The name “Safety Pen” reflects that all retractable nib pens were designed primarily to allow for a good seal to prevent ink leakage. The nib retracts into the barrel. Inside the cap is a flat plug that tightly seals the end of the barrel when the cap is screwed down. The most common type of retractable nib safety pen is shown here. Waterman is quite well known for this, but it is seen from many makers. Since I am a Mabie Todd Swan fan, here is my Swan that utilizes this mechanism. Since Swan labelled its early screw top pens “safety pens”, indicating that there was in inner cap to prevent leakage, this Swan pen is further described on the barrel as “propelling pen” to indicate that it has a retractable nib.
The nib carrier has a cross pin at the base. The cross pin first engages a helical rod attached to the end knob of the pen. The ends of the cross pin then sit in linear grooves inside the barrel. Twisting the knob rotates the helical rod, which pushes the nib carrier forward or pulls it back.