Improved Positive-Displacement Rotary Hydraulic Machine

An improved rotary positive-displacement hydraulic machine comprising two gear wheels, which mesh including a driving gear wheel and a driven gear wheel respectively, enclosed in a pump housing having a delivery opening and an intake opening for a fluid, wherein said two gear wheels include a plurality of teeth, which mesh, wherein said two gear wheels include respective shafts each support by bushings housed inside said pump housing, wherein a material of which said bushings are made has a thermal expansion coefficient equal, or differing by less than a factor of +/−15%, to the thermal expansion coefficient of a material of which said two gear wheels are made, characterized in that said pump housing includes:

• a radially internal portion adapted to face said plurality of teeth of said two gear wheels, said radially internal portion of said pump housing being made of a material with hardness lower than 160 HB, and
• a radially outer portion made of a material which has a thermal expansion coefficient equal, or differing by less than a factor of +/−15%, to the thermal expansion coefficient of the material of which said two gear wheels are made.