An apparatus comprising: a first ring having an open annular space and a series of variable-width cutouts (204A-F) disposed on an interior peripheral surface of the first ring; a second ring rotatable within the open annular space of the first ring, wherein the second ring has a respective series of variable-width cutouts (210A-D) disposed on an exterior peripheral surface of the second ring; and a plurality of rollers (220A-E) disposed between, and configured to roll on, the interior peripheral surface of the first ring and the exterior peripheral surface of the second ring and rotatable therebetween, wherein: the first ring has a total number of variable-width cutouts (204A-F) and the second ring has a total number of variable-width cutouts (210A-D), with the total number of variable-width cutouts (210A-D) of the second ring being smaller than the total number of variable-width cutouts (204A-F) of the first ring, and a total number of the plurality of rollers (220A-E) being less than the total number of variable-width cutouts (204A-F) of the first ring and greater than the total number of variable-width cutouts (210A-D) of the second ring; as a given roller of the plurality of rollers (220A-E) traverses a variable-width cutout of either the variable-width cutouts (204A-F) of the first ring or the variable-width cutouts (210A-D) of the second ring, a radial distance between a center of the first ring and the given roller varies; each of the variable-width cutouts (204A-F; 210A-D) starts with a first width at a first end of the cutout, increases to a second width (d) larger than the first width at a center of the cutout, and narrows back to the first width at a second end of the cutout; and each roller (220A-E) is symmetric about its axis of rotation. a first ring having an open annular space and a series of variable-width cutouts (204A-F) disposed on an interior peripheral surface of the first ring; a second ring rotatable within the open annular space of the first ring, wherein the second ring has a respective series of variable-width cutouts (210A-D) disposed on an exterior peripheral surface of the second ring; and a plurality of rollers (220A-E) disposed between, and configured to roll on, the interior peripheral surface of the first ring and the exterior peripheral surface of the second ring and rotatable therebetween, wherein: the first ring has a total number of variable-width cutouts (204A-F) and the second ring has a total number of variable-width cutouts (210A-D), with the total number of variable-width cutouts (210A-D) of the second ring being smaller than the total number of variable-width cutouts (204A-F) of the first ring, and a total number of the plurality of rollers (220A-E) being less than the total number of variable-width cutouts (204A-F) of the first ring and greater than the total number of variable-width cutouts (210A-D) of the second ring; as a given roller of the plurality of rollers (220A-E) traverses a variable-width cutout of either the variable-width cutouts (204A-F) of the first ring or the variable-width cutouts (210A-D) of the second ring, a radial distance between a center of the first ring and the given roller varies; each of the variable-width cutouts (204A-F; 210A-D) starts with a first width at a first end of the cutout, increases to a second width (d) larger than the first width at a center of the cutout, and narrows back to the first width at a second end of the cutout; and each roller (220A-E) is symmetric about its axis of rotation. the first ring has a total number of variable-width cutouts (204A-F) and the second ring has a total number of variable-width cutouts (210A-D), with the total number of variable-width cutouts (210A-D) of the second ring being smaller than the total number of variable-width cutouts (204A-F) of the first ring, and a total number of the plurality of rollers (220A-E) being less than the total number of variable-width cutouts (204A-F) of the first ring and greater than the total number of variable-width cutouts (210A-D) of the second ring; as a given roller of the plurality of rollers (220A-E) traverses a variable-width cutout of either the variable-width cutouts (204A-F) of the first ring or the variable-width cutouts (210A-D) of the second ring, a radial distance between a center of the first ring and the given roller varies; each of the variable-width cutouts (204A-F; 210A-D) starts with a first width at a first end of the cutout, increases to a second width (d) larger than the first width at a center of the cutout, and narrows back to the first width at a second end of the cutout; and each roller (220A-E) is symmetric about its axis of rotation.