516 



EEPOET 1879. 



resistance of the blocks exceeds the adhesion between the wheel and the rail if the 

 speed is kept up ; or, if the speed is slackening, when it exceeds the adhesion 

 between the wheel and the rail, plus the effort required to retard the rotation of 

 the wheel equally with the retardation of the train ; and the excess of resistance 

 then acts as an unbalanced force, tending to destroy the momentum of the wheel. 



Usually there are in a train a certain number of vehicles braked and a certain 

 number unbraked. If the brakes acted on all the wheels, then the rotating momen- 

 tum of the wheels does not add to the distance in stopping a train, because that 

 momentum can be acted upon by the brakes directly, without in any way affecting 

 the adhesion of the wheels to the rails. It simply requires an additional amount of 

 brake-block pressure. 



With the unbraked portion of a train the rotating momentum of the wheels is 

 an addition to the momentum due to the weight of the train (including therein the 

 actual weight of the wheels), which cannot be utilised for retardation; and it 

 is therefore important that there should be brakes on every wheel of a train. 



As it is the adhesion which governs the retardation which the brake-blocks can 

 exert upon wheels, it is manifest that the pressure brought to act on the brake- 

 blocks should never give an amount of friction which exceeds the adhesion. At a 

 high speed, however, the pressure required to produce a degree of friction equal to 

 the adhesion is much greater than what is required at a low speed. 



The following table gives approximately the proportion which the pressure to 

 be applied to the brake-blocks should bear to the weight upon the braked wheels, 

 with coefficients of adhesion between wheel and rail, varying from - 30 to "15 of 

 the weight on the wheels : 



Ratio of Brake-Block Pressure to Weight on Wheels. 



It will be seen that, when the adhesion equals '30 of the weight, a pressure 

 equal to 1-2 of the weight would skid the wheel at 7£ miles per hour, whilst a 

 pressure equal to 4-14 times the weight would be required to do so at 60 miles per 

 hour. 



On the other hand, if the adhesion is only -15, the pressure required to skid 

 the wheel would be only -60 of the weight at 7£ miles per hour, and 2 - 08 of the 

 weight at 60 miles per hour. 



Thus the efficiency of a brake depends upon the pressure being proportioned to 

 the speed and to the adhesion. If the adhesion were always uniform, the rule 

 would be very simple ; but this is not the case. 



The adhesion of the wheels to the rails varied according to the materials, that 

 is, whether the train was travelling upon iron or steel rails ; and according to the 

 state of the rail, whether dry, wet, or sanded. 



On dry rails it was found that the coefficient of adhesion of the wheels was 

 generally over "20. In some cases it rose to '25, or even higher. On wet or 

 greasy rails, without sand, it fell as low as "15 in one experiment, but averaged 

 about '18. With the use of sand on wet rails it was above -20 at all times ; and 

 when the sand was applied at the moment of starting, so that the wind of the 

 rotating wheels did not blow it away, it rose up to '35, and even above •40. Con- 



