The sheer variety of fasteners can be mindboggling, especially if in doubt as to which fastener you need to use for your project. Though all fasteners create a bond between two objects of the same or different materials, how they do it also matters. For light to medium applications, screws are the preferred choice as they create a secure connection that should last. But when you need the highest holding strength, there’s no substitute for bolts.
Combined with washers and nuts, bolts spread the load over a larger area ensuring less wear and a more permanent connection. As opposed to screws, they can be easily removed by unscrewing the nut that holds everything in place. The torquing of the nut as opposed to the head on the screw is one basic distinction between bolts and screws. The other is that bolts are driven outside of the head and screws are operated by a central drive. All this leads to the higher holding strength in bolts, making them the fastener to use in heavy-duty applications.
Parts of a Bolt
Bolts consist of three distinct parts – the head, the shank and the threading. The head is the upper portion of the bolt. The head can have a different shape and lends to different types of bolts. The shape of the head also determines which tools you need to tighten or loosen the bolt. The shank is the smooth gripping part just below the head, and is also referred to as the grip length. This is the part of the bolt that accommodates the parts that need to be joined. The bolt then extends into the thread length, or the part to which threaded nuts are attached. When shopping for bolts, bear in mind the nominal length of the bolt, or the combination of shaft and threading, and the diameter denoted in millimetres.
Types of Bolts
There are dozens of bolt types, each with different designs, particularly the shape of the head and with different uses. Bolts hex are some of the most common.
Hex bolts get their name from the six-sided hexagonal head. They can be fully or partially threaded with a visible grip length. Bolts hex are widely used in numerous industries and in securing different types of materials. Common uses include construction, machinery assemblies, engineering applications and in the automotive industry for heavy-duty fastening and fixing applications. They can carry heavy loads in wood, various metals, concrete and other materials. The hex head allows for easy tightening and loosening with spanners or hex bolt wrenches.
Hex bolts, like other bolt types, come in different materials. Steel comprises over 90 per cent of fasteners, and various steel and steel alloys determine overall strength. The higher the grade of steel, the higher the tensile strength. This often appears as clear markings on the head. Stainless steel is commonly used outdoors because of its high resistance to corrosion, while zinc-plated steel bolts are often the preferred bolt type for indoor applications. Other materials, like brass, are less frequent but have better corrosive qualities and also a higher price. Coatings and heat treatment also matter for overall wear and tear. Typically, treatment is done with zinc in various processes allowing for armoured bolts, hot-dip galvanised, zinc-coated and yellow-passivated bolts. Chrome-plated bolts with black oxide coatings are also frequently used, especially where friction and hardness levels are involved.
Sizes of hex bolts vary. The length is measured from the chamfer or end to just under the head. The diameter is that of the threaded part marked with an “M” and a number in millimetres. Threads can also be coarse or fine. Bolts hex with a higher thread pitch or fine threads are better for more demanding applications and can carry heavier loads.
Other Bolt Types:
- Anchor bolts – these are used in masonry anchors to attach structural elements in wet concrete during construction, also known as pre-installed anchors, or to attach objects to existing structures made of concrete, brick or stone.
- Carriage bolts – also called cup head bolts because of the shape of the head, are used where the head either cannot be tampered with or needs to provide a smooth finish. These bolts include a square section under the head, and are tightened with nuts along the thread. Washers reduce friction and increase strength. Carriage bolts are more often found in wood, but the snug finish also complements metal projects. Roundhead bolts are similar but don’t have the square tapering beneath the head.
- Eye bolts – have a full or partial loop instead of a traditional head. The loop can be used for various purposes, like hoisting or lifting, as well as a securing point for cables or rope in attaching various items to walls, ceilings or objects. Similar in design are hook bolts, which have a hook-shaped head and are used in hanging a variety of items.
- Flange bolts- these have a skirting stemming outwards from and around the head. Its purpose is to better clamp the joining materials and evenly distribute weight. Flange bolts are used in the car industry, plumbing, and electronics where the use of separate washers is impractical.
- Squarehead bolts – these were frequently used before the arrival of hex head bolts. Today they’re found in railway applications, and where there’s the need to replicate an old-fashioned look to the joining parts. They’re available in most steel grades as with hex bolts and come in similar designs in terms of threading and size.
- T-head bolts – are used with nuts to join objects with T-shaped slots. When installed, the head of the bolt sits below the material surrounding it. T-bolts are used in such things as channels and guide rails.
- U-bolts – are partially threaded at both ends and used in piping and automotive applications, or in fastening and fixturing circular objects.
Other bolt types include track bolts, hollow bolts, banjo bolts, bolt ploughs and roofing bolts.
Bolts offer advantages in terms of strength, longevity, clamping force and versatility over other fasteners, and are usually used along with nuts and washers in the same diameter and materials.