Mechanical fasteners are hardware devices that hold two or more parts together using physical force rather than chemical bonding like adhesives or welding. They range from simple pins and rivets to precisely engineered bolts rated for thousands of pounds of tensile load. Nearly every manufactured product uses them, from furniture and electronics to bridges and aircraft, because they create reliable joints that can often be inspected, tightened, or replaced without destroying the surrounding structure.
Threaded vs. Non-Threaded Fasteners
The broadest way to sort mechanical fasteners is by whether they have threads, the helical ridges that spiral around a cylinder and allow it to grip material or mate with another part.
Threaded fasteners include bolts, screws, nuts, and studs. A bolt is a cylindrical fastener with a head on one end that typically passes through a hole and mates with a nut on the other side. A screw either fits into a pre-tapped (pre-threaded) hole or cuts its own threads as it’s driven into material. Nuts are the internally threaded counterparts that clamp down against a bolt. Set screws, which are usually headless, lock a component like a pulley or gear onto a shaft.
Non-threaded fasteners skip threads entirely and rely on friction, interference fit, or mechanical locking. The most common types are pins, keys, retaining rings, and rivets. Each solves a different problem: pins align and secure parts, keys prevent rotation on a shaft, retaining rings limit movement along an axis, and rivets create strong permanent joints in sheet material.
Temporary and Permanent Joints
Fasteners also split into two functional camps: temporary (removable) and permanent.
Temporary fasteners, primarily bolts and screws, are designed so you can take a joint apart for repairs, upgrades, or inspection without damaging the parts. That removability is their biggest advantage and their biggest vulnerability. Vibration, thermal cycling, and shifting loads can gradually loosen a threaded connection over time. Locking washers, nylon-insert nuts, and thread-locking compounds exist specifically to counteract that loosening.
Permanent fasteners are meant to stay put for the life of the assembly. Rivets are the classic example. They have high tensile and shear strength, install quickly, resist vibration naturally, and work especially well with sheet metal because their low profile matches thin material. Unlike welding, rivets don’t warp the surrounding metal or create new corrosion points from heat exposure. The trade-off is that removing a rivet means drilling it out and replacing it.
Some fasteners blur the line. Panel fasteners, for instance, permanently attach a threaded insert to one component so a mating panel can be removed and reinstalled repeatedly without loose hardware. Clevis pins secured with cotter pins are technically removable but require tools, making them semi-permanent in practice.
Common Non-Threaded Types
Pins
Pins are thin, cylindrical fasteners that hold parts in alignment or prevent movement. Semi-permanent pins, such as dowel pins, taper pins, spring pins, and shear pins, require a press fit or tools for installation and removal. Quick-release pins have a built-in mechanism (often a spring-loaded ball or detent) that lets you insert and remove them by hand, which is useful for jigs, fixtures, and equipment that needs frequent reconfiguration.
Keys
Keys are small metal blocks that fit into matching slots (called keyways) cut into a shaft and the hub of a component like a gear or pulley. They prevent the component from spinning freely on the shaft. Square keys have flat, parallel sides. Woodruff keys have a semicircular bottom that seats into a rounded pocket in the shaft, which makes them self-aligning but slightly weaker. Gib-head keys have a protruding head so you can pry them out for removal.
Retaining Rings
Retaining rings, also called snap rings, are flat circular clips with an opening on one side. They snap into a groove on a shaft or inside a bore to act as a shoulder, preventing parts from sliding off. E-rings are a variation that clip on from the side rather than over the end, which is useful when you can’t access the tip of a shaft.
Fastener Materials
The material a fastener is made from determines its strength, weight, corrosion resistance, and cost. Low-carbon steel is the default for general-purpose fasteners because it’s inexpensive and easy to manufacture. Medium-carbon and alloy steels are heat-treated to reach higher strength grades for structural and high-load applications. Stainless steel trades some raw strength for excellent corrosion resistance, making it a common choice for outdoor, marine, and food-processing environments.
