Filter device for a hydraulic system

Abstract

Filter devices are useful in hydraulic systems for removing contaminants or foreign particles from the oil in the hydraulic system. One of the problems encountered with such filters is that they are subjected to high pressure drops thereacross when the oil is cold. The subject filter device of one embodiment includes a pump driven by a two-speed electric motor. When the oil temperature is below a predetermined temperature, the pump is driven at a normal speed for increased efficiency and the pressure drop across the filter is within an optimal range. When the temperature of the oil is below a predetermined temperature, the pump is driven at a low speed so that the pressure drop across the filter remains within the optimal range. In another embodiment, an electric motor driving the pump is stopped when the oil temperature is below the predetermined temperature. In a third embodiment, the pump is driven at a substantially constant speed and a laminar flow restriction is positioned upstream of the pump to induce slight cavitation when the oil viscosity is high while permitting normal fluid flow to the gear pump when the oil viscosity is low.

Dependencies are inferred from the claim text.