| 摘要 |
A ground piercing tool as according to the invention has a front head assembly mounted on a bit shaft. A mid-portion of the bit shaft is mounted between front and rear chambers. Compressed fluid is supplied to the front chamber to form a gas spring that prevents the bit shaft from impacting against a front stop when it receives a blow from the striker. A valve is provided that includes a passage that permits communication between the front chamber and the rear chamber when the bit shaft is in a forwardmost position wherein it contacts the front stop. The valve permits compressed air to pass from the front chamber to the rear chamber, negating the gas spring when the bit shaft is in the forwardmost position. |