Beyond steel, aluminum fasteners save weight in aerospace and automotive applications, brass offers electrical conductivity and corrosion resistance, and titanium delivers an exceptional strength-to-weight ratio for demanding aerospace and medical uses.
Coatings add another layer of protection. Zinc plating is the most common corrosion barrier for steel fasteners. Electroplating applies a thin metallic layer (zinc, nickel, or chrome) for durability and appearance. Silver coatings provide high electrical conductivity for specialized electrical connections. Dry lubricant coatings reduce friction and wear during installation and service. Cadmium plating offers strong corrosion protection in harsh environments but has fallen out of favor due to environmental and health concerns.
Strength Grades and Sizing Standards
Fastener strength is communicated through a grading system stamped or marked on the head. The two major systems are imperial (used primarily in the United States) and metric (used nearly everywhere else), and they are not directly interchangeable.
Metric grades use a two-number code separated by a decimal point. The first number, multiplied by 100, gives the minimum tensile strength in megapascals (MPa), which is the maximum pulling force the fastener can withstand before breaking. The second number indicates the ratio of yield strength to tensile strength. A grade 8.8 bolt, for example, has a minimum tensile strength of 830 MPa (about 120,000 psi) and a yield strength of 660 MPa. Grade 10.9 jumps to 1,040 MPa tensile and 940 MPa yield. Grade 12.9, the highest common metric grade, reaches 1,220 MPa tensile.
Imperial fasteners use ASTM or SAE grade numbers. SAE Grade 5 and ASTM A325 are rough equivalents to metric 8.8, while SAE Grade 8 and ASTM A490 approximate metric 10.9. These equivalents are approximate because the standards differ in how they handle larger diameters and specific alloy requirements. Substituting one system’s grade for another without checking the exact specifications for your diameter and application can create a dangerous mismatch.
How Vibration Affects Fastener Joints
Vibration is the leading cause of fastener failure in dynamic assemblies. A fastener can experience axial vibration (along its length), transverse vibration (perpendicular to its length), orbital vibration, or random combinations of all three. Axial vibration alone can reduce a bolt’s clamping force, known as preload, by 30 to 40 percent over time. Severe transverse vibration is even worse: it can cause complete loss of preload, at which point the fastener is effectively just sitting in the hole doing nothing.
Loosening also happens through embedment, where the metal surfaces under the bolt head or nut deform slightly after installation, relaxing the clamp. Mismatched friction between the fastener and joint materials, corrosion, and unbalanced loads all contribute as well.
Engineers test vibration resistance using the Junker test, a standardized procedure that subjects a fastened joint to 2,000 cycles of transverse vibration in about 62 seconds. The test measures how quickly preload drops, giving a clear comparison between different fastener designs and locking methods.
Several fastener designs specifically target vibration resistance. Trilobular screws form their own threads with low driving torque while generating strong thread engagement that resists back-out. Some screws use dual-angle thread forms where a steep tip angle creates stored energy that fights loosening. For plastic assemblies, specialized thread-forming screws use a steep helix angle to drive in quickly, then transition to a shallower angle that resists backing out under dynamic loads.
Choosing the Right Fastener
Selecting a fastener comes down to a handful of practical questions. First, does the joint need to come apart? If yes, threaded fasteners are the starting point. If the joint is permanent and involves sheet material, rivets are likely the better fit.
Next, consider the loads involved. A fastener in pure tension (being pulled apart) needs adequate tensile strength, while one loaded across its body needs shear strength. Many joints experience both simultaneously. The strength grade you choose should leave a comfortable margin above the expected service loads.
Environment matters just as much as load. A fastener in a climate-controlled factory has very different corrosion demands than one on an offshore platform or under a car. Material and coating choices follow directly from the exposure conditions.
Finally, think about vibration and weight. The industry trend toward lighter, smaller products means assemblies vibrate more, and smaller fasteners handle less vibration. If your application involves motors, engines, vehicles, or any repeated motion, vibration-resistant designs or secondary locking features should be part of the specification from the start.