TRANSACTIONS OF SECTION G. 



517 



sequently, the retarding effect of the brakes would be greatly increased, were means 

 devised for placing sand under every wheel to which a brake is applied, during the 

 progress of a stop. 



The effect in stopping a train is greatest when the friction between the brake- 

 blocks and the wheels amounts to a quantity just short of the resistance caused by 

 the adhesion, because as soon as the brake-block friction exceeds the adhesion, the 

 wheel becomes fixed and begins to slide. In order, however, to secure the best 

 results in stopping, it is obviously necessary that the brake-block pressure should be 

 regulated to give a friction about equal to the adhesion of the wheels at every 

 stage during the progress of a stop. 



There is no reason why, in the progress of mechanical science, these conditions 

 should not be regulated by a self-acting arrangement. 



As the adhesion varies it is necessary to consider what amount of adhesion for 

 purposes of retardation can be safely calculated upon. 



The following table shows the distances required to stop a train on a level line 

 from a speed of fifty miles per hour, with a retarding force of from 5 to 30 per 

 cent, of the total weight of the train : — 



If the brakes act upon each wheel, then a retardation of 10 per cent, of the 

 load carried by each wheel — counting the rotating momentum as part of the weight 

 — will stop a train in 277f yards. 



If the brakes act upon only half of the weight of a train, a retardation of 20 

 per cent, would have to be exerted upon the braked half to produce the same 

 result. As pointed out, 20 per cent, adhesion is rather above the average obtain- 

 able, while 25 per cent, is the highest result obtained under the most favourable 

 circumstances at any considerable speed, or except when sand was applied to 

 wheels moving slowly. 



The above table should be carefully noted, for it will be seen that, even if 

 brakes act upon all wheels, 25 per cent, retardation will only give twenty-eight 

 yards better result than 20 per cent., or if half of the train only be braked, it will 

 give fifty-nine yards advantage. 



A consideration of this feature of the brake problem points out (1) that the 

 advantage to be gained by trying to obtain above 20 per cent, retardation on each 

 wheel is greatly overbalanced by the risk of ' skidding ; ' and (2) that it is far 

 easier and safer to make a stop in 250 yards from fifty miles per hour with the 

 whole train braked, than with brakes upon only half of the train. 



The following diagram, No. 10, shows the advantage of applying brakes to every 



No. 10. 



Retardation 



jjCarriage. 



'Its. eod&i 



300 



6000 

 4000- 



3000- 

 200O- 



sooYards ebo 



wheel of a train. The line AB shows the stop which a train would make from 

 fifty miles an hour with the retardation of *20 shown by the horizontal lineJDDjf 



