There are a number of different types of engagement couplings. The automatic coupling is of the inertia type. The small gear is mounted on the armature shaft through a steep spiral thread. The rotational inertia of the gear prevents it from turning when the motor armature begins to rotate, and it is therefore moved toward the fly-wheel ring gear. The teeth are so chamfered both on the pinion gear and on the ring gear that engagement is quite positive. When the engine starts under its own power, its speed increases and the small gear on the armature shaft rotates faster than the shaft. This action screws the gear out of mesh with the fly-wheel and thereby prevents injury to the starting motor through excessive speed. In another type of coupling the pinion is forced against the ring gear by means of a foot lever and held tightly in place by means of a spring. Further motion of the foot lever compresses this spring until the lever closes the start ing motor switch. The pinion then revolves slightly until en gagement is effected and cranking of the motor follows. An over-running clutch disengages the starting motor when the engine starts. Fig. 13 shows a typical starting motor.
Lighting.—Automobile head-lighting practices differ very ma terially in the different countries. These differences arise chiefly from the fact that no system of head-lighting has yet been de veloped which satisfactorily meets all of the requirements of the various driving conditions. In localities where roads are straight and traffic is light, cars may be operated at relatively high speeds, and drivers will be interested primarily in seeing obstructions, turns, etc., far down the road. In other localities where the roads twist and turn and traffic is heavy, much lower speeds will prevail. Here drivers do not need to see nearly as far ahead but require more light to either side to illuminate roadsides and ditches and to show the way around turns. Some satisfactory means is also needed to pass other cars safely and without dangerous glare. Where cars are driven from one part of the country to another and encounter a wide range of driving conditions, the head-lighting system adopted for general use must take the form of a compromise designed to provide for reasonable safety and comfort under all conditions of driving.
In the United States, where the traffic problem has been difficult for some years, head-lighting has received a large amount of attention. The trend of development has been influenced to a con siderable extent by the recommendations of technical societies and by the laws in the various States. These systems of head lighting in general use require that the beam be adjusted in such a manner as not to permit the high intensity rays to rise above the level of the head-lamps under ordinary conditions of operation. A small amount of light is required above the head-lamp levci to reveal overhanging obstructions and objects, the lighter coloured or bulkier parts of which are above the level of the lamps, but this part of the light should be closely limited so that it will not dazzle approaching drivers to a dangerous extent. In the
United States head-lamps were designed for many years to pro duce a single beam which had the general form of a horizontal band of light when projected on a vertical surface. The beam was aimed so that the top of the beam would not be above the horizon tal when the car was fully loaded, the highest intensity being near the top of the beam where it was needed to reveal objects as far ahead as possible without causing glare.
Even on smooth level roads, head-lamps adjusted in this man ner were unsatisfactory, because of the limited seeing distances they provided. Moreover, all roads are not smooth and level. The angular difference between the eyes of an approaching driver i ooft. ahead and objects which one must see on and above the road is so very small that it does not require a great change in gradient to lift the blinding part of the beam into the other driver's eyes.
On rough roads the action of the springs raises and lowers the beam. Also in cars having both front and rear seats, loading must be considered.
When the rear seat is loaded, the beams may be raised as much as 2ft. at a distance of i ooft. ahead.
For this reason, it has been customary to aim the beams with the car loaded.
The tendency toward lower drivers' seats, softer springs, and lower-pressure tires in the United States and elsewhere has aggravated these troubles.
Because of these inherent deficiencies in fixed-beam head-lamps, multiple-beam head-lamps came into general use a number of years ago.
With the double-filament incandescent lamp used in practically all multiple-beam head-lamps on American cars, the driver may raise or lower the beam at will by switching from one filament to the other.
The latest form of multiple-beam system, adopted simultane ously by almost all American producers on 1940 models, embodies two beams and a reflector, light source and lens made into a single permanently sealed unit, as a result of which the lamps lose only a small fraction of their efficiency throughout their entire life. About 5o% more light is projected than by previous systems and emphasis is placed on use of the lower or "traffic" beam in all places where there is any movement of other vehicles, the upper or "country" beam being reserved for use only on the open high way when there is no approaching traffic. Developed by the auto mobile industry with the co-operation of the American Association of Motor Vehicle Administrators, important modifications in exist ing laws and regulations were obtained to permit taking full ad vantage of dual-beam lighting.