292 



NATURE 



[July 24, 1879 



GENERAL RESULTS OF EXPERIMENTS ON 

 FRICTION AT HIGH VELOCITIES MADE IN 

 ORDER TO ASCERTAIN THE EFFECT OF 

 BRAKES ON RAILWAY TRAINS 



THE experiments were made on the Brighton Raihvay, 

 with the assistance of Mr. George Westinghouse, 

 with a special four-wheeled van constructed for the pur- 

 pose ; it Mas attached to an engine, and was run at various 

 speeds, during which time various forces were measured 

 by self-recording dynamometers. The principle of these 

 dynamometers is that the force to be measured acts on a 

 piston fitting in a cylinder full of water, and the pressure 

 of the water is nieasured by a Richards indicator con- 

 nected by a pipe to the cylinder ; thus, as the drum of the 

 indicator revolves, diagrams are obtained, giving the force 

 acting on the piston. The advantages of this method are 

 obvious, because the indicator can be placed at any 

 convenient point and the inertia of the water tend 

 to make the pencil keep a position corresponding to 

 the mean force. A detailed description of the con- 

 struction of the dynamometers has been given in the 

 Proceedings of the Institution of Mechanical Engineers, 

 but would occupy too much space in this rhume of the 

 experiments. 



Brake blocks were applied to both pairs of wheels, 

 but the dynamometers were attached to one pair of 

 wheels only. The greater number of experiments were 

 made with this latter pair of wheels, the second pair being 

 reserved for special experiments when the van was slipped 

 from the engine. 



The levers for bringing the brakes into operation were 

 so arranged that the brake blocks were applied on both 

 sides of each wheel, and the pressure was equally distri- 

 buted between the four brake blocks, acting on the pair 

 of wheels. 



The dynamometers above mentioned registered (i) The 

 pressure applied to force the brake blocks against the 

 wheels. (2) The friction which took place between the 

 brake blocks and the wheels, due to that pressure, mea- 

 sured by the effort made by the revolving wheel to cause 

 the blocks to revolve. (3) The weight on the springs 

 over the braked wheels at each moment during the ex- 

 periment, which, added to the weight of the wheels, axles, 

 and springs, gives the weight for calculating the adhesion. 

 (4) The tractive force exerted by the draw-bar. (5) Two 

 self-recording speed-indicators were used, designed by 

 Mr. Westinghouse, one instrument being attached to each 

 pair of wheels. This instrument has been repeatedly 

 tested, and was used at the brake trials on the North 

 British Railway and on the German State Railways. It 

 consists of a small dynamometer made on the same 

 principle as that just described ; it measures the centri- 

 fugal force of two weights, which are made to revolve by 

 a strap from a pulley on a shaft driven by friction-gear 

 from the pair of wheels to which the brake was ap- 

 plied ; a Richards indicator is used, as with the other 

 dynamometers. As the centrifugal force varies as the 

 square of the velocity, the speed is got by taking the 

 square root of the ordinate at any point of the indicator 

 diagram. 



The diagrams from one speed indicator showed the 

 speed of the pair of wheels to which the brake was applied, 

 and therefore the velocity of the train at the moment of 

 applying the brake and subsequently, provided there was 

 no slipping. Any variation in the speed-diagram was 

 due to the wheels slipping, and shows to what extent and 

 in what way the brake acted to stop the wheels. The 

 diagrams from the other speed indicator showed the 

 speed of the unbraked wheels. A Bourdon gauge, with 

 the face divided in such a way that the hand showed the 

 speed in miles per hour, was attached, for convenience, to 

 the Westinghouse speed-indicator. As a check upon 

 these, two of Mr. Stroudley's speed-indicators were fixed 



side by side in the van ; one attached to the axle be- 

 longing to the braked wheels, the other to the axle which 

 was running free. These indicators do not record the 

 speed. 



The indicators were all placed on a table in the centre 

 of the van, and their drums were made to revolve by the 

 cords being wound up on pulleys on a shaft, which was 

 turned at a uniform rate by a water clock. This clock 

 merely consisted of a plunger sliding in a cylinder through 

 a water-tight packing, and loaded with a heavy weight ; 

 it was wound up by connecting it with the accumulator 

 which supplied the dynanometers, and at the beginning of 

 each experiment a small cock was opened which allowed 

 the water to run out and the weight to fall, thereby 

 turning the indicators round at an ascertained uniform 

 speed. Thus, while the ordinates of the diagrams taken 

 from these several indicators shov/ the various forces, the 

 abscissae show the time occupied in the experiments. 



In most of the experiments the tyres were of steel, and 

 the brake-blocks of cast iron. Some experiments were 

 made with wrought iron blocks, but the results were not 

 uniform or satisfactory. 



Numerous diagrams were taken with this apparatus, 

 but it will suffice here to give the general results 

 arrived at. 



It is convenient in looking at the question of railway- 

 brakes to consider first, what is the operation of a brake ? 



A train through the adhesion of the wheels of the 

 locomotive acting on the rails, slowly accumulates energy, 

 and for each ton of weight in the train, the accumulated 

 energy is equal to 120 foot-tons at 60 miles per hour, 53 

 foot-tons at 40 miles per hour, and 30 foot-tons at 20 

 miles per hour. Thus, for a train of fifteen vehicles, 

 weighing 200 tons, the energy at 60 miles per hour is 

 equal to 24,000 tons falling a distance of one foot. 



After a train has attained the desired speed, the reasons 

 for stopping it may be of two kinds: (i) at prearranged 

 places for convenience ; and (2) for the prevention of acci- 

 dents or for mitigating the consequences if accidents are 

 unavoidable. 



To stop a train for the first reason requires but a 

 limited amount of force, which may be applied in any 

 crude manner. 



For the prevention of accidents, however, there is 

 required : — 



a. The instantaneous application of the greatest pos- 

 sible amount of retarding force. 



b. The continuous action of this force until the momen- 

 tum of the train is destroyed. 



The retarding force used in practice is that due to the 

 friction resulting from the forcible application of pieces 

 of metals or wood (brake blocks) to the tyres of the 

 wheels ; this friction impedes the rotation of the wheels, 

 and tends, through the adhesion of the wheels to the 

 rails, to destroy the energy stored in the train. The re- 

 tarding force is therefore limited to the resistance obtain- ,^m 

 able between the wheels and rails. j^^ 



It was at first customary to attach to a train, for pur- ^^ 

 poses of retardation, a certain number of vehicles with 

 extra weight, to which the brakes were apphed ; but since 

 the question of retardation has become better understood, 

 brakes have been applied to every vehicle, the means of 

 applying these brakes being placed in the hands of both 

 the engine-driver and the guard. The reason for this is 

 that the maximum amount of retarding force can be 

 obtained only by applying brake blocks to every wheel in 

 the train, each block being pressed with suflicient force 

 to produce a resistance to the rotation of the wheel just 

 equal to the greatest possible friction between the wheel 

 and the rail. This greatest possible friction occurs when 

 the adhesion of the wheel to the rail is just about to be 

 overcome by the superior effort of the brake blocks, which 

 effort, if further increased, immediately begins to stop 

 the rotating movement of the wheel, and thus causes it 



