Abstract: The present invention relates to an improved mechanism for clamping means comprising a pair of split nuts resiliently retained in place inside a hollow cylindrical body by spring means, which mechanism can either slidably hold the screw spindle to allow the latter to be rapidly pushed to its working position or retracted to dismount the workpiece when the threads of split nuts are not in mesh with the screw spindle, or offer a snug, complete engagement between the split nuts and screw spindle in which the load is equally distributed, thereby largely reducing the wear thereof, and allowing the device to be fabricated relatively small in size.