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THE INDIA RUBBER WORLD 



[-May 1. 1915. 



New Devices for Rubber Testing. 



By A. F. Shore. 



VULCANIZED rubber is divided into two general classes 

 that bear the trade names, hard rubber, or vulcanite, and 

 soft rubber. Under the last-named class is grouped an 



infinite variety of g Is that vary greatly in hardness and in 



elasticity. Many instruments have heen designed to measure 

 their physical properties 



Difficult) is, of course, experienced in this because of wide 

 variation in structure and in chemical composition as well as 

 frequent exaggeration of one property at the expense of the 

 other. The factors thus concerned are what might he termed 

 fibrous as opposed to non-fibrous formations, extreme brittleness, 

 elasticity or plasticity. The range is so wide that it is difficult 

 to make a single instrument which in its range of sensitiveness 

 would cover it entirely. 



Crude rubber shows unexampled elasticity, but little hardness; 

 hard rubber, any degree of hardness up to glass brittleness or 

 any degree of elasticity down to that common to other brittle 

 hard bodies. 



It thus follows that an instrument suitable for measuring the 

 hardness of soft rubber, will be useless for brittle hard rubber; 

 also, one that will determine elasticity of such soft robber will 

 be useless on the more brittle hard rubber. To successfully meas- 

 ure the properties named, the Elastometer for elasticity, and the 

 Durometer lor hardness, have been devised by the Shore Instru- 

 ment & Manufacturing Co. 



The Durometer when applied to soft rubber indicates its re- 

 sistance to the penetrating force of a blunt pin. This pin pro- 

 jects from the instrument three thirty-seconds of an inch and is 

 held by a carefully calibrated spring. On the harder grades it 

 is pushed in most of its length against the tension of the spring. 

 The extent of the compression, and, conversely, the deformation 

 of the rubber, are indicated on the dial, expressing units of hard- 

 ness. The size and position of these units, since the value 50 is 

 the average hardness for soft rubber, have been carefully chosen 

 and obviously will remain constant in the future. 



With the Durometer is provided a standardized spring balance 

 upon which the hardness it represents is marked and with which 

 it is checked up from time to time, as is customary with all 

 accurate testing instruments. The hardness of crude rubber 

 varies all the way from 25-30, on the scale adopted, to 75-80 after 

 vulcanizing. When a hardness of 70 is exceeded, there is ob- 

 isly a rapid drop of elasticity, but an increase in strength or 

 resistance to stretching, so that it is safe to say 70 is ordinarily 

 the limit of hardness where the service value of the rubber de- 

 pend^ equally upon the accompanying elasticity. 



Where rubber cushions have to withstand the greatest possible 

 load, as in solid tires, the highest hardness is called for. with the 

 necessary quota of elasticity. Where the rubber itself is not 

 depended upon for sustaining loads, as in pneumatic tires, the 

 rubber used is given a more moderate hardness, with due com- 

 pensation by greater elasticity. The hardness measure thus may 

 give information as to the suitability of vulcanized rubber for 

 various purposes. This means that in the cheapest solid tires, 

 ample, the desired hardness will be shown, but there would 

 be a lack of elasticity, and such a tire would fail to give the 

 desired cushion effect and soon wear out. 



Rubber may be said to have a total value. This would be 

 the figures representing hardness and the elasticity multiplied 

 together. As an example, a solid tire showing a hardness of 70 

 units and an elasticity of 55 per cent., would have a total value 

 of 70 times 55, or 3,850, which would be considered a high 

 figure. Another solid tire, having a hardness of 70, but an 



elasticity of only 30 per cent., would show a total value of 70 

 times 30, or _',100. 



In general practice, however, it would not be proper to refer 

 indiscriminately to rubber as having a total value, for a given 

 product may be obtained in widely different ways, as for example, 

 one rubber is 50 hard and has an elasticity of 70, while another 

 is 70 hard and has 50 per cent, elasticity. The product in each 

 casi is the same, but the rubber is adapted for widely different 

 purposes. The former will be better for pneumatic tires, and 



The Shore Elastometer. 



The Shore Durometer. 



the latter for solid tires. When hardness and elasticity meas- 

 urements are made to ascertain the total value of the rubber, 

 it is necessary to state the specific degree of hardness, and also 

 the percentage of elasticity. 



Elasticity i^ more difficult to measure than hardness, because 

 its manifestations are not the same under different methods of 

 testing. 



The simplest though not the most convenient way of testing 

 the elasticity is by stretching a band of uniform thickness. 

 Another way is to drop a ball on a sample of rubber from a 

 certain height and note the rebound. The rebound values will 

 not agree perfectly with the stretch values, for conditions enter 

 tin rebound test, in regard to time and distance penetrated, which 

 would not enter in a stretch test. 



In testing the elasticity of rubber with an instrument that is 

 to be applied to the surface without damage, the stretch test is 

 most closely imitated by one involving a tearing or cutting stress. 

 Rubber having 100 per cent, elasticity will resist the penetration 

 of a knife or a sharp point for a given depth without cutting in 

 the slightest degree. Should, however, the elasticity be some- 

 what imperfect, the cutting will take place to the extent that the 

 elasticity is deficient, until at last the elasticity is so low at a 

 given hardness that cutting will occur almost the entire distance 

 penetrated. Cutting, after all, in the absence of a saw action, 

 consists of imposing a stress on the fibers in contact with the 

 edge, which will more readily be overcome as the rubber fails 

 to elongate or stretch away from it. The Elastometer has been 

 devised by applying this principle. The action is as follows: A 

 medium sharp pin, three thirty-seconds of an inch long, is locked 

 and caused to penetrate its entire length into the rubber. After 

 a few seconds the pin is unlocked and allowed to be pushed 

 back by the rubber, according to its power to recover its original 

 form or its elasticity. The pin actuates a very delicately balanced 

 indicating needle, reading in percentages of elasticity. If it is 



