Abstract: A rotary mechanism (10) has an annular (chamber 12) defined by an inner wall (16) of housing (11). A symmetrical two lobed rotor (15) has opposing side faces (21a, 21b) a longitudinal axis between apices (22). A drive shaft (50) eccentrically rotates rotor by a block (51) and slot (52) receprocating arrangement and a second supporting means (53). The centre of the rotor follows a circular orbit in the chamber (12). The apices (22) continuously sweep the inner wall (16) creating cavities (25) of successively increasing and decreasing volumes with associated fluid inolet and exhaust port (31, 35).