3^8 



NATURE 



Vy^ly zi, 1879 



I 



other experiments it was found that the best results -n-ere 

 obtained in cases where the pressure applied at first ivas 

 from about if to twice the weight on the wheels, and 

 where the reduction of the pressure was effected with 

 sufficient rapidity towards the end of the stop to prevent 

 the friction being sufficient to skid the wheels. 



The necessity for the instantaneous application of the 

 maximum brake-block pressure throughout the train is 

 evident from the fact that, at a speed which is frequently 

 obtained, namely, 60 miles per hour, a train passes over 

 88 feet each second ; therefore the loss of two or three 

 ■seconds in applying the brakes means often the difference 

 between safety and danger, and the rapidity of a stop 

 largely depends upon the rapidity with which all the 

 brake-blocks can be brought to act against the wheels of 

 a train. 



This points to the advantage of being able to move the 

 brake-blocks with great rapidity from their position of 

 inaction to that of contact with the wheels ; because it is 

 essential to provide that the brake-blocks, when out of 

 use, shall be removed to a distance from the wheels suffi- 

 cient to prevent the possibility of their dragging against 

 the wheels, and thus retard the progress of the train. The 

 question of the rapidity with which brakes can be applied 

 in practice is thus one of much importance. 



Some experiments were made in October, 1878, upon 

 the North Eastern Railway, on a train fitted with the 

 vacuum brake, and one fitted with the Westinghouse 

 brake to ascertain the time which was required after 

 moving the brake-handle to set the brakes with various 

 degrees offeree in different parts of the train. The fol- 

 lowing table shows the result arrived at : — 



A long interval of time between brakes coming on at 

 the front and rear of a train may become a source of 

 danger ; and improvements have been introduced in both 

 the vacuum and Westinghouse apparatus since that date 

 to reduce the interval as shown by the experiments. 



In the Westinghouse brake a simphfied triple-valve has 

 been adopted, the friction has been reduced by the use of 

 an enlarged pipe and by the removal of bends in the con- 

 nections between the carriages ; by these alterations the 

 interval of time required to put on the brakes, as shown 

 in the above table, has since been reduced by nearly one- 

 haif, and an experiment recently made on the application 

 of the brake in rear of a train of twenty-four vehicles on 

 the Western Railway of France showed that the pressure 

 commenced to be brought on in one second, and was fully 

 on in two and a half seconds from the time of first moving 

 the brake lever. 



The importance of simultaneous action of the brakes in 

 every part of a train arises from the fact that the train is 

 not a rigid mas?, but is made up of separate vehicles con- 

 nected by means of spring draw^-bars and buffers. The 

 length of the train can thus be modified to a certain e.\tent 

 by the degree of compression of these springs. In a recent 

 experiment on the North-Eastern Railway the train con- 



sisted of twenty-four carriages, and the whole extent to 

 which the buffers could be compressed amounted to 35 

 feet. A train travelling at 60 miles an hour moves at 88 

 feet in a second. If the brakes act on the front part of 

 the train before they affect the hind part the speed of the 

 front carriages maybe diminished by to to 18 feet in a 

 second, whilst the hind part moves on with undiminished 

 speed ; thus the hind part may press against the front 

 part with a force of from 12 to 20 foot tons for every ton 

 weight of the hind vehicles. The buffer springs would be 

 compressed by this force and remain so till the brakes 

 acted equally on all the wheels, when a reaction of the 

 buffer springs would take place ; this reaction creates the 

 violent jerks often felt with continuous brakes, and occa- 

 sionally results in fractures of couplings and draw-bars. 

 In a perfect brake the application would be instantaneous, 

 and simultaneous on all the wheels of a train. 



It is beyond the scope of this paper to enter fully into 

 the merits of different kinds of brakes, but it may be con- 

 venient to sum up what seem to be the requirements of a 

 perfect brake. 



1. It should be fitted to act upon each wheel of the 

 engine, tender, and every other vehicle in a train of any 

 length. The brake-blocks, when out of action, must be 

 kept a certain distance away from the wheels, in order to 

 prevent any liability to drag against the wheels ; and this 

 distance, after being once adjusted, gradually increases 

 by the wear of the blocks, and often exceeds three-quarters 

 of an inch ; while the springing of the brake-gear under 

 great strain also adds to the extent of movement required 

 in the brake force before the blocks are fully applied. 

 Hence the brake gear should be so adjusted as to be 

 capable of moving the brake blocks instantaneously 

 through a space of one inch. 



2. However brought into action, it should be capable 

 of exerting upon the blocks of each pair of wheels, within 

 two seconds, a force of twice, or at the very least one- 

 and-three-quarter times, the load on those wheels. 



3. The brake-block pressure acting on each wheel 

 should be regulated so that the friction between the 

 brake-block and the wheel may always be limited so as 

 not to exceed the adhesion between the wheel and the 

 rail ; by which means it will produce the maximum effect 

 at each moment of its application. 



4. The brake-block pressure should be capable of being 

 applied by engine-driver or by guards. 



5. The engine, tender, and vehicles should each carry 

 its own store of brake-power, which should be indepen- 

 dent of the brake-power on any other vehicle. 



6. The brake-block pressure should be automatically 

 applied to every vehicle by the separation -of the train 

 into two or more parts, and it should also be applied by 

 the act of the wheels of any carriage leaving the rails. 



7. The brake-block pressure should be automatically 

 applied by such failure of the connections or appliances- 

 as would render it afterwards incapable of application 

 until the failure had been remedied. 



8. The brake-block pressure should be capable of ap- 

 plication with any degree of force up to the maximum, 

 and it should be capable of continued action on inclines,, 

 or, of repeated applications at short intervals at junctions 

 and stations. 



In addition to these requirements, the questions of 

 cost, durability, convenience in operation, and other 

 essential points, will of course come under consideration. 



The experiments which have been here described were 

 made on trains travelling under conditions which were 

 necessarily continually varjung, both in respect of the- 

 condition of the rails and other matters ; and they there- 

 fore contained many elements beyond the reach of calcu- 

 lation. It is hoped that some opportunity may arise, ere 

 long, for resuming experiments on friction at high velo- 

 cities under conditions whence these elements of dis.- 

 turbance may be eliminated. Meanwhile it is evident 



