| 摘要 |
The invention provides a down-the-hole pneumatic hammer drill assembly. A backhead is secured to a wear sleeve for an impact piston and a drill bit is mounted in a chuck in the wear sleeve for limited reciprocation remote from the backhead and providing an anvil for the piston. The piston has a stem of reduced diameter guided in a piston stem bush for impacting the bit. An exhaust flow passage is provided passed the piston stem bush into a bore through the bit to vent air from a chamber around the pisto n stem when the bit is lifted from a working surface. The exhaust flow passage is either provided through the piston stem bush or by a recess partway along the piston stem. T he assembly also has an annular passage with spaced apart ports communicating with an air supply passage through the backhead at one end and with the interior of the wear sleeve at the other end. In one embodiment, the annular passage extends between the outer surface of a part of the wear sleeve having a reduced diameter and the inner surface of a shroud radially spaced from the surface of reduced diameter and secured to the wear sleeve. The annular passage is alternatively provided between th e inner surface of part of the wear sleeve having an increased diameter and the oute r surface of a skirt depending from the backhead with the skirt screw-threaded to enga ge within a screw-thread at the end of the wear sleeve for attachment of the backhead to the wear sleeve.
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