Abstract: A constant speed hydraulic drive comprising a variable displacement hydraulic pump controlled to maintain a constant flow of hydraulic fluid even though a prime mover therefor is subject to speed variations. The constant flow rate delivered by the hydraulic pump is at a rate sufficient to drive a load hydraulic motor at a predetermined speed when the load hydraulic motor is subject to any given load. A pilot operated relief valve proportionally bleeds hydraulic fluid from ahead of the hydraulic motor in accordance with the speed and load of the hydraulic motor.