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.本發明可容易更換地保護連結部之外側面與後端面之邊界部分。於轉子3之連結部之後端面與外側面之邊界部分，分別可裝卸地設置第1防護構件34及第2防護構件35。第1防護構件34具有覆蓋連結部30a之後端面與外側面之邊界部分之第1防護部34a。第2防護構件35具有於與第1防護部34a對向之位置覆蓋連結部30a之後端面與外側面之邊界部分之第2防護部35a。
