Of the different types of sliding gears, the first two subdivisions are not very closely marked, but blend somewhat into one another. The only real difference between them is the method of operation, the names serving to indicate the distinctive characteristics. Thus, in a selective gearset, it is possible to "select" any one speed and change directly into it without going through any other. So, too, in the progressive form of transmission the act of changing gears is a "progressive" one, from the lowest up to the highest, and vice versa.
With the selective method of changing gears, it is possible to make the change at once from any particular gear to the desired gear without passing through any other, Fig. 68. Of course, the car will not start on the high gear any more than in the other case, but shifting into low for starting purposes is but a single action, accomplished quicker than it can be told. So, too, when the car has been started, it can be allowed to attain quite a fair speed and the change to high made at once without going through the intermediate gears.
Progressive gears are said to operate on the Panhard system, called after the originator of the system. This method leads to a number of troublesome occurrences; thus, in stopping it is necessary to gear down through all the higher speeds into low. If this is not done, when it is next desired to start the car it will be necessary to start the engine, throw in the clutch, drop from the gear in mesh to the next lower, from that to the next, and so on down to low, throwing the clutch out and in for each change of speed. When first is reached, the car may be started. After starting, it is then necessary, in order to obtain any measurable speed with the car, to change back up the list, from low to second, from second to third, and so forth. In this way the progressive gear is disadvantageous, since its use means much gear shifting; but, on the other hand, the shifting is very easy for the novice to learn, as it is a continuous process, all in one direction.
In Fig. 69 is shown the Panhard transmission for a car of 22 horsepower." This is a four-speed transmission, and to show graphically one big disadvantage claimed against this form as in favor of the selectively operated gears, refer again to Fig. 68. This represents the four-speed transmission used in the Mercedes cars of approximately equal power to the Panhard just described. The difference in the over-all length of the two is immediately noticeable, yet the only other difference is that the German cars are operated selectively and the Panhards progressively. Panhard and Mercedes have been consistent advocates of these two forms, the former adhering to its special type on a few models, even when this type passed out of popular favor. The latter, too, has always used the selective form of gear on all its cars, since the inception of this by Herr Maybach. In the United States, the progressive form has slowly hut very surely gone out.
Fig. 70. Cadillac Transmission and Housing.
To present some modern selective types of gear boxes, and point out their various differences, advantages and disadvantages, refer to Fig. 70. This type shows the three-speed selective gear used on the Cadillac cars, this being but slightly modified from the type which has been used by this maker for three years. This change should be noted, however: the lay shaft, which formerly was on the same horizontal level as the main shaft, is now placed directly below it. This makes a higher but narrower gear box - that is, instead of being wide and fairly fiat, it is now high and narrow. The placing of the shifting levers on the cover, directly over the center, has aided in making the gearset more compact than formerly. In it there are two shifting gears, one gear carrying a set of dogs cut into its face, which mesh with a similar set on the main driving gear to give high speed, which is the direct drive. The gear portion of this member meshes with another gear for second.
Fig. 71. English Transmission without Direct Drive.
The second shifting member meshes with one gear on the lay shaft for low speed and with another on the third shaft for reverse. This last-named gear is at all times in mesh with the fourth lay-shaft gear, so that on reverse the drive is through five gears instead of four. On high gear the drive is through the dogs, the lay shaft being driven, of course, but silently, as it transmits no power.
As will be noted, in all the modern gears presented, the drive is off the rear end of the main shaft, the lay shaft serving only as a medium for speed reduction. It will be noted also that in all the original forms first shown, the drive was off the lay shaft; that is, the engine drove one shaft, while the other drove the wheels. This form was widely used in the early days, in fact, it is now used to some extent, but mostly in England and on the Continent. An up-to-date English gear box with this form of drive is shown in Fig. 71, this being of the three-speed selective type, the same as those previously shown.
Fig. 72. Four-Speed Gear Box and Shaft Brake of Austin (English) Car.
Obviously, this construction makes it impossible to have a direct drive, and on all speeds the drive must be through gears. This makes all speeds noisy, which is the real reason why it has gone out of use. As will be noted in Fig. 71, this has both the shifting members on the main shaft, the gears on the lay shaft being of the bolted-on type. This is done to reduce manufacturing costs. Note the double-row ball bearing to take thrust at the driving end of the main shaft and the driven end of the lay shaft.