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The Friction of Kailway Brake Shoes Under Various Con- 

 ditions OF Pressure, Speed and Temperature. 



By R. a. Smart. 



Information concerning the friction of unlubricated rubbing surfaces 

 is, unfortunately, limited in quantity, and it is believed that the data pre- 

 sented herewith, although relating particularly to the friction of brake 

 shoes for railway cars, may be properly offered to the Academy as a 

 contribution to the general subject. 



The bralie shoe is an important factor in the chain of mechanism 

 popularly known as the air brake. It is not, strictly speaking, a part of 

 the air brake, but is the immediate agent through which the air brake 

 accomplishes the stopping of the train. It is the block of metal which 

 is pressed against the tread of the car-wheel and which creates, in contact 

 with the wheel, the friction which brings the wheel and hence the train 

 to rest. It will at once be seen that the effectiveness of the whole air 

 brake system on our railways is dependent directly upon the efficiency 

 with which the brake shoe does its work. For instance, we can conceive 

 of the brake shoe being made of some substance like glass, so hard that 

 its friction would be practically nothing, in which case the air brake would 

 be powerless to stop the train. 



In fact, so important is the brake shoe in the eyes of railway officials 

 that the Master Car Builders' Association has caused to be built an 

 elaborate machine to be used exclusively for the testing of brake shoes. 

 The need of such a machine will be understood when it is stated that 

 the tendency of brake shoe manufacturers is, in order to be able to 

 guarantee long life for their shoes, to make them so hard as to seriously 

 impair their frictional qualities. 



The Master Car Builders' Brake shoe testing machine, which has been 

 deposited by them in the engineering laboratory of Purdue University, 

 consists of a heavy revolving weight whose kinetic energy at any speed 

 is equal to that of one-eighth of a loaded 60,000-pound freight car. On 

 the same shaft as this weight and revolving with it is an ordinary car 

 wheel. By a series of weighted levers, the shoe to be tested is pressed 

 against the moving car wheel, thus bringing the wheel and, hence, the re- 

 volving weight to rest. When it is remembered that the freight car has 

 eight wheels, each fitted with a brake shoe, it will be seen that the ma- 



