a scroll - type fluid displacement apparatus includes a pair of scrolls having a circular end plate and a spiral wrap extending from an axial end surface of the circular end plate . a pair of scrolls is maintained at an angular and radial offset to make a plurality of line contacts between the spiral curved surfaces , which define fluid pockets . a driving mechanism is operatively connected to one of the scrolls to effect relative orbital motion with respect to the other scrolls to thereby change the volume of the fluid pockets . the spiral element of each scroll member has a stepped cross - section . as a result , the mechanical strength of the spiral elements of the scroll member of the fluid displacement apparatus is increased and the fluid displacement apparatus has increased durability and efficiency .