Abstract: A hand-held power tool, e.g. a high speed file or polisher has a single rotary drive shaft (8) to which there is locked a first eccentric member (1) in which a second eccentric member (2) can be rotated and locked forming a first assembly (10). The second eccentric member has an eccentrically placed crank pin in the form of a hollow needle bearing ring (2a) through which the drive shaft passes. The outer ring (7) of the needle bearing reciprocally drives a connecting rod (1.2) to a tool holder (9). The drive shaft also passes through an identical assembly (10f) of first and second eccentric members (1?, 2?) disposed upside down on the first assembly (10) and reciprocally driving, in the opposite direction and in counterphase, a counterweight, to cancel the reactive oscillating forces from the tool holder during operation, which would otherwise have to be absorbed by the hand and aim of the operator.

Abstract: A hand-held power tool, e.g. a high speed file or polisher has a single rotary drive shaft (8) to which there is locked a first eccentric member (1) in which a second eccentric member (2) can be rotated and locked forming a first assembly (10). The second eccentric member has an eccentrically placed crank pin in the form of a hollow needle bearing ring (2a) through which the drive shaft passes. The outer ring (7) of the needle bearing reciprocally drives a connecting rod (1.2) to a tool holder (9). The drive shaft also passes through an identical assembly (10f) of first and second eccentric members (1?, 2?) disposed upside down on the first assembly (10) and reciprocally driving, in the opposite direction and in counterphase, a counterweight, to cancel the reactive oscillating forces from the tool holder during operation, which would otherwise have to be absorbed by the hand and aim of the operator.