1. Field of the Invention
The present invention relates to a cutter bit abrasion detecting device of a shield machine for detecting the abrasion of cutter bits provided on a cutter head.
2. Prior Art
A conventional shield machine for excavating a tunnel comprises a cutter head rotatably provided in front of a shield body and a plurality of cutter bits for excavating a working place which are radially arranged and mounted thereon.
When such a shield machine as set forth above excavates gravel or sandy beds, the cutter bits are rapidly worn away.
Accordingly in case of the conventional shield machine, the time for exchanging the cutter head is estimated beforehand based on an empirical formula corresponding to the soil to be excavated and the worn-out cutter bits are replaced with new ones from inside the shield body at the time of exchange during the excavation.
This method, however, has a drawback in that an error occurs between the actual time of exchange and the estimated one if the excavated soil is different from supposed one.
In order to improve such a drawback, an abrasion detecting device for detecting the abrasion of cutter bits using optical fibers is disclosed in the Japanese Utility Model Laid-Open Publication No. 60-51996. The abrasion detecting device set forth above detects the stop of signal caused by the breakage of the optical fibers due to the abrasion of cutter bits, wherein a plurality of optical fibers are folded back in a U shape and the tip end of the U-shaped optical fibers are inserted into a hole formed in one of the cutter bits. The optical fibers need to have a large radius of curvature at the folded back portion thereof in order to prevent them from being broken. As a result, the hole needs to have a large diameter to accommodate the U-shaped optical fibers therein, which lowers the strength of the cutter bit per se so that the cutter bit is liable to be broken in excavation.
Moreover, the conventional abrasion detecting device also has a drawback such that it cannot detect the abrasion of a shank to which the chip is fixed since the number of holes into which the optical fibers can be inserted in one cutter bit is limited due to the large diameter thereof.
Another cutter bit abrasion detecting device equipped with a reflector plate at the tip end of the optical fibers for reflecting light thereby is also proposed in order to improve such drawbacks.
The sensor head equipped with a reflector plate at the tip end of the optical fiber, however, has a drawback in that the amount of light S.sub.1 received thereby is remarkably reduced compared with the sensor head comprising a U-shaped optical fiber due to the attenuation of light as it is propagated in the air as illustrated in FIG. 11.
In FIG. 11, denoted at (a) is the amount of light S.sub.1 U received by the sensor head comprising the U-shaped optical fiber, (b) is the amount of light S.sub.1 H received by the sensor head equipped with the reflector plate and S.sub.2 represents the variation of light received by the sensor head equipped with the reflector plate as the sensor head is worn away.
When the cutter bit is worn away, the amount of light received by the sensor head varies according to the colors or ingredients of soil due to irregular reflection thereof in the working place.
That is, as illustrated in FIG. 11, the amount of light received by the sensor head increases according to the whiteness of soil, decreases according to the darkness of soil, increases when the soil contains much quartz or clay and decreases when the soil contains little quartz or clay.