December 1, 1918.] 



THE INDIA RUBBER WORLD 



135 



placing any test piece in the machine ready for stretching, the 

 piece shall be drawn just taut and the stop watch started at 

 the instant of the beginning of the stretch. Tlie piece shall 

 then be held for ten minutes at a specified distance, and the time 

 shall be again noted at the moment the piece is released. This 

 moment is simultaneously the beginning of the period of rest. 

 The measurement is then to be taken at the instant of expira- 

 tion of the second 10 minutes. 



(2) Measurement of Elongation. — Marks two inches apart 

 shall be placed on the test piece by means of a marker. These 

 marks shall be at right angles to the direction of pull of the 

 piece in the machine. Great care shall be taken: (1) That the 

 marks are not too wide, and (2) that at the time of marking, 

 the piece shall have been lying for a sufficiently long time to be 

 completely at rest on a wooden table which has been at the 

 temperature of the room mentioned in paragraph 2 herein. The 

 marks shall be placed on the smooth side ; that is, in no case 

 on tlie side which is corrugated, due to its impression taken 

 from the fabric. 



After clamping the test piece in the jaws of the machine the 

 movable jaw shall be so adjusted with the pointer reading zero 

 on the scale that the test piece is just taut, but not under ten- 

 sion. The operator shall throw on the power to start the screw 

 and, when ready, throw in the engaging lever to start the jaws. 

 He shall keep the elongation scale pointers opposite the outside 

 edges of the marks on the piece. To stop the motion at the 

 desired elongation or upon the break of the piece, the jaws 

 shall be disengaged from the screw. 



The accuracy with which the elongation measurements are 

 made will depend upon the accuracy with which the operator 

 keeps the two pointers opposite the outside edge of the marks 

 on the test piece. 



The elongation shall be reported in inches, including the origi- 

 nal two inches ; that is, if the rupture occurs at eleven inches, or 

 twelve inches, or thirteen inches, it will indicate that the stretch 

 has been two to eleven, two to twelve, or two to thirteen. The 

 piece shall be removed from the machine, but not snapped back; 

 in no case shall the time consumed in removal from the machine 

 exceed two seconds. 



(c) Measurement of Permanent Set. — After the piece has 

 been removed from the machine the permanent elongation or 

 recovery shall be measured by laying it upon a wooden table 

 which is of the temperature of the room, and allowing it to rest 

 for ten minutes. Immediately upon the expiration of the ten 

 minutes a rule graduated to 1/32 inch shall be laid upon the 

 piece and the elongation read in 1/32 of an inch, measuring the 

 outside of the marks. 



The per cent of elongation of the test piece above the original 

 two inches shall represent its permanent elongation. 



Tests for the determination of permanent set will be made 

 upon a machine similar in principle to the one illustrated on 

 the last page of these specifications. 



TENSILE STRENGTH. 



9. (a) The tensile strength shall be determined by stretch- 

 ing a test piece not previously tested in the tensile machine until 

 it breaks. If the test piece breaks outside the marks, or in the 

 wider portions of the piece, and the tensile strength is much 

 below that called for in the specifications, it is probable that 

 this piece is faulty and that another would meet the require- 

 ments. If the piece breaks outside the marks and yet shows 

 a tensile strength above that called for in the specifications, it 

 is probable that the piece is faulty and that its true tensile 

 strength is higher than indicated. Since its recorded tensile 

 strength exceeds that called for in the specifications, however, 

 it shall not be necessary to retest. 



Before any tests are made, the width of the test piece shall 

 be determined at three points equidistant between the marks. The 

 backing or irregularities of fabric impression shall be stripped 



or buffed off and the thickness measured with the backing re- 

 moved. It shall be determined at three points equidistant between 

 the marks on the test piece, by means of a standard spring- 

 gage micrometer (exerting a pressure of eight to twelve ounces), 

 the disks of which are ^i inch in diameter. The measurements 

 used in the computation of tensile strength shall be those read 

 nearest the point of break. The disk of the micrometer shall be 

 '4 inch in diameter when measuring thickness of the tube of all 

 hose which has an inside diameter of one inch or under. 



(b) Initial Stress.— During the elongation and recovery 

 test the initial stress shall be taken by connecting a spring 

 balance with the piece under test. The number of pounds read 

 on the balance at the maximum stretch shall then be computed 

 in pounds per square inch and expressed as "initial stress." 

 PRESSURE TESTS. 



10. (a) The hose shall be stretched out for inspection, con- 

 nected to the pump, and filled with water, leaving the air cock 

 open to allow the air to escape. The air cock shall then be 

 closed and a pressure of ten pounds per square inch applied. 

 The test shall then begin by taking original measurements with- 

 out releasing the pressure. 



(b) All pressure tests shall be made by using a hand or 

 power water pump, and a standardized gage. The increase in 

 pressure shall be made at the rate of 300 pounds per minute, 

 and the hose under test shall be held for measurement not 

 more than two minutes, unless otherwise called for in the speci- 

 fications. 



COMPOSITION. 



11. (a) Friction,— Wherever, in the detail specifications, 

 friction is mentioned it shall be made from a compound which 

 will neither yield to acetone, any organic constituent foreign to 

 Hevea rubbers, nor contain more sulphur than is necessary for 

 vulcanizing, so that the pecentage of sulphur in the rubber lay- 

 ers shall not be raised beyond the permissible amount. 



(b) Material.— The (article) shall be properly vulcanized 

 and shall be made from and shall have all the characteristics 

 of a compound containing not less than .... per cent of washed 

 and dried fine Para rubber, not more than .... per cent of 

 sulphur, with the remainder suitable dry inorganic mineral 

 fillers. The mineral fillers may contain barytes, but shall be 

 practically free from sulphur in other forms and from any sub- 

 stance likely to have a deleterious effect on the rubber compound. 

 The sulphur in barytes will not be included in the allowable 

 sulphur content. 



(c) Sample for Chemical Analysis. — A sample taken for 

 chemical analysis shall be sent intact to the laboratory for tests. 



AVERAGE READING. 



12. Since the physical properties of rubber vary noticeably 

 in any given product, it may occasionally happen that tests are 

 made upon a sample which will be of poor quality. The hose, 

 belting, or packing will, as a whole, meet the requirements of 

 the standard, but the particular piece taken may fall some- 

 what below it. To reject or accept a lot of hose because of 

 its failure to meet one test under specifications 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, e. g., 

 cases in which the tensile strength is low on account of a small 

 flaw in the article or the friction is low on account of a small 

 flaw in the friction part. In other words, the intent of the 

 specifications is to insure a high-grade article in every particu- 

 lar, and the intent of the methods of testing is to determine 

 whether the article as a whole is of this high standard. 



Deliveries of hose, packing, etc., which regularly meet certain 

 provisions of the specifications, but quite as regularly fail to 



