Brake Tests

247 MEAN Coefficient of FRICTION. 476. The application of the brake shoe to the revolving car wheel results in the generation of a force between the surfaces of the shoe and the wheel which acts so as to retard the motion of the wheel. The re- sistance is termed “Friction” and can be defined as the force of resist- ance which opposes the relative motion of two surfaces in contact. Re- ferring to Fig. 149 which shows a number of characteristic brake shoe test machine dynamometer cards, it will be noted that these cards give a continuous record of the force of resistance or friction produced be- tween the brake shoe and the wheel on a distance base throughout the stop. 477. The friction developed between the surface of the wheel and the shoe will always be proportional to the normal pressure which forces the shoe against the wheel and to the condition of the wheel and shoe surfaces in contact. The ratio between the force of resistance or the friction and the normal pressure is termed the coefficient of friction of the two surfaces in contact. This ratio depends on the nature and conditions of the contact surfaces, or in other words, on the ability of these surfaces to generate a force of resistance, opposed to the motion of the wheel. It is a percentage factor and can be regarded as expressing the combined frictional qualities of the two rubbing surfaces. 478. The cards (Fig. 149) show that the friction between the brake shoe and the wheel was changing although the normal pressure on the brake shoe remained constant after the shoe had been fully applied to the wheel 2.5 seconds from the start. The fact that the friction developed was variable, while the normal pressure on the brake shoe was constant, means that thè ratio between the resisting force and the normal pres- sure on the shoe (coefficient of friction), was varying during the stop; which in turn indicates that the nature or condition of the two surfaces in contact were changing. 479. The mean value of the resisting force can be obtained by dividing the area under the force-distance curve by the length of the card from the point of equivalent instantaneous application to the stop and when this mean resisting force is divided by the normal pressure on the shoe the result is the Mean Coefficient of Friction. This factor ex- presses the average frictional quality of the brake shoe surface in con- tact with the wheel tread surface during the entire stop and is convenient for use in comparing tests made on different types of shoes at various normal shoe pressures. 480. Previous to the starting of the tests the dynamometer was calibrated and a large number of cards were integrated. It was found that the efficiency of the rigging through which the weights were applied to the shoe of the test machine was, practically, 100 per cent. It was decided therefore that much time could be saved by computing the