Here's a buyer's guide to getting the right bolt for your job.
There are many types of bolt on the market, but choosing the right one for the job is so important. Bolts are a type of mechanical fastening, manufactured with external threads and when used with nuts, they make a strong fastening.
Bolts are made to be inserted into pre-drilled holes, which makes them different from screws. Most bolt fastenings also require a nut to combine the objects together in a tight and strong fix, although not all bolts require nuts.
Fasteners like bolts can provide a strong and stable fastening and come in a range of materials suitable for all types of connections and weather uses.
Advantages of Nut & Bolt Fastenings
The advantage of using nuts, washers and bolts as a fastening method means you can unscrew them anytime, making them easier to replace. Unlike riveting fasteners, bolts and nuts are far more flexible. Riveting is a more permanent method of fixing, whereas bolting items together with a nut and bolt still makes a good, solid fixing, but is far more adaptable.
A wide variety of surfaces can be connected together, including; wood, steel, iron, plastics, concrete and sheet metals. Using a washer means you spread the tension over a much wider area and can also protect the surface from being damaged.
Materials & Grade Options For Metric Bolts & Nuts
Depending on your required use, bolts come in a variety of finishes so you have to chose the right bolt material for the intended job. All bolts, nuts, screws and washers are assigned a grade which sets out the minimum acceptable standard of performance and mechanical properties of the particular fastener.
Watch this video to understand more about bolts and fixings:
About Bolt Grades
Grade identifications for bolts appear on the head of the bolt and the larger the grade the higher the strength characteristics of that bolt. The most popular grade for metric bolts is ISO 898-1 which is for nuts, bolts and studs made from carbon steel and alloy steel. This grade, or property designation, has a two digit number separated by a dot stamped onto the head of the bolt. For example 8.8. The figures mean:
- 1/100th of the parts nominal tensile strength (Megapascals or MPa’s)
- 1/10th of the percentage of the parts nominal yield strength compared to the parts nominal tensile strength
What does this all mean? This property class bolt has a tensile strength of 800 MPa and a yield strength of 80% of that tensile strength - 640 MPa.
Bolt Materials
Steel is the most common material for fasteners as it has so many variations depending on your requirements. The surface treatment of steel means it can be galvanised, or plated with zinc or chrome.
With enhanced corrosion characteristics, stainless steel is an alloy of low carbon steel and chromium, making it highly resistant to corrosion even if scratched or damaged during installation. However, stainless steel is not as strong as hardened steel as it cannot be hardened during the heating process due to its alloy content.
Stainless steel fixings are also less magnetic than regular hardened steel fasteners.
What Does 18-8 Stainless Steel Mean?
18-8 stainless steel refers to the mix of steel and alloys. In this case, 18% chromium and 8% nickel, which happens to be the most popular content for stainless steel fasteners.
A4 stainless steel is often called marine grade stainless steel as it less corrosive and ideal for many outdoor projects especially in salt water areas. A4 stainless steel is often more expensive but over time they will repay you in dividends as they will not need replacing as often.
Bolt Grades & Their Meanings
The most common grades for steel fixings are 2, 5, 8 and Alloy Steel. These are the ones that are used most regularly when it comes to fixings and fasteners. Grades 2, 5 and 8 usually look more 'silver-ish' in or yellow in colour due to their plating, whereas alloy steel that has not been plated will look darker.
Types of Bolt Fixings
Once again, depending on your desired usage, bolts come in different varieties, each with different characteristics, making them a very adaptable and purposeful fixing. Here’s a short list of the many varieties of bolts available.
- Anchor Bolt Fixings
- Socket Head Bolt Fixings
- Hex Bolt Fixings
- Carriage Bolt Fixings
- Flange Bolt Fixings
- Double End Bolt Fixings
- Eye Bolt Fixings
- Penta Head Bolt Fixings
- Shoulder Bolts
- U-Bolts
Here’s a description of some of the bolt fixings available and in which scenario they would be ideal to use as a fixing.
Anchor Bolt Fixings
Anchor bolt fixings can be used for highly structural work, such as fixing a metal structure to a concrete slab or foundation. They can be preset whilst the concrete is being poured and the structure bolted into place once the concrete base has set. Or you can simply drill a hole into the concrete base and set the anchor bolt in place, usually with setting resins, as this will provide a really solid and secure fixing. The top plate or structure is then bolted into place using washers and nuts.
Socket Head Bolt Fixings
Socket head bolts come in a variety of head shapes, including domed, countersunk, cylindrical and hex head. They have a recessed head socket that allows tightening using an Allan key or hex head socket tool, the deeper the recess the less chance you have of stripping the hex opening and producing a stronger tightening.
Hex Bolt Fixings
Hex head bolts come with, as the name suggest, a hexagon-shaped head, making it ideal for tightening with a wrench or spanner. They do not always come with a fully threaded shaft, it really depends on the manufacturer, so always make sure you know what you order. The threaded part of the hex bolt protrudes beyond the jointed pieces and allows tightening with a nut or washer and nut. This type of bolt helps prevent against water damage to the joint as water will travel up the thread and into the jointed pieces, it stops at the end of the thread.
Carriage Bolt Fixings
Often classed as a self-locking bolt due to its under head shape and its flush-mount domed head. This square under head section can be compressed into wood or a cut-out section of the material being fixed, to provide an anchor section for the bolt head to fix into. This means the coach bolt can be tightened from the underside using a nut with the same M value as the thread diameter. For example, a M10 coach bolts require an M10 nut.
Flange Bolt Fixings
Flange bolts look as if they come ready with a washer installed just below the head allowing for an even and wider spread of pressure on the fixing surface during the tightening process. The underside of the flange part of the bolt can either be serrated or smooth, depending on your requirements. The rougher serrated surface will provide a slighter tighter grip during tightening.
Double End Bolt Fixings
Sometimes called stud bolts, these double ended bolts have a screw section and a traditional bolt thread section. The screw section goes into a pre-drilled hole and tightened, whilst the other end protrudes to allow the attachment of a washer and nut.
Eye Bolt Fixings
The head of the eye bolt comes either fully connected as one continuous loop or partially closed with a slight gap in the loop. They can be bolted in place and used to thread cables for lifting, fixing garden gates to posts, or for running cables through and keeping them in place. Some structures also use eye bolts to attach wire cables to strengthen the fencing. Other options include having a base plate underneath the head loop to allow for more pressure to be spread evenly whilst tightening in place.
Penta-Head Bolt Fixings
Penta head bolts are usually used in security fixings where a tamper proof fixing is required. The head requires a specially designed socket tool to tighten and loosen the joint.
Shoulder Bolt Fixings
Shoulder bolts have a plain cylindrical shaft directly underneath the head to allow the unthreaded part of the bolt to be used as an axle containing a rotating part or bearing. The shoulder bolt is often referred to as a stripper bolt.
U-Bolt Fixings
As the name suggests, a U-bolt is a 'U' shaped bar with screw threads at either end for the attachment of washers and nuts. Not being fully threaded allows for the U bolt to clamp or support specific parts, such as pipes and cables. They can also be square shaped.