TC Bolts

Tension Control Bolts

Tension control bolts are often called TC bolts or twist-off bolts. They are commonly used in construction applications as a means of connecting steelwork and other heavy elements within a structure. As fasteners, they are considered high strength and are most frequently made from heavy-duty industrial materials like tempered alloy steel and carbon steel. 

Their endurance capabilities may be further enhanced with the addition of finishes, including hot-dipped galvanization (HDG), mechanical galvanization, and zinc-coating. These additions can improve the corrosion resistance of TC bolts, which is critical when they will be installed in applications that might be subjected to moisture and temperature fluctuations. If tension control bolts are required for an interior or enclosed setting, then plain steel bolts will usually perform reliably. They are best used within a temperature range of 40 and 90 degrees F. 

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Although they are known as a bolt, tension control bolts are actually a fastener assembly that consists of a nut, washer, and bolt with a spline, also called the “twist-off”. They are often sold pre-assembled.

The spline is the portion of the component that serves as a tension control feature. During installation, it’s easy to determine when the proper tension is achieved once the spline snaps off the assembly. This provides a reliable tension indicator that prevents over- and under-tightening of the bolt.

The installation of TC bolts is completed with an electric shear wrench. This tool will rotate the nut and the spline simultaneously and in opposite directions. In addition to ensuring proper torque, this installation method can be completed by one person and in an efficient manner.

A propriety lubrication on these bolts also ensures a smooth and controlled clamping force, which is integral to achieving a suitable tension level. Tension control bolt manufacturers add this lubrication through a specialized process. If re-lubrication is required, it is often necessary for a manufacturer to perform this service. 

Tension control bolt specs are defined under ASTM International (American Society for Testing and Material) standards. The most common varieties of TC bolts are ASTM: A325 (F1852) and ASTM: A490 (F2280). ASTM: A325 may be plain or mechanically galvanized, while A490 are uncoated.

What Is A Twist-off Bolt?

A twist-off bolt is another way of referring to a tension control bolt or a TC bolt. The “twist-off” name is derived from the snapping of the spline, which occurs as the bolt is tightened or twisted to an optimal tension level. 

Are TC Bolts Slip Critical?

TC bolts are considered slip critical as they are used to secure a connection through optimal tension or friction. This tensioning is achieved as the bolt is tightened to the point at which the spline separates from the assembly.

What Are High Strength Bolts?

High strength bolts describe threaded fasteners that are made from high strength materials like tempered alloy steel or carbon steel, which are specifically designed to secure and connect critical and heavy-duty parts. These parts include steel elements within a structural application. Tension control bolts are considered high strength bolts.

Can A490 Bolts Be Galvanized?

Per ASTM International standards, A490 (F2280) bolts cannot be galvanized and must remain plain or can be treated with a zinc/aluminum coating. This is a precaution against hydrogen embrittlement, or a loss of ductility, which can compromise load-bearing capability.   

What Is The Difference Between A325 And A490 Bolts?

Though both types of TC bolts are considered high strength and are very similar in structure, A490 bolts offer extended strength and load-bearing capabilities compared to A325 bolts. According to ASTM International standards, A325 bolts may be kept plain or can be galvanized while A490 bolts cannot be galvanized.