An Engineer’s Primer on the Fastener Component

At a glance:

Fastener types and materials—from steel bolts to plastic rivets
Fastener durability under common stresses
Fastener standards and the evolving “right to repair” market

When the goal is to join materials, options are seemingly endless: welding, crimping, gluing cementing and so on. Fasteners represent the tool option.

Simply put, a fastener is a mechanical device that holds or joins materials together. Fasteners can be broadly categorized as permanent or non-permanent.

A permanent fastener, such as a nail or rivet, is designed for secure single use. These fasteners require force to remove, potentially damaging the materials they have joined.

A non-permanent fastener, such as a screw or bolt, is designed for reusability. Non-permanent fasteners should be easily removed without causing damage to the hardware or joined material.

Common Fastener Types

Drilling down further, fasteners are often grouped according to the presence of threads. While non-permanent fasteners generally have threads and permanent fasteners are typically non-threaded, there are notable exceptions.

Common Non-Threaded Fasteners

Nails. A nail consists of a head, a smooth shank and a point. Nails are designed to be driven into material easily with a hammer or nail gun.
Rivets. Durable and designed for axial loads, rivets have a head, shank and tail. The force used during installation of a rivet expands the tail, creating a shop head that clamps the materials together. Types include solid rivets, semi-tubular rivets and split rivets.
Pins. With a range of styles and sizes, pins are typically designed to fit into premade holes and require friction or tension to keep them in place. Pins are an example of a non-threaded fastener that is made for reuse.

Common Threaded Fasteners

Bolts. A bolt has a head on one end and is threaded on the other end. Once fed through the material it is joining, a nut with internal threads secures the bolt in place. Bolts are further categorized according to their shape and the type of head.
Screws. A screw has a head and a threaded end and requires internal threading in the material it is driven into. Most screws are self-threading, which means they create their own threads as they are driven in.
Studs. A stud is headless, threaded on both ends and joins material typically by using nuts on either end.

When choosing a fastener type, keep in mind load, clamping force and securing and tightening methods.

Fastener Material

The right fastener material for the job comes down to environmental conditions, weight, magnetic properties, stresses, reusability and longevity.

Opting for standard materials—typically steel—is generally the cheapest route. Heat treating, cold working or coating can add strength or improve the finish. Find a glossary of fastener platings and coatings here.

Steel represents an estimated 90% of the fastener market. Steel is strong, workable and inexpensive. It is available as low carbon, medium carbon or an alloy such as stainless steel. The chromium in stainless steel creates an oxidation-resistant film that self-repairs in the presence of oxygen, which makes the material ideal for corrosive conditions.

Aluminum is a lighter, weaker and often cheaper material option. Aluminum is also corrosion resistant and can be hardened by alloying. It weighs about a third as much as steel and some grades have similar strength to mild steel.

Other less common fastener materials include brass, copper, nickel, Monel (nickel copper alloy), Inconel (nickel chromium alloy), titanium, beryllium and plastic. The latter—typically nylon, PVC, polypropylene and polycarbonate—is ideal as a lightweight, cost-effective and non-rusting option.

More details about fastener materials can be found here.

Factors Affecting Fastener Durability

All fasteners face similar stresses that affect their durability—a key performance factor.

For example, moisture combined with elements such as sulfur, sodium and chlorine can form acid that will lead to corrosion on fastener surfaces, causing failure. Fasteners with recessed drives can be damaged or can accumulate debris, both of which can take them out of use. And threaded fasteners can seize from friction, heat or corrosion.

Other stresses include vibration, metallurgical fatigue, UV damage and thread wear. More details on durability factors are highlighted in James A. Speck’s article “Making Fasteners Reliable and Reusable.”

Ultimately, most premature fastener failures are the result of misuse rather than poor quality. Users need clear instructions to avoid failures that result from misuse, such as over or under tightening, overloading, improper material and improper installation.

Fastener Standards

With fasteners used worldwide, a plethora of industry standards are in place to encourage uniformity and, ultimately, safety. The standards cover, at minimum, terminology, specifications and dimensions.

Organizations that govern fastener standards include ASME, ISO, SAE International, ANSI, DIN and ASTM International.

While fastener manufacturers are not required to make standard products, some countries have laws governing the use of standard-compliant fasteners. In 1990, the U.S. enacted the Fastener Quality Act in response to high-profile fastener failures in the 1980s. The Act requires certain fasteners to be accredited as conforming to standard specifications.

Evolving Market

With increasing consumer pressure for sustainable products, a growing number of countries and states are adopting “right to repair” legislation. In general, these laws are designed to make it easier for consumers and independent repair technicians to fix broken products.

Non-permanent fasteners are an ideal design option for products where welding, gluing or cementing failures render them useless. The movement may also inspire manufacturers to design fasteners that meet more rigorous requirements, such as a recent one-piece sealing screw design for the automotive industry.