One should not under estimate the size of the gearbox in heavy utility vehicles. It is in fact, bigger than most six-cylinder in-line motor car engines. Its weight is more comparable to an eight-cylinder engine. The above
drawings should depict only the principle and not the size of the gearwheels. Today's truck gearboxes must endure up to 3000 Nm and perhaps more of input torque, and need, because of the narrow usable RPM-
band of truck diesel engines, a large amount of gear-speeds. The figures show a 16-speed gearbox, which apart from the 12-speed gearbox in road-vehicles, counts as the most frequently found.
The gearbox layout, from the engine side on the left, to the cardan shaft on the right, is less confusing if one splits it into single sub-assemblies. It consists in the middle, of a four-speed transmission (2nd to 5th
gearwheel pairs from the left) to which, on the right, a 2 speed planetary gearbox is attached as a downstream group transmission or also a range group (range-change gearbox). In addition, there is also a pre-shift
group consisting of two gearwheels to be seen on the left.
The four-speed transmission is completely shifted through four times, one time each with- and without large reduction in the downstream group on the right. In addition, the small gear reduction also changes in the
pre-shift group on the left. One could speak of a 2x4x2 gearbox.
The two-speed downstream group transmission was previously a conventional gearbox, it has however, for a long time now, been replaced by a planetary gearbox. To better explain the principle, in the above drawing
it is equipped with two sliding clutches which execute in each case the same movement. In this case, in the gears 1-8, the ring-gear is connected with the casing, and in the gears 9-15, with the drive shaft. The
planetary set revolves as a block with direct transmission ratio.
Should a truck gearbox have only 12 speeds, a 3-speed gearbox forms the basis. Here by the way, the reverse gear has been left out for reasons of clarity. As mentioned before, only the principle should be depicted
here. In reality, e.g., the downstream group is formed by a planetary gearbox. In principle, in this type of a gearbox, four reverse gears are possible from which in practice however, only two are shiftable. In the above
drawings these have been left out for reasons of clarity.
The shifting is still, to some extent, done with a conventional gearshift, comparable in its
function to that of the motor car, except that to begin with, one can select 8- and a reverse gear in a total of five shift lanes, instead of 5 or 6 gears. This lever, basically operates only the middle four-speed transmission,
not the pre-shift- and downstream group. They can be changed pneumatically, the downstream group with the shifting from 4th to 5th and vice versa, the pre-shift group through a small lever in the gearshift. Actually,
one hardly ever shifts through all the gears. More than likely an intermediate gear is used, e.g., on a steep hill. When without load, only four straight gears are often sufficient.
Of course this type of gearbox needs a synchronisation. It needs heavier shafts and gearwheels, and also one large- or two small clutch discs to do its job. Therefore, the Synchronisation rings are not made, as
opposed to the passenger car, of brass rings, but from molybdenum-coated steel. All together the force needed for shifting is also not comparable. It is reduced through the pneumatic assistance which is controlled
through small micro-switches. The lubrication in truck gearboxes is also different. The planetary gearbox in the downstream group, through its external sealing would hardly take up any oil. Therefore, an oil pump is
nearly always flanged onto the, which supplies, either through the countershaft or through the main shaft, the single functional elements directly. Sometimes there is also an oil-line with small drillings above the sub-
assemblies. An important factor, is the retention time of the oil, which can be directly extended, e.g., through restraining discs, and also the ensured supply to the auxiliary drives from the gearbox. The lubricant itself
hardly differs from that used in the motor car. It is a full-synthetic oil for low friction resistance and high lubrication performance. 07/09