> For the complete documentation index, see [llms.txt](https://monasheng.gitbook.io/zerotohero/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://monasheng.gitbook.io/zerotohero/cots-parts-zero-to-hero/cots-parts/fasteners/bolted-connections.md). # Bolted Connections ## Working Principles Bolted joints work by leveraging the elasticity of both the fastener and the clamped medium. When a bolt is tightened, either into a nut or a tapped hole, through a clamped medium, it is forced to stretch by the relative motion of the threads. Due to the [mechanical advantage](https://en.wikipedia.org/wiki/Mechanical_advantage) that the thread angle provides, the applied tightening torque is multiplied many times into an effective clamping force.

From: https://www.metalartspress.com/books/machine-shop-essentials/chapters/chapter-6-threads-threading

Like a spring, a stretched bolt wants to snap back to its original length, and will bear down on the clamped medium with an equal and opposite force, compressing the joint. This joint compression first takes up any gaps, and then, once fully seated, establishes an amount of so-called *pre load* to the fastener and joint.
Pre-load obviously acts to oppose outside forces that might want to pull the joint apart, but it also serves an important purpose in preventing shearing of the clamped media. There is a common misconception that bolted joints rely on interference/fouling of the bolt shank in the hole to oppose shear loads - this is not the case in most joints. By design, the applied pre-load generates enormous friction between the bolt head and media, nut head and media, and media interfaces, and this friction provides lateral load capacity in most joints. This is in contrast to pinned joints, where the pin directly carries shear through bearing and material strength.

From: https://www.boltscience.com/pages/The-Importance-of-Friction-in-Friction-Grip-Joints.htm

## Anatomy of a Bolt The key features of a bolt include: * A *Head,* which both provides an interface for tightening and bears against the joint (usually through a *washer*), * A threaded, smaller-than-nominal-diamter section called the *thread*, that engages with a nut. These threads are usually formed by [rolling](https://www.youtube.com/watch?v=RNTTUvK9Lb0), which, like forging, upsets the metal and strengthens it. * An unthreaded, larger-than-nominal-diameter section called the *Shank*, which forms the primary load-bearing section of the bolt. Some bolts, called *screws* have no shank, and are threaded from tip to head.

From: https://www.essentracomponents.com/en-us/news/solutions/fastening-components/what-are-the-different-types-of-nuts-and-bolts

## Specifying Bolts In designing a bolted joint, you will typically need to decide between: * The bolt *system* - i.e. the thread geometry and nominal sizes, * The bolt *type* - i.e. what head and other features it has, * The bolt *grade* - i.e. what material it is made from, * The bolt *geometry* - i.e. diameter, length, and sometimes pitch. * Whether to use an integrated or separate *washer*, * Whether to use a *nut* or *tapped hole*. ### Bolt Systems Bolts are manufactured at enormous scale and are remarkably inexpensive given their complex geometry and material properties. This is a result of decades of optimisation in mass-manufacturing processes. Consequently, bolts can be purchased in one of a handful of coexisting standard *systems*, which define the available types, geometries, grades, and accessories. The two most common systems in use today are *Metric* and *Imperial*, which provide standardised, “even” size ranges within their respective measurement systems.
Imperial fasteners remain common in the United States, but are less prevalent in Australia. From a mechanical standpoint, neither system offers significant performance advantages; selection is typically driven by legacy, convention, industry norms, or regional practice. In Australia, we generally design around metric fasteners. This ensures good availability of parts and service tooling. More important than national convention, however, is maintaining internal consistency within a design or organisation. Mixing systems increases the number of required tools, spare parts, and the potential for assembly error. {% hint style="info" %} 🧙‍♂️ **Key Point:** Unless you have a *very good* reason, use metric fasteners in your design. {% endhint %} Metric bolts are commonly available in even-number nominal diameters - 6mm, 8mm, 10mm, and so on. These are referred to as M6, M8, M10, etc. bolts. At smaller sizes, single-millimeter bolts are available but less common. There are other bolt systems that exist - either as legacy relics - the Whitworth fastening system was common from \~1840-1950 in the UK - or as variants on the metric/imperial standards for specialist applications (e.g. tapered pipe threads, and army/navy (AN) threads). Note also that there are some industries/applications where even in Australia, use of imperial fasters is still more-or-less standard (e.g. in automotive brake systems, or for timber or metal screws). ### Bolt Types Bolt type refers to the physical form of the fastener, primarily the head geometry and any additional features intended to aid installation, service, or alignment. A non-exhaustive list of common bolt types includes hex head bolts, socket head cap screws, button head screws, countersunk screws, shoulder bolts, and set screws. The choice of head style influences not only the tools required for assembly, but also load distribution, achievable tightening torque, clearance requirements, and sometimes aesthetics.

