Motor-car principles; the gasoline automobile

by Roger B. Whitman

The revolution of the square shaft is transmitted to the driving wheels, the speed of the car therefore corresponding to the speed at which the square shaft is driven by the gear combinations between it and the countershaft. To obtain the reverse, which enables the car to be backed without reversing the engine, a third gear is introduced between the low-speed gears of the square shaft and countershaft. When two gears are in mesh, they revolve in opposite directions, but when one of them is in addition meshed with a third gear, the first and third will revolve in the same direction, and opposite to the direction in which the middle gear revolves. When the car is going forward, the square shaft and countershaft revolve in opposite directions, but when the reverse gear is introduced between them, the square shaft is revolved in the same direction as the countershaft, reversing the rotation of the driving wheels.

The ends of the teeth of the gears are chisel-shaped, instead of being flat, as in ordinary gears, so that they will go into mesh easily.

The greatest economy in the operation of a gasoline engine results from its running at as nearly constant a speed as possible, and the gear is therefore changed when the resistance of the road to the movement of the car decreases the speed of the engine, or permits it to increase.

The selective type of sliding change-speed mechanism as shown on Fig. 34 is in use on many of the high-grade cars, and its control differs from that of the progressive type described in that the lever has only a short movement forward and back, and in addition may slide sideways. To the lower end of the lever is attached a shaft that rocks in its bearings as the lever is moved forward or back, and slides lengthways when the lever is moved toward or away from the car.

Fig. 34.—Selective Type. A, Sleeve driven by engine; B, fixed gear on sleeve; C, fixed gear on countershaft; D, low-speed gears; E, second-speed gears; F, third-speed gears; G, clutch for direct drive; H, rod and arm for third-speed and direct drive; J, rod and arm for low and second speeds; K, rod and arm for reverse, L, rocking-shaft finger in groove; M, guide plate and control lever; N, bevel gears on square shaft and jack shaft; O, idler gear for reverse.

The arrangement of the countershaft and square shaft is the same as in the progressive type, but there are two sets of sliding gears instead of one, and these are moved by means of arms that extend from rods sliding in bearings at the side of the gear case. When these rods are slid endways, the gears attached to their arms slide on the square shaft to correspond, and go in or out of mesh with the countershaft gears. Across the ends of these rods are grooves, which when the gears are in the neutral position are in line with the rocking shaft attached to the control lever. From the inward end of the rocking shaft a finger (L) projects downward into the groove; when the grooves are in line, the rocking shaft may be slid endways, the finger passing from one groove to the next without affecting the rods. When the shaft is rocked, however, the finger in engaging one of the grooves slides the rod endways, shifting the gears controlled by its arm. Moving the control lever into such a position that it may enter the middle slots of the guide plate (M) slides the rocking shaft so that its finger projects into the groove of the central sliding rod (J), and if the control lever is then pushed forward so that it enters the front half of the slot, the sliding rod will be moved by the finger in the opposite direction, and the low-speed gears (D) will be brought into mesh. Bringing the lever back to the central position will separate the gears, and moving it to the back half of the slot will slide the same gears in the opposite direction, meshing the second-speed combination (E). Moving the control lever outward so that it is in line with the outside slot brings the finger into the groove of the sliding rod (H) that moves the third and high speeds (F and G), the latter being direct drive, and the reverse is obtained through the movement of the sliding rod (K) that is engaged by the finger when the control lever is in the inside slot. The movement of this rod brings a third gear (O) into mesh with the low-speed gears on the square shaft and countershaft, and the rotation of the countershaft is reversed.

While this type is in general use, the gears are often so arranged that the direct drive combination is reached when the control lever is in the third-speed position, the gears meshed by the fourth-speed position driving the square shaft at a still higher speed. This high speed can only be used for running under the best road conditions.

The advantages of the selective type over the progressive are the shorter movements of the control lever and the ability to pass from one speed to any other without the necessity of first meshing and unmeshing those between, or, in other words, there is a neutral position between every combination of gears, and from neutral any desired combination may be obtained directly without reference to the others.

In starting up a car fitted with either of these types of change-speed mechanism, it is necessary to withdraw the clutch before sliding the gears, and this is also necessary in changing from one combination to another. The square or corresponding shaft of a change-speed mechanism is always connected with the driving wheels, and is at rest when the car is standing. With the engine running, the countershaft will be revolving, and it will obviously be difficult to slide the stationary gear into mesh with a gear that is revolving. When the clutch is withdrawn, the countershaft moves only through momentum, and will be brought to a stop by the contact of the teeth of the sliding gear as that is moved against it, the two then easily going into mesh. The clutch is then thrown in slowly, and will bring the speed of the countershaft to the speed of the crank shaft.