Brake Tests

264 The resulting higher average temperature of the working metal due to the action of warping also resulted in a greater rate of wear. All the data of shoe wear show that shoes of the same type and hard- ness had a high rate of wear per unit of energy absorbed when a low coef- ficient of friction was developed and likewise a lower rate of wear when a higher coefficient of friction was developed. In other words when the coefficient of friction is high, the average temperature of the working metal must be comparatively low and therefore the metal torn from t e surface of the shoe works more effectively because it is at a lower temperature and consequently less metal is required to do a given amount of work. (C) AVAILABLE AREA. The shifting of the bearing area will tend to be more rapid if the size provides more available area for shoe bearing. The average working temperature will also be reduced because a shoe o large area, such as a flange shoe provides better facilities for radiating and conducting heat away from the working surfaces. This is borne out by the better performance of the flange shoes in comparison wit the plain shoes, both solid and slotted. VARIATIONS IN SUCCESSIVE TESTS MADE WITH THE SAME SHOES AND UNDER SAME BRAKING CONDITIONS. 515. In all brake shoe machine tests considerable variation is found in the results obtained on successive tests and under supposedly similar conditions (Fig. 159). From what has gone before it is now pos- sible to account for these variations on the basis of the changes in the bearing surface conditions as the mean force of retardation depends directly on the mean average temperature of the working metal and as the chief factor affecting the temperature of the working metal is tlie manner in which the bearing area shifts from point to point on the surface of the shoe, it follows that when this shifting is accomplished with the greatest promptness (and therefore the least amount of excessive heating at one spot) the average temperature of the working metal will be lower and the mean coefficient of friction higher. The rate at which the bear- ing area will shift depends, as has been stated, on the existing tendency for the shoe to warp. Warping is caused primarily by the uneven heat- ing of the shoe but so many other factors, dependent on the quality and structure of the metal, are involved that the effect of warping will vary a great deal, even though the initial speed and braking powei are the same throughout a series of tests. 516. Furthermore, the heating effect may be distributed over the face of the shoe in a different manner during one test than during another test, and so on. Consequently when successive tests are made