May 1. 1915.] 



THE INDIA RUBBER WORLD 



431 



pushed back only half way, SO per cent, is shown; if it is pushed 

 back all (he way, as when the rubber suffers no injury at all, 

 100 per cent, elasticity is shown. The depth the pin is required 

 to penetrate and the sharpness of its point have been determined 

 by experiments. The most elastic rubber, according to tin- stretch 

 test, just barely escapes cutting. 



Soft rubber will lose about 14 per cent, of its hardness when 

 the temperature is raised from 32 to 212 degrees. Such heating 

 does not materially decrease the total value of the rubber in so far 

 as its stress and deformation resisting properties go. This is 

 explained by the fact that there is an increase of elasticity at 

 about the same rate as there is a decrease in hardness. The 

 only danger that would appear to exist is where the rubber is 

 exposed to friction with other rubber. The heat increases the 

 cementing affinity and causes rapid wear. Solid tires having 

 the right proportion of hardness and elasticity at normal at- 



10 20 W 'VC 50 60 TO 80 90 IM 



Chart Showing Tests of Elasticity and Hardness. 



mospheric temperatures may become too soft at 212 degrees, 

 and show compression, internal friction and further development 

 of heat. On ordinary road temperatures, however, in hot weather, 

 the efficiency of the rubber should not visibly be impaired. 



When rubber is stretched its elasticity may or may not be 

 constant when the duration of the stretch varies. In other words, 

 if rubber is stretched only for a moment it may show a higher 

 elasticity or hardness than if it were stretched for some length 

 of time. Consequently, in the less elastic rubber goods the per- 

 manent set will be developed under the continued pressure of the 

 indicating pin. This is shown by the gradual recession of the 

 indicating hand. 



To meet this peculiarity in testing, a special holding stand has 

 been provided. The same stand also serves to avoid possible side 

 friction on the sensitive pin, due to not holding the instrument 

 straight when used free handed. 



The following table of rubber values may be of interest in this 

 connection : 



Average and Extreme Values of Pliable Rubber Articles. 



Percentage of 



Lowest. . 

 Average 

 Highest 



Low- 

 Average 

 Highest 



Pneumatic Tikes. 



Low 35 65 3,000 . . 400 



Average 42 80 3,360 .. 800 



Highest 70 100 4,800 .. 1,000 



Ki i i;i b Heels (some only hard compositions). 



Low 55 5 425 . . 



74 28 1,683 .. 



Highi t i , mi, i , 85 60 3,600 



Ri i Bands. 



Low 35-45 80-95 75-85 700-1,500 



Study of th,- figures in this table will disclose several interest- 

 ing facts, among them a limit of what has been referred to as 

 tli. total value of rubber. The two gages here described have 

 eai h a scale of 100 units, and if 100 is multiplied by 100 a pro 

 of 10XXX) will be obtained. In rubber this is not possible. The 

 highest value found in the course of our experiments was in a 

 pneumatic tire, which showed a hardness ol 48 and 100 per cent. 

 elasticity, which gives a product of 4,800. The highest value noted 

 in solid tires of a high grade was only 4.000. thus showing that 

 too much vulcanizing or compounding deprives rubber of elas- 

 ticity and toughness. The limit appears to have been reached 

 in the processes hitherto used for solid tires. J u m to what ex- 

 tent rubber can be vulcanized or compounded without materially 

 detracting from its characteristic elasticity, which is here repre- 

 sented as 100 per cent., remains to be shown. The many ex- 

 periments upon which these figures are based would appear to 

 indicate a total value of 5,000 as about the limit, or just half of 

 the range of measurement provided for by the two instruments. 



RUBBER PLATES FOR IRONCLADS. 



■PHIS publication has referred a number of times to the 

 suggestion made by various people interested in naval 

 construction that the resistance to attack of an ironclad would 

 be greatly increased by a suitable covering of rubber. The 

 claims made by Mr. Bowler, an English inventor, for the rub- 

 ber plates which he had devised for application over the iron 

 plates of warships were mentioned on page 290 of the Feb- 

 ruary issue. He has been trying to persuade the British Ad- 

 mirality to experiment, at least, with this method of rubber 

 protection. In a recent number of a British daily, he cites 

 further instances coming under his own observation of the 

 superior resistance of rubber to the effect of explosions over 

 any other material. 



"In 1906," he writes, "I witnessed the effect of an explosion 

 of about 200 tons of dynamite at Bramfontein, Johannesburg. 

 It occurred on a siding used for shunting dynamite-laden 

 trucks. Special precaution had been made at the head end 

 in a rubber buffer, so that if the trucks did run loose the 

 rubber would lessen the shock and perhaps prevent an ex- 

 plosion. The trucks dashed with such force into the In: 

 that a terrific explosion took place, killing more than .300 

 people. Trucks and siding were blown into the air, and on 

 examining the spot half and hour after the explosion I 

 noticed that nothing was left but the rubber buffers, which 

 remained intact, barring some rents and cuts made by flying 

 projectiles and scorching by the heat. This was proof of 

 the extraordinary resisting powers of rubber. 



"In an experiment made to show the recoiling strength of 

 rubber a piece of rubber 2 inches thick and 1 foot square was 

 laid under a steam hammer and a 6-inch round shot was 

 placed on it. The hammer fell with tremendous force and 

 broke the shot to pieces, while the rubber remained elastic 

 and unimpaired." 



The estimated number of automobiles owned in the United 

 States in 1914 was 1,754,570. Assuming that these cars aver- 

 age not less than five tires each during the year, 8,772,850 tires 

 were necessary for their equipment. 



