The Door Chain Claim Examined: Why a Chain Assembly Cannot Hold What Its Packaging Suggests

Door chains are sometimes sold with language suggesting they withstand hundreds of kilograms of force. This study examines a claim of 750 kg, about 7,350 N, by computing the strength of each element of a typical chain assembly from stated assumptions: the chain itself fails in tension at 4,200 N, its keep plate bends at 1,800 N, and the anchor plate's four small screws, embedded 16 mm into the edge grain of a timber door, pull out as a system at about 676 N. An assembly fails at its weakest element, so the fitted chain resists about 676 N, which is 9.2 percent of the claimed figure. Even doubling the screw embedment to 32 mm raises the assembly only to 1351 N, at which point the bending plate governs. The claim is therefore not sustained for the assembly as fitted, whatever the chain link alone may hold on a test machine. A door chain should be understood as a device for identifying a caller while the door is ajar, not as a forced-entry barrier, and households relying on it for resistance should redirect the same money to the strike and frame. SIMULATED, NOT MEASURED.