Pump unit with adjustable piston stroke length

A pump unit comprising a pair of reciprocating pumps communicating with a common inlet conduit and a common outlet conduit, said pair of reciprocating pumps each having a working piston mounted for reciprocating movement in a pump cylinder between an extended and a retracted position relative to said pump cylinder and a displacement piston and a hydraulic control piston, each of said pumps having a hydraulic control cylinder cooperating with said hydraulic control piston, pipe means interconnecting the hydraulic control cylinders of said pair of pumps to form a closed control system that includes a control fluid for synchronizing of said working pistons for alternating movement between said extended and retracted positions, pneumatic power means for providing a driving fluid for moving each of said displacement pistons alternately toward said retracted position, wherein said pipe means is communicable to an oil supply through a valve such that a predetermined quantity of oil may be introduced into said control system in order to adjust the length of stroke of each working piston, said predetermined quantity being selected according to a desired length of stroke of each working piston and said predetermined quantity of oil remaining substantially unchanged for a particular desired length of stroke during operation of said pump.