Relief pressure controlling device used in vehicular hydraulic

Abstract

A relief pressure controlling device comprises a driving side relief pressure controlling valve, a capacity detector which detects the capacity of a hydraulic pump, and an engine throttle opening detector. When the hydraulic pump and the hydraulic motor which are connected by a first oil passage and a second oil passage constituting a hydraulic closed circuit are driven by an engine, the relief pressure controlling valve controls the relief pressure of the oil passage on the driving side that is either the first oil passage or the second oil passage whose pressure becomes higher than the other, to a target relief pressure. The target relief pressure is determined in correspondence with the engine output and corrected on the basis of the pump efficiency which is predetermined in relation to the capacity of the hydraulic pump and to the throttle opening, and this corrected target relief pressure is set as a driving side target relief pressure. The driving side relief pressure controlling valve performs a relief pressure control so that the relief pressure of the driving side oil passage will become the driving side target relief pressure, which is set in correspondence with the capacity of the hydraulic pump and the engine throttle opening.