Bolting Terms

Clamp Force: The compressive force on the joint members that results from the tensile force developed in a tightened bolt. This force, not torque, is what holds your bolted joint together.

Design Tension: The tensile force on the bolt, measured in klbf or kN, at which SmartBolts® are designed to indicate the “Tight” color. “Tight” shows the bolt has been properly installed and Preload is established. All of the SmartBolts® products listed in this catalog have an indicating Design Tension of 70% of fastener Modified Proof Strength.

Elastic Interaction: In a bolted joint with multiple fasteners, the variation in individual fastener preload due to tightening of the other fasteners.

Embedment: Localized yielding of bolted joint components resulting in a change of grip length consequently causing relaxation of the bolted joint.

Grip Length: The combined thickness of all components joined together between the bolt head and nut. Given in inches for Inch series SmartBolts® and millimeters for Metric series SmartBolts®.

KIPS: A non-SI unit. 1,000 pounds-force, used primarily by American architects and engineers to measure engineering loads

klbf (kilopounds-force): A measure of tensile force or clamp force on a bolt as it is tightened. 1,000 pounds-force. The symbol KLBF is used when it is necessary to clearly distinguish it as a unit of force rather than mass. The name comes from combining the words “kilo” and “pound”; it is occasionally called a kilopound.

Design Tension, Proof Strength, and Ultimate Strength are expressed in klbf for Inch series SmartBolts®.

kN (kilonewtons): A measure of tensile force or clamp force on a bolt as it is tightened. Design Tension, Proof Strength, and Ultimate Strength are expressed in kN for Metric series SmartBolts®.

Minimum Grip Length: With respect to SmartBolts® in this catalog, if a Minimum Grip Length is denoted, there is a limit to how far the nut can be run up the shank during installation before it begins to impact the accuracy of the SmartBolts® indicator. Customers may contact the factory to learn what the specified minimum grip length is for a particular SmartBolts® model, if it applies.

Modified Proof Strength: The proof strength of the bolt after conversion into a SmartBolt. In all but the shortest bolt lengths listed in this catalog, this strength is identical to the bolt proof strength before modification.

Nut Factor: A constant that attempts to model all of the many variables (such as friction) that affect the torque-tension relationship. Torque charts depend on an assumption of the nut factor, but it is very difficult to predict and measure. SmartBolts® eliminate the nut factor from the bolting equation, measuring and displaying Tension directly.

Preload: The tensile force developed during installation (tightening) of a bolt. This load can be directly measured and displayed by the SmartBolts® indicator.

Proof strength: The tensile strength of a bolt beyond which permanent elongation of the bolt may occur. SmartBolts® are designed to operate in tensile loads from zero up to proof strength. This is called the elastic region of the bolt.

SI: The International System of Units, an evolution of the metric sytstem.

Strain: With respect to bolts under tensile force, strain is the measure of elongation of the bolt as it is stretched.

Stress: An engineering value that is defined as force per unit area. For bolts, it is often expressed in PSI (pounds per square inch). The metric equivalent measure is MPa (megapascals).

Stress Area: The effective cross sectional area of the threaded region in a bolt.

Tension: The tensile force developed by tightening a bolt. It is equal and opposite to the clamp force – the force holding your bolted joint together. Tension is the force that SmartBolts® measure and display.

Torque: A twisting or rotational force given in units of pound-feet (lb-ft) or Newton-meters (Nm). It is a measure of how much force is required to turn the head or nut of a bolt. Torque is related to bolt tension by the nut factor, which is difficult to predict. SmartBolts® allow proper bolted joint installation without concern for torque or nut factor, eliminating the requirement to use a torque wrench.

Ultimate Strength: The maximum tensile load that a bolt can withstand before breaking.

USCS: The U.S. Customary System, an evolution of the British Imperial System