Brake Tests

262 steel wheel under relatively heavy normal pressure. Consequently the resistance or retarding force developed has frequently been spoken of as the friction of abrasion.. 509. This force bears some relation to the ultimate strength o e metal which undergoes abrasion, because the action of abrasion con- sists of the pulling apart, or crushing of the particles of the contact surfaces. Practically none of this abrasive action takes place on the wheel, due to the harder and tougher nature of the wheel surface an the fact that the surface of the wheel is not continuously in contact with the brake shoe, while the brake shoe is in continuous contact with the wheel. Inasmuch as the tearing or abrasion of the metal partic es can take place only in the thin layer of the shoe metal which is in con tact with the wheel and as the actual contact area is, as has already been shown, but a portion of the total face area of the shoe, it s evident that the generation of the retarding forces and consequent absorption of the energy of the moving train is dependent upon but a very small quantity of brake shoe metal. 510. The result of this condition is that the metal in a state of abrasion undergoes a very rapid rise in temperature, and the tempera ture of the working area of the brake shoe is changing continuous y because when one set of particles is torn off, energy is dissipated in the form of heat so that the next particles to be torn off are at a hig er temperature than the first. This process continues until the surface of the metal in abrasion reaches such a temperature that the force re- quired to tear the particles away is greatly reduced, l he abrasive action is then extremely rapid until the bearing of the shoe shifts to some other and cooler spot on its surface, not in the same degree o contac with the wheel. The action will then be repeated, the bearing area thus shifting continuously from one portion of the surface to another during the stop. 511. Unquestionably the constantly changing temperature of the contact surface has an important relation to the force of retardation developed by the tearing down of the particles, lhe resistance ue to abrasion is dependent on the ultimate strength of the cast iron. It is well established that above a critical temperature (approaching 900 degrees Fahrenheit) the ultimate strength of cast iron decreases rapidly and that at or above red heat temperatures (1400 degrees to 1800 degrees Fahrenheit) its ultimate strength is greatly reduced. 512. The force of retardation due to abrasion and the corre- sponding mean coefficient of friction will therefore be a function of the constantly changing, but always high, temperature of the working metal. Obviously the lower the mean temperature can be maintained, the higher will be the mean coefficient of friction as long as this average temperature is above the critical temperature at which the ultimate strength of cast iron is a maximum.