Oil pump

An oil pump comprising:

• a rotatable inner rotor including a vane-housing unit that houses a plurality of vanes such that the plurality of vanes is slidable in a radial direction;
• a rotatable annular outer rotor including a plurality of vane-connecting parts connected with tip ends of the plurality of vanes located radially outward;
• a first volume-changing part, which is provided between the inner rotor and the outer rotor, a first volume of which is changed in response to eccentricity of the inner rotor with respect to the outer rotor, thereby providing a pumping function;
• and
• a second volume-changing part, which is provided in the outer rotor, a second volume of which is changed by a change in a distance between the vane-connecting parts adjacent to each other in a circumferential direction in response to the eccentricity of the inner rotor with respect to the outer rotor, thereby providing a pumping function, wherein
• the outer rotor includes a plurality of outer rotor pieces annularly connected to each other,
• each of the plurality of outer rotor pieces includes a first engaging part provided in a first end surface in an axial direction and a second engaging part provided in a second end surface in the axial direction and being capable of engaging with the first engaging part of an adjacent one of the outer rotor pieces, and
• the plurality of outer rotor pieces is circumferentially arranged in a state where the first engaging part and the second engaging part of the outer rotor pieces being adjacent to each other engage with each other such that a distance therebetween in the circumferential direction is variable.