Hydromechanical transmission

Abstract

A hydromechanical vehicle transmission system for an engine most efficiently operated at a single speed is disclosed providing an infinitely variable reduction ratio for a given constant input speed requiring minimum operator skill in operation of the vehicle speed control and the engine torque control in both full and part load operation. The transmission consists of a multi-speed hydrostatic mechanical arrangement using a variable displacement hydrostatic pump-motor in conjunction with a fixed displacement hydrostatic pump-motor providing two power paths through the transmission, one hydrostatic and one mechanical. The preferred embodiment discloses four gear ranges although three gear ranges are sufficient to fulfill the objects of the invention. A split torque recycling planetary unit is coupled with the hydrostatic units and two conventional planet trains, the low and reverse range being fully hydrostatic, the intermediate ranges a combination of hydrostatic and mechanical drive, the high range being hydrostatic and mechanical in the lower output speed portion and fully mechanical in the higher output speed area when the hydrostatic path is locked out and engine speed is increased. Control of the transmission is provided through a hydraulically operated automatic system allowing minimum override capability on the part of the operator. Automatic shifting of reduction ratio is accomplished by comparing actual engine speed to an operator commanded engine speed with resulting upshifting or downshifting of the variable displacement unit and if required a selection of a different gear range occuring without operator intervention except for a capability to limit the highest ratio selected to one of the four gear ratios. The fully hydrostatic low and reverse range may be fully controlled by the operator limiting the displacement of the variable displacement hydrostatic pump-motor to a selected level.