Rubber for Truck Tires.
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By J. Traxler (Goodyear Tire and Rubber Co.).
Centuries ago, long before the discovery of America by Columbus. rubber or caoutchouc was known to the South American Indian. As far back as 1500, Pineon, the Spanish explorer, told of these Indians of the dense forests bordering the banks of the great Amazon, who tapped the caoutchouc trees, and extracted a milky fluid. He did not explain its use. Columbus noticed natives playing ball with a curious substance grown in the primeval forests and prepared according to Dative ways. Little did the Spanish explorer of the 16th century dream what an important part those immense forests were to play in 20th-century commercialism.
In 1770 Priestly, an Englishman, found that this milky white fluid, when hardened, could be used in effacing pencil marks, and in the early part of the 19th century Hancock discovered that caoutchouc could be used in the manufacture of articles of dress. A few years later Charles Mackintosh, a Scotchman, rendered two fabrics waterproof by uniting them with a solution of rubber in coal naphtha., hence the name " mackintosh" for the waterproof coat.
_Not until about 1839, however, 16 years later than the advent of the mackintosh, did Charles Goodyear, an American, discover the method of vulcanization, and make rubber fit to take its place among the most important commercial products of the world.
The recent increase of rubber-tired vehicles, and the introduction and the development of the automobile for pleasure and the motor truck for commercial use, have increased enormously the market for crude rubber.
In 1898 the price of crude rubber, owing to the demand created, suddenly advanced. Some years before it had been foreseen that possibly the supply of South American gum would run out, and, through the efforts of American and English planters, seeds were shipped to almost every tropical climate, and cultivated rubber trees were grown with success. The best rubber, however, still grows wild in the forests on the Amazon, and, according to F. A. Sieberling, president of the Goodyear Tire and Rubber Co., Akron, 0., who made an extended trip down the Amazon two years ago, the supply will more than equal the demand for some years to come. In the meantime, rubber plantations are springing up in many tropical countries. Some of these are already large producers of high-grade gum, and the future of the supply of raw materials seems to be assured.
Rubber-producing trees, shrubs, etc.. are found mostly in northern South America, Central America, Mexico, Central Africa, and Borneo.
The finest rubber obtained is fine Para, gathered in the Amazon regions of South America. Selecting .a rubber tree, the natives cut "V "-shaped grooves in the bark with a special knife made for the purpose, these grooves being cut in herring-bone fashion diagonally around the tree, with one main groove cut vertically down the centre like the-main vein in a leaf. The latex of the tree, from which the rubber is taken, flows from these veins and down the centre vein into a little cup at the bottom of the main vein. When the cups are filled they are gathered, and brought into the rubber camp, and there the latex is coagulated by means of smoke. This is done by the use of a paddle, which is alternately dipped into a bowl of the latex and then revolved in the smoke from a wood or palm-nut fire. This smoke seems to have a preservative effect on the rubber as well as drying it out and causing it to harden on the paddle,, each successive layer of the latex causing the size. of, the rubber ball or biscuit to increase. When a biscuit of sufficient size has .been thus coagulated, it is removed
from the paddle and is ready for shipment to various countries where rubber products are manufactured.
Crude rubber as it is received at the factory is more or less dirty. Sand, leaves and twigs sometimes constitute as high as 40 or 50 per cent. of the weight.
These foreign substances are removed by washing the gum ; one or two tons being soaked in warm water and then taken to a machine called a "cracker." The cracker consists of two large rollers, covered with pyramidal projections, which revolve very closely together at different surface speeds. It is a powerful machine, and as the gum is fed between the rolls the projections tear it to pieces. During this process a continual flow of water from perforated pipes plays upon the rubber, and as it goes through these rolls repeatedly the water gradually washes away most of the foreign substances.
After this the gum is shovelled into boxes and taken to what is known as a "washer." This also consists of rolls, but the projections are relatively small andthe rolls are nearer together. The work of removing the foreign matter is completed here, and in addition the gum is "sheeted out." Sheeting is accomplished by using warm water as the washing finishes. The heat is just enough to make the gum sticky, so that from 15 to 25 pounds of rubber a sheet 18 in. wide,
several feet long, and in. thick results. These sheets are then dried for from three to four weeks.
Mixing the crude rubber with the compounding ingredients is the next step. Each of the constituents of a 100 lb. " batch " is carefully weighed and taken to the milling room. Here the rubber is warmed and softened before the compound is added, by passing it between the warm rolls of a machine similar in design to the washer, but much larger and with highlypolished rolls. The front roll revolves more slowly than the back one, and the gum soon forms an envelope around this. The compound is sprinkled on the gum, a little at a time, and is gradually worked in by the constant kneading effect of the rails on the softened gum. Finally, the rolls are opened a little and the stock cut away in slabs about half an inch thick.
Once again it is allowed to rest—this time 48 hours —when it is ready to be shaped for the rolls. In the case of rubber heels, pieces of suitable size are cut from the milled stock, but in making solid tires, where the length is many times the other dimensions, it is necessary to make us of another ma-chine. This is done by running through what is known as a tubing machine. This resembles a meat chopper on a large scale, with a die opening in place of the small holes. This turns out a continuous length of rubber of proper cross section as long as the milled stock is fed in.
Then follows the weighing of each tire, cementing to the base band, laying in the mould, solting the mould, and placing the whole in a boiler heater. The length of time required to " cure " or "vulcanize " a tire of medium size is about three hours.
Taken from the heater it is cooled, removed from the mould, trimmed of the rinds at the line of contact of the halves of the mould, and the tire is ready for shipment.
Tire manufacturers have had to be chemists, inventors, analysts, and keen observers of conditions as well as mere craftsmen. They have had to establish their own precedents, to solve problems of construction and service that men have never faced before.. It is a long step from the Amazon jungle, where natives slash trees and gather the sap, with chattering monkeys and brilliant parrots as an audience, to the sturdy, rumbling truck that is revolutionizing traffic in the cities of the world.