101 



chine reproduces the conditions surrounding one-eighth of a freight car, so 

 far as the forces involved in stopping the cai- are concerned. The machine 

 provides a complicated recording mechanism by which the performance of 

 the shoe while under test may be determined. 



The present tests were undertaken to determine the effect upon the 

 coefficient of friction of variations in tlu-ee factors, viz.: The normal 

 pressure between the shoe and the wheel, the speed of the wheel at the 

 time the shoe is first applied, and the temperature of the rubbing surfaces. 

 The effect of the first two variables was determined by maliing stops 

 from various initial speeds and under different braking pressures, and 

 calculating for each test the mean coefficient of friction for the stop. 

 The limits of the variable elements under which the tests were made 

 were as follows: Initial speed, 10 to G5 miles per hour; normal pressure, 

 from about 2,800 pounds to about 10,700 pounds, these limits being the 

 ones found in ordinary road service. In making a stop, the method of 

 procedure is as follows: The weight and car wheel are brouglit to the 

 desired speed of rotation by an engine. The engine is then disconnected 

 from the revolving Aveight by a clutch and the brake shoe is brought in 

 contact with the car wheel with the desired braking pressure. As the car 

 wheel and weight are being brought to rest under the action of the brake 

 shoe, the recording mechanism attached to the latter draws an autographic 

 record of certain elements in the performance of the shoe, from which 

 the mean coefficient of friction during the stop may be calculated. 



The effect of the third variable mentioned above, namely, the tempera- 

 ture of the rubbing surfaces, was more difficult to determine. The tem- 

 perature of the shoe only was observed, and this was found by imbedding 

 in each end of the shoe the thermo-electric joint of a Le Chatelier pyrom- 

 eter. This joint, in connection with a D'Arsonval galvonometer, gave 

 continuous readings of the temperature of the face of the shoe near each 

 end. The tests were made by making continuous runs at constant speed 

 and noting simultaneously the temperature of the shoe and the coefficient 

 of friction. The limits of temperature under Avhich the tests were made 

 were from about 60° F. to about 1500" F. 



The results from the tests may be summed up as follows: 



1. The coefficient of friction of brake shoes decreases with increase of 

 pressure. The values are approximately as follows: 



