A first clamping part and a second clamping part, which are movable relative to one another and to a base. A spring fixed between the first and second clamping parts. The spring applies force to the first and second clamping parts in a direction of the reciprocal movement thereof. A first locking part detachably locks the first and second clamping parts to each other when the spring is tensioned. A second locking part detachably locks the second clamping part to the base. The locking parts intermesh by forming a joint with limited motion.