Hydraulic control for hydromechanical transmission

Claim 1

A hydromechanical transmission connectable between a prime mover and power-using device: said transmission comprising an input shaft (36); an output shaft (64); a hydrostatic pump-motor unit connected to the input shaft; control means (44, 46) associated with said pump-motor unit for varying the pump displacement and motor displacement; an epicycloidal gearing system connected to said shafts and to the pump-motor unit, said gearing system including two planet gear carriers (62 and 56); a first hydraulically-operated clutch (72) for exerting a locking action on one of the planet carriers to establish a low output speed ratio drive path through the gearing system; a second hydraulically-operated clutch (80) for exerting a locking action on the other planet carrier to establish a high output speed ratio drive path through the gearing system; an error signal means (88) for comparing prime mover speed and user demand speed; said signal means including a hydraulic input (32, 98) representing prime mover speed, a mechanical input (24, 94) representing user demand speed, and two separate hydraulic output signals (118 and 120) representing plus or minus differentials between user demand speed and prime mover speed; a clutch selector valve (150) having an inlet (164) connected to a hydraulic pressure source, a first pressure outlet (178) connected to said first clutch for operating same, a second pressure outlet (170) connected to said second clutch for operating same, a third drain outlet (172) for the first clutch, a fourth drain outlet (176) for the second clutch, and a flow diverter piston (152) movable in a reciprocatory fashion between (1) a first position wherein the inlet is connected to the first pressure outlet and the secnd pressure outlet is connected to the fourth drain outlet, and (2) a second position wherein the inlet is connected to the second pressure outlet and the first pressure outlet is connected to the third drain outlet; means for operating the flow diverter piston comprising a spring means (154) and hydraulic means (148) controlled by a pilot valve (138) having a power piston (136); said power piston having opposed end faces (136a and 136b) located to receive the aforementioned hydraulic signals (118 and 120), whereby said pilot valve is closed when user demand speed exceeds prime mover speed and open when prime mover speed exceeds user demand speed; means (166) for depressurizing the first clutch, comprising a piston (177) that is operated by a pressurized condition of the aforementioned second outlet to form a drain path from the third drain outlet; means (168) for depressurizing the second clutch, comprising a piston (169) that is operated by a pressurized condition of the aforementioned first outlet to form a drain path from the fourth drain outlet; and means (146) responsive to motion of the aforementioned power piston (136) for operating the aforementioned pump-motor control means (44, 46).