Fasteners 101: Types of Fasteners and How to Choose Them

Fasteners are mechanical components used to join two or more objects together. Engineering fasteners are specially designed and manufactured to provide a secure and reliable connection between two or more objects, often in high-stress or high-temperature environments.

Fasteners are used in a wide variety of applications, including aerospace, automotive, construction, and industrial machinery.

There is a huge range of types of fastener, each with its own properties and applications. Some common types of fasteners include bolts, rivets, screws, nuts, washers, and rivets. Each of these is a family of products of divergent capabilities, applications and sizes.

Each type of fastener is designed to perform a specific function, such as providing a secure hold  distributing applied load evenly, or shearing off when overloaded. Some fasteners are built with explosive cores, to allow them to be broken remotely at a controlled moment, fircexample in stagecseiaration of a rocket.

The selection of the appropriate fastener for a particular application is critical to ensuring a safe and reliable connection. Factors such as material, size, strength, and environmental conditions must be considered when selecting a fastener.

Additionally, proper installation and torque specifications are essential to ensuring that the fastener performs as intended.

Engineering fasteners play a critical role in many industries, and advancements in materials and manufacturing techniques continue to improve their strength, durability, and performance. The smallest fasteners require magnification to be fitted (watch screws for example) and the largest potentially weigh several tons (large ship propeller nuts).

I. What Is a Fastener?

A fastener is a mechanical part or parts that is/are used to attach two or more other objects or parts together. As a rule, fasteners are considered to be temporary or removable jointing methods, although some fastening systems are more permanent (e.g. rivets) or distort/damage the fixed objects to create the joint (e.g. PEMS or rivnuts).

There is a huge diversity of applications for fasteners and a nearly equal diversity of solutions applicable to any one application.

Examples of fastener use abound in engineering, product manufacture, construction, vehicles, homes, DIY, medical applications and more.

Materials for fasteners are equally diverse, as virtually every material used in making human things is or has been used as a fastener.

Fasteners are not generally considered to include jointing methods that are chemical in nature, such as materials that change from a liquid to a solid (glues and adhesives, solders) or jointing methods that cause two like material components to merge by melting (welds).

II. Applications of Fasteners

Fasteners have a wide range of applications in various industries and fields, including:

• The construction sector uses fasteners extensively in the joining of various materials, such as wood, concrete, glass, plastics and metals. They are used to secure roofing and wall panels, framing, rebar, finishing components and other fixtures and fittings.
• The automotive sector uses fasteners to join different parts of vehicles, such as engines, transmissions, and chassis. They are also used widely in automotive interiors and exteriors, including seats, dashboards, and body panels.
• The aerospace sector makes massive use of fasteners in critical roles in aircraft construction, where they are used in airframes and control surfaces, engines, and throughout the vehicles. The use of lightweight, high-strength materials and specialized designs is essential to ensure the safety and reliability of aerospace fasteners.
• The industrial machinery sector uses fasteners in diverse ways, including heavy equipment, conveyor systems, and manufacturing equipment. They are used to join components and to ensure the stability and strength of the machinery.
• The electrical supply sector uses fasteners to construct and/or secure electrical switchgear, cables, transformers, generators, turbines and much more.
• The consumer products sector makes extensive use of fasteners of all kinds, across audio, whiteware, toys, tools and more.
• The medical devices sector uses fasteners in the manufacture of medical devices and equipment, surgical instruments, and diagnostic equipment as well as in dental and orthopedic devices to install surgical implants into patients.

Fasteners are essential components in virtually all systems, and their selection, installation, and maintenance are critical to ensuring safe and reliable operation.

III. Fastener Materials

Fasteners are made from almost all classes of materials and draw on all aspects of materials science and technology.

• Mild, medium and high Carbon steels are the most commonly used materials for making fasteners due to its strength, durability, and affordability. It is used to make bolts, nuts, screws, and washers. It’s also used to make various rivet types, wood and paneling screws, knock-down joints, nails and more.
• Stainless steels are used in exactly the same applications as steels, where corrosion resistance is a factor that is significant enough that zinc plating will not suffice.
• Brass is commonly used to make decorative fasteners such as screws, rivets plus nuts and bolts. It is also used in electrical equipment for its toughness and good conductivity.
• Bronze is used extensively in marine applications, for resistance to salt corrosion.
• Aluminum is used to make fasteners that need to be lightweight and corrosion-resistant. It is commonly used in the aerospace and automotive industries, primarily for rivets but sometimes for nuts and bolts.
• Titanium is used in applications that require high strength and low weight, such as in the aerospace industry, for rivets and all kinds of threaded fasteners. It is also highly resistant to corrosion.
• Polymers such as nylon are used to make plastic fasteners such as cable ties and clips and also to make low stress and insulating nuts and bolts. It is lightweight, durable, and resistant to corrosion. ABS is used to make various snap and push fit fasteners, for light applications
• Wood is widely used to peg wooden parts together, for more traditional furniture and construction applications.

