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Bolts (and nuts) are a ubiquitous form of fastener that we are all familiar with. But selecting the right bolt for a job takes some consideration. In this post we provide everything you need to know when choosing your bolts.
The most significant developments in bolt and screw production and use have taken place over the last 150 years. Although experts have various views regarding the origins of nuts and bolts the use of helical threads can be readily traced back to 400 BC. Archimedes (287 BC&#;212 BC) is known to have developed the screw principle and used it to construct devices to raise water. But there is evidence that this form of water screw may have actually originated in Egypt, before the time of Archimedes. Screw threads are known to have been historically used in various devices such as the presses for grapes in wine-making.
It has been argued that the threaded bolt with a matching nut, used as a fastener, dates back to the 15th century. But a form of unthreaded bolt is known to have been used by the Romans who are also credited with developing the first screws, made from bronze or silver.
Bolts are a form of threaded fastener with an external male thread that mates with a matching pre-formed female threaded nut. Bolts have three primary sections: the head, the shank and the threading. Some bolt descriptions also refer to the chamfer, which is the tip of the bolt that&#;s sometimes bevelled to make it easier to insert.
The bolt head is the widest diameter section of a bolt, available in a variety of designs. The bolt head provides the surface that bears much of the load when bolts are tightened. Bolts are commonly used in conjunction with various forms of washer that distribute the load and can help prevent bolts from becoming loosened.
The shaft of the bolt is called the shank. This is sometimes threaded over its entire length or it may be split into two sections: an unthreaded &#;body&#; section along with a threaded section.
The threading on a bolt is the helical groove that mates with a matching nut. Helical threading is what enables bolts to be tightened and loosened, as required.
Bolts and nuts operate using friction and torque. When tightened, by applying torque, nuts and bolts are held together due to friction between the two components. Nuts and bolts may loosen due to vibration or the loads they are supporting. This can be overcome using locknuts or lock washers.
A question that&#;s often asked is: what&#;s the difference between a bolt and a screw? The widely accepted differentiation is that a bolt is a non-tapered fastener that uses a nut (and sometimes a washer) to hold objects together. While a screw is a fastener that mates with an existing thread or creates its own thread in a material as it turns.
Bolt vs Screw - Image Source: Engineer Explains
Another area of confusion exists regarding the difference between bolts and machine screws. Machine screws, like other fastenings, are used to securely join two or more objects. Although they are widely used, there is no single, agreed definition for machine screws.
Characteristics of machine screws include:
The term &#;set screw&#; is often used in reference to bolts. In the UK, the term "set screw" or "setscrew" simply refers a bolt which is fully threaded along its entire shank. In the U.S. this type of fastening is called a cap screw.
Set screws are most commonly used without nuts, which is what distinguishes these fasteners from bolts. They attach to pre-drilled and tapped holes in just one of the two objects being fixed together. Some forms of set screw are entirely headless enabling the fastening to be secured without any protruding head. This type of set screw can be called a grub screw or blind screw.
Nuts and bolts are used just about everywhere. Bolts, along with a wide variety of other fasteners, are widely recognised as a reliable fastener and used in a huge variety of applications and environments. They are available in various shapes, sizes and materials.
The largest bolt ever made was in by the Penrith Engineering Works company in Scotland. These fasteners were manufactured for SWISSOIL in an experiment to boost the capacity of oil tankers by bolting two of them together. These massive bolts were 8.3m (27.4 feet) long, 1.27m (4.2 feet) in diameter and weighed in at 12.6 tonnes.
At the other end of the scale, micro-fasteners are becoming increasingly tiny. Scientists have developed a mechanism to guide the deposition of gold atoms on pre-made rod shaped nanoparticles in a spiral structure. These "nano screws" are described as screws smaller than dust.
Some of the areas in which nuts and bolts are typically used include:
When considering the bolts needed for a particular application its vital to correctly determine the bolt dimensions. Primary bolt dimensions are the diameter, length and thread pitch.
