Hydraulic circuitry for the hoist ram and the like of the building machinery

Claim 1

In a hydraulic system comprising hydraulic ram means having a piston to be loaded in a single direction, hydraulic pump means communicating with said ram means for supplying a liquid medium to said ram meanS for driving said piston thereof, a pair of pipelines for transferring said pumped liquid medium alternatively to a pair of spaces on opposed sides of said piston, respectively, a controlling valve means for interchanging said pair of pipelines with each other so as to direct said liquid medium to either of said pair of spaces, reservoir means from which liquid medium is supplied to said pump means and to which liquid medium is returned from said ram means, a return pipeline extending between said controlling valve means and said reservoir means, and means comprising an additional pipeline for supplying one of said pair of spaces with said liquid medium under a positive pressure automatically, when a pressure subjected to said liquid medium within said one of said pair of spaces is reduced, said additional pipeline being connected between that one of said pair of pipelines which is connected with said one space and said return pipeline, and said additional pipeline carrying a check valve for directing liquid medium only to said one space while said return pipeline carries a throttle valve means for directing liquid medium under pressure through said additional pipeline and check valve to said one space when the pressure therein is reduced.