Pump for hydraulic steering unit

Claim 1

A pump mechanism for operating an unbalanced cylinder comprising a housing, a pump body contained within said housing, said pump body having a plurality of circumferentially sPaced axially extending cylinder bores, cylinder ports in the inner end of said pump body communicating with said cylinder bores, means for rotating said pump body to cause reciprocation of said pistons within said cylinder bores, a porting plate secured to said housing having an end face in sliding sealed engagement with the inner end of said pump body, a pair of arcuate grooves in said end face of said porting plate which alternately communicate with said cylinder bores during each revolution of said cylinder body, an annular make-up groove in said end face radially inwardly of said arcuate grooves, a radial groove in said end face between said arcuate grooves on the suction side of said pump mechanism, said radial groove intersecting said annular make-up groove and extending radially outwardly beyond said cylinder body to provide communication with the interior of said housing, said housing comprising a reservoir containing make-up fluid which is supplied to said cylinder bores during rotation past said radial groove, a transverse bore in said porting plate, first and second passages in said porting plate communicating said arcuate grooves with said transverse bore in axially spaced relation along the length of said transverse bore, a third passage in said porting plate communicating said annular make-up groove with said transverse bore intermediate said first and second passages, and a spool valve axially movable within said transverse bore between a first position blocking fluid communication between said passages and a second position establishing fluid communication between said first and third passages while still blocking communication with said second passage for bypassing excess fluid entering said arcuate groove communicating with said first passage to said reservoir.