After many years of development, CLAAS Industrietechnik GmbH (CIT) has introduced what it calls the transmission technology of the future. It says its new HM continuously variable transmission (CVT), which is based on hydrostatic/mechanical power-split technology, provides easier operation; offers reduced fuel consumption; and provides full integration capability into powertrain management via CAN bus control.

The primary components of the HM transmission (see Figure) are the reverse unit (A), hydrostatic unit (B), double planetary gear (C) with low- (D) and high-speed (E) coupler shafts, and a multistep reduction gear (F) downstream of the coupler shafts. The following describes how these elements interact.

It is in the reverse unit that power is split into mechanical and hydrostatic paths. The rotational direction of the mechanical path is selected by the driver, while that of the hydrostatic path remains unchanged.

The hydrostatic drive wheel drives the variable component, which hydrostatically operates the drive gear through the constant unit. The hydrostatic unit is driven in only one rotation direction. The variable unit allows the constant unit to cover a variable speed range in either direction. Similar hydrostatic units are used in forklift trucks and combine harvesters.

Several transmission gear drives are available on the low- and high-speed coupler shafts. In the hydrostatic unit, sophisticated synchronization ensures coordination of the gear drives on the high- and low-speed coupler shafts. Power flow is transferred from one coupler shaft to the other without jerks or interruptions in power flow. The end of one driving range represents the beginning of the next one, making it possible to travel continuously through the entire transmission range. Shifting gears at identical speeds allows the use of the low-loss dog clutches employed in conventional gearboxes—one of the reasons for the transmission's high efficiency.