Fluid: Oil or any suitable fluid (Hydraulic or pneumatic)
Piston: Piston with holes
Outer cylinder is used for excess oil. By changing piston & hole diameter we can use this design concept in various place in many doors.
Spring is mainly used to auto-returning purpose.
If spring has more stiffness, definitely operation of the doors will be critical.
So to avoid that, spring material must have less stiffness and the diameter of the cross-section should be little.
Spring is used to store & release the energy given by the operator.
By using spring and damper together, the wind speed is restricted within a limit corresponding to the piston hole diameter & spring stiffness.