a transmission generates the travelling and pto outputs of a farm tractor . the transmission can easily generate the travelling output of a 15 - step forward movement and a 9 - step backward movement and the output of a 4 - step pto by fractionalizing the number of speed changing steps . the transmission uses a large number of identically standardized gears and hub sleeves but reduces the number of built - in parts in each part of the transmission , raises the productivity by using a large number of the same parts and controls the equipment efficiently due to easy disassembly for maintenance .