CONTROLLED PRESSURE UPSTAGING AND FLOW REDUCTION

An improvement in a hydraulic system (10) having a variable displacement pump (12) and being capable of delivering pressurized fluid from the pump (12) to selectively engage at least three work circuits (18) with a first (20) of the circuits (18) being normally operable up to a first pressure and a second (22) of the circuits (18) being normally operable up to a second and lower pressure. The improvement comprises circuitry (80, 92, 94, 98, 108, 120, 122, 124, 126) for selectively operating a third (24) of the circuits (18) atone of the first or second pressures or at an upstaged pressure which exceeds the first pressure and circuitry (128) for reducing the displacement of the pump (12) in response to the third circuit (24) being operated at the upstaged pressure. Such upstaged pressure is available in a controlled manner for working against particularly heavy loads. Thus, a third and higher operating level is added to a system which already operates at at least two pressure levels. Systeme hydraulique (10) ayant une pompe a deplacement variable (12) et pouvant fournir un fluide sous pression a la sortie de la pompe (12) pour attaquer de facon selective au moins trois circuits de travail (18), le premier (20) des circuits (18) pouvant etre normalement actionne jusqu'a une premiere pression et le deuxieme (22) pouvant etre normalement actionne jusqu'a une deuxieme et plus basse pression. L'amelioration comprend un reseau de circuits (80, 92, 94, 98, 108, 120, 122, 124, 126) servant a actionner de facon selective un troisieme (24) de ces circuits (18) a une pression egale a la premiere ou a la deuxieme ou a une surpression excedant la premiere pression et des circuits (128) pour reduire le deplacement de la pompe (12) en reponse a l'actionnement du troisieme circuit (24) a la surpression. Une telle surpression peut etre obtenue de maniere commandee dans le cas de charges particulierement lourdes. Un troisieme niveau de fonctionnement plus eleve s'ajoute donc a un systeme fonctionnant deja a au moins deux niveaux de pression.