PURPOSE: To prevent a rotor body from being damaged and at the same time to make inspection work of a wear point unnecessary by properly detecting the timing to replace a chip plate due to the wear proceeding to a specified degree or to repair a rotor due to the hardening of the rotor' s circumference and to repair the cladding parts by welding.

CONSTITUTION: Countless small holes 29 are formed in the circumferential board 27 of a chamber 28 which is prepared as a closed chamber and in which compressed air is sealed and a hardened type cladding part by welding 30 is formed in the outside. On the other hand, a vertical hole which is communicated with the chamber 28 is formed in a chip which is attached to the body in a freely detachable way, wherein the main body composes an installation seat of a chip plate and thus the pressure of the chamber 28 decreases when the holes are opened due to the wear of the hardened cladding part 30 by welding or the chip. Also, an axial hole which is communicated with the chamber 28 is formed in the axial center of a driving shaft of a rotor and works in the case the pressure of the compressed air of the cylinder installed in the shaft end lowers and an adjacent cylinder detects the lowering and generates alarming sound.