The very best way to go would be a polygonal shaft - quote "Polygon precision, non-round shaft to hub mechanical connections are a direct replacement for applications that would otherwise use keyed or splined shaft to hub connections. They have been proven superior to splines and keys for High Speed, High Torque Drives and Close Fits. They have 35% greater torque with twice the shear strength. Polygons are Self-Centering and Self-Aligning connections with High Precision Fits. We provide advanced mechanical connections for demanding power and motion transmission."
There's no mention of a continually changing axial engagement, but it shouldn't be a problem. Industrial loads shift frequently, just not as much as say when Nick's drilling for oil.:Banane09:
The downside is that manufacturing these designs is not commonly practiced, and specific information regarding the geometric profile can be ~tricky~ to obtain. Here's a company that specializes:
http://www.generalpolygon.com/
See "What is a Polygon?" for more info.