Bolt Dimensions - Image Source: Davis EngineeringThe diameter refers to the width of the bolt shank. This important dimension doesn&#;t refer to the bolt head.
The bolt length generally refers to the length of the shank, from the underside of the head to the tip of the bolt. But its important to note that bolts with a countersunk head are measured over their entire length.
And the thread pitch defines the distance between the apex of adjacent threads on the bolt shank. Pitch is sometimes defined by the number of threads per unit length.
These dimensions are measured in both metric and imperial units and its important to remember they cannot be mixed. Metric bolts don&#;t mate with imperial nuts and vice versa. Bolt dimensions are an important aspect of their strength and load bearing capacity.
In the UK, Europe, China and other manufacturing centres such as Taiwan, metric bolt sizes are most commonly used. But in the USA and Canada they use Unified Thread Standards (UTS), based on imperial measurements.
Another important bolt attribute that affects their strength and application is the material they are made from. Bolts can be made from a variety of materials each with varying properties and applications.
Plastic bolts have a variety of low load applications and may be used in environments involving water, such as swimming pools. They are relatively inexpensive and don&#;t corrode.
Aluminium bolts are also low cost, lightweight and corrosion resistant. They are also non-magnetic and are often used in consumer electronics.
Brass bolts are strong, corrosion resistant and conduct electricity. Brass is also often a component in corrosion resistant alloys.
Copper bolts are used in electrical applications, medical equipment, plumbing and marine applications. Copper alloys have a good load capacity and are suitable for use near magnets.
Mild steel bolts are strong, but uncoated steel is vulnerable to corrosion. Steel bolts are commonly available in galvanised or bright zinc plated (BZP) finishes that resist corrosion.
Hardened steel bolts are stronger than mild steel, but more brittle. Steel is hardened by tempering and quenching.
Stainless steel bolts are chemical and corrosion resistant and provide an aesthetically appealing appearance. A2 stainless steel is easy to machine and A4 stainless steel is a harder, marine grade option.
Titanium bolts are lightweight, hard and strong. They are corrosion resistant and when alloyed with other metals, titanium increases strength and durability.
Bolts made from various superalloys provide excellent mechanical strength, corrosion resistance, temperature tolerance and surface stability.
A variety of bolt finishing options also available including various forms of plating (as noted) along with brushed and polished.
British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in and later adopted as a British Standard. It was the world`s first national screw thread standard and is the basis for many other standards, such as BSF (British standard fine) and BSP (British standard pipe). Whitworth bolts and nuts are still commonly used by classic car enthusiasts for their vehicle restoration projects.
In the UK, Europe and most of the world the metric system is used to specify fasteners. Metric fasteners are usually classified as either coarse or fine, referring to their thread pitch, although some fastener sizes are available in an extra fine thread.
The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. This was one of the first international standards agreed when the ISO (International Organization for Standardization) was established in . Metric fasteners are commonly manufactured to meet standards set by DIN (German Institute for Standardization), or ISO (International Organization for Standardization).
M: Indicates this is a metric thread.
12: Refers to the diameter of the bolt shank (12mm)
25: Refers to the shank length (25mm). This form of specification refers to a coarse threaded bolt which is the standard and most commonly used thread pitch.
M: Indicates this is metric.
8: Defines the shank diameter as 8mm.
1.0: Is the pitch (distance between threads) of 1mm.
20: Is the shank length in mm.
Bolts are available with a wide variety of head shapes and in various designs, often specific to certain applications. The most common bolt head shapes and types include hex, square, slotted and socket bolts. Here`s a handy bolt type reference that will help when selecting the right bolt for a job.
Hex Bolts are characterised by their hexagonal head with threads that mate with a nut or tapped hole.
Allen bolts are distinguished by their hexagonal socket head. They require a matching Allen key to tighten and loosen them.
Masonry Bolt
Self tapping masonry bolts are used to securely attach things to masonry and concrete.
Also known as carriage bolts, coach bolts have a smooth rounded head with a small square section underneath.
Tri-head bolts are also known as triangular head security bolts, triangular head security screws or security recess screws. The head features a distinctive flange and requires a matching tool for use.
