A Short Article on Mechanical Fasteners

A mechanical fastener is a device that is used to mechanically join (or fasten or affixes) two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or dismantled without damaging the joining components.

A welding joint or riveted joint can be termed as a permanent joint, which if required to be removed tends to damage or destruct both the joining components as well as the joint itself.

Mechanical fasteners are usually made of stainless steel, carbon steel, or alloy steel. There are many different types of mechanical fasteners being used in many different industry sectors. Some of them that are extensively being used in the process piping industry will be detailed out in this article.

Bolts

Bolts are mechanical fasteners used for the assembly of two non threaded components, with the aid of washer and nut. A bolt has a head on one end and threads on the other end. When the objects are fastened, the bolt holds the objects together, and washer and nut fasten from the other end that holds it firmly.

Bolted joint is a combination of the nut applying an axial clamping force and the plain non threaded length (known as shrank) of the bolt pinning the joint against sideways shear forces. The non threaded grip length should be chosen carefully, to be around the same length as the thickness of the material and washers through which the bolt passes. An overly long non threaded length prevents the nut from being tightened down correctly. An insufficient non threaded length results in the threads extending into the hole, and places the shear load onto the threads, which may cause wear on the hole. No more than two turns of the thread should be within the hole.

Screws

Screws, similar to bolts, have head on one end and threads on the other end. However, screws usually have longer threads than bolts, also they can be made with slotted heads. Screws are used in components which contain their own thread or the screw may even cut its own internal thread into them. Screws are used to fasten objects by digging in and wedging into a material when turned, while the thread cuts grooves in the fastened material that may help pull fastened materials together and prevent pull-out.

A screw will usually have a head on one end that allows it to be turned with a tool. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. The cylindrical portion of the screw from the underside of the head to the tip (known as the shank) may be fully threaded or partially threaded.

In the industry, Bolt and Screw are commonly mixed terms so sometimes you will see something that is called a screw or a bolt that is actually the opposite. Bolts are termed as headed fasteners consisting consistent external threads so as they would only get fitted with a non-tapered nut. On the other hand, Screws are headed; externally-threaded fasteners that don’t get fitted with a non-tapered nut and are usually get fitted into the material they generally hold. A screw must always be turned to assemble the joint whereas bolts are held fixed in place during assembly by a tool and only the corresponding nut is turned.

There are many different types of bolts and screws where each is suitable for different types of applications. The figure below shows some of the most commonly used types of bolts and screws.

Nuts

Nuts are flat steel pieces, usually hexagonal in shape with internally cut threads. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together. The two partners are kept together by a combination of their threads and compression of the parts to be held together.

The most common shape for nuts today is hexagonal, for similar reasons as the bolt head. Six sides give a good granularity of angles for a tool to approach from (good in tight spots), but more (and smaller) corners would be vulnerable to being rounded off. It takes only one sixth of a rotation to obtain the next side of the hexagon and grip is optimal.

A wide variety of nuts exists, from household hardware versions to specialized industry-specific designs that are engineered to meet various technical standards.

Bolts and nuts can be made of various materials such as steel, titanium, bronze etc. The finish or plating on a metal bolt or nut affects its look and durability. Below are some common finishes and benefits:

• Zinc – Most common, low cost, resists corrosion and rust
• Nickel –  Very hard finish, higher investment, good corrosion resistance
• Chromium – Bright finish, good rust and corrosion resistance
• Chromate – Adds color, shine, superior rust resistance
• Anodizing –  Aluminum, hard oxide surface, excellent corrosion resistance

Washers

A washer is a thin plate (typically disk-shaped) with a hole (typically in the center) that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Washers can also relieve friction, prevent corrosion or leakage, maintain tension and may serve as a spacer. High-quality bolted joints require hardened steel washers to prevent the loss of pre-load due to  permanent indentation after the torque is applied. Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces.

Washers can be categorized into three types;

• Plain washers, which distribute the load to a relatively larger area minimizing the stress present in the bolt and nut while preventing damage to the surface being fixed.
• Spring washers, which have axial flexibility and are used to prevent fastening or loosening due to vibrations.
• Locking washers, which prevent fastening or loosening by preventing unscrewing rotation of the fastening device. Locking washers is a type of spring washer.

Anchor Bolts & Screws

Anchor Bolt is a type of heavy-duty fastener that’s designed to allow objects to be attached to concrete surface. The bolt head is usually embedded in concrete leaving the threaded end exposed. The threaded end is attached to the connecting object with the help of nuts.

Anchor bolts are used widely for foundation purposes. They are used for anchoring and erecting structures and/or industrial equipment where durability and stability is required. Concrete forms, columns, foundations and pads are the most popular mediums where anchor bolts are used.

Studs

A stud is an externally threaded headless fastener. A stud rod is threaded on both ends with a non threaded section in between. Sometimes full stud rod is also threaded with no non threaded section in between. A nut may be used on one end or both the ends. A stud is similar to a bolt but has no head. Typically studs are chamfered on both ends to allow for nuts to be screwed on. It is not necessary for the stud to have the same diameter at each end, although having the same diameter is the most common. Mostly threaded stud is used in hanging things where the length isn’t readily known until the installation has begun.

The length of stud bolt should be enough to cover entire nuts and joining objects, plus 1.5 to 3 threads exposed. Long studs must be avoided as it increases the chance of corrosion and other damage to exposed threads, which would make subsequent removal difficult. For a performance or heavy-duty application, the use of studs is preferred whenever possible instead of main cap bolts, in at-least those instances where a choice is available. Studs provide the ability to obtain much more accurate torque values because the studs don’t twist during tightening as do bolts. Because the studs remain stationary during nut tightening, the studs stretch in one axis alone, providing much more even and accurate clamping forces. Also, because the use of studs results in less wear applied to the block’s threads, this extends the life of the threaded holes in the block over periods of servicing/rebuilding.

Stud bolts are used in a number of industrial applications to join parts, equipment, flanges and permanent fixtures. They are available in a wide range of sizes and lengths, in double end and tap end configurations. They are also available with full body and cut thread bolts. U-Bolt-It’s stud bolts are strong, reliable and high performing.

• Double-end Studs, have equal-length threads on each end to accommodate a nut. Length of stud is measured overall. Double-end studs are used for flange bolting or other applications where torching from both ends is necessary or desirable.
• Tap-end Studs, have threads with unequal thread engagement length, called the tap end. This end is for screwing into a tapped hole. The other or nut end is threaded. Length of the stud is measured overall.

Fasteners Selection Criteria

When selecting a fastener for industrial applications, it is important to consider a variety of factors. Few of them all listed below.

• Material of construction (stainless steel, carbon steel, or alloy steel).
• Materials to be joined.
• Environment, including temperature, water exposure, and potentially corrosive elements.
• The applied load on the fastener.
• The stiffness of the fastener.
• Special process conditions (special coatings or plating).
• Installation process.
• Re-usability.
• Weight restrictions.