On the 20th of August 2011, the US Air Force investigated the crash of the Global Hawk RQ-4 Air Force Drone just outside of Kandahar in Afghanistan. The report concluded that the cause of the crash was an improperly torqued fastener. This ended up costing them 74 million USD.

Torque tools are precision tools that are used to apply a specific torque to a fastener such as a nut, bolt or screw. Torque tools are used where the tightness of screws and bolts are crucial to avoid loose parts and ensure that gaskets are sealed tight.

Tensioning of bolts can be performed in several different ways. The most common method is using a calibrated torque wrench. The torque that is necessary is calculated from the prestressing force in the bolt. Another method of bolt tension is rotational tightening where the tensioner is rotated to directly produce the preload. The final method is using hydraulic torque tools. Prestressing force in bolts can reduce over time and this is primarily due to overload or fatigue load.

In order for bolt connections to withstand static and dynamic loads over time, the bolts have to be fastened with a certain pretension. Lack of this can result in dynamic loads that can result in fatigue of the bolts. Necessary prestressing force depends on the quality of the bolts and the surface treatment, as well as the friction between the screws/nuts and base material.

Prestressing force must not exceed the yield stress of the bolts. Tensioning of bolts can be performed in these ways:
- Using a calibrated torque wrench
- Turn-of-the-nut rotation
- Combined method

A calibrated torque wrench allows fasteners to be tightened to the proper tension, helping to avoid damage from over tightening or joints coming apart from under tightening. This means applying the correct torque measure is important for safety, assembly and performance of equipment. The torque wrench is calibrated so that the wrench “releases” when the correct amount of torque is achieved.

The “turn-of-the-nut” method is performed by rotating the nut or bolt of a fastener assembly a specific turn angle based on the fastener's length, and diameter while restraining the unturned element from rotating.

The US Military concluded in their report that the RQ-4 Drone had cap screws that were not tightened to the torque specifications, which resulted in the cap screws loosening during the flight.

kNm Hydraulikk offer a number of tools within bolt tensioning from both Enerpac & Gedore. Find out more about these product by clicking the links below:

Enerpac Torque Tools
Enerpac-Hydratight Bolt Tensioners
Gedore Torque Tools
Enerpac Torque Checker
Enerpac Mobile Calibration System

Call us on +47 952 51 310 to find out more or send us an email
The drone before the crash. 
The drone after the crash.