One of the tendencies of recent years has been the gradual change toward more speeds, as shown by the increasing use of four-speed gear boxes. Other indications of this have been the two-speed axle, which gave double the number of gear-box speeds, with the ordinary three-speed and reverse transmission; and the electric transmission, which affords seven forward and two reverse speeds.
An excellent example of the four-speed selective gear is the Austin (English), shown in Fig. 72. As will be noted, the two shafts are set side by side, running entirely on ball bearings. A notable feature is the introduction of an extra bearing in the center of each shaft, which, with the web of the case, forms the bearing supports and practically divides the case into two parts. In addition, an extra bearing-support will be noted on the main driving gear at the left. High speed is a direct drive by means of dogs on high gear and on the first shifter. For second, this is shifted to the right. For third, the second shifter is moved to the left; and for low, to the right. Reverse is gained by setting the second shifter in the neutral position as shown, and shifting another pair of gears beneath the third speed and the small end of the shifter. An interesting feature of this unit is the spherical ball joint at the drive nend and the transmission brake, which is outside of this, and which is supported from the sub-frame at that point.
The form of final drive alters the construction of the transmission very materially. Formerly, when all final drives were of the double-chain form, it was customary to include the differential, bevel gears, and driving shafts in the gear box. Now that the chain has gone out, this construction is found only when the gear box is a unit with the rear axle.
Just for comparison with the domestic product, the transmission of the Horch, a famous German car, is shown in Fig. 73. High speed is obtained with direct drive by meshing gears A and D, through a gear not shown, cut in the face of A. Second is obtained by the combination A, B, C, and E. Third is given by A, B, F, and G. Fourth is produced by A, B, H, and I, while a still further movement to the right of this second train gives the reverse. Attention is specifically called to the device for throwing the lay shaft out of gear when the high speed is engaged, this resulting in no gears running on the direct drive, that is to say no gears meshing face to face. The result is an absolute lack of gearbox noises on high speed. High speed is effected by the final movement of the shifting rod to the left, which actuates the high gear.
Fig. 73. Horch (German) Four-Speed Selectlve Transmission.
The partial worm engages a sector so pivoted in the middle as to slide the gear B to the right, and enough to throw it out of mesh with A, when the shaft is moved so as to introduce gear, or rather, jaw clutch D into its mate within A.
In all the transmissions shown and described thus far, the direct drive has been the highest speed. By referring back to Fig. 55, which showed the Winton four-speed gear box, as well as the clutch, a point of difference will be seen. This has the direct drive on third speed, fourth being a geared-up speed for use only in emergencies, when the very highest rate of travel is required, and when a little noise more or less would make no difference. This arrangement of the direct drive and silent speed has long been a debated point, some designers favoring the type just shown with an over-geared speed for occasional use, while the opponents of this say that this construction practically reduces the transmission to a three-speed basis, the fourth being so seldom used that it is practically negligible. They say, also, that the modern motor can attain a high enough speed on the one hand and is flexible enough on the other to permit its being used with the high-gear direct drive upon almost all occasions.