DESCRIZIONE
A first guard member 34 and a second guard member 35 are detachably disposed to the boundary between the rear end surface and the outside surface of the coupling portion of a rotor 3. The first guard member 34 has a first guard portion 34a that covers the boundary between the rear end surface and the outside surface of the coupling portion 30a. The second guard member 35 has a second guard portion 35a that covers the boundary between the rear end surface and the outside surface of the coupling portion 30a at a position that opposes the first guard portion 34a.