In an endeavor to find an engineering justification for the use of the airbrake on automotive vehicles, an investigation was first made as to what actually causes a car to stop when the brakes are applied; and it was ascertained that nothing that can take place within the car itself can directly influence the motion of the automobile as a unit, that its motion can be changed only by some force external to the car itself.
Four such forces are normally present, namely, wind resistance, road resistence, gravity, and the adhesion of the road to the wheels. The first two are negligible. Grades have a measurable effect on the stopping distance, but the force that actually stops the car is the last named: the force that is applied from a point external to and in a direction opposite to that of the motion of the automobile. This frictional force is called into existence by the resistance that the brake-bands offer to the continued rotation of the wheels, the maximum possible road adhesion occurring when it is equal to the coefficient of adhesion multiplied by the weight carried by the wheels. If the brake-band pressure is greater than the maximum possible adhesion, the wheels will lock and a transfer of energy will take place between the sliding tires and the surface of the road. The shortest possible stopping distance is obtained when all braked wheels are held just short of the point of locking throughout the duration of the stop.
Friction is defined as the resistance of two bodies in contact that opposes a change in their relative positions. When the friction is greater than a force that tends to produce motion, the friction is termed “static” friction, or friction of rest; when the impelling force is greater than the resistance of friction and one body slides over the other, the friction is termed “kinetic” friction, or friction of motion. Rolling friction between a rolling wheel and the road is static friction, for the point of contact does not move. For the sake of simplicity, this is termed “adhesion,” to distinguish it from the kinetic friction that exists between the brake-band and the brake-drum mounted on the wheel.
Every surface, no matter how highly polished it may be, contains humps and depressions that tend to interlock or mesh with those of other surfaces, like the teeth of two gears. Static friction is always greater than kinetic friction because these humps and depressions have a greater opportunity of becoming interlocked. When in relative motion, two surfaces have not time in which to become interlocked, and each surface hits only the high spots of the other. Consequently, the friction becomes less as the velocity increases.
After these preliminary observations, the author discusses the variations of the coefficient of friction of different kinds of brake-lining under varying conditions, develops formulas to show the forces necessary to lock the wheels of a car under given conditions and determines the amount of push of the pedal or pull of the lever that would be necessary to produce this effect with various arcs of contact between the brake-shoe and the drum.
With consideration for operating and maintenance requirements, the best practice in heavy-vehicle design is said to have determined that the maximum total multiplication between the pedal input and the cam output is about 36, and between the hand-lever and the cam, about 50. Inasmuch as an average man can exert a push of approximately 200 lb. on the pedal and a pull of 150 lb. on the hand-lever, the statement is made that, under the best possible operating conditions, either hand or foot-actuated rear-wheel brakes are inadequate to produce the shortest possible stop in any vehicle having a gross weight of more than 7500 lb. Under the worst combinations of operating conditions, it is probable that the weight that could be so controlled would not exceed 3500 lb. If a braking pressure sufficient to lock the rear wheels is not available, the addition of front-wheel brakes operated from the same actuating source will not reduce the stopping distance, but, in fact, will increase it.
The limitations of propeller-shaft service or foot-brakes, or internal and external-band brakes, and of mechanically actuated and self-energizing servo-brakes are outlined, and the conclusion is reached that, in order adequately to brake vehicles weighing more than 5000 lb., a power brake is essential.
With the object of applying more force with greater flexibility to existing brake-riggings, the Westinghouse automotive airbrake has been developed, the details of which are described with copious illustrations.