| 摘要 |
A reverse circulation down-the-hole drill includes an outer wear sleeve (1) and a backhead (2) assembly located at one end of the outer wear sleeve (1) for connecting the drill to a double-walled drill string and to a source of pressure fluid to actuate the drill. A fluid diverter (5) is mounted inside the outer wear sleeve (1) adjacent to the backhead (2). An inner tube (9), concentric with the outer wear sleeve (1) and extending into the fluid diverter (5), defines at least part of a central return passageway in the drill. A bit (10), located by a chuck (3) at the other end of the outer wear sleeve (1), is slidably mounted on the inner tube (9) in an annular chamber (28, 30) defined by the outer wear sleeve, by the inner tube, by the diverter at one end (28), and by the bit at the other end (30). An inner cylinder (6) mounted inside the outer wear sleeve (1) towards said one end (28) of the chamber adjacent to the diverter defines a fluid communication passage (8) between the diverter and the chamber via porting elements (26, 27). A piston (14) is slidably disposed in the annular chamber (28, 30) with respect to the inner cylinder (6) so as to co-operate with the porting elements (26, 27). The piston is mounted over the inner tube (9) to reciprocate within the annular chamber and repeatedly strike the bit (10). Improved performance is provided, wherein the face (18) of the piston (14) adapted to strike the bit (10) is defined by a reduced diameter front end portion of the piston. The piston (14) defines a shoulder (29) facing said one end (28) of the chamber opposite the strike face of the piston to which a substantially continuous supply of pressure fluid is applied via the porting elements (26, 27). The inner cylinder (6) extends only part-way into the annular chamber. The porting elements (26, 27) are adapted to apply pressure fluid to the piston so as to maintain a force on the shoulder (29) in the direction of the bit (10) of approximately 30 to 45% of the lift force on the lift face 32 during the lift stroke.
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