US Patent 7451833 Vibration dampening mechanism

A hammer drill includes a body, a motor; a centre of gravity, and a hammer mechanism driven by the motor in reciprocating movement along a hammer axis at a first distance from the centre of gravity. A counter mass is mounted within the body for sliding movement along a slide axis at a second further distance from the centre of gravity. A biasing member biases the counter mass to a mid-position along the slide axis. The biasing means may be a leaf spring or a helical spring. The counter mass may be slideably supported on rods and may be able to twist about a number of axes.