an apparatus includes a feed cylinder having a first end port and a second end port . a feed cylinder is located within the feed cylinder and is movable in a drive direction in response to fluid pressure applied to the first end port , and is movable in a return direction in response to fluid pressure applied to the second end port . a feed pump selectively displaces fluid between the first and the second end ports . a variable overcenter device controls fluid pressure offset , independent of the feed pump , between the first end port and the second end port . an adjustment device automatically adjusts the variable overcenter device during travel of the feed cylinder in the drive direction to a value corresponding to an amount of force required to displace the feed cylinder in the return direction . the variable overcenter device may be either microprocessor based or mechanically based including an accumulator reservoir which builds to a certain pressure based upon the amount of weight of the drill string when the feed cylinder was last displaced in the return direction .