Both hydrostatic transmissions and geared transmissions are used in agricultural and construction equipment to transmit power from engines to equipment for accomplishing a desired task. For example, transmissions are used to properly transmit power to the wheels of a vehicle, or to a vehicle implement such as a rotor on an agricultural combine. However, depending upon the application, it is seldom that either a geared transmission or a hydrostatic transmission will provide the best performance for all of the transmission requirements. Two important considerations in selecting transmissions are their efficiency and range of input and output speed variability. In general, hydrostatic transmissions provide extremely high speed variability between the input and output, but are less efficient than geared transmissions.
Turning to the rotor of a combine, such rotors usually are relatively high in mass, and as the size of combines increases, become increasingly larger. Typical combines now use variable pulley belt drives to transmit power to the rotors. However, the power required to rotate the larger rotors of larger combines cannot be effectively transmitted through conventional belt drives. Also, the current machines have a provision to manually rotate the rotors in reverse, after they become clogged with excess material. This is not only cumbersome but also less effective with larger machines. Accordingly, in the larger combines there is a move toward using geared transmissions to drive the rotor and provide an engine-powered reversing feature. However, one of the problems with geared transmissions is the limited variability in speed. This limitation restricts the operator's ability to optimize rotor speed to obtain peak harvesting efficiency. Another problem with geared transmissions is the amount of energy which must be dissipated by the input clutch when engaging the power source to initiate rotation of the rotor. Due to the high inertia of large combine rotors, initiating rotation of the rotor when the combine engine is rotating at operating speed results in substantial wear of the input clutch to the transmission.
In view of the need for an improved transmission having improved variability between transmission input and output speed while maintaining efficiency, it would be desirable to combine the features of a hydrostatic transmission with a geared transmission.