A driveshaft assembly, comprising: a driveshaft including a longitudinal shaft axis, a first end, a second end opposite the first end, and a radially outer surface, wherein the first end includes a plurality of recesses extending radially inward from the radially outer surface, the recesses each comprising a planar engagement surface; a first end housing including a longitudinal housing axis, and an axially extending receptacle, wherein the receptacle includes a plurality of planar receptacle surfaces; and a torque transfer assembly configured to transfer torque between the driveshaft and the first end housing, the torque transfer assembly including: a plurality of torque transfer keys each including a planar key surface and a convex key surface; and an adapter including a plurality of concave adapter surfaces and a plurality of planar adapter surfaces; wherein: the planar engagement surface of the each recess engages the planar key surface of one of the torque transfer keys; the convex key surface of each torque transfer key directly and slidably engages one of the concave adapter surfaces of the adapter; and each of the planar adapter surfaces of the adapter engage with one of the planar receptacle surfaces; characterized in that each convex key surface is a cylindrical surface concentrically disposed about an axis of curvature, wherein the axis of curvature of the cylindrical surface of the convex key surface is aligned with a first pivot axis oriented orthogonal to the longitudinal shaft axis, wherein the driveshaft is configured to pivot about the first pivot axis relative to the first end housing. a driveshaft including a longitudinal shaft axis, a first end, a second end opposite the first end, and a radially outer surface, wherein the first end includes a plurality of recesses extending radially inward from the radially outer surface, the recesses each comprising a planar engagement surface; a first end housing including a longitudinal housing axis, and an axially extending receptacle, wherein the receptacle includes a plurality of planar receptacle surfaces; and a torque transfer assembly configured to transfer torque between the driveshaft and the first end housing, the torque transfer assembly including: a plurality of torque transfer keys each including a planar key surface and a convex key surface; and an adapter including a plurality of concave adapter surfaces and a plurality of planar adapter surfaces; a plurality of torque transfer keys each including a planar key surface and a convex key surface; and an adapter including a plurality of concave adapter surfaces and a plurality of planar adapter surfaces; wherein: the planar engagement surface of the each recess engages the planar key surface of one of the torque transfer keys; the convex key surface of each torque transfer key directly and slidably engages one of the concave adapter surfaces of the adapter; and each of the planar adapter surfaces of the adapter engage with one of the planar receptacle surfaces; the planar engagement surface of the each recess engages the planar key surface of one of the torque transfer keys; the convex key surface of each torque transfer key directly and slidably engages one of the concave adapter surfaces of the adapter; and each of the planar adapter surfaces of the adapter engage with one of the planar receptacle surfaces; characterized in that each convex key surface is a cylindrical surface concentrically disposed about an axis of curvature, wherein the axis of curvature of the cylindrical surface of the convex key surface is aligned with a first pivot axis oriented orthogonal to the longitudinal shaft axis, wherein the driveshaft is configured to pivot about the first pivot axis relative to the first end housing.