in a co - rotational scroll apparatus having two interleaving scroll wraps secured to end plates rotating about parallel , non - concentric axes to produce a relative orbital motion , a coupling for causing concurrent rotation and for enhancing the nutational stability of the scroll element . in the preferred embodiment , the nutation reducing means is comprised of an oldham - type coupling engaging extension members on one of the scroll end plates and drive keys on the other respective end plate . the angular disposition thereof is optimized to cause the oldham coupling to generate during rotation a moderating moment load in opposition to the tipping moment load of at least one of the scroll elements and thereby enhance the nutational stability of the scroll elements . the moderating moment load of the coupling imposed on the scroll elements may be altered by varying the angular and radial disposition of the center of gravity of the coupling .