Abstract: A hydraulic transmission system comprising a housing having at least an oil tank containing oil, an oil pump mounted in the housing and having an inlet port and an outlet port, an oil motor mounted in the housing and having an inlet port and an outlet port, a high-pressure oil passage defined in the housing and interconnecting the outlet port of the oil pump and the inlet port of the oil motor, a low-pressure oil passage defined in the housing and interconnecting the outlet port of the oil motor and the inlet port of the oil pump, an oil supply passage defined in the housing and interconnecting the low-pressure oil passage and the oil tank, for supplying oil pumped from the oil tank by the oil pump for circulation between the oil pump and the oil motor through the high-pressure oil passage and the low-pressure oil passage, and a return oil passage defined in the housing and connecting at least a portion of the low-pressure oil passage to the oil tank, for removing air trapped in the oil circulating between th

Abstract: A hydraulically operated power transmission apparatus transmits the output power from a power source at a selected speed ratio. The power transmission apparatus includes a casing having an opening in one end thereof, a base plate attached to the end of the casing in covering relation to the opening, the casing and the base plate jointly serving as a transmission case defining a closed space therein, a hydraulic pump having a pump drive shaft adapted to be operatively coupled to the power source, the hydraulic pump being disposed in the closed space, a hydraulic motor having a motor output shaft and drivable by a hydraulic pressure generated by the hydraulic pump, the hydraulic motor being disposed in the closed space, a pair of output shafts extending through the casing and the base plate, respectively, and disposed coaxially with each other, a differential mechanism disposed in the closed space and operable by the motor output shaft, the differential mechanism being coupled to the output shafts.