A non-exhaustive spread of bolt types. From: https://www.linkedin.com/pulse/types-fasteners-nuts-bolts-washers-rivets-manufastin-qanaf

#### Bolts and Screws In practice, the terms *bolt* and *screw* are often used interchangeably. The distinction is not strictly defined and varies by context. Some use “bolt” to describe a fastener used with a nut, and “screw” for one used in a tapped or blind hole. Others distinguish based on geometry; for example, calling partially threaded fasteners with a shank “bolts” and fully threaded fasteners “screws.” Head style also influences language, with hex-head fasteners commonly referred to as bolts and socket or countersunk variants referred to as screws. **There is no universal rule.** You will need to become comfortable with this informal terminology and rely on specification rather than naming convention when selecting fasteners. #### Hex Head Bolts Hex head bolts are the most common, and are typically tightened with sockets or spanners. They are robust and tolerant of misuse, making them common in structural and general-purpose applications.

A typical hex-head bolt, in black finish, with a class 8.8 stamp on the head. Note the unthreaded shank. From: https://www.securefix.com.au/product/m36-4-0-coarse-x-110mm-high-tensile-steel-8-8-black-hex-head-bolt/

#### Socket Head Cap Screws Socket head cap screws (SHCS) are tightened using internal hex (Allen) keys. They allow higher tightening torque for a given head diameter and are well suited to confined spaces where external wrench access is limited. They are widely used in machinery, motorcycles, and precision equipment. Incidentally, SCHS are most commonly available in high-strength [#grades](#grades "mention"), and are sometimes used to explicitly signpost to inspectors, at a glance, that a high quality fastener has been used.

A pair of stainless-steel socket head cap screws. Note that the shorter length is fully threaded, and the longer one has a shank section. From: https://www.boltandnut.com.au/1-4-x-20-tpi-unc-coarse-g304-stainless-socket-3-16-key-head-cap-screws

#### Shoulder Bolts Shoulder bolts are special fasteners that feature a [precision-ground](https://en.wikipedia.org/wiki/Cylindrical_grinder) unthreaded shank between the head and the threaded portion, designed to be used a pin. Unlike standard bolts, shoulder bolts intent to load their shank in shear, and are often used as a locating or pivot element. Shoulder bolts are commonly used in linkages, rotating assemblies, and sliding mechanisms where a controlled bearing surface is required. Because the shoulder diameter is tightly toleranced and smooth, it provides better wear characteristics and alignment than a threaded shank passing through a clearance hole. They are also often used as a precision pins in other applications, due to their relatively low cost (compared to custom turned parts). When specifying shoulder bolts, care must be taken to ensure the shoulder length matches the thickness of the clamped components; if the shoulder bottoms out before clamping is achieved, the joint will not preload correctly.

A typical shoulder bolt. Note the precise, cylindrically ground 'shoulder', designed to be used as a pin. Also note the hex head, and that the threaded section is typically one size smaller (i.e. a 10mm shoulder bolt will have an M8 thread). From: https://au.rs-online.com/web/p/shoulder-bolts/0292366

#### Countersunk Screws Countersunk screws are also tightened using an internal hex, but sit flush with the surface of the clamped part and are often used where a smooth mating surface, sliding interface, or aesthetic finish is required. To achieve this, material must be removed from the joint to create the countersink, which reduces net cross-sectional area and concentrates stress at the throat of the fastener. For this reason, countersunk screws should be used with care. They are generally unsuitable for heavily loaded structural joints unless specifically designed for that purpose. In critical applications, preserving material and load path integrity is usually more important than achieving a flush surface.

A typical countersunk machine screw. From: https://www.miamistainless.com.au/product/m6-x-10-countersunk-machine-screw-hex-drive-304-grade/

