A chuck is provided with a body, a plurality of jaws and an adjustment ring. The adjustment ring is threadably engaged with the jaws. When the adjustment ring is rotated in one direction, the jaws tighten by moving closer to each other. When the adjustment ring is rotated in the opposite direction, the jaws loosen by moving away from each other. An impact member is also provided for engagement with the adjustment ring. When the drive shaft of the power tool is rotated, the adjustment ring impacts against the impact member. As a result, the adjustment ring and the body of the chuck rotate relative to each other causing the jaws of the chuck to loosen or tighten depending on the direction the drive shaft is rotating.