Patent ID: 8336613

Claim:
Apparatus comprising a) a hydraulic cylinder assembly comprised of a cylinder sealed by an upper head and a lower head and carrying a piston which divides the hydraulic cylinder assembly into an upper chamber and a lower chamber; b) a downward load connected to the piston urging it toward the lower head, c) a reversible hydraulic pump assembly, d) a pressure vessel containing a pressurized supply of hydraulic fluid compatible with the hydraulic pump assembly, wherein the pressure vessel contains hydraulic fluid in a lower portion thereof and a head of pressurized gas in an upper portion thereof, e) a first conduit for hydraulic fluid connecting the reversible hydraulic pump assembly in fluid flow communication with a lower chamber of the hydraulic cylinder assembly, f) a second conduit for hydraulic fluid connecting the reversible hydraulic pump assembly in fluid flow communication with the pressurized supply of hydraulic fluid, and g) a control system operably associated with the reversible hydraulic pump assembly to cause hydraulic fluid to flow back and forth between the lower chamber of the hydraulic cylinder assembly and the pressure vessel containing the pressurized supply of hydraulic fluid, wherein the pressurized supply of hydraulic fluid is maintained at an adequate pressure by the head of pressurized gas to counterbalance the downward load on the piston so that'the reversible hydraulic pump assembly demands similar peak power to operate the piston during the downstroke and during the upstroke, said apparatus further comprising h) an auxiliary pressure vessel containing a pressurized supply of hydraulic fluid compatible with the hydraulic pump assembly, wherein the auxiliary pressure vessel contains hydraulic fluid in a lower portion thereof and a head of pressurized gas in an upper portion thereof; i) a reservoir of hydraulic fluid and a third conduit means including an auxiliary pump connecting the reservoir of hydraulic fluid with the auxiliary pressure vessel, said auxiliary pump for pumping fluid into the auxiliary pressure vessel to pressurize the head of gas in the upper portion thereof; j) a fourth conduit for connecting a lower portion of the auxiliary pressure vessel into flow communication with the second conduit, said fourth conduit including a check valve which opens in response to the pressure in the pressure vessel dropping beneath the pressure in the auxiliary pressure vessel, so that hydraulic fluid flows from the auxiliary pressure vessel through the second conduit and to the reversible hydraulic pump assembly when the pressure in the pressure vessel dropping has fallen beneath the pressure in the auxiliary pressure vessel.