Why Bolt Tension Safety is Important


On August 20th, 2011, the U.S. military investigated the crash of a Global Hawk aircraft drone outside of Kandahar, Afghanistan. The subsequent report concluded that the cause was an improperly torqued fastener. In total, it cost them 73 million dollars.

This is just one example of why bolt tensioning safety is so crucial to manufacturers and why accountability and liability are of increasing concern. Let’s look at the basics of how to measure torque first.

What is Torque?

Torque is caused by applying a force at one end of a lever, causing a turning motion at the pivot point (the opposite end of the lever), and is measured by multiplying the force times the length of the lever. For example force in pounds multiplied by the length in inches will give torque in inch pounds. Typically, the lever length would be the length of the wrench (say 2 feet) and your hand would apply force (example 10 pounds of force), resulting in 20 foot-pounds of torque (2 foot X 10 pounds = 20 foot pounds).

Torque is used to create tension. Bolts are used to affix two components so that they can resist tensile (pulling apart) and shearing (sliding apart) forces. After the nut has been seated & pulls the assembly together, additional turning due to the ramp angle of the threads causes the nut to stretch the bolt, creating tension in the bolt and a clamp load on the assembly.

When the bolt stretches, it becomes a solid spring that clamps the components together. As long as the clamp load is not exceeded, the components won’t be pulled apart. Under shear loads, the bolt diameter resists lateral movement of the components.

Determining how much torque is needed depends on two related questions:

  • The material properties of the bolt (i.e. the metal and heat treatment of the bolt)
  • The separating forces in the assembly

In short, you need the right bolt materials for the job.

You can both over, and under torque a bolt. According to the U.S. military’s report, their drone had cap screws that were not torqued to 18-22 inch-pounds above prevailing  torque, as per specifications, allowing the cap screws to loosen in flight.

Typically, an under torqued bolt will be unable to provide as much clamping force as needed. An over torqued bolt will not provide clamping force required due to elongation or will break.

On a small scale, hand torque wrenches will measure the amount of torque being applied in either analog or digital formats. To function reliably they must be treated with care. Unlike a hammer, or a screwdriver, a torque wrench is a precision instrument and must be handled properly to avoid inaccurate results, and the possibility of failure of the joint.

bolt tensioning

The Importance of Bolt Torque Reporting

On a larger scale, it’s now more important than ever to be able to measure and record torque data to help with accountability and liability in every day applications.

In many industries, a number of jobs require a record and report of torque or tension attained on each bolt. Ultra Torq provides tools to help capture final torque data, record it, and also offers software for downloading report data into a proper bolt torque report format. This is much more efficient than having to manually record data.

Products that will measure and record torque on the tool for later download, are:

  • ERAD nut runners
  • RAD Transducer nut runners
  • Stahlwille wrenches
  • SCS Freedom 3 wrenches

These tools are very accurate, and require no extra labour, such as a manual audit would require.

In addition, products are also available to help control hydraulic tools, made by Torq-Comm. These tools help one person remotely operate both the hydraulic wrench and the pump, as well as record data for download afterwards. Wind towers are one example of the need to have proper records of bolting measurements. Overall, these tools also assist with liability by providing a record showing that the bolts were tightened to their specified torque.

Whether you’re creating a multi-million-dollar drone, or something more every day, attention to bolt tension safety and measurement is crucial to proper function and accountability. Our selection of products will allow you to measure and apply the proper torque for the results you need.