Because of the way an internal combustion gasoline engine breathes, it can produce torque, or twisting force, only within a narrow speed range. Most engines covered by this guide must turn somewhere between 2000-4000 rpm to produce their peak torque. By 4500 rpm, they are producing so little torque that continued increases in engine speed produce no power increases.
The transmission and clutch are employed to vary the relationship between engine speed and the speed of the wheels so that adequate engine power can be produced under all circumstances. The clutch allows engine torque to be applied to the transmission input shaft gradually, due to mechanical slippage. The truck can, consequently, be started smoothly from a full stop.
The transmission is used to change the ratio between the rotating speeds of the engine and wheels by the use of gears. 4-speed or 5-speed transmissions are used on these vehicles. The lower gears allow full engine power to be applied to the rear wheels during acceleration at low speeds.
The clutch driven plate is a thin disc, the center of which is splined to the transmission input shaft. Both sides of the disc are covered with a layer of material which is similar to brake lining, and which is capable of allowing slippage without roughness or excessive noise.
The clutch cover is bolted to the engine flywheel and incorporates a diaphragm spring which provides the pressure to engage the clutch. The cover also houses the pressure plate. The clutch driven disc is sandwiched between the pressure plate and the smooth surface of the flywheel when the clutch pedal is released, thus forcing it to turn at the same speed as the engine crankshaft.
The transmission housing contains a mainshaft which passes all the way through the transmission, from the clutch to the driveshaft. This shaft is separated at one point, so that front and rear portions can turn at different speeds. Power is transmitted by a countershaft in the lower gears and in reverse. The gears of the countershaft mesh with gears on the mainshaft, allowing power to be carried from one to the other and then back again to the rear half of the mainshaft. All the countershaft gears are integral with that shaft, while several of the mainshaft gears can either rotate independently of the shaft or be locked to it. Shifting from one gear to the next causes one of the gears to be freed from rotating with the shaft, and locks another to it. Gears are locked and unlocked by internal dog clutches which slide between the center of the gear and the shaft. The forward gears usually employ synchronizers: friction members which smoothly bring gear and shaft to the same speed before the toothed dog clutches are engaged. The clutch is operating properly if:
- It will stall the engine when released with the vehicle held stationary.
- The shift lever can be moved freely between 1st and reverse gears when the engine is running, the vehicle is stationary and the clutch disengaged (the pedal held downward).
A clutch pedal free-play adjustment is incorporated in the linkage. If there is about 1-2 in. (25-50mm) of motion before the pedal begins to release the clutch, it is adjusted properly. Inadequate free-play wears all parts of the clutch releasing mechanisms and may cause slippage. Excessive free-play may cause inadequate release and hard shifting of gears.