IV. The Most Common Types of Fasteners

The family of fasteners is to wide for a complete analysis and review in a simple article like this, but a broad overview can point towards areas you may wish to study more deeply.

Bolts:

Bolts are threaded (generally but not exclusively right hand) fasteners that are generally made from steels, brass and occasionally bronze of Titanium. The threaded section runs from the tip to part way along the shaft, which uses then unthreaded to the underside of the head.

There are many forms of thread for equivalent nuts, there are also threads for wood and other soft materials.

There are also many patterns of head, adapted to various cosmetic, functional and driver types. Driver adaptations suited to AF, hex and ring spanners are parallel hexagonal and this is the most common driver type. They can also be domed, though this type often gets defined as a screw.

Heads can include a flat underside, or countersunk, or possess a square element to bite in and prevent rotation. They can also include a smooth or serrated flange or integrated washer, fir lead spreading and to prevent  accidental loosening.

Screws:

Screws encompasses a very wide family that includes devices adapted to; tapped holes; thread cutting (self tapping) and thread forming in metals, sheet materials and plastics; thread forming/cutting in wood and more.

Screws are differentiated from bolts in that they generally have head types that contain a recess for one of a wide spectrum of driver bits such as cross head, torx, Allen or Robertson drives, to name a selection.

Nuts:

Nuts form a small decorum group that are generally hexagonal in shape, with a threaded interior hole that matches a size of bolt.

They can be plain hex nuts and half nuts; or carry a cosmetic cap/done; or contain a nylon grip ring (nylok); or castellations to accommodate a split pin for locking. They can also be long tubular as in a Chicago or sex nut.

Washers:

Washers are not technically fixings, but many fixings use/require them for improved liad distribution, to prevent bolt heads/nuts from cutting into the parts and to reduce rusk of loosening.

Washers are generally flat steel, brass or plastic discs but can be toothed, or sprung (domed or split) for a variety of anti vibration or force control purposes.

Rivets:

Rivets are a broad family of generally metal parts that are forged or otherwise distorted to form a ‘head’ on either end and apply a compressive force to restrain two or more components at a joint.

These can be formed from any malleable metal for cold distortion/forging – including Aluminum, brass, bronze, steel, stainless steel etc. Wrought iron and steel rivets used to be commonly applied hot, to allow head formation. This was typical of heavy engineering for ships, construction and steam equipment until welding processes displaced this method.

Many types of industries now use blind rivets that use a central pull pin to form the obverse side head, allowing single sided access with high compression force. These ‘pop’ rivets were developed for aircraft manufacture but have become universal and are available in Aluminum, steel, stainless steel, monel, titanium and more metals.

V. Different Types of Fasteners and Their Uses

The applications for fasteners are universal across all industries and product sectors.

Bolts Types

There are various types of bolts, each designed for a specific application.

Here are some of the most common bolt types and their applications:

• Hex bolts have a hexagonal head and are used for a wide range of applications, including automotive, construction, and machinery. They can be made from mild steel, stainless steel, high tensile steel, brass and even copper. This is by far the largest bolt family, with huge range of size and application.
• Carriage or coach bolts have a smooth, domed head and a square shoulder beneath the head, to lock into materials to prevent rotation, making them ideal for use in timber construction.
• Eye bolts have a circular loop at the head end and are used for lifting and securing heavy objects.
• U-bolts have a U-shaped design with threads at both ends and are used to secure pipes and tubes.
• J-bolts have a J-shaped design and are commonly used for anchoring objects to concrete or other materials, placing the bolt into the wet concrete to provide a strong anchor point.
• Shoulder bolts have a cylindrical shoulder between the head and the thread which serves as a precise location and a bearing point.
• Flange bolts have a wide flange under the head and are used in applications where a wide load distribution is required, such as in automotive and industrial machinery.

Screws Types

Screws of many different types are each designed for specific and often quite narrow applications. Each classification is generally available with various drive recesses (cross head, slot head, torx, Robertson etc) and often with various head types (cheese head, countersunk, dome etc).

• Wood screws are suited to use in wood or man made wood fiber materials. They typically have a sharp point and a threaded shank that grips the wood.
• Machine screws fasten metal parts together. They have a straight shank and are threaded for the entire length.
• Sheet metal or self tapping screws are used for attaching to thin metal sheets. They are generally hardened and have a variety of points and thread types.
• Lag bolts (or coach screws) are coarse threaded and taper tipped, for use in wood and are used for heavy-duty applications, such as securing lumber and other materials to a wall or post.
• Concrete screws for attaching objects to concrete or masonry surfaces. They have a special thread design that allows them to grip these harder, brittle masonry type materials.
• Drywall screws are specific to attaching drywall to wood or metal studs. They have a special thread design that prevents the paper from tearing.
• Hex cap screws for high-stress applications such as heavy machinery or automotive parts. They have a hexagonal head that can be tightened with a wrench.
• Socket head cap screws are similar to hex cap screws, but with a cylindrical head and a recessed hexagonal or torx socket for tightening.

