Inch Fasteners

The current American standard refers to inch threads for Unified fasteners, designated by the letters UN. Generally, each thread is associated with a coarse pitch (UNC) and a fine pitch (UNF). The pitch is defined by the number of threads per inch.

(*) it expresses the length up to which the hex head screws are fully threaded.

In European standards there is a standard for fully threaded hex head screws. It does not exist as such in inch screws. The only fully threaded screws are those with 1/4" thread diameter or smaller, from this size on, depending on the length of the screw, it will be fully threaded or not, so it must be taken into account for the design of parts joints.

In Europe there are 4 grades of carbon steels and 2 grades of stainless steels for fasteners.

For carbon steels, the former is not used for high strength fasteners and the latter is difficult to find, as it can suffer hydrogen embrittlement when the coating is applied, so the most used are the intermediate ones. In stainless steels, the A2 grade has the particularity of rusting in marine environments due to high salinity.

Equivalent materials for European and American fasteners are compared.

Various types of coatings are usually used for fasteners. In this case it is shared for European and American. The fastener coating will depend on the application and the environment in which it is installed.

For comparison of coatings, saline mist chamber tests are usually performed according to ASTM B-117, ISO 9227 and DIN 50051, which are equivalent.

Common coatings such as electrolytic zinc plating withstand less than 200 hours to the appearance of red rust, which is usually low for demanding applications. In these cases, surface treatments from the automotive industry such as Geomet B are used which resists about 1000 hours.

Another treatment that holds out against this number of hours is Dacromet but it is in disuse because of its hexavalent chromium content which is carcinogenic and is currently being replaced by other treatments with trivalent chromium such as Geomet.

Hot-dip galvanizing can be a good option for durability. If compared in salt spray chamber tests it is usually around 900 hours until the appearance of red rust. But the results can be misleading, as the zinc in the galvanizing alloys with oxygen from the environment repairing any small cracks. Salt spray chamber tests are performed without oxygen so, in a real application, hot dip galvanizing will tend to last longer than the tests say it will. In contrast, hot-dip galvanizing is a coating of several tens of microns so it can clog small metric threads. The standard ISO 10684 regulates hot-dip galvanizing for fasteners

Tests can be a good option to compare different surface treatments but the results of these tests are difficult to correlate with real application environments, so experience becomes the best ally to decide the fastener coating for our application.