TurbineD:
torsional load on the shaft and on the spline coupling comes form hot gases from burning process. Bearfoil calls it fluid drive but the fluid is still very compressable compared to any real fluid. This has significant smootering efect. The burning process creates vibrations but these are small compared to overall value of forces acting on a blade (and additionally dampened by the dovetail blade fixing). And than we have a relatively heavy disc where this vibrating loads from many blades are totalled into significantly smoother torque on the coupling