Rotary percussive coal drill with hydraulic motor, piston, rod controlling cam. percussive motor and a system of ports coupled to pump and offtake respectively and with hydraulic accumulators for feed and bleed lines. The rotary motor consists of a ring whose inside orifice is mounted on the cylindrical sleeve. The control coupling is connected to the piston through an adjustable connector and adjustable connections are provided between the motor and the piston. A stepped dia. axial bore is pref. fitted into the piston with the smallest dia. at the rod end, and the bore links the space between piston and connector, and similarly the inside of the motor through a channel to space. This is at the far end of the piston within the cylinder. Pref. a nonreturn valve in the channel between the space and the bleed line via accumulator permits flow from this space to the bleed line as required.
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