| 摘要 |
Three air hammers are housed and arranged in a hammer case, and have bits at their respective ends. Each of the air hammers can be activated independently. In a normal excavation, the earth is excavated by activating all the air hammers. When correcting an excavating direction, only an air hammer positioned in the correct direction is activated first so that the earth in the correct direction is excavated in a predetermined amount. Next, all the air hammers are activated to excavate the earth. Thereby, a leading hammer can direct the excavation toward the correct direction, and thus the excavating direction is corrected.
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