could have been avoided by a dead-weight test before calibration. It is considered that most of this change occurred in the thin ground-donx portions of the last coil a t each end of the spring. Another question has been raised as to the variation between calibration at room temperature and a t the standard test temperature (70" C.). I n Figure 5 it is shown that a t 182 kg. (400 lbs.) there is less than 0.038 cni. (0.015 in.j lyeakening a t i o " C. as compared with the yalue a t 24" C.. whereas a t 120 kg. (265 Ihs.) there is practically no error. This is quite within the experimental error in mcst spring clamps in use. It has been claimed (private communication to the author)
155.
i n Spring Load D u e to Failure of Test Piece ~
CLAMP
1 2
s&YpGI;*
OF S P R I S G I N C R E A S 5 A T 136 IN
SET
Cm.
%
0.965 0.966
11.1 9.2
~
Cm.
Cm.
7.62 7.6
7.7 i.65
Cm. 0 08 0.05
1
1 '
Cm.
5
2.135 2.135
3.0
-___
It was shown in Part I that the load falls when test pieces stand under constant distortion due to the fatigue in the rubber. Even greater fatigue occurs in the constant-load spring clamp, because the load is greater than that necessary for a constant-distortion test. A large change in distortion of the test piece a l l o w the spring to expand slightly with an appearance of stiffening: hence the load in a spring clamp is continually decreasing with increasing fatigue. but this load drop is quite negligible. Since the load drop is doubled if two samples are tested in the same clamp, only one test should be made a t one time in each clamp. The accuracy of spring calibrations as determined by data obtained from tests on 1.9-cm. pieces tebted a t 42 kg. per kq. em. for 24 hours at TO" C. in different clamps is qhoan in Table IV. All the disks were cut from the same slab of stock (KO. 15 cured 20 minutes a t IGO" C.) and readings were checked independently by two observers. Final gage measurements were made by a small-foot ( S o . 2) B. &- S. micrometer caliper.
d S d L Y TICAL E DI TIOS
156
T a b l e 1%'-Comparison of Accuracy of Calibration of Various Clamps GAGE
CLAXP
A- 1 A-2 B-l B-2 B-3 0-4
SET
Loss
Original
Final
Cm.
Cm.
0,702 0.695 0 691 0.695 0.675 0.678 0.678 0.695
0.372 0 562 0.550 0.565 0.542 0,542 0.526 0.555
0.030
0.033 0.041 0.030 0.033 0.036 0.052 0.040
1 1
v /'O 18 5 19.1 20 6 19 0 19.9 20.2 22.5 20.4
These data indicate that the calibration curves for the various springs used were very accurate. The duplicate tests ill clamps -4-1 and A-2 likewise show very satisfactory agreement when tests are made a t different times in the same clamps. The slope of the spriiigI I calibration curve has a definite effect on the accuracy possible with the constant-load clamp. It is doubtful if any spring should be used a t test loads of 182 lig. (400 lbs.) or under if the slope of its calibration curve is greater than that noted in this paper. On-ing to the increased r e l a t i v e distortion a t low loads due to the ground-down spring ends, the slope is arbitrarily set for data between 120 and 182 kg. (265 and 400 lhs.). Thus the original slope of the calibration curve (Figure 4) for the springwas 3.86, ,,,' 0 2 04 06 08 IO/?% whereas the filial value DIS TOR TI ON (Inches) m s 3.97. A niasimuni Figure 5-Effect of H e a t o n Spring T-alue of 4.5 might be set, Calibration b u t i s probably high, and any value lover than 3.75 prevents use over a nide load range. For high-load clamps this laboratory is using springs calibrated t o 455 kg. (1000 Ibs.) n i t h a maximum slope of 6 5 . Sole-Since presentatxon of this paper clamps of 1800 kg (4000 lbs 1 capacity have been secured.
xu. 2
Accordingly compression-set tests were made to determine the effect of a lustrous chromium plate on polished steel. I n one experiment two test disks of 1.9 cm. diameter neie separated by a bright chromium-plated spacer during test, 111 poliqhed steel clamps. I n each case the final diameter o f the surface in contact n i t h the steel plate was 1.9 cm., v.hcrea\ that in contact with the chromium surface v a s 2.16 ern. This represents over 22 per cent increased area with the chromium surface. Practically no slippage occurred a t the polished steel surface. Table V-Influence
I
of Gaae Variation on Compression-Set
1 251
0 415 361 0 445
32 6
3 5i
~
43 30 0
34 4
30 0
Table T? s h o w set data obtained when both surfaces of the test, pieces were in contact with polished steel and bright chromium-plated surfaces, respectively. In this laboratory briglit chromium-plated contact surfaces have been macle a standard because it is easier to keep them in a uniform condition. A11 constant-load clamps have been chromium-plated all over. The results are so satisfactory that such a procedure is reconiinended for adoption as standard. Influence of Steel v s . C h r o m i u m - P l a t e d Surfaces i n Set Tests A . S. T. hl. hardness stock S o 1. Load, 120 kg. Test specification, 20 hrs. at 70° C. Diameter of disk, 1.9cm.
Table VI-Relative
CLharp
X-l -1.2
I1
GAGE
,
~
Crig.
I
Final
Loss
Cm.
Cm.
Cm.
726 0 726 0
0 596 0 615
0.13 0 11
1
SET
SWRPACE
178
Chromium plate Steel
15.4
IZirkitt and Dralceley (3, Part I) reported more nearly ideal slippage iii deflection tests when pieces were lubricated with vaseline but they made no nieiition of set tests. Table 1-11s h o the ~ increased set rcsiilting from the use of a thin film of xhite vaseline on the round l.0-cni. test disks subjected to 32 kg. per sq. cm. for 24 hours at 70" C. as conipared with normal tests.
Influence of Gage and Slippage and Factors Influencing Slippage
Variation in the gage of the test piece has a n enormous effect on permanent-set tests under compression. The effect on single disks of variable gage is magnified in tests conducted on pieces formed by "piling" disks. Table T- gives data of permanent-set tests conducted similar to deflection tests recorded in Table YII, Part I, except that the set tests lvere of 24 hours' duration at 70" C. under 188 kg. (414 lbs.) in the compressetometer Greater slippage (distortion) occurred a t the rubber-torubber surfaces and evidently the increase in area of the middle disk on test 3 alone accounts for the increased set, for it was subjected to less load per unit area and also per unit volume than specimens in tests 1 and 2. I n Part I it was shop-n by percentage-deflection data that lubricated samples distorted the most, as a result of surface slippage. Evidently control of slippage is of prime importance in true set tests as aell as true deflection tests. Various surfaces affect the slippage to different degrees.
1-01. 2,
Table VII-Effect
of Lubrication of T e s t Piece COJIPRESSIOK-SET
C U R EAT 148" C.
-
Korrnal surface
Vaseline
~
Mi 11 i d e s 30 45 60
75
1-