#### Set Screws/Grub Screws Set screws, sometimes called "grub screws", are headless fasteners designed to secure one component within or against another — commonly a gear, pulley, or collar onto a shaft. They are typically fully threaded and tightened using an internal hex or similar drive, allowing them to sit flush with or below the surface of the outer component. Set screws do not clamp two components together in the same way as conventional bolts; instead, they apply localised pressure at a point or small surface area. This makes them suitable for positioning and light torque transmission, but generally unsuitable for high-load or high-vibration applications unless additional design measures are taken. Tip geometries (cup point, flat point, cone point) influence how the load is transferred into the mating shaft. In many applications, particularly where significant torque must be transmitted, relying solely on a set screw is poor practice. Keys, splines, or clamping hubs often provide more reliable and repeatable torque transfer. Set screws are best viewed as positioning or retention devices rather than primary structural fasteners. \[img] ### Grades, Classes and Materials Most structural bolts are made from carbon or alloy steels, and within [#bolt-systems](#bolt-systems "mention"), are distinguished in their relative strength by a so-called "class" in Metric, or "grade" in imperial. Steel is a good choice for bolts, due to not having an [endurance limit](https://en.wikipedia.org/wiki/Fatigue_limit) and thus being less succeptible to fatigue than e.g. Aluminium. It is also relatively cheap, and easy to form. Higher class bolts are usually made from a hardened/heat treated steel. These are stronger and harder than lower class bolts, though can see reduced strength in service if exposed to high temperatures. It is also possible to purchase bolts made from more exotic materials - stainless steel (304 or 316) is common in marine, outdoor, or otherwise corrosive environments, titanium bolts occasionally appear in weight-critical applications (e.g. aerospace), and copper and/or plastic bolts are used in electrical assemblies where conductivity (or lack thereof) is important. {% hint style="info" %} 🧙‍♂️ **Key Point:** Higher class bolts are usually not *that* more expensive, and thus, if uncertain about the load capacity required, opt for a higher class bolt. {% endhint %}

Bolt classes, materials, and properties. From: https://www.runsom.com/design-tips/a-guide-to-different-bolt-grades-bolt-grades-chart/