Nuts Types

Nuts generally come in a limited number of styles and are usually hexagonal for AF/ring/socket drivers. They’re commonly made from steel, stainless steel, brass and plastic and can be painted, galvanized or bright zinc plated for corrosion resistance. They can also be Chromium plated for cosmetic purposes.

• Full nuts are simple hexagonal outside profile with a threaded hole through
• Half nuts are full nuts but half height
• Nyloks are full nuts with a nylon ring crown to bind on the thread and prevent free spinning
• Castellated nuts are full nuts with a castellated end to accommodate split pin locking
• Dome nuts are full nuts with a closed end that is covered by a cosmetic dome
• Anti-backlash nuts are a full nut which has been cut part through on the transverse midline and then partially crushed, to provide a binding force on the thread to take up any slack

Washers Types

Considering that washers are by definition simple metal discs, there is a surprising amount of variations for specific purposes. Like nuts, washers can be steel, stainless steel, brass, plastic and occasionally Aluminum.

• Plain washers are simple metal discs with a bolt hole in the middle
• Penny washers are similar but tend to have a larger outer diameter, for load spreading and bridging slotted bolt holes
• Spring watchers are a simple hardened steel ring that has been cut through at one point and distorted to offset the cut edges. These are an attempt to provide a claw to the head and the substrate to reduce vibration loosening
• Bellevue washers are spring steel domes that provide some spring force under the head
• Grip washers are hardened steel plain washers with a variety of features cut into both surfaces to add traction to prevent vibration loosening

Rivets Types

Rivets are a wide family of fasteners that are used to permanently join two or more materials together.

• Solid rivets are the most basic type, used in structural applications where strength is important. They are made of a single piece of metal, usually with one upset forged head. They are installed by hammering or using a hydraulic press to upset forge the second head and induce the clamping force.
• Blind or pop rivets are used in applications where access to both sides of the material is limited. They consist of a hollow body with a mandrel that is pulled through the body to expand the rivet and secure the materials. At a preset compression force, the mandrell shears off and the rivet is formed.
• Tubular rivets or eyelets are used in leatherwork. They consist of a hollow tube with a roll-formed head at one end. The shank is inserted into the material and the obverse head is formed using a compression tool.
• Drive or push rivets are used in sheet metal and plastics applications. Made of metal or plastic, they have a short mandrel that is driven into the body of the rivet after insertion, to expand it and secure the materials.
• Split rivets are used in fabric and leatherwork. They consist of a cylindrical body with two legs that are split at the end. The legs are pushed through the material and then bent over or rolled to secure the materials in place.
• PEMS or nutserts are used in sheet metal and plastic applications, they swaged and press fitted into one component to be fixed, so a screw/bolt can be fitted to them, to secure joint.
• Flush rivets are used in aerospace applications. They have a smooth (often countersink) head that lies flush with the surface of the material, creating a smooth and aerodynamic surface.

VI. Advantages and Disadvantages of Fasteners

Fasteners of various types are as old as human technology. The range, precision and flexibility constantly improves, along with the spectrum of applications. Fasteners offer a spectrum of advantages that are easy to understand and benefit from and disadvantages that are generally easy to compensate for.

Representative advantages are:

• Convenience – fasteners allow for quick and low cost assembly (and often disassembly) of parts.
• Strength and durability – fasteners provide a strong and secure connection between two or more parts.
• Versatility – fasteners come in a wide range of sizes, styles, and materials.
• Cost-effective – fasteners generally require no finishing after fitting, making them a low labor option for fixing materials.
• Maintainability – fasteners mainly allow easy removal and replacement, and in case of the need for maintenance.
• Resistance to corrosion – most fasteners types are available in materials that are resistant to corrosion, making them ideal for outdoor applications or in aggressive/corrosive  environments.
• Aesthetics – some fasteners can be designed to add aesthetic value to a project.

Typical disadvantages are:

• Potential for loosening – over time, fasteners can become loose due to vibration, changes in temperature, or other factors.
• Damage to materials – fasteners can cause damage to the materials they are attached to, such as holes or scratches. This can weaken the material or reduce its aesthetic appeal.
• Stress concentration – poorly designed fastener solutions can accelerate material failures because they involve fitting holes and highly localized stress points.
• Corrosion – fasteners can accelerate corrosion, especially in environments with high humidity, salt, or acids.
• Size limitations – fasteners have size limitations, and in some cases, they may not be strong enough to support heavy loads or withstand high stress.
• Cost – fasteners are generally cost-effective, but some specialized or high-performance fasteners can be expensive.
• Complexity – fasteners such as rivets, rivnuts etc require specialized tools or techniques to install properly.

