The Duplex governor shown in Fig. 68 was designed on the principle of a dual actuating influence. This dual influence consists of a motor influence, as to its speed, which is imparted to the governor, and a vehicle influence which is also imparted to the governor. The motor speed is conveyed from some revolving part of the motor to one of the speed terminals of the governor, and the vehicle speed from the propeller shaft or jack shaft. The conveying means consists of a steel cable revolving in a hard fiber or metallic casing. The governor is so constructed that it may be mounted between the intake manifold and the carburetor. Instead of the ordinary butterfly valve a grid valve is used, through which the entire gas supply must pass. This grid valve consists of a fixed part set into the upper part of the valve chamber, which is provided with a series of elongated slots, with flaring walls from its upper surface downward. When the openings of both fixed and movable parts coincide, the valve is open and when the bars of one part cover the openings of the other part, the valve is closed. The movable part is held open by spring tension and its possible motion in cither direction is limited by adjusting screws.
Within the governor there are two automatically acting oneway clutches, the floating members of which consist of gears in mesh with a third gear mounted on the centrifugal governor spindle. These clutches are so designed as to impart to the centrifugal member that of the two speeds which is the higher. With the motor running idle, the motor speed will actuate the governor, and the motor is always under governor control. When the vehicle is propelled by the motor on the higher gears, the speed imparted to the governor by the vehicle will be the higher speed and will govern the motor, whereas on the low gears the motor speed will be the higher and will govern the motor.
The influence of the centrifugal member, as a result of the speeds imparted to it, is to develop a pressure sufficient to overcome the spring pressure tending to hold the valve open, and to close it. As soon as the pressure is removed, the movable valve part is released and returned to its full open position. Provision is made for adjustment, which is enclosed and provided with a lock so that it cannot be tampered with.
These various types of governors have their advantages and disadvantages, while there may also be certain disadvantages to their omission. When commercial vehicles are not equipped with governors, the owner is forced to rely entirely upon the discretion of the operator, while the manufacturer must rely on careful management of them by the owners to prevent the evils of motor racing and excessive speed. This also applies on hills where it is asserted that governors are of no use in preventing excessive vehicle speeds.
There are some who claim that there is no adequate method yet devised which is capable of governing a truck or its engine, without handicapping the driver somewhat in the operation of the vehicle. However, the writer opines that it is worth while fool-proofing the commercial car even in a crude way, particularly in speed. With solid tires and the tendency to overload commercial vehicles, the speed becomes a large factor in obtaining long life.
It may be said that certain governing devices do limit the power of the motor on the lower car speeds. However, this may not be as serious as some think, as these vehicles are operated on the higher speed the greater portion of the time.
Some also claim that the intelligence and general ability of the commercial vehicle operator exceeds that of the touring car operator, but as the conditions of both are vastly different, it is a difficult matter to determine this fact. Truck drivers as a rule are paid a certain wage for a stipulated number of hours per week, and if they are delayed long enough in their daily trips to make their work exceed this stipulated time, they will try and make up this discrepancy by speeding up the vehicle whether loaded or empty.
However, it should be remembered that there is a certain class of conservative operators who can operate a vehicle safely without limiting its speed, but as operators of this class are few, some means must be resorted to for limiting the destruction of trucks through excessive speeding.
Many manufacturers equip their vehicles with governors admitting their shortcomings, arguing that even though the econ-omv and efficiency of the truck may be slightly affected, this loss is more than made up for by the saving in depreciation resulting from the unskilful operation of careless drivers.
Fig. 68. The Duplex Governor.