U-bolts have this distinctive `U` shape. They are partially threaded at either end and typically used to support pipework, poles or similar structures.
J Bolts are characterised by their distinctive `J` shape. Threaded on the longer length and typically used in structural applications.
Roofing bolts, commonly used to secure roofing materials, have this distinctive round head.
Saddle bolts have this distinctive curved head shape. Commonly used to secure various forms of fencing.
Expansion bolts are typically used in masonry substrates. As the bolt is tightened the anchor section expands in the predrilled hole, securing the bolt in place.
Asphalt fixing bolts are a special type of expansion bolt specifically designed to securely attach objects to asphalt.
Chemical resin studs or anchor studs are used to achieve high strength fastening to concrete using Polyester or Vinylester Resin.
Similar to a standard allen socket head bolt but with the addition of a security pin that prevents the fastening from being tampered with using standard Allen keys. Also known as Tamper Pin Hex bolts.
Pin Torx bolts are also available with the Torx head type along with a central security pin.
Scroll bolts are high security fastenings that don`t offer any sharp edges that might be used to grip and loosen the bolt. These bolts can only be used with matching Scroll driver socket tools.
TRM supply professional and honest service.
Pignose security bolts, also known as snake-eye security bolts or two-hole security bolts, are another security fastening that require a special tool.
Shear bolts are a high security fastening that`s ideal when robust, permanent, high security fixing is required. Once tightened the hex section of the head simply shears off under torque.
A shouldered bolt is characterised by the shank that has a larger diameter that reduces to a smaller diameter for the threaded length.
When selecting the most appropriate bolt for a specific task its important to consider safety critical factors. Sometimes it will be necessary to consult with relevant experts to determine the required bolt specifications. The criteria that need to be considered include:
In most cases answering the following questions will determine the best bolt for the job.
For example, indoor or outdoor or possibly marine, where it will need to withstand the corrosive impact of salt water.
Its important to get the dimensions right. Consider the bolt shank length and diameter as well as the threading. When replacing bolts its important to match the required bolt dimensions.
As noted, bolts are available in a variety of materials suitable for a range of applications and environments. Its important to consider exactly which bolt material is right for the job and be aware of how the bolt material affects the load the bolt can take.
As demonstrated in the previous section, there are many bolt types so its important to select the right type for the job.
The bolt material, size and design are all important factors affecting a bolt&#;s ability to withstand stress and load. Its always important to ensure selected bolts provide the strength required.
Some bolts, such as pig-nose and scroll bolts, require specialist tools to install and undo them, making them less susceptible to tampering.
Shear bolts offer the ultimate in security as once they are tightened the hex head is sheared off, making them very difficult to remove. If the bolt is a permanent fastening then it may be sensible to consider shear bolts.
Aesthetic aspects of bolt selection often need to be considered. As noted, bolts are available in wide variety of designs and finishes to suit all applications.
If you have any questions about bolts or fastenings and which options are appropriate for your needs, remember we are here to help. Give us a call on and we&#;ll provide you with free, expert advice.
This message was added on Thursday 23rd March
This is Page 2, continuing the Bolts 101 article. Here we discuss choices for bolts for an application. The previous post, Page 1 of Bolts 101, gives a ton of good background info, so we recommend reading that first, then this one on making a good bolt choice. Both pages focus on the practical side rather than on all the theoretical detail.
The hard part is defining what bolts to choose for a given job. It&#;s the hard part because the rules of thumb have lots of exceptions. Also, it&#;s your judgement and responsibility for the bolt choice. If the joint is critical &#; especially if it&#;s for safety &#; perhaps some engineering help is appropriate. However, in most cases you can follow these guidelines and feel quite comfortable in making the decisions. Just note the exceptions.
See the Bolts 101 article for the basics of bolts.These pages have a practical focus. A Google search will give in-depth detail, so no need to repeat it here.
When mounting an object that comes with bolt holes, use the size of bolts recommended by the manufacturer. If the item doesn&#;t have specific instructions telling you what bolts to use, then use a bolt size that fits nicely, and fills in the holes.
