346 APPLIED MECHANICS 
always used on railway trains, tram-cars, and vehicles on common roads. 
The block is made of a softer material than the rita or tyre of the wheel 
against which it is pressed, so that the wear is confined mainly to the 
. block, which is easily renewed. When the block is made of wood, a hard 
and strong wood, such as elm, oak, or beech, should be used. For heavy 
and rapid running vehicles, cast-iron brake blocks are the most common. 
If P is the normal force pressing the brake block on the wheel whose 
radius is R, then the resisting torque set up by the brake on the wheel 
is #PR when the wheel is rotating. 
When a brake block is applied to a rolling wheel an additional load 
is thrown on the bearing or journal of the wheel or axle, but if two 
blocks are applied at opposite ends of a diameter of the wheel, there is no 
such additional load. The braking action is also doubled by the use of 
two blocks, and the two blocks may be operated by practically the same 
force which will operate one. 
Fig. 536 shows the Milnes-Daimler differential block brake as used 
on motor omnibuses.* A is the brake drum fixed to the final driving 
shaft, BB are the brake blocks, C is a bracket fixed to the frame of the 
chassis, D is the operating lever, E is the pull-off spring, and F is the 
brake adjusting screw. 
Fig. 537. 
Fig. 537 shows the James and Browne block brake, as used on motor 
cars.t A is the sprocket wheel and brake drum, BB are the brake blocks 
which act on the inside of the rim of the brake drum, C is an arm jointed to 
the frame of the chassis, D is the operating lever, and E is the pull-off spring. 
295. Action of Railway Brakes.—The block brakes used on the 
wheels of railway vehicles are found to be most effective when the forces 
pressing the brake blocks on the wheels just prevent the wheels from — 
skidding on the rails. The explanation of this is that the coefficient of 
sliding friction between the wheels and the rails is less than that between 
the brake blocks and the wheels, and also that the coefficient of friction 
between the wheels and the rails just before skidding begins is greater 
than the coefficient of friction of skidding. 
For example, at a speed of 50 miles per hour the coefficient of sliding 
* Proceedings of the Institution of Mechanical Engineers, 1907, p 432. 
+ Ibid., 1902, p. 730. 
