716 



THE INDIA RUBBER WORLD 



[August 1, 1920. 



(5) Buffing: It is recommended that for buffing test pieces, 

 the wheel shown in Fig. 3 be used. The wheel is operated with 

 a %-h.-p. motor. The rubber to be buffed is clamped to a carriage 

 which is moved back and forth under the 5 by 1-inch emery 

 wheel (No. 60) running at about 3.000 revolutions per minute. 

 The central portion of the carriage just under the rubber is 

 slightly raised, by which means it has been found that the 

 operation is more easily performed and with less danger of in- 

 jury to the rubber. The wheel has a vertical adjustment and a 

 thumb-screw serves to lower the wheel by very small amounts as 

 the buffing proceeds. Shields are provided for the purpose of 

 keeping the fine particles of rubber of? the guide. The starting 

 base, though not necessary, is desirable in bringing the wheel 

 gradually up to the maximum speed. 



(6) Measuring Width and Thickness: A micrometer of the 

 rack and pinion 

 and dial microm- 

 eter type, Fig. 

 4, shall be used, 

 using a known 

 weight (225 

 grams), namely, 

 just enough to ac- 

 tuate positively 

 the rack and pin- 

 ion and bring the 

 disk positively on 

 the face of the 

 test piece without 

 compressing the 

 rubber. The shoe 

 and base which 

 presses against 

 the rubber shall 

 be 1 cm. in di- 

 ameter. 



(7) In case of 

 asbestos packing, 

 which is very 

 stiflf, it is neces- 

 sary to use a 

 greater weight to 

 obtain the correct 

 thickness : 3 kg. 

 on the foot 1 cm. 

 diameter should 

 be used. 



(8) Before any 



tests are made, the width and thickness shall be determined 

 by taking several readings on the constricted portion of the 

 specimen. The cross-sectional area shall then be determined 

 on the basis of the readings which show the smallest cross 

 section. 



(9) Conditions for Making Tests : .\11 tests of the rubber 

 parts shall be made in a room the temperature of which is be- 

 tween 15 and 35 degrees C. The tests shall not be made until 

 the test pieces have stood long enough to attain room tempera- 

 ture (not less than 4 hours). In case it should be impossible 

 to perform the tests in room temperature within the limit of 

 15 degrees and 35 degrees C, a box or conditioning chamber is 

 provided which can be kept uniformly at 24 degrees C, in which 

 the pieces should be placed before testing, for a period of 4 

 hours, and the test performed immediately after removal from 

 the chamber. 



(10) Aging after Curing Before Testing: The slabs from 

 which the strips are cut shall have been cured at least 24 hours 

 before the test is made. 



(11) Data on Reclaimed Rubber: The report of tests on 



--{^gtf^O 



I'lG. _'. .\rBUR PRE.■^ 



TRic Drive. Fig. 4. 



FOR Cltting Test Pieces. !■ 

 Dial and Hand Micrometer 



FOR Details of Construction). Fig. 6. Special jaws for Pure Gum Tests. 



vulcanized reclaimed rubber shall be accompanied by a statement 

 in regard to the cure of the slabs from which the test pieces were 

 cut; this statement shall include time and temperature of the 

 curing heat and the amount of sulphur used. 

 PHYSICAL TESTS. 



(1) Tensile: Tensile may be defined as the force required 

 to break a piece of unit cross-section area. It should be ex- 

 pressed in kilograms per square centimeter. 



(2) Elongation : This term is used to express the increase in 

 the length of the lest piece measured between two marks placed 

 on that portion of the test piece which is of uniform cross sec- 

 tion. It should be expressed in per cent. 



(3) Set: This term is used to express the increase in the 

 length of the test piece measured between two marks placed on 

 that portion of the test piece which is of uniform cross section, 



after it has been 

 stretched to 60 

 per cent of its 

 lireaking elonga- 

 tion for three suc- 

 cessive times, 

 holding it under 

 this elongation 

 for ten minutes 

 each time, and 

 permitting five- 

 minute intervals 

 of rest between 

 each interval of 

 stetch, and finally 

 allowing it to rest 

 ten minutes be- 

 fore the final 

 measurement is 

 taken. It is meas- 

 ured in per cent, 

 based on the 

 original length. 



(4) Gage 

 Marks: The gage 

 marker, Fig. 5, 

 shall be in the 

 form of a stamp 

 having thin steel 

 blades which are 

 strictly parallel 

 and 50 mm. apart; 

 another marker 

 having blades 25 mm. apart shall be used for the high-grade 

 stocks such as pure gum. The blades shall be kept clean 

 and free from accumulation of ink, in order that the lines 

 marked on the test pieces shall be very fine. 



(5) Number of Tests: At least four strips shall be tested in 

 every case and the average of these tests taken. 



(6) Report of Results: Since the physical properties of rub- 

 ber vary noticeably in any given product, it may occasionally 

 happen that tests are made upon a test piece which will be of 

 poor quality. The material as a whole meets the requirements 

 of the standard, but the particular piece taken falls somewhat 

 below it. To reject or accept a lot of, say, hose, belting, packing, 

 etc., because of failure of just one test piece to meet the speci- 

 fications, would, therefore be unfair. For this reason, acceptance 

 or rejection of an item offered for delivery shall be based on 

 the average of at least four determinations for each quantity. 

 In arriving at these averages no weight shall be given to tests 

 which are obviously in error, and do not represent true average 

 conditions ; namely, cases in which the tensile strength is low on 

 account of a small flaw in the article. If the tensile strength of 



;. 3. lirFFiXG WnFFL .\ND Klec- 

 FiG. 5. Gage M.\rker (See Fig. 



