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Tightening torque, or torque value, is the force used to tighten a screw so that the joint stays securely fastened without overtightening or causing damage. The correct torque depends on the screw’s material, size, threads, lubrication, and the coating used (e.g. zinc flake coating).

When carrying out maintenance repairs, screw fasteners are generally replaced with new ones. In such cases, it is important to understand the fastener markings and follow them.

The atmospheric exposure class determines the conditions in which a screw fastener should be used to ensure the best possible durability of both the fastener and the screwed joint. There are six classes, defined in accordance with the SFS-EN ISO 12944-2 standard, covering conditions from dry indoor spaces to harsh marine climates and industrial environments.

Fasteners made of stainless steel and acid-resistant steel can seize, especially during rapid machine tightening, which can make them difficult to remove. Seizing can be prevented by using lubricants or waxing the fasteners before use.

A bolted joint can be tightened either by turning the nut or by stretching the bolt. The most common method is turning the nut, which subjects the joint to both tensile stress and torsional stress, affecting the bolt’s load-bearing capacity. Reducing the coefficient of friction improves axial tightening performance.

In a bolted joint, the nut or screw must be secured after pre-tightening so that the joint cannot loosen when the parts move relative to each other.

The properties of the materials used in fasteners give them special characteristics. The requirements of the chemical and offshore industries place specific demands on fasteners, so the materials used in them must meet these requirements.

Flange and non-flange hex nuts differ in that on a flange nut, the flange acts as an integrated washer, which reduces friction during tightening. DIN 6932 nuts are quick to install and remove, and they provide an unobstructed solution in some applications.

In Finland, the fastener market mainly uses metric threads, but inch threads have been used especially in Swedish-speaking regions and in machinery imported from Sweden. Since the 2000s, the use of inch threads has decreased significantly, and today they are mainly found in machines manufactured in the UK and the USA.