Countershaft transmissions having ten or more forward speeds typically consist of a five-speed main section or box and a two- or three-speed auxiliary section. The main and auxiliary gearing can be contained in a single housing or each section can have separate casings that are coupled to each other. The auxiliary gearing is usually located behind the main box.
The main box shown in the ten-speed transmission has a five-speed ratio; the auxiliary section that couples to it has two ranges or speeds. There is a direct (or high) range and a low range in this transmission. Shifts in the main section are effected mechanically by the driver using the shift stick. However, selection of the gears in the auxiliary section is made by a driver-actuated, air-operated piston. The driver uses a pneumatic switch, usually located on the gear lever, that moves the auxiliary section into low- or high-range ratios. The driver controls this range selection mechanism through the use of a master control valve switch mounted on the gearshift tower in the operating cab. Follow the powerflow through the transmission; it is operating in low range. The following is true:
1. Torque from the engine flywheel is transmitted to the transmission input shaft.
2. Splines on the input shaft mesh with internal splines in the hub of the drive gear.
3. Torque is split between the two countershaft drive gears that mesh with the main shaft drive gear.
4. Torque is transferred by the two countershaft gears to whichever main shaft gear is engaged. Shifting and powerflows in the main section are similar to those in the five-speed transmission discussed earlier in this chapter under the heading Mechanical Shift Mechanisms. A notable difference is the use of a power take-off (PTO) gear located between the countershaft drive gears and fourth gears.
5. Internal splines in the hub of the main shaft gear transfer torque to the main shaft by means of a sliding clutch gear.
6. The main shaft transfers torque to the auxiliary drive gear through a self-aligning coupling gear located in the hub of the auxiliary drive gear.
7. Torque is split between the two auxiliary countershaft drive gears.
8. Torque is delivered by the two countershaft lowrange gears to the auxiliary low-range gear.
9. Torque is transferred to the output shaft through a self-aligning sliding clutch gear that locks the low-range gear to the output shaft.
10. The output shaft connects by means of a yoke to the driveline.
In this range, the sliding clutch gear locks the auxiliary drive gear to the output shaft. The low-range gear on the output shaft is now allowed to freewheel.
The ten forward speeds are selected by using a fivespeed shifting pattern twice, the first time with the auxiliary section engaged in low gear or low range; the second time engaged in high gear or high range. By using the same shifting pattern twice, the shift lever position for sixth speed is the same as first, seventh the same as second, eighth the same as third, ninth the same as fourth, and tenth the same as fifth.