May 1, 1916.] 



THE INDIA RUBBER WORLD 



405 



Hard Rubber in Automobile Construction. 



THE following interesting and instructive paper by McCon- 

 nell Shank, manager of the hard rubber department of 

 The B. 1". Goodrich Co., Akron, Ohio, was read at the 

 meeting of the Cleveland Section, Society of Automobile Engi- 

 neers, held at Cleveland, Ohio, December 17, 1915: 



Although rubber of the hardness of an automobile bumper 

 or of a solid tire is often referred to as "hard rubber," that term 

 as used in this paper refers to rubber compounds hard enough 

 to be machined and polished. In Europe hard rubber is known 

 as "ebonite." "Vulcanite" was originally a trade name for a 

 certain compound, but is now synonymous with "ebonite" or 

 "hard rubber." "Vulcanized rubber" is a term often applied to 

 such compounds, but speaking accurately, soft as well as hard 

 rubber is vulcanized. 



Vulcanized rubber compounds may be hard by reason of 

 either a more complete chemical combination of the rubber with 

 sulphur or the presence of ingredients other than rubber and 

 sulphur. While the degree of hardness is controlled in both 

 ways, the completeness of the combination of the rubber with 

 sulphur is usually the greater factor. The finest grade of soft 

 rubber, such as is found in a good rubber band, may contain 

 95 per cent rubber and 5 per cent sulphur and may have been 

 vulcanized two hours, at a constant temperature. The finest 

 grade of hard rubber such as is found in a good comb or foun- 

 tain pen may contain 75 per cent rubber and 25 per cent sulphur 

 and may have been vulcanized ten hours at the same tem- 

 perature. 



PROPERTIES OF PURE H.\RD RUBBER. 



For the many purpose^ for which hard rubber may be used 

 in automobile construction there are many qualities. The prop- 

 erties of hard rubber vary with the quality, of course. An idea 

 of the properties of the various qualities can best be gained by 

 taking as a standard and noting the variations of what might 

 be called "pure hard rubber," which contains practically no in- 

 gredients other than rubber and sulphur. 



Pure hard rubber ranges in specific gravity from 1.12 to 1.25. 

 On account of the large percentage of sulphur necessary for 

 proper vulcanization, it is impossible to make hard rubber with 

 a specific gravity much less than 1.12. Hard rubber of fair 

 commercial qualitv will usually have a specific gravity of 1.25 

 to 1.40. 



The finest grades of hard rubber have a tensile strength as 

 high as 7.000 pounds per square inch of cross section. Battery 

 manufacturers who make tensile tests on their jars usually re- 

 quire a tensile strength around 3.500 pounds. 



The United States Navy hard rubber sheet specifications which 

 require a fair commercial quality, call for a dielectric strength 

 of 10,000 volts per 1-32 inch of thickness. Some battery manu- 

 facturers test their jars electrically with a pressure of about 

 25,000 volts alternating current for J-s inch (if thickness. The 

 finest hard rubber will withstand twice that voltage. 



Hard rubber compounds, except some that contain organic 

 substances other than rubber, are moisture proof. Pure hard 

 rubber is unaffected by most chemicals. Concentrated nitric and 

 sulphuric acids, carbon bisulphide, aniline and benzol are the 

 only chemicals used largely in a commercial way that aflfect it 

 at ordinary temperatures. As compared with soft rubber, hard 

 rubber deteriorates slowly. 



Hard rubber compounds before they are vulcanized are usually 



of about the same consistency as soft rubber compounds, that 



is. they are of about the same consistency as stiff chewing gum. 



EFFECTS OF TEMPERATURE VARI.\TIONS. 



Pure hard rubber has a greater coefficient of expansion than 

 most other substances, either organic or inorganic. In cooling 

 from the ordinary temperature of vulcanization to 70 degrees F. 

 it shrinks about 2 per cent. In molding, this shrinkage is not 

 altogether uniform, so that regardless of how accurately the 

 cavities of a mold are finished, the goods have to be turned, 

 ground, buffed or polished after they are molded according to 

 the accuracy of dimensions and character of polish required. 

 The coefficient of expansion of pure hard rubber after it has 

 once cooled is .00004278 per degree F., or about six and a half 

 times that of steel. 



