The disclosure concerns a hydraulically driven impactor in which the operating cycle of a reciprocating piston is determined by the retraction velocity and displacement of the piston itself. Energy is conserved and heat dissipation minimised by the utilization of hydraulic spring accumulators to decelerate the piston. The drive to the piston is switched into a forward direction at such a point in each displacement cycle that the rebound energy of the piston is always re-absorbed into the drive fluid. The impactor operates at optimum acceleration and frequency at all times, whether or not impact occurs, and this feature combined with high energy conservation makes the impactor particularly suitable for use in a full-face tunnelling machine where high power density is required and individual impactors are often constrained to operate without impacting the rock face for considerable periods.
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