A drill chuck has a body carrying jaws displaceable between inner and outer positions and an adjustment ring operatively connected to the jaws for movement from outer to inner positions on rotation of the adjustment ring in a tightening direction. This adjustment ring has teeth engageable by an antiloosening detent and is urged by a spring into the teeth. The antiloosening detent is angled relative to the adjustment ring such that when engaging the teeth it blocks rotation of the adjustment ring opposite to the tightening direction and permits rotation of the adjustment ring in the tightening direction. An antitightening detent engageable with the teeth of the adjustment ring is urged by a spring outward into the teeth and is angled relative to the adjustment ring such that when engaging the teeth it blocks rotation of the adjustment ring in the tightening direction and permitting rotation of the adjustment ring opposite to the tightening direction. A lockout element engageable with both of the detents can move between a holdback position holding the detents out of engagement with the adjustment ring and a freeing position permitting the detents to engage with and lock in the respective directions on the adjustment ring. A guide surrounds the antitightening detent and has a stop against which the antitightening detent is urged by a side spring so that when the antitightening detent is engaged with the teeth it can move in the tightening direction with the adjustment ring only until it engages its stop.
