Fig 17.1 shows a typical constant mesh gear box in an agricultural tractor. This gear box has 8+2 gear ratios i.e. 8 forward speed gear ratios and 2 reverse speed gear ratios.

In this gear there are 5 shafts and a number of gears. There are also four sliding dog clutches – 3 on the upper shaft axis and 1 on the lower shaft.

Gear meshing