Hydraulic control for variable displacement pumps

In a hydraulic system for controlling the output of at least one variable displacement pump driven by a single prime mover, the combination of:

• a source of pilot fluid under pressure including a fixed displacement pump adapted to be driven by the prime mover proportional to the speed thereof;
• means defining an underspeed control valve connected to said source and having an inlet and an outlet adapted to be connected to the variable displacement pump (s) for controlling the displacement thereof;
• and
• load sensing means having an input connected to said source and adapted to be associated with said variable displacement pump(s) for sensing the loading thereon, and having an output connected to said inlet for directing a fluid signal thereto proportional to a sensed load;
• whereby changes in speed of the prime mover will not affect displacement of the variable displacement pump (s) unless it is loaded in excess of a predetermined amount.