Abstract: Rotor having a pair of parallel side surfaces and a cured perimeter surface therebetween formed of contiguous mutually tangential curves each including a major portion defining a first major arc subtending a predetermined angle at a predetermined center of rotation, and having a first radius; a minor portion defining a first minor arc subtending a predetermined angle at the center of rotation, and having a second, shorter radius, the major and minor arcs arranged along an axis of symmetry; and intervening curves extending tangentially between major and minor arcs, each formed of a second major arc and a second minor arc, of predetermined radii; the curved perimeter shaped for coplanar, non-touching, and same-directional rotation with another identical rotor, and having mutually parallel orientations at the start of rotation and rotated at the same angular velocity, and separated from the curved perimeter of the other rotor by a predetermined fixed distance.