An apparatus comprising: a first outer ring having an open annular space and a first-outer-ring variable-width groove disposed on an interior peripheral surface of the first outer ring ; a first inner ring (600; 806) rotatable within the open annular space of the first outer ring , wherein the first inner ring (600; 806) has a first-inner-ring variable-width groove disposed on an exterior peripheral surface of the first inner ring (600; 806); a first plurality of rollers (604; 608A, 608B) disposed between, and configured to roll on, the interior peripheral surface of the first outer ring and the exterior peripheral surface of the first inner ring (600; 806) and rotatable therebetween while engaging the first-outer-ring variable-width groove and the first-inner-ring variable width groove ; a second outer ring having an open annular space and a second-outer-ring variable-width groove disposed on an interior peripheral surface of the second outer ring ; a second inner ring (600; 808) rotatable within the open annular space of the second outer ring , wherein the second inner ring (600; 808) has a second-inner-ring variable-width groove disposed on an exterior peripheral surface of the second inner ring (600; 808); and a second plurality of rollers (604; 608A, 608B) disposed between, and configured to roll on, the interior peripheral surface of the second outer ring and the exterior peripheral surface of the second inner ring (600; 808) and rotatable therebetween while engaging the second-outer-ring variable-width groove and the second-inner-ring variable width groove, wherein the first outer ring is coupled to the second outer ring and the first inner ring (600; 806) is coupled to the second inner ring (600; 808), and each roller (604; 608A, 608B) is symmetric about its axis of rotation, characterized in that a width of each variable-width groove varies in a spatially periodic manner between a first width and a second width larger than the first width. a first outer ring having an open annular space and a first-outer-ring variable-width groove disposed on an interior peripheral surface of the first outer ring ; a first inner ring (600; 806) rotatable within the open annular space of the first outer ring , wherein the first inner ring (600; 806) has a first-inner-ring variable-width groove disposed on an exterior peripheral surface of the first inner ring (600; 806); a first plurality of rollers (604; 608A, 608B) disposed between, and configured to roll on, the interior peripheral surface of the first outer ring and the exterior peripheral surface of the first inner ring (600; 806) and rotatable therebetween while engaging the first-outer-ring variable-width groove and the first-inner-ring variable width groove ; a second outer ring having an open annular space and a second-outer-ring variable-width groove disposed on an interior peripheral surface of the second outer ring ; a second inner ring (600; 808) rotatable within the open annular space of the second outer ring , wherein the second inner ring (600; 808) has a second-inner-ring variable-width groove disposed on an exterior peripheral surface of the second inner ring (600; 808); and a second plurality of rollers (604; 608A, 608B) disposed between, and configured to roll on, the interior peripheral surface of the second outer ring and the exterior peripheral surface of the second inner ring (600; 808) and rotatable therebetween while engaging the second-outer-ring variable-width groove and the second-inner-ring variable width groove, wherein the first outer ring is coupled to the second outer ring and the first inner ring (600; 806) is coupled to the second inner ring (600; 808), and each roller (604; 608A, 608B) is symmetric about its axis of rotation, characterized in that a width of each variable-width groove varies in a spatially periodic manner between a first width and a second width larger than the first width. <EOS>