Brake Tests

273 input becomes less than this the coefficient of friction rises and con- tinues to rise more and more rapidly as the heat generated decreases until the end of the stop. 527. The maximum resistance limit encountered in the brake shoe action would seem to be accounted for by the more rapid abrasion which takes place at the higher temperatures. In other words, when the heating of the working metal exceeds a certain value the effect of the molten condition thus produced is chiefly to increase the rate at which the metal is worn away, but for this very reason, to maintain a sub- stantially uniform condition of the surfaces successively presented to the action of abrasion. Effect of Severe BRAKING Conditions. 528. If the influence of the rate at which energy is being absorbed has been correctly described in the foregoing, it follows that the higher the rate the greater the heating of the working surface and the more rapid the abrasion and that this effect would be most marked under severe braking conditions, such as obtained when heavy cars equipped with one brake shoe per wheel are stopped by the application of high braking powers from high initial speeds. In such cases the rate of energy absorption and consequent temperature rise at the working surfaces becomes very high and the coefficient of friction is corre- spondingly low. The conditions are further aggravated by the inability of the shoe body metal to conduct heat away from the working surface with anything like a proportionate rate at which such conduction can take place when the total heat developed is much less. Abrasion in such cases occurs at a very rapid rate and when the time of such action is prolonged, as in high speed stops, instances have been observed when early in the stop the entire shoe surface becomes red for some distance from the surface and metal is discharged from the shoe at such a rate and in such a plastic condition that it is found deposited on the car body above the shoe in the form of recast metal. 529. That this action is a function of the amount of shoe metal available at any instant for the absorption of heat follows from the fact that under clasp brake conditions, and especially when flanged brake shoes were used, there was only moderate sparking, while on the other hand, under single shoe conditions at high braking powers and high speeds the sparking was vigorous and in many instances, especially when the shoe had only a partial bearing on the wheel, the molten metal was discharged as mentioned above. The latter action in many cases resulted in wearing through a shoe newly applied to the back and some- times into the brake head in a single stop with high braking power from high speeds.