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 . <EOS>