(57) A fluid operated hammer comprising a cylindrical casing (11) having a bore for slidably
receiving a piston (26), a top sub (12) mounted at one end, a drill bit (13) mounted
to the other end by means of a chuck (14), a feed tube (20) concentrically mounted
within the upper end of the bore of the casing and extending downwardly into the casing,
said feed tube being connectible to a fluid source through said top sub. The piston
is slidably mounted within the bore of said casing over said feed tube (20). The feed
tube (20) is provided with at least one aperture (24) in the walls of said feed tube,
and at least one port (27, 29) provided at each end of the piston. Each port opens
at spaced locations into the bore of the piston to periodically communicate with said
aperture (24) as a result of sliding movement of the piston over the feed tube to
alternately drive the piston towards and away from the drill bitto impact thereon
and to drive the piston away from the drill bit. A further aperture (40) is provided
in said feed tube (20) to communicate with the space between said drill bit (13) and
said piston (26) when said hammer is in the "blow-down" position. The drill bit (13)
is mounted in the chuck (14) in such manner that when in the "blow-down" position
there is communication between said space and the exterior of the hammer through the
zone between the drill bit and the chuck.
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