VII. How to Choose the Right Fastener for Your Applications

Fastener choice is often difficult – because there are many options and design freedom can allow for a variety of approaches. A clear understanding of the implications is required at the design specification stage, to guide the process to a generally cost effective and acceptable solution.

Know the Application

The first step in this is to have a clear understanding of the product, its usage patterns, stresses and environment. From that understanding can develop a design approach that selects and integrates the most appropriate methods of fixing and assembly, the requisite tools, the fit with the production facility and more.

Select the Right Thread

For threaded fasteners, the selection of thread type and form is often critical. Increased pull strength to resist correct tightening, over tightening and potential shock loads can result from the selection of the right coarseness, profile and overall diameter of the threads.

Screwing into soft materials places very different demands on a fastener than screwing into a tapped hole or a tapped threaded insert.

Screwing into sheet metal requires a different form of thread that can cut the distorting edge of a sheet hole, or form a thread into a pressed hole that has more engagement.

Threads that can resist overtightening have significant benefits, as do threads that offer tight control of torque by having a fine pitch.

Threads to hold plastic parts together are particularly demanding of care and precision. The consequent damage of using a thread cutting screw as opposed to a trilobular thread forming screw can have serious impacts on product resilience and life expectancy.

There are extensive design guides that can help you in this surprisingly delicate process, when you need the deepest understanding of your fixing issue and the options in satisfying it.

Consider the Materials of Construction

Many options exist in the material options for fixings:

• Bolts, screws and machine screws and machine screws (plated with Zinc or Chromium, black oxide coated, hot Zinc dipped etc):
  • Mild steel – for low costHardened and tempered steel – for higher strength and better thread cutting/formingStainless steel – for corrosion resistance and cosmetic reasonsMonel – for high strength and salt resistanceBrass – for conductivity, aesthetics
  • Plastics – for insulation, chemical resistance and fail-safe

• Rivets:
  • Pop rivets in Aluminum, stainless steel, monel, Titanium, mild steel depending on application needs in strength, cost, bimetallic corrosion resistance, general corrosion resistance etc
  • Solid rivets in mild steel, stainless steel, brass, Copper and Aluminum depending on strength, thermal/electrical conductivity and corrosion resistance needs

• Washers (plated with Zinc or Chromium, painted):
  • Mild steelStainless steelSpring steelHardened steelBrass
  • Plastic

Type of Exposure for the Fastener

Fasteners can experience a wide range of environmental exposures that influence choice of materials and finishes:

• Marine – no plating realistically survives marine exposure, so selection is limited to Nickel Aluminum Bronze, 316 stainless steel and monel
• Outdoor environment – not acidic, non salt environments can utilize bright Zinc and hot dip Zinc coatings on steel fasteners. Brass and stainless steel offer greater environmental resilience
• Hot environments – plating generally suffers poor heat resistance, so stainless steel is required in wet heat, where mild steel with zinc plating can be used at lower temperatures and dry heat environments
• Cold environment – aerospace p[arts can be subject to extreme temperature ranges, so low temperature behavior is a critical factor in material choice

VIII. Fabrication Methods for Metal Fasteners

Fasteners are diverse in nature, application, size and materials and call upon a wide range of manufacturing methods

CNC Machining

Many fasteners are manufactured on specialist CNC equipment. Capstan lathes, CNC press tools, CNC thread rolling machines and CNC cold heading systems are all used.

Automation is important in fastener manufacture, as the volume of parts is huge. This usually results in highly specialized special purpose machines that are essentially CNC equipment with narrow and specific capabilities.

Cold Heading

Cold heading, or upset forging is the most common method for forming a ‘head’ onto a metal rod. The application of pressure without heating the material results in a work hardening process that forms a strong head with low effort. Cold heading is commonly used to create screws, bolts, and other threaded fasteners.

IX. Types of Metals for Fastener Fabrication

The great majority of fasteners are made from a small range of materials:

• Mild steel for bolts, nuts, screws, machine screws, rivets, washers and more
• Stainless steel for bolts, nuts, screws, machine screws, rivets, washers and more
• Brass for specialist bolts, nuts, screws, machine screws, rivets, washers
• Aluminum for rivets and specialist washers
• Copper for specialist rivets, washers and some bolt and machine screws
• Titanium for specialist bolts, nuts, rivets and washers
• Bronze (generally Nickel Aluminum types) and Monel for marine fixings of all types

Conclusion

Kemal can help at all stages of your product development, transition to production and mass production. We have great experience and specialist knowledge in fastener selection and use and we will be happy to assist in easing your path to mass production.