The caster shown in the photo is a good example. It has 4 slots that accept 3/8&#; or 10 mm bolts nicely. Either size works as a bolt choice. Note: Because the holes are slots, washers support the nut.
If the holes are not defined for the bolt size, there are lots of other considerations to select the size.
Use a bolt that works well with the thickness of the materials to connect. If you&#;re bolting thin sheet metal, then big diameter bolts will not make it more secure. In fact, when push comes to shove, the material will often fail near the bolts, so it&#;s better to have several smaller fasteners than one large one.
A typical rule of thumb . . . Use a bolt diameter that is 1.5 &#; 2.5 times (up to three times) the thickness of the thinner material you are bolting together. So, for 1/8&#; material, a 1/4&#; diameter is often a good bolt choice. For 1/4&#; thick material, perhaps a 3/8&#; or 1/2&#; bolt.
This guideline falls apart for very thin materials, and for really thick stuff. In both these cases &#; if the joint is critical &#; it&#;s better to use more fasteners rather than bigger ones. Note: For thin materials, it&#;s usually best to back the joint with thicker material, like washers, or face plates, then the rules can apply again.
What do the bolts need to do? Are they through bolts (bolt with a nut)? Or does the bolt go into a threaded hole? Maybe the threaded hole goes through? Or maybe it is blind (go into a threaded hole that does not go through)?
Most of the discussion in this article shows bolts with nuts (and washers), but there are a lot of applications where the threads exist in another part. Really, the discussion is almost the same either way. Trailer wheel bolts are a good example where some are bolts into a threaded hub, while other times they are studs with a nut. Read more in the Trailer Wheel Bolts article.
Anyway, function matters, and it is often connected with the other things on this page like Thread Engagement and Loading (both topics below). What do the bolts need to do? Hold stuff, yeah, but do think about what they are holding? How they are holding it? And, perhaps most important, how critical is it? The bolts you choose &#; grade, size, and number &#; should reflect what the bolts need to do.
Have you ever noticed that bolts rarely appear with just one? How many bolts do you need?
When you bolt something, you are constraining it in one direction. Argumentatively, it constrains more with friction and extended face contact, but if you really want to hold the piece, it requires more bolts. 2 bolts constrain rotation, and at least 3 bolts (not in a line) for bending constraint. We often see sets of 4 because it&#;s an easy pattern, and it constrains many things very well.
Place bolt patterns so the bolts are spread &#; within reason. Generally, more distance between them makes the connection stronger. From the standpoint of holding things down, more is merrier, but it also makes things harder to work with.
The other reason for more bolts is safety. If the connection involves safety, better to over-kill.
The image here of the hitch illustrates some of these bolt choice factors. Bolt size is 1/2&#; (defined by the hitch holes) &#; which is a little more than 2.5 times material thickness (less than 3 times), so that&#;s good in 3/16&#; thick material. They are Grade 8 for strength because safety is definitely a concern. Fine thread with Nylock nuts for security in a vibration environment. The main connection forces are in shear, and they have sufficient distance for joint stability.
In the hitch example, there are 4 connections. Each bolt connects the materials twice, once on each side. The 4 connections are not linear, so it&#;s good for stability in bending.
Choose the bolt strength based on what you&#;re asking the bolt to do. For safety, think about higher strength bolts like Grade 8 or more. In shear forces, think about a diameter that will easily take the loads. With tension forces, thick about fine threads as well as higher grade material.
There are 2 ways to achieve higher strength. The first is the bolt material &#; the bolt grade. Second is the bolt diameter.
As a perspective, from the chart in Part 1, we see that (generalized) Grade 5 is about twice as strong as Grade 2. That&#;s a great way to increase strength. Another big improvement is diameter. When we consider diameter, a 3/8&#; bolt is more than twice the strength of a 1/4&#; bolt. In fact, it takes a high grade socket head 1/4&#; bolt to match a 3/8&#; grade 2 bolt. Truly, diameter is also a major factor. Consider this when using Stainless Steel bolts.
