It quite often makes sense, to go out from a simple, basic framework. The above shown framework however, is still a little too complicated. Actually, one should leave out the components for the synchronisation. If this was done, then we'd be at the point, that for a long time, the gearbox technology on the American continent was all about.
Indeed, the number of gear-speeds should of course, not be considered typical. However 8 gears should be enough to start with. Let's assume the far left hand side of the engine, you can look at the left hand part of the gearbox as a pre-change group, as described here. What's left over, on the right, would then be the basic four-speed gearbox.
Up to now, we haven't dealt with what is actually special about this gearbox, the two countershafts, here at the top and at the bottom, in trucks, on the left and on the right, you may ask yourself, why two countershafts are needed, when basically, one would be enough. At this point, a longer debate about the differences between the torque to be transferred in a truck and that in a motor car would begin.
For a torque of approx. 1000 to 3000 Nm you could use particularly dimensioned gearwheels and only one countershaft, or two shafts, thus saving greatly as far as the layout of the gearwheels is concerned. Supposedly, the center drive/output shaft could almost manage without bearings, because as far as strain is concerned, it lies favourably between the other two.
Oddly enough, the Fuller Company, who originally designed this gearbox, did not plan any synchronisation, There are still any number of trucks in North America which suffer from this situation. This putative shortcoming was remedied by their successors, the Eaton Company. From then on, the gearboxes looked roughly like the one shown above. 03/13