In a hammer drill and/or percussion hammer 1 having an adjustable percussion mechanism eccentric clutch, the total eccentricity of the percussion mechanism drive element 13 is varied by axial movement of an inner eccentric part 20 relative to an outer eccentric part 21 which is held axially immovable on a shaft 4. Rotation of shaft 4 causes arm 14 of a driver element 11 to move up and down this causing the driver element to pivot about axis 12 and thereby cause the drive lever (15 Fig. 2) to move the striker 6. The percussion mechanism clutch having this configuration has the advantage that the pressure force to be applied by the operator for the purpose of bringing the percussion mechanism into action is kept low and the no-load behaviour of the machine is improved. <IMAGE>
