Circular rotor 1 rotates about eccentrically located fixed axis A, coinciding with the geometric axis of housing 2. Sliding abutment 3 reciprocates in fixed guide a and has roller 4 which rolls and slides along slot b of loading lever 5. Lever 5 turns about fixed axis B and its weight 6 can be adjusted to the required position along the axis of the lever to hold abutment 3 against rotor 1. When rotor 1 rotates, it slides along the internal surface of housing 2 and delivers fluid in the direction of the arrows. Abutment 3 separates the suction and discharge chambers.