A swing drive system (11) for an excavator is provided which utilizes a prime mover (20) mechanically connected to a first hydraulic pump/motor (30) and a second hydraulic pump/motor (32) mechanically connected to a swing mechanism (70). The system includes a hydraulic circuit (31) connecting a hydraulic fluid reservoir (42), a hydraulic accumulator (40), the first hydraulic pump/motor (30), and the second hydraulic pump/motor (32). The system is operable in one mode where the second hydraulic pump/motor acts as a pump to retard movement of the swing mechanism and pressurized hydraulic fluid from the second hydraulic pump/motor is pumped into the hydraulic accumulator. The system is operable in another mode where the pressurized fluid from the hydraulic accumulator is used to assist the prime mover in driving hydraulic consumers (62,64,66), including the swing mechanism.