In substance, the electrically operated transmission has all the hand levers, rods, and other levers replaced by a series of push buttons. When it is desired to change speeds even before the actual change is necessary, in fact, the driver presses the button marked for the speed he thinks he will require. Then, when the actual need becomes apparent, he throws out the clutch and immediately drops it back again, all this forming but a single forward and back movement of the foot. During the slight interval while the clutch is out, the electrical connections shift the gears automatically, so that when the clutch is let back, the gears are meshed ready to drive.
To explain this action briefly, the gears are moved by means of solenoid magnets, which are nothing more than coils of wire, through which an electric current from a convenient battery is allowed to pass. Through the center of each one of these coils passes an iron bar. When a current passes through the coil, it is converted into an electromagnet and draws the iron bar inward. As the other end of the bar is connected to the gear to be shifted, this movement of the bar shifts the gear. Consequently, when the button is pressed so that current flows through one of the coils, that action shifts the gear for which the button is marked.
By referring to Fig. 74, this action will be made more clear. The diagram shows but one pair of gears to be meshed, and the battery, push button S, coil D, iron bar P, and clutch connection M are all shown as simply as possible. When button S is pressed, current through the coil D will draw the bar P and mesh the gears, as soon as the clutch has been thrown out, thereby closing the circuit at M. The application of this to an actual transmission is shown more in detail in Fig. 75. This shows the clutch pedal and its connection to the six solenoids necessary to produce four forward speeds, one reverse, and a neutral point.
Fig. 74. Sketch Showing How a Solenoid Moves a Gear When Current Flows.
On the steering wheel, Fig. 76, the control group of six buttons will be noted on the small round plate at the center, with the addition of the horn button in the center. In Fig. 77 is another arrangement.
In the 1916 forms of electric control systems, the buttons are grouped in one case; on the top of a small box about four or five inches square, which is placed on the steering post below the wheel in another on the dash, and in a third on a rod connecting post and dash.
Fig. 77. Another Arrangement of Buttons for Gear Shifting.
The pneumatic system of gear-shifting is along lines somewhat similar to the electric system, air under pressure being used to move the gears, instead of a hand lever and rod combination. For this purpose it is necessary to add to the car an air compressor, a tank to carry the compressed air, and what is called the "shift" - really a complicated valve and a series of plungers. The valve and plungers respond to a finger lever on the steering wheel, the same as the electric system responds to the buttons, air being admitted behind the plungers, which move the gears as soon as the clutch is depressed. It is seen, therefore, that this system, like the electric shifter, permits the anticipation of the car's needs.
Fig. 78. Farm of Gear for Gasoline Railway Car.
All transmissions previously presented have had but one reverse. For gasoline railway cars, the inability to turn requires as many reverse speeds as forward, which means special gearing, as shown by Fig. 78. This is the type used by the Sheffield Car Company, Three Rivers, Michigan, in their gasoline-driven railway cars. The driving bevel, faintly seen at A, instead of driving but one bevel, as is usual, drives two, C and D. Each one of these is free on the shaft, being bushed to reduced wear, but between the two is a sliding member B, with jaw clutches formed on both faces. This slides on a squared shaft, and the jaw clutches match jaw clutches formed in the inside face of the two driven bevels. When slid to the right, then, B engages with jaw clutches in gear D, and thus drives it. If, then, the gear box gives three speeds, all three may drive through this combination, giving the forward speeds. If it is desired to drive in the other direction, the clutch member B is slid out of mesh with D and into mesh with C, thus reversing not only the direction of motion but also the direction of all speeds.
Aside from the railway form, the location of the transmission varies its form. The front location for the engine is now universal, but this cannot be said about the gear box. This unit, on the contrary, is placed in every conceivable position, sometimes forming an integral part of the rear end of the engine and even being removed to form a part of the rear axle and differential housing.