Classification of variable displacement hydraulic pumps that automatically adjust discharge volume according to load. These pumps enable energy-saving operation in manufacturing equipment such as machine tools, press machines, and injection molding machines by variably controlling flow rate according to system requirements. They feature structures including axial piston type, radial piston type, and variable vane type, with control methods such as electronic control, pilot control, and load-sensing control. Unlike fixed displacement pumps, they incorporate pressure compensation and flow control functions, enabling system efficiency optimization. They are ideal for applications requiring precise flow control such as spindle drive in NC machine tools, slide control in press machines, and injection control in molding machines.