Proper bolt choice for strength comes with a balance of the other factors on this page. Sometimes we want a less strong bolt for impact or yield considerations.
By the way, when calculating bolt strength, use the root diameter of the threads (smallest diameter inside the threads), not the full diameter of the bolt. Also, think safety factors &#; and use them generously.
Fine Thread or Coarse Thread? That&#;s a good question without a definitive answer. Here are some considerations.
When choosing bolts, there is not a definitive criteria, so make a call based on the advantages.
For me, I tend to use Coarse threads for smaller bolts (1/4&#;, 6mm and smaller) most of the time. With larger bolts, I use fine threads in vibration situations and when clamping force is critical. On the folding trailer tongue, for instance, the clamping bolts are 1/2&#;-20 (fine thread). Grade 8 for strength over-kill. The pivot bolts are not tight (or it wouldn&#;t pivot), so they don&#;t enter the calculation, but they are the same size and thread so everything matches. (And, use the same wrenches on everything.)
How long is enough? How long is too much?
Long enough is when the threaded portion sticks through the nut with a couple threads out. Too long is when the unused end of the bolt interferes with something &#; or, when the bolt can&#;t completely tighten. Just right might be the bolt you have in the bin (because you don&#;t have to go buy something).
With that said, don&#;t be afraid to cut a bolt. If it&#;s a little too long, cut it to be right, then finish the end. Likewise, if you need a long or custom bolt, make it with threaded rod. Read this full article on working with threaded rod.
Don&#;t forget the washers (if you need them) when thinking about length. Also, special nuts, like Nyloc nuts, require a little more length than standard ones.
For bolts threading into something (not a nut), the concept of &#;long enough&#; must also take into account the thread engagement which is our next bolt choice factor.
It&#;s easy to think about bolts with nuts for fastening &#; especially for DIY. That makes things easy. However, once in a while, you need to tap a hole. Taps are pretty easy to use and, when done right, make great threads for screwing a bolt into. When making your own threads, here are some things to consider.
The rules of thumb for thread engagement involve the material the bolt is threading into as well as the bolt diameter. It is a ratio with thread engagement length as a function of the bolt diameter. For instance, 2x means the depth of thread engagement is twice the diameter of the bolt. In this case, for a 1/4&#; bolt, thread engagement is 2x the diameter, or 1/2&#;. That&#;s just an example.
In general, use 1x for steel bolts into steel threads. 1.5x minimum and preferably 2x or more for aluminum. Magnesium and high strength plastics are 2.5x or 3x.
Again, these are rules of thumb, and alloys of the materials act differently. For Bolts 101 just use these numbers as a perspectives in your bolt choice.
You&#;ll note that most nuts are less than the 1x stated above. That has a lot to do with the way threads are formed. Cutting threads with a tap is not as strong as roll forming threads. Again, just a perspective.
Where do you put the bolts when you&#;re building something? While there are several ways that forces might interact with a bolt, some directions are definitely better than others. If you have a choice, think about bolt positions and force direction as well as convenience and access.
First choice is putting the bolts as Locators only. This works when the parts are pushing into each other and the bolts are only there to locate and hold the position. Not much bolt strength is required for this kind of joint. Although this is ideal, it&#;s often not how the forces direct.
Second choice is putting the bolts in Shear. That means the forces want to cut the bolts. Look above at the hitch photo. Forces of pulling the trailer and braking are perpendicular to the bolts. That is shear. Bolts are very strong in shear, and even when not fully tight, they are still strong in shear. That said, the bolt must be sized for the forces, so strength and diameter are both important.
The last choice is bolts in Tension. I say it&#;s last choice, but it&#;s often the only option &#; or in the case of the blue trailer tongue hinge, it&#;s the best option to meet other goals. Read the section about bolting in our review of the folding tongue for more explanation. In tension, the bolt relies on the the threads to carry the loads. When this is needed, use larger bolts, stronger bolts, and multiple bolts for best security.
