The standard door closer operates by using fluid in a rack-and-pinion spring-loaded piston mechanism. When the door is opened, the arm assembly rotates a geared spindle that moves a gear driven piston, compressing the main spring and forcing fluid into the area where the piston originally was. The result is the door closer builds up spring pressure as the door is opened. The farther the door is opened, the greater the amount of spring pressure. The location of the door closer and arm assembly determine the maximum opening.
NOTE: Most door closers are pre-loaded during installation, so the closer exerts spring pressure even when the door is closed. This way there is sufficient pressure to close and latch the door even though the arm assembly cannot rotate any further.
When the opened door is released, the built up spring pressure moves the gear-driven piston, rotating the geared spindle and causing the door to swing back towards the closed position. At the door open position, the fluid is on the side of the closer cylinder where the piston has moved out of and the expanding spring pressure wants to return the piston. When the door is closed, the spring has expanded, the piston is at the rest position and the fluid is on the spring side of the closer body. When there is sufficient spring pressure, the door closes and the lock's latching mechanism engages, securing the door.
The fluid in the door closer is designed to control the speed at which the closer arm assembly rotates during opening and closing. Without the fluid, the spring would expand rapidly and the door would slam. There are several types of door closer fluid. Most door closer fluids have the consistency of lightweight oil. Depending upon the manufacturer and the application, door closer fluids are available for extreme weather conditions, and fluid types can vary by the function of the door closer.
In a closer where the door is closed, almost all of the fluid is in the area containing the main spring. As the door is opened, this spring compresses and the fluid is forced around the piston through two or more portholes into the other side of the piston. The adjustable valves control the flow of the fluid. Depending upon the door closer; there can be a closing valve, latching valve, backcheck valve, delayed action valve, etc. The location of each channel's openings and the valve determines the functionality, as the piston moves from the closed to the open and back to the closed position.
Each valve controls the rate at which the fluid flows from one side of the piston to the other. The fluid flow rate controls the rate of spring expansion. Controlling the fluid flow permits the closer to operate smoothly.
The functionality of the modern rack- and-pinion type of door closer operates on average at approximately two-thirds efficiency. This means that for every three pounds force exerted on a theoretically frictionless door, only about two pounds of force affects the opening and closing. In the real world, when we add in butt hinges and door alignment friction, rack and pinion door closer efficiency drops even more.
The reason that the efficiency of the door closer is important is that federal and state laws determine the maximum force permitted for operating a door. Years ago, there was a federal law of eight pounds for operating an exterior door. The federal law was struck down; however, a number of states had and still have their own accessibility laws. Most require a minimum of eight pounds.
California Building Code (CBC) 2001 and 2007 in section 1133B.2.5, can be read as the effort to operate an exterior door shall not exceed 5 pounds. However, this code section does not appear to address an exterior entrance where multiple door leafs are provided at one location. Check with your local authority having jurisdiction (AHJ) before installing a door closers on an exterior door.