Boom crane centering

Claim 1

A hydraulic system for centering and controlling the rotational movement of a boom on a crane, said hydraulic system comprising a pump to pump hydraulic fluid; a reservoir for said hydraulic fluid which is fluidly coupled to said pump; a hydraulic swing motor capable of rotating the boom clockwise and counterclockwise; a swing control valve fluidly coupled to said pump, said reservoir and said hydraulic swing motor, said swing control valve being designed to control the rotational direction of said hydraulic swing motor and said boom; free swing means hydraulically coupled between said swing control valve and said hydraulic swing motor to permit fluid to flow from said pump through said swing control valve and a portion to flow through said hydraulic swing motor to said reservoir back through said swing control valve for controlling the rotational speed of said swing motor and said boom during centering by controlling the flow of hydraulic fluid through said hydraulic swing motor, while the remaining portion of hydraulic fluid flows through said free swing means, through said swing control valve, to said reservoir; said pump including a high pressure delivery line for delivering hydraulic fluid to said free swing means and said swing motor; said reservoir including a low pressure return line for returning to said reservoir said hydraulic fluid from said swing motor and said free swing means, said swing control valve being fluidly coupled both to said high pressure delivery line and to said low pressure return line; said free swing means including means for controlling the flow of hydraulic fluid through said swing motor, said means for controlling the flow of hydraulic fluid including a fluid coupling means fluidly coupled to said high pressure delivery line and to said low pressure return line to bleed-off a portion of the hydraulic fluid from said high pressure delivery line and to transfer said bleed-off fluid to said low pressure return line; said means for controlling the flow of hydraulic fluid further including a poppet valve having a spool with an adjustable stop to bleed-off said portion of said hydraulic fluid when said poppet valve is open; said free swing means further including an on-off valve for actuating said free swing means, said on-off valve being coupled to said high pressure delivery line through said poppet valve and being coupled to said reservoir so as to drain any hydraulic fluid which accumulates behind said spool to said reservoir; and said means for controlling the flow of hydraulic fluid additionally including an adjustable pressure compensating valve coupled between said poppet valve and said low pressure return line so as to maintain a constant pressure drop across said poppet valve.