Power transmission

A hydraulic system comprising

• a full flow supercharged variable displacement pump including displacement varying means,
• a fixed displacement charge pump connected to the variable displacement pump such that it is driven by the same prime mover as said variable displacement pump,
• a fluid supply line for supplying fluid from said fixed displacement pump to said variable displacement pump,
• orifice means in said supply line to said variable displacement pump,
• feedback valve means responsive to the position of said displacement varying means,
• means providing a circuit from a point downstream of said orifice means to tank pressure,
• said feedback means comprising a body including a spool providing a pair of mutually and oppositely adjustable flow restrictions in series with said circuit,
• means connecting said restrictions to said displacement varying means to vary them equally and oppositely in proportion to the movements of said displacement varying means,
• a speed sensing control valve including a spool responsive to the pressure level between the two restrictions and the pressure level upstream of said orifice means,
• said speed sensing control being connected to the outlet of said variable displacement pump and operable to operate said displacement varying means to reduce the displacement of the pump when the pressure differential between the pressure at the outlet of said feedback means and the pressure upstream of said orifice means falls below a predetermined value.