At a temperature of 150 degrees F. pure hard rubber softens 

 perceptibly. At 212 degrees F. it becomes so soft that it can 

 be easily bent, and at 240 degrees F. it becomes leathery, so that 

 it can be readily cut with a knife. Upon cooling, it remains 

 in the shape in which it was bent and shrinks slightly from its 



original size. When warmed again it tends to return to its 

 original shape. Each time it is heated and cooled it shrinks 

 less than the previous time until after a few times the shrinkage 

 is negligible. While this is the effect of heat upon most hard 

 rubber compounds, it is possible to make them withstand the 

 action of heat to a much greater extent by scientific com- 

 pounding. 



On account of its resilience, hard rubber machines less readily 

 than would be expected, but with good grades fine work can 

 be done. In general, the better the grade, the more easily it 

 can be worked. A good quality of hard rubber readily takes 

 a fine jet-black polish. This color and polish are fairly perma- 

 nent, but not absolutely so, for even the finest quality takes on 

 a slight greenish tint, especially after it has been exposed for 

 a long time to the bright sunlight. When the finish becomes 

 dull or discolored, it can be restored by buffing and polishing. 



On account of the large percentage of sulphur in hard rubber 

 compounds, and the comparatively long time and high tempera- 

 ture under which they are vulcanized, it is impossible to make 

 hard rubber in the large variety of colors in which soft rubber 

 can be made, for the high temperature and sulphur tend to 

 discolor pigments. Hard rubber can be made in a commercial 

 way in black, red and intermediate shades. It cannot be made 

 of a clear white color at all, and most of the other colors are 

 not usually attempted. 



The quality of hard rubber may be determined roughly by 

 superficial examination in many ways — by its color and texture 

 as shown by a polished or freshly-broken surface, by its strength, 

 by the ease with which it machines, and by the toughness, color 

 and grain of a shaving. The best way is to whittle it slowly 

 with a knife, noting the color and toughness of the shaving. 

 With practice, one can judge hard rubber fairly accurately in 

 this way. 



The parts of an automobile that may be made wholly or in 

 part from hard rubber may be divided into three classes- 

 handles, parts for the ignition system and battery parts. 



RliBBER HANDLES FOR AUTOMOBILE SERVICE. 



In the first classification may be included handles for starting 

 cranks and controller levers, caps for radiators and gasolene 

 tanks, and steering wheels. For these parts hard rubber is de- 

 sirable on account of its strength, attractiveness and permanence 

 of finish. Hard rubber for handles is usually molded except 

 where the quantity required is So small as not to warrant the 

 expense of a mold. 



Hard rubber parts for automobiles are molded in two ways — ■ 

 from soft-metal or from hard-metal molds. Soft-metal molds 

 are made from a matrix. The molds are stackea and clamped 

 in racks and the vulcanization is done in a heater of the type 

 formerly used for tires. Hard-metal molds for hard rubber are 

 usually made from iron or steel the same as are those for 

 bumpers and other soft rubber goods. The vulcanizing is also 

 done in hydraulic presses. 



For hard as compared with soft rubber, a long time is re- 

 quired for vulcanization so that for large production a great 

 many mold cavities are required. With a comparatively inex- 

 pensive matrix it is an easy matter to make as many molds 

 as may be required, but cavities in an iron or steel mold are 

 expensive. Soft-metal molds have the advantage, therefore, of 

 low cost of mold equipment, but they warp and stretch so that 

 it is impossible to do nearly as accurate work with them as 

 with hard metal mulds. Furthermore, they will not withstand 

 pressure and the result is that goods made of the same com- 

 pound from hard metal moWs under hydraulic pressure 

 show greater strength. With hard-metal molds there are less 

 defective goods and the work can be better arranged, so that 

 iron or steel molds are the best, except where the quantity re- 

 quired is small. 



Hard rubber steering rims have been used for the past ten 

 years. The first ones were made entirely from hard rubber and 

 screwed on the spiders. The screws did not hold well enough 

 in the hard rubber and this led to the use of a hard rubber cov- 

 ered steel tube, also fastened to the spiders with screws. Objec- 

 tion was then raised to the projections beneath the rim where 

 the arms of the spider joined it. This resulted in the use of the 

 integral construction in which the hard rubber is molded on 

 the rim of the spider. This type of rim is a little heavier than 

 wood and more expensive, but it looks well when new and re- 

 mains practically unchanged after years of use, without atten- 

 tion of any kind. 



