Abstract: A spline coupling comprises a male spline having a plurality of spline teeth separated by keyways. An insert ring comprises a plurality of U-shaped segments each having a pair of side walls separated by a gap and interconnected by a proximal end wall. Each U-shaped segment is configured to be received in one of the keyways of the male spline with the side walls being against surfaces of the keyway and the gap leaving a portion of the keyway exposed. Each U-shaped segment is connected to adjacent U-shaped segments by a distal end wall to leave a portion of the spline teeth exposed between each pair of adjacent U-shaped segments. A female spline has a shape complementary to that of a combination of the male spline and insert ring for coupling engagement therebetween.