A mechanical fastener is any hardware device that holds two or more parts together using physical force rather than chemical bonding or heat. Bolts, screws, nails, rivets, clips, and clamps all fall under this umbrella. They work by creating clamping force, friction, or interference between components to keep an assembly locked in position. Mechanical fasteners are the most common joining method in manufacturing, construction, and repair because they can be installed with simple tools and, in most cases, removed without destroying the parts they connect.
How Mechanical Fasteners Work
At their core, mechanical fasteners use force and deflection to align and secure parts within a system. When you tighten a bolt, for example, stretching the bolt shank creates tension that presses the joined pieces together. That clamping force is what actually holds the joint tight, not the threads themselves. Rivets work differently: a rivet is deformed during installation so it expands and locks into place, creating a permanent grip.
This distinction between removable and permanent fasteners is one of the most important in engineering. Threaded fasteners like bolts and screws enable assembly and disassembly without damaging the parts. Rivets, press-fit pins, and certain clips are permanent: removing them means destroying the fastener or the surrounding material. Your choice between the two depends on whether the joint will ever need to come apart for maintenance, inspection, or repair.
Types of Threaded Fasteners
Threaded fasteners are the largest and most versatile family. The three you’ll encounter most often are bolts, screws, and studs, and the differences between them matter for selecting the right one.
- Bolts pass through a clearance hole in the parts being joined and are secured on the other side with a nut. The nut and bolt head squeeze the parts together. Bolts are ideal when you need a strong, removable joint and have access to both sides of the assembly.
- Screws create or engage threads directly in the material they enter. A wood screw cuts its own threads into lumber; a machine screw threads into a pre-tapped hole in metal. Because screws don’t need a nut, they work well when you can only access one side of the joint.
- Studs are threaded on both ends with no head. One end threads into a component permanently, and the other end accepts a nut. Studs are common in engines and flanged pipe connections where repeated disassembly is expected but the base component shouldn’t be disturbed.
Beyond these three, the threaded family includes set screws (which lock one part against another using only compression at the tip), eye bolts (with a loop for attaching cables or hooks), and U-bolts (shaped to clamp around pipes or tubes).
Non-Threaded Fasteners
Not every fastener relies on threads. Rivets are among the oldest and most reliable non-threaded options. A rivet is a smooth shaft with a head on one end. During installation, the opposite end is deformed (bucked or pulled) to create a second head, permanently locking the parts together. You’ll find rivets throughout aircraft fuselages, bridges, and older steel structures. Their limitation is permanence: you can’t remove a rivet without drilling it out.
Pins serve a different purpose. Dowel pins align parts with high precision, while cotter pins and split pins prevent nuts or clevis pins from backing out. Spring clips, retaining rings, and snap rings fit into grooves to hold components on shafts or inside housings. These small fasteners often go unnoticed, but they’re critical in everything from bicycle wheels to industrial machinery.
Fasteners vs. Welding and Adhesives
Mechanical fasteners aren’t the only way to join parts. Welding fuses materials together with heat, and adhesive bonding uses chemical compounds to create a bond. Each method has trade-offs that determine when fasteners are the better choice.
The biggest advantage of threaded fasteners is disassembly. A bolted flange can be taken apart for inspection, cleaned, and reassembled. A welded or adhesive joint is permanent. If something fails in a welded joint, repair typically means grinding out the old weld and starting over. Adhesive joints rate even worse for repairability. Fasteners also concentrate stress at known, predictable points, which makes engineering calculations more straightforward. And unlike welding, fasteners don’t introduce heat that can warp thin materials or weaken heat-sensitive alloys.
Welding and adhesives have their own strengths, though. Welding distributes load across the entire joint rather than through a few bolt holes, which can be an advantage in structural applications. Adhesives bond dissimilar materials (like metal to plastic) cleanly and spread stress across a large surface area. In practice, many assemblies combine methods: an automotive body panel might be both bonded with adhesive and fastened with rivets for extra security.
Why Fasteners Fail
Fatigue is the most common failure mode for bolts and screws. Every time a joint experiences loading and unloading (vibration from an engine, wind loads on a structure, thermal expansion and contraction), the fastener goes through a small stress cycle. Over thousands or millions of cycles, microscopic cracks form and gradually grow until the fastener breaks. Fatigue failures often trace back to installation problems: a bolt that wasn’t tightened enough, or one that lost its clamping force over time due to settling or vibration loosening.
High-strength bolts face an additional risk called hydrogen embrittlement. Despite the name, the bolt doesn’t become brittle in the traditional sense. Instead, hydrogen atoms trapped inside the metal cause cracks to develop slowly over days or weeks while the bolt is under tension. This is most common in zinc- or cadmium-plated bolts above a certain hardness threshold. The hydrogen can be introduced during manufacturing (specifically during the plating process) or during service through certain types of corrosion. Proper baking after plating drives out trapped hydrogen, but if that step is skipped or insufficient, delayed cracking can occur well after installation.
Other common failures include shear overload (when sideways forces exceed the bolt’s capacity), thread stripping (when the threads in the mating material tear out instead of holding), and corrosion that gradually eats away at the fastener’s cross-section.
Fasteners in Demanding Environments
Not all fasteners are created equal. In aerospace, fasteners must perform under extreme vibration, rapid temperature swings, and enormous mechanical loads. Every pin, collar, and threaded fastener in an aircraft is engineered to meet strict performance standards, because a single loose bolt at 35,000 feet is not an acceptable risk. Aerospace fasteners are also designed for maintainability in tight, hard-to-reach spaces, since aircraft require frequent inspection cycles.
Automotive engines present a similar challenge on a smaller scale. Cylinder head bolts, connecting rod bolts, and exhaust manifold studs all operate in high-heat, high-vibration conditions. These fasteners are typically made from alloy steels with specific strength grades and require precise tightening procedures (often involving torque plus a specified angle of rotation) to achieve the correct clamping force.
In marine and chemical processing environments, corrosion resistance becomes the primary concern. Stainless steel, titanium, and specialty nickel alloys replace standard carbon steel fasteners to survive constant exposure to saltwater, acids, or caustic chemicals.
Materials and Strength Grades
Fasteners come in a wide range of materials, each chosen for a combination of strength, corrosion resistance, weight, and cost. Carbon steel is the most common and least expensive, suitable for general construction and machinery. It’s usually plated with zinc or another coating to resist rust. Stainless steel offers much better corrosion resistance at a higher price. Brass and bronze fasteners show up in marine and electrical applications. Aluminum fasteners save weight in aerospace and consumer electronics. Titanium combines light weight with extreme strength and corrosion resistance, but at a premium cost that limits it to aerospace and medical applications.
Strength grades tell you how much load a fastener can handle. In the inch-based system used in the United States, you’ll see grades like Grade 2 (low-strength, general purpose), Grade 5 (medium-strength, the most common in automotive and machinery), and Grade 8 (high-strength, used in demanding structural applications). The metric system uses a property class numbering system: 8.8 is roughly equivalent to Grade 5, and 10.9 is comparable to Grade 8. The markings are stamped on the bolt head, so identifying the grade of an installed fastener is straightforward if you know what to look for.