Hammer drive mechanism

A hammer drive mechanism is provided for converting rotary drive from a motor to reciprocatory movement of an impact member of a hammer drill. The mechanism comprises a rotatable plate adapted to be rotated by the motor, an input drive member associated with the rotatable plate in an eccentric position with respect to the axis of rotation of the rotatable plate, an output drive member associated with the impact member, and a crank shaft having a respective driver adjacent each of its ends. Each driver engages with, and is complementary to a respective one of the drive members. At least one end portion of the crank shaft comprises a lubricating aperture which opens into the adjacent driver to provide a lubrication path to the engaging surfaces of the drivers and the drive members.