How to Choose a Suitable Fastener

Choosing a fastener looks like a small decision, yet the joint it creates must hold pressure, resist vibration and survive the medium it touches. Good selection follows the same logic as choosing flange gaskets: define the joint, the load and the service environment first, then work back to the type, material grade, size and surface treatment.

Fastener Families and Typical Uses

Industrial connections are built from five fastener families. Each one transfers clamping force in a different way, so the joint design decides which family to use.

• Bolts with nuts — the bolt passes through aligned holes and is tightened against a nut; this is the standard arrangement for thick plates, pressure boundaries and flanged joints.
• Stud bolts — a fully threaded rod with a nut at each end, chosen where clamping force must be symmetrical and the joint is opened for inspection.
• Screws — machine, self-tapping and wood screws form or cut their own thread in thinner material, where no nut can be fitted from the back.
• Washers — spread the clamping load over a larger area, protect the bearing surface, and as spring or locking types they resist self-loosening.
• Rivets — permanent fasteners for sheet metal and structures that are never disassembled.

If the joint will be opened again, stay with bolts or stud bolts and nuts; for a permanent assembly on thin sheet, a rivet or a self-tapping screw is usually simpler and lighter.

Material Grade, Size and Load Capacity

The material sets strength, toughness, corrosion resistance and temperature limit, and the bolting is part of the pressure envelope of the joint. In a typical pipe flanges connection an ASTM A193 B7 stud bolt works with ASTM A194 2H nuts, while stainless grades take over where chlorides or acids are present.

Table 1. Fastener materials and typical applications. The values are indicative — confirm the grade and property class against the applicable material standard.

Size decides how much load the joint can carry and how it behaves in service.

• Diameter — section area grows with the square of the diameter, so one size up raises capacity sharply; also check the bearing strength of the plate or the flange hub.
• Length — allow full thread engagement plus nut and washer; too short a grip strips the thread, too long a bolt wastes material and can bend under vibration.
• Thread series — metric (ISO 965) and inch (UN/UNR) threads are not interchangeable, so mating parts must share the pitch and the tolerance class.
• Load type — static, pulsating and impact loads behave differently; pulsating pressure needs a fatigue-rated grade and controlled preload.
• Safety factor — allow between 1.5 and 3 on the design load and adjust it to the actual working conditions, including temperature and possible overload.

Preload is what keeps the joint closed under working load, and torque is only a proxy for it. Tighten in a cross pattern in several passes so the clamping force spreads evenly across the face.

Surface Treatment and Service Environment

The coating decides how long the fastener lasts after it leaves the workshop. Common treatments are:

• Zinc plating or galvanizing — economical rust protection for outdoor and humid service; the layer is thin and does not cover chemical atmospheres.
• Black oxide — a thin conversion layer giving a clean dark finish and limited indoor corrosion protection.
• Painting or powder coating — a thicker barrier with possible colour coding, but the film must not hide thread damage.
• Passivation — required after machining stainless steel so the chromium oxide layer can reform.

Then read the environment: high temperature calls for heat-resistant alloys; low temperature rules out grades that turn brittle; chemical and marine service favours stainless steel or nickel alloys; and joints exposed to vibration need locking washers, prevailing torque nuts and periodic torque checks.

Frequently Asked Questions

How do I choose between a bolt and a stud bolt for a flanged joint?

Use a stud bolt where the joint is opened for inspection or maintenance: it stays in place, the clamping force is symmetrical and thread wear is easier to control. A hex head bolt suits the case where one side of the joint is inaccessible or the plate is thick enough for a tapped hole.

Which material grade should I use in a corrosive environment?

Match the material to the medium, not only to the load. Austenitic stainless steel covers most wet and chemical service, carbon and alloy steels need a coating, and for acids or seawater nickel-based alloys are the reliable choice.

Why does a fastener loosen in service?

Vibration, uneven thermal expansion and loss of preload. Spring or locking washers, prevailing torque nuts and correct preload reduce the risk; check the torque of critical joints after the first thermal cycle.

How do I set the safety factor?

Start from the design load and the strength of the chosen grade, then apply a factor of 1.5 to 3 depending on how well the load is known. Impact loads, pulsating pressure and high temperature push the factor toward the upper end.