A truck standard transmission usually consists of a main shaft and one, two, or three countershafts. These shafts are mounted parallel to one another. Torque from the engine is transmitted from a transmission input shaft at the front of the transmission through gearing to the countershaft(s). This torque path travels through the countershaft(s) until it reaches the selected gearing. This gearing routes the torque path into the transmission main shaft and from there out to a compound (auxiliary gearing) and the driveline components.
Single-countershaft transmissions are used in a few heavy-duty applications but are more common in medium-duty trucks. Most heavy-duty trucks use a double- or twin-countershaft design. In a twin-countershaft transmission, torque delivered to the transmission input shaft is divided equally between two shafts. This means that each countershaft gear set carries only half the load of a similar gear set used on a single-countershaft design.
One design of truck transmissions features triple countershafts. Note that the countershafts are equally spaced around the main shafts. This design evenly distributes the torque load around the three countershafts, minimizing deflection and containing gear tooth loading to a minimum. The principles are the same as in a twin countershaft transmission. The difference is that torque is divided between three instead of two countershafts.