In such a gearbox, the gear pairs on the intermediate and main shafts are constantly in mesh, while the main shaft wheel can rotate freely on it. The coupling of this gear wheel with the main shaft is made by a gear coupling.
The principle of operation of the tooth clutch. A sliding ring is placed on the spline of the main shaft. Narrow internal toothed rings are made on both sides of this ring, corresponding to the outer rims on the toothed wheels. The torque from the engine is supplied to the gearbox by a clutch shaft, and then through a pair of gears - it is transferred to the intermediate shaft. The constantly meshed pair of gears causes the gear to rotate, but does not move the main shaft due to the gear bearing. Only by shifting the sliding ring to the right and the meshing of the gear rims, the gear is engaged with the main shaft and the torque is transmitted through the gearbox. The torque on the main shaft is multiplied thanks to the ratio of toothed gears. Shifting the sliding ring to the left and the meshing of the gear wheels causes direct engagement of the clutch and main shafts. In this case, the torque transmitted by the shafts does not change, although all the gears in the gearbox are constantly in motion.
The permanently engaged gears may have helical teeth, thanks to which they work much quieter.

At the moment of coupling the rim of the toothed clutch with the rim of the toothed wheel, shocks occur due to the difference in the rotational speed of the coupled elements. These impacts cannot be avoided by using gear couplings combined with additional friction clutches. The task of the friction clutches is to synchronize (leveling) rotational speeds of the coupled elements before engaging the tooth clutch.