The difference in the residual magnetism of the fields or in the magnetic qualities of the fields of the two motors is primarily the cause of the generation of the current. The motors at first act in opposition, but one of them generates the higher voltage and forces a current through the other. This current overcomes the residual magnetism of the second motor, thereby changing its polarity and both motors then act in series to send the current through the low resistance path afforded by the windings. Any current passing increases the strength of the fields and consequently the voltages, so that abnormal currents are generated and the braking action is consequently severe.
This generating action does not take place before the reverse lever is thrown because the connections of the armatures and fields are such that any current generated by reason of the residual magnetism of the fields, flows in such a direction through these that this magnetism is destroyed. The current then ceases to flow. This explains why current is not generated in No. 2 motor with a K type of controller during the change-over period when it is short-circuited, or in equipments when the trolley flies off and the controller is turned on.
Fig. 57. Pneumatic Sander.
Brake Shoes. The subject of brake shoes is of very little importance on the smaller cars traveling at slow speeds and controlled alone by hand brakes. On the larger high speed interurban cars, the brake shoe question becomes an important item because of the rapidity with which they are worn away. On such cars shoes sometimes last but about one week. This means eight shoes per week per car or an expense of about $4.00 per car per week.
Brake shoes are usually of soft gray cast iron with inserts of steel, although some companies use very hard iron. They are usually fastened by means of a key to a brake shoe head permanently attached to the brake rigging. The brake levers are so adjusted that the shoes clear the wheels about ³⁄₁₆-inch when the brakes are released. This distance increases as the shoes wear, so that the brakes must be adjusted frequently to take up the slack and prevent waste of air.
Track Sanders. A sprinkling of sand on the rail increases wonderfully the adhesion of the rail and wheel. There is usually on cars some provision made for scattering sand on the rails immediately in front of the leading wheels. From sand boxes placed under the seats in the smaller cars, or on the truck of the larger ones, flexible hose or pipes drop within an inch or two of the rail in front of the leading wheels. A valve under the control of the motorman regulates the flow of sand to the rail. Sometimes air pressure is used to blow the sand out of the sand box into the hose. In this case air pressure is obtained from the air brake system, and an air valve leading to the sand box is placed in the motorman’s cab. A section through a pneumatic sander of this kind is shown in [Fig. 57].
Fig. 58. Curves of Braking Tests.
Coefficient of Friction. It has been found by experiment that the coefficient of friction between the car wheel and rail is about 25 per cent of the weight on the wheel when the rails are dry; that is, a car wheel having a weight of 2,000 pounds upon it would not be able to exert either an accelerating or a retarding force exceeding 25 per cent of this, or 500 pounds. This is when the wheel is rolling. There is apparently a kind of locking or inter-meshing of the rough surfaces of wheel and rail when the wheel is rolling, because it is found that when a wheel begins to skid or slide, the coefficient of friction falls off about two-thirds. The maximum braking or retarding force that can be obtained, therefore, in a dry rail, amounts to 25 per cent of the weight of the car. If the rail is slippery this is much reduced; or if the wheels are allowed to slide it is also much reduced. If more retarding force than can be obtained through the medium of a wheel rolling on the rail is desired, it must be obtained either by the track brakes or by magnetism.