The truth is, most bolting connections are not just one of these forces, they are a combination. And, often the forces change at different times, so take that all into account.
As an example, look at the caster photo. In a practical sense, the bolts are there primarily as locators. The wheels carry the loads to the flange, then the flange is against the steel support. In reality, there are also shear forces when the brake is on, or when twisting the caster. Then there are some tension forces when the wheel hits something (like a seam in a concrete floor).
Finally, sometimes forces reverse, like from Shear one way, to Shear the other. These oscillating or vibrating situations are the worst for bolt choice because they pose a challenge in keeping things tight. In these cases, we recommend using more bolts, fine threads, and perhaps especially, a method of vibration resistance. (Like locknuts or wires or cotter pin nuts, . . . .)
While it&#;s not really the last thing to consider, the environment for the bolt is pretty important. See Page 1 of Bolts 101 about finishes for more information. Also, please see Page 3 for information about Stainless Steel bolts if your environment will include significant moisture or chemicals.
We often think of bolts in a classic way like so many of the images on this page. Bolt through a hole, and clamps tight. But there are other applications like the very top &#;feature&#; image. That bolt is sticking out using nuts to anchor it in place. The extending part uses the head and top portion of the bolt as a place to hook a spring.
The spring hook bolt as pretty normal clamping &#; with nuts on either side of the structure. But what about more difficult situations like bolting a trailer safety chain? What about chain held by a bolt? Note the intersecting chain links that will not allow the bolted chain link to set fast against the metal surface. It&#;s hard to clamp the bolt solid because adjacent links are in the way. Bolt choice here is more about size and locking than about getting it properly tight.
In this case, washers are stacked to provide a foundation for clamping forces. (Washers with some grinding to fit the chain.) This can work, but when the bolts are not &#;tight&#;, they will move when yanked into service. Such situations are not necessarily bad, but certainly require added bolt strength (and size).
It&#;s not just a bolt choice, but also a bolting choice. Sometimes re-thinking function is the better way, so the bolts are solidly tight. Here&#;s an example. (See another example in the Chain Anchor System Free Plans Article.)
How much space do you allow for a bolt hole? A lot of DIY projects suffer from bolts that are either too loose in their holes, or too tight and hard to assemble. What&#;s the happy medium?
The problem, honestly, is not usually the size of any given hole, but rather, the alignment of holes with respect to each other. If you do things DIY with a scribe and a drill press, sometimes the holes don&#;t line up perfectly. Yeah, I&#;m right there, so here&#;s a post on Aligning Misaligned Holes.
Typically, I add something like 10% to 15% to the bolt diameter for the hole size. Example: 1/4&#; bolt + 15% = 0.287&#; so round to a 9/32&#; drill = 0.281&#; diameter. I tend to round down as the bolts get larger. Example: 1&#; bolt + 15% = 1.15&#;. That would suggest a 1-1/8&#; hole. To me, the 1/8&#; hole is a little too loose (sloppy fit), so for bigger bolts, I make it more like 10%. Example: 1&#; bolt + 10% = 1.1&#; or with rounding = 1-3/32&#; or even 1-1/16&#;.
A good rule of thumb is to add as much tolerance to the hole as you are able to hold in tolerance on the hole location.
As simple as bolts seem, there is actually a lot to know about them. This is a fairly simple overview as a Bolts 101 kind of DIY introduction. Yet, there is so much more to know. Fortunately, this is also a subject with a lot of great information on the web. Just search on the topics that you need to know. Things like &#;Bolt Thread Pitch&#; or &#;Bolt Shear Strength&#;. You&#;ll find a ton of great stuff in deeper detail.
Again, this article is an introduction with a practical spin rather than a scientific one. It&#;s about bolt choice, specifically because the practical side seems to be missing on the bolting sites we&#;ve seen. Let us know if we&#;ve skipped something important.
Here is some related reading. Nuts and Bolts 101 is a great overview of all sorts of bolting information. Then, when you need fasteners, see how to save money on bolts and where to shop. Good luck with your projects.
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