### Geometry ### Finishes Bolts are commonly available with a range of surface finishes. In this context, “finish” refers to the coating applied to the fastener, typically for corrosion resistance, appearance, or both. These coatings influence not only durability and aesthetics, but also cost, with 'better' finishes generally attracting a modest price premium. #### Uncoated/Black So-called “uncoated” bolts are typically supplied with at least a light film of oil to prevent corrosion during storage and transport. Many are also treated with a black oxide (sometimes referred to as a ["blue" finish](https://www.youtube.com/watch?v=OAA_cEZswU8)), which provides minimal corrosion resistance but a uniform dark appearance. Black fasteners are inexpensive and well suited to indoor or controlled environments. Their lack of heavy coating makes them suitable for applications where welding may be required (for example, captive fasteners), although welding high-strength bolts should always be approached with caution. High-grade fasteners derive their strength from controlled heat treatment, and localised heating can significantly degrade their mechanical properties. #### Zinc Plated Zinc-plated fasteners are the most common general-purpose option. The zinc coating provides a reasonable level of corrosion resistance for indoor and light outdoor use, while maintaining good dimensional consistency and a relatively clean appearance. They are only marginally more expensive than black fasteners, which makes them a sensible default choice for many applications. Zinc plating strikes a practical balance between cost, protection, and appearance. #### Galvanised Galvanised bolts are coated with a thicker layer of zinc, typically through hot-dip galvanising, and offer substantially greater corrosion resistance. They are commonly used in outdoor and infrastructure applications such as fencing, structural steelwork, and agricultural equipment. Galvanised fasteners are however generally unsuitable for precision assemblies. The coating is thicker and less uniform, which can affect thread tolerances and lead to inconsistent fit. Threads may feel “gummy” during assembly, and torque–tension relationships become less predictable. For high-precision machinery or fine-thread applications, galvanised bolts are usually not appropriate. ### Washers In any bolted joint, is important to use a washer (a small, cylindrical, flat disc) under the head of the bolt and/or nut to distribute load, minimising bending of the bolt head. Washers help prevent tear-out, and localised damage to the clamped surface from the bolt head. Washers for high-load joints are typically made from steel, and are available in a variety of thicknesses and diameters. Exotic washers are also avaiable for special applications. For example, plastic, rubber, or soft-metal (copper, aluminium) washers are used for sealing (e.g. on car oil pans). {% hint style="info" %} 🧙‍♂️ **Key Point:** Except in *very* special circumstances, you should *always* use washers in your joints. {% endhint %} ### Nuts and Tapped Holes A bolted connection is made possible by a threaded body for the bolt to interface with. Two arrangements are common - nuts, or threads formed directly into one side of the joint. #### Nuts Nuts are small, inexpensive, and usually-hex-shaped components designed to be used in bolted joints. They are generally available in the same range of systems, classes, and finishes that bolts are. The same provisos apply in selection of nuts as bolts, and one will almost always select a nut to 'match' the corresponding bolt. In most structural and general-purpose applications, a through-bolt with a nut is preferred. This arrangement allows the bolt to be fully supported by the clamped parts while the threads reside in the nut, which is easily replaceable if damaged. Because nuts are manufactured in high volume with rolled threads, their thread strength and durability are typically excellent. #### Threaded/Blind/Tapped Holes In some cases, threads are formed directly into one of the clamped components. This is commonly done via a so-called "blind" hole, allowing the component itself to function as the “nut” and enabling tightening from one side only. This arrangement reduces part count and can simplify assembly, particularly in enclosed structures or where only one face of the joint is accessible. However, it introduces important trade-offs. First, manufacturing complexity increases. Tapping threads into a component adds machining operations and cost. Second, threads cut directly into the parent material are generally weaker than rolled threads in a nut, particularly in softer materials such as aluminium. Repeated assembly and disassembly can degrade these threads, leading to stripping or loss of preload. Tapped holes are entirely appropriate in many engineering applications, particularly where space or access constraints dominate. However, they should not be viewed as automatically equivalent to a nut; the parent material will often become the limiting factor in the strength and durability of the joint. ### Positive Locking Bolted joints subjected to cyclic loading or vibration will loosen if no additional locking mechanism is provided. This occurs not because the bolt “unscrews itself” spontaneously, but because small transverse motions at the joint interface gradually reduce preload and allow relative thread movement. Once preload is lost, rapid loosening can follow. This process can occur surprisingly quickly. Many environments that do not appear to “vibrate” in an obvious sense are subject to continual low-amplitude cyclic loading, such as road vehicles, machinery frames, or even enclosures mounted to structural members. Assuming that a joint is “not a vibration environment” is usually wrong. Where joint integrity is critical, designers should employ deliberate positive locking strategies rather than relying solely on friction. #### Locking Adhesives (e.g. Loctite) Thread-locking adhesives are a common and convenient solution. These cure in the absence of air and bond the threads together, increasing resistance to loosening while still allowing disassembly with tools. They are inexpensive, easy to apply, and provide a clear visual indication that locking has been considered. However, they are subject to user error — insufficient cleaning, incorrect grade selection, or inadequate curing time can render them ineffective. Many formulations also have temperature limits and are unsuitable for high-temperature environments. #### Locking Washers and Nuts So-called “locking washers” are frequently misunderstood. Conventional split (spring) washers are often assumed to prevent loosening, but in many applications their effectiveness is limited. They may provide some resistance to rotation, but they do not reliably maintain preload under significant cyclic loading. Locking nuts are generally more effective. Nylon-insert (nyloc) nuts provide a simple and inexpensive all-purpose solution for many applications. The nylon insert increases prevailing torque and resists rotation by plastically deforming, but is unsuitable for high-temperature environments where the insert may degrade. They are also generally single-use, and should be replaced if a joint is disassembled. All-metal locking nuts (such as cone-lock or distorted-thread types) provide improved performance in higher temperature or more demanding applications. These nuts rely on controlled thread interference rather than polymer inserts and are more robust under harsh conditions. #### Safety Wire, Tab Washers, Locking Plates Physical locking methods provide the highest level of security. Safety wire (lockwire), commonly used in aerospace and motorsport applications, physically prevents rotation by tying fasteners together or anchoring them to fixed points. It is highly effective but requires appropriate bolt preparation, specialised tooling, and operator skill. Tab washers and locking plates achieve similar outcomes through simple mechanical restraint. Once bent into position, they physically block rotation of the nut or bolt head. These methods are robust, visual, and relatively “idiot-proof,” making them attractive in safety-critical or high-vibration environments. ## Suppliers and Purchasing Fasteners and their accessories can be purchased from a wide range of suppliers. Many dedicated fastener suppliers (e.g. [Newcastle High Tensile Bolt](https://nhtb.com.au/)) will allow you to browse the full range online, and filter their stock by system, type, class, geometry, and finish, and compare corresponding prices. Designing with commonly available stock in mind is prudent - some parameter combinations are more common than others. In a pinch, hardware stores also stock a limted range of fasteners. Care should be taken here however - as the applications they cater to typically trend toward classes and finishes that are unsuitable for serious engineering applications. As always, carefully consider the specifications of any fastener you purchase. In general, ordering from a dedicated fastener supplier (e.g. [United Fasteners](https://www.unitedfasteners.com.au/), or [Keables](https://www.keablesbolts.com.au/) in Melbourne), will provide the greatest range of available stock, lowest price, and highest confidence in quality (which here means 'properties accurate to specification').