Grades 6–8 · Machines
How does a door closer pull a door shut without slamming?
The small box above a self-closing door is doing two jobs. Opening the door moves an arm and loads a spring inside the box. When the door is released, the spring returns energy through the arm and pulls the door toward its closed position.
In the diagramThe spring closes; flowing oil slowsA closing-stroke cutaway and a separate spring-force exampleDoorSmall oil passageSpringPistonClosing direction: left20 mm compression × 1,000 N/m = 20 N.
A spring alone could return that energy too quickly. In a hydraulic closer, the motion also drives a piston through oil. During closing, displaced oil must flow through a restricted passage. The resistance to that flow opposes the motion and turns mechanical energy into thermal energy.
The spring supplies the closing force; the oil regulates how quickly the mechanism moves. For a simple spring example with stiffness 1,000 newtons per meter, compressions of 10, 20 and 30 millimeters correspond to forces of 10, 20 and 30 newtons. A real closer adds linkages, valves and changing leverage, so those numbers do not specify the force at a door handle or its closing time.
About this explanation
Starting knowledge: Understand force, millimeter-to-meter conversion and stored spring energy.
Sources
- ASSA ABLOY — Door Closer catalog: Operation of Door Closers
- ASSA ABLOY — Door Closer: spring, checking mechanism and drive