US Patent 7469536 Series-parallel hydrostatic transmission

According to the present invention, an improved hydraulic transmission system is provided which includes a fluid supply adapted to supply hydraulic working fluid and having a hydraulic adjustment device for adjusting the flow rate of the hydraulic working fluid. A hydraulic control has an actuated condition in which the hydraulic control generates a hydraulic control fluid having a control pressure, which is adjustable between first and second control pressures. A first connection control operable by the hydraulic control fluid supplied from the hydraulic control is adapted to direct, upon actuation of the hydraulic control, the hydraulic control fluid to the hydraulic adjustment device. The first connection control includes a distributor having a control position in which it directs the hydraulic fluid to the adjustment device and a neutral position in which it connects the adjustment device to a tank.