In the progressive type of transmission all sliding gears are moved simultaneously when a speed change is made. The several speeds are arranged in a fixed succession as the combination of sliding gears is progressively shifted. It is thus necessary to shift into the low-speed gear first and progress to the high.
Fig. 78 is a diagrammatic view of a three-speed and reverse progressive transmission which is used on a number of heavy commercial cars.
High speed is obtained by meshing the jaw clutches formed integral with the sliding member and the constant mesh gear F. Second speed is obtained by moving the sliding member back want and meshing gears D and E, so that the drive is through gears F, I, E and D. For low-speed the sliding member is moved backward so that the gear B will mesh with the gear C on the countershaft, and the drive is through the gears F, /, C and B. For reverse, gear B of the sliding set is meshed with the reverse pinion G, which is constantly in mesh with the gear H on the countershaft, and the drive is through gear F, I, H, G and B. This also illustrates a unit construction of jack shaft and transmission.
Fig. 79 illustrates the non-direct type of change gear, the principle of operation being similar to the above, excepting that the high speed is obtained by meshing gears instead of jaw clutches, while the upper shaft forms the drive and carries the fixed gears. The lower is the driving shaft and carries the sliding gears. Since the high speed is obtained by meshing gears, the drive shaft may be constructed in one piece.
The principal objection to the progressive type is that it requires long shafts, which are likely to be inefficiently rigid and to spring and bend under the thrust of the gear teeth, causing noisy and inefficient operation. The great length of this transmission is mainly due to the fact that that the gears on each of the shafts must be spaced relatively far apart, so as to avoid interference.
Fig. 78. Diagram of Three-Speed and Reverse Progressive Sliding-Gear Transmission.
Fig. 79. Non-Direct Progressive Type.
This objection is overcome in the selective sliding type, as two sliding sets are used. By comparing the two types it will be noted that the latter is much more compact. It also has an advantage in that the operator may change directly from one speed to any other without passing through the intervening gears.
Fig. 80 shows a three-speed forward and reverse selective type transmission. The primary shaft is squared and carries two sliding gears, which are operated by independent shifter rods. The countershaft is driven through constant mesh gears A and B, A being driven by the drive shaft extending from the clutch. In effecting the different speeds, gear C is moved forward and meshed with gear D for low speed, while for reverse it is moved backward and meshed with the reverse pinion E, which is in constant mesh with the reverse gear H on the countershaft. For second speed the gear F is meshed with gear G, while for high speed gear I, which is integral with gear F, is moved forward and meshed with the internal gear formed integral with the constant mesh gear A. This forms another type of jaw clutch, and in some cases the jaw clutch described above is used for effecting the high speed.
A typical unit power-plant transmission is shown in Fig. 81. This transmission is intended for low-powered delivery cars and is provided with ball bearings for the main shaft while the four counter-shaft gears are cut in one and are provided with plain bearings. This shows the method of mounting the clutch on the forward main shaft.
Fig. 82 depicts a typical four-speed transmission for amidship mounting. The main shaft is mounted on ball and roller bearings, while the countershaft is mounted on roller bearings. The shifter rods are mounted in the cover and are provided with locks for the various speeds. High speed is obtained by meshing two jaw clutches this is the direct drive, third by meshing gears A and B, second by gears C and D and low speed through gears E and F. For reverse speed two idler gears G and H are used, which are so arranged that they may be moved along their shaft and meshed with gears E and F, the latter being held in its neutral position.
The positive clutch type of transmission is somewhat related to the selective sliding gear type. However, the gears remain constantly in mesh, and the gears on the main shaft are normally free to turn thereon, but may be fixed to the shaft by positive clutches. These clutches, when of the jaw type, are similar to those mentioned above, while internal and external gears may also be used as positive clutches. The gears on the main shaft are fixed while the clutches are free to slide upon keys or squared portions of the shaft.
In this type the speed changes are obtained by the individual clutches, but since the high speed is direct through the case and the speed of the countershaft is reduced through the constant mesh gears, a provision must be made so that the latter shaft cannot turn on its bearings. This is accomplished by disengaging a clutch on the countershaft, simultaneously with the engagement of the high-speed clutch. Fig. 83 illustrates this type of transmission. It also presents a method of driving all four wheels by the addition of another shaft and silent chain.
Fig. 80. Three Speed and Reverse Selective Sliding- Trunsmission.
Fig. 81. Conventional Type of Unit Tower Plant Transmission and Control Mounting.
Fig. 82. Four Speed Selective Sliding Gear Transmission for Amidship Mounting.
Fig. 83. Constant Mesh Type of Selective-Sliding Gear Transmission.
Fig. 84 illustrates the type of jaw-clutch transmission used on the Vulcan trucks. The clutches are shifted automatically and are slightly undercut so that they will not release until the driving pressure is removed, while the change from one speed to another is made by means of springs. This action is accomplished as follows: moving the gear-shift lever compresses a set of springs which control the arms which move the jaw clutches. However, as the clutches are undercut this spring action has no effect, but just as soon as the engine speed is momentarily reduced there is a tendency for the vehicle to drive the engine, which instantly frees the jaw clutch, which, under the action of the compressed springs already set by the movement of the gear-shift lever, forces the clutch to take up its new position, engaging the desired speed. This means that slightly throttling the engine and releasing the clutch will always effect the desired change of gears, but this may not take place until any desired moment after the gear-shift lever has been moved. By this arrangement it is possible for the driver to approach a hill and before reaching it, if he thinks it too steep for the highest speed, he can set the lever for the next speed. The jaw clutch will not shift as long as the engine is driving until at the desired moment on the hill, by releasing the clutch the change will be automatically effected.
Transmission gears are usually lubricated by a non-fluid oil. For easy introduction of the lubricant, a hole is provided in the cover plate, which is enclosed by a screw plug, while a drain plug is usually placed at the bottom so that the stale lubricant may be washed out with kerosene or gasoline. The bearing caps are invariably provided with felt washers, while all other parts are provided with paper gaskets, to prevent the lubricant from working out of the case.
Fig. 84. Type in which Clutches are Shifted Automatically.