How to Choose the Right Fastener

In the process industries, such as oil, gas, and petrochemicals, the selection of the right fastener in the right application can be critical. The bolts used to connect companion flanges, for example, or the screws used to connect a pipe standard to a load bearing wall, or the nuts and bolts used to connect the dished ends of a reactor are all critical to production. Picking the right fastener is also critical to safety, both for the process and for the plant workers. Picking the right fastener will not be noticed. But failing to pick the right fastener for an application may cause problems from a small leak to a disastrous failure, and that will be noticed for sure. We are going to look at how to pick the right fastener every time.

Know the Application

Before you call your fastener supplier, you need to know what the application is. There are so many applications for industrial fasteners that even a list is out of the scope of this paper, but the kinds of fasteners and their suitability for applications and service are often the same in many applications. Whether you use mild steel or Chrome Moly, or even something exotic will depend of the requirements of the application. You should also know how to use the standards that cover fasteners and their uses. ASTM A307, for example, covers carbon steel bolts, studs, and threaded rod up to 60,000 PSI tensile strength. ASTM F593 covers stainless steel bolts, hex cap screws and studs. ASTM A193/A193M covers alloy steel and stainless steel bolting for high pressure or high temperature service and other special applications. You can often find your application, or an application that is quite similar to your own, mentioned in these and other standards.

Select the Right Type of Fastener for the Application

There are some basics here. Will your fastener fit? Sometimes more than one type of fastener will appear to work. That’s where your experience and that of your fastener supplier will help you decide on the right fastener for the application. Typically, a bolt consists of a head, a shaft of some length, and a threaded end.

The head of the bolt can be hex, screw head, socket head, or have another design. The design of the bolt head can be important for the amount of torque the bolt will take, and other issues, so it is important to select the bolt head properly. The length of the bolt is based on the application. For example, a flange bolt needs to be long enough to hold a washer, the flange, the flange gasket, the companion flange, another washer and the nut, with several threads left over for safety.

Threads are part of the application, too. Whether coarse (UNC), fine (UNF), or 8 thread (UN—mostly used in oilfield applications) the threads need to be chosen to provide the best load handling characteristics. There are also Metric standard threads, BSW (British Standard Whitworth), and others. Thread selection is important: coarse threads will allow for quicker assembly. Fine threads take longer to assemble, due to the number of revolutions required to travel the same distance as a coarse thread. However, they provide better thread engagement, and more of the mating surfaces are in contact, which provides a more secure connection that can accommodate greater tension in the connection.

Materials of Construction

The most common fasteners in use are the hex head Grades 2, 5, and 8. Grade 2 is a standard hardware-grade steel. This is the most common grade of steel fastener, and is the least expensive. Grade 2 bolts are typically used where high strength is not a concern, such as handrail installation, and on pipe clamps and hangers. Grade 5 hex head bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions, and equipment assembly.

Other materials can also be used for fasteners: brass, bronze (especially for marine and offshore applications), and occasionally aluminum. These are often chosen for the same reasons as stainless steel. These metals and alloys have good corrosion resistance. In the case of brass and bronze they may also have an aesthetic appeal. With aluminum, you also have a very light weight fastener. Similar to stainless, these fasteners lack the strength of most carbon steel bolts. Their strength is typically that of an un-hardened Grade 2 fastener.

If the device or pipe or flange for which you are selecting a fastener has a specification for the fasteners, you should always adhere to that specification. Otherwise, you need to understand the parameters of the application and select the fasteners accordingly. Most of the information you need can be found in the ASTM specifications. The most common bolting specifications are:

ASTM A193 - Alloy steel and stainless steel bolting materials for high temperature or high pressure service.

ASTM A307 Carbon steel bolts and studs, 60,000 psi tensile strength.

ASTM A354 Quenched and tempered alloy bolts, studs, and other externally threaded fasteners.

ASTM A449 Quenched and tempered steel bolts and studs for general use.

ASTM F593 Stainless steel bolts, hex cap screws, and studs.

ASTM F1554 Anchor bolts designed to anchor structural supports to concrete foundations.

ASTM F3125 New, unified structural bolt specification replacing A325, A325M, A490, A490M, F1852, and F2280

ASTM A194 Carbon and alloy steel nuts for bolts for high pressure and high temperature service.

ASTM A563 Standard specification for carbon and alloy steel nuts.

ASTM F594 Stainless steel nuts.

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