3F/H25T Spline Servo to 1/4" Shaft Coupler (Set Screw)

This 25T shaft coupler is intended to mate a servo spline to a ¼” diameter shaft.  While it will accept a round shaft, the most common setup is to install a d-shaft so that the set-screw can apply pressure to the flat section.  The cupped point of the set-screw will actually start to burrow into the flat, creating excellent resistance to the shaft moving or the set-screw backing out.