Dave Read explains how tensioning large bolted joints can be made simple.

Large bolted joints are a common site in safety-critical applications across the oil & gas industry. Ensuring the correct tensioning of these joints is a complicated but essential process.

When tensioning bolted connections, the turning movement of tightening results in axial elongation and preload of the bolt due to the inclined plane of the bolt threads. This leads to thread deformation that reduces the pre-load, durability and re-usability of the bolt.  The torsion stress from thread friction and deformation also reduces the axial load bearing capacity of the bolt. 

Tensioning of large bolted joints usually requires the use of electric, hydraulic or pneumatic tools, a factor that adds additional costs and delay to any project. When tightening a bolt with a hydraulic cylinder the bolt is elongated and the nut is then tightened free of load.  When the hydraulic pressure is released the bolt contracts, resulting in the proper tension in the bolt being achieved. The disadvantage of this process is that as the nut was not pre-tensioned while the bolt was elongated, a percentage of the elongation of the bolt is lost in joint settlement, meaning only around two thirds of the bolt strength can be used.

A new approach

The Heico-Tec tension nut has a novel design that eliminates the requirement for specialist tools as the user only requires a hand-held torque wrench to achieve the required tension, resulting in savings in both time and money in comparison to specialised tooling. And, as the product meets all the requirements of DIN EN ISO 898-2, it can easily be used as a direct replacement for any hex nut from the same strength class whilst also offering all the advantages of a hand-tightened nut with no need for time-consuming calculations and formulas to determine suitability.

