71. Non-standard semi-circular keys are a type of key, with a flat upper surface and a semi-circular arc on the lower surface, the side surfaces being parallel. Commonly known as crescent keys.

It is essentially the same as a flat key connection, but it's easier to manufacture and assemble. It's particularly suitable for connecting conical shafts to hubs. The downside is that it reduces the shaft's strength, so it's typically used only in areas that experience less force.

71. The non-standard semi-circular key transmits torque through the side. The key can swing around the center of the arc curvature at the bottom of the shaft groove, making assembly easy. The keyway is deeper, resulting in greater reduction in shaft strength.