The pressure system, or pressure bars, is a mechanical system that presses the door very firmly into the door frame. This ensures the door can be locked tightly without any gaps that could allow explosives (such as nitroglycerine) to be poured in. It also helps seal the door against fire, smoke, and flooding. It can also mitigate problems from normal wear on the hinges that cause the door to sag, by pulling the door up against its taper and thus lift it upward. And when it comes time to open a tightly pressed-in door, the pressure bars can exert force to help break it free of the door frame.
Pressure systems can generate very great forces, many tons in most cases.
The system usually consists of one, two, three, or four bars across the door that rotate less than one turn. The bars are rotated by a large hand wheel and gears. Each bar has an offset pin that engages an anchor with a curved slot.
The pressure needs to be evenly applied across the door. Hence in the recent computer-generated stock image, the two bars on the left side of the door would not occur on a real-world door.
