Brake Tests

227 at a satisfactory relation between cause and effect) the non-uniformity of brake rigging and brake shoe conditions (which can not be as satis- factorily controlled on a train of a number of cars as on a single car) and the variable effect of the locomotive. With this in view, Figs. 138 to 141 have been plotted to show the speed-time, and speed-distance curves plotted from the track chronograph records of the various stops chosen. From the speed-time curve, the time-deceleration and re- sistance curves have been plotted, the ordinate at any point on the deceleration curve being proportional to the slope of the tangent to the speed-time curve at the corresponding point. The fact that the same curve, referred to different scales, shows both deceleration of the train and the instantaneous resistances to the motion of the train at any point during the stop, follows from the fact that the relation be- tween resisting forces and resultant deceleration is constant, so long as the mass of the train is unchanged. 431. It will be noted that the curves showing deceleration and resist- ance are not horizontal lines beyond the point where the brake becomes fully applied. The resistance is not constant but changes more or less as the speed of the train is reduced, especially toward the end of the stop, where a considerable and continual increase in retardation is ex- perienced. As pointed out above, this occurs during the time the brake cylinder pressure is constant, and consequently the resultant normal brake shoe pressure is substantially constant. The conclusion, there- fore, follows that the other factor, viz., the coefficient of brake shoe friction is changing and that the character of the changes which it undergoes is accurately portrayed by the deceleration curves derived as explained. This characteristic change in the coefficient of brake shoe friction has been a matter of common observation in every instance where train tests or laboratory tests of brake shoes have been studied from this point of view. A further analysis of the influences which bring about the characteristic variations in brake shoe friction will be found in Chapter VII, but it may be stated here that with constant brake shoe pressure, the observed changes in retardation are brought about by changes in the character of the bearing surface between the brake shoe and the wheel. This in turn depends upon the temperature of the metal doing the work, which is dependent upon the rate at which energy is being absorbed; and this varies with the decreasing speed of the train. 432. Having the deceleration and resistance curve thus plotted on a time basis, the value of the resistance at different distances from the point of brake application can be determined by the aid of the speed- time and speed-distance curves. In this way, the distance-deceleration and resistance curves were plotted. The work done during any portion of the stop is proportional to the area under the distance-resistance curves.