What general part of the breathing does an iron lung replace?

Iron lung=muscles: An iron lung is a device (negative pressure ventilator)you lie in up to your neck. Air is pumped in and out around you, so your lungs expand and contract; it breathes for you. It replaces the mechanical part of breathing that your muscles perform, so it was used for polio patients. It doesn't help if you can't exchange gases efficiently. You need oxygen therapy for that.

Weakness: An iron lung provides negative pressure to the chest wall to help move it if you are too weak or sick to do so on your own. They are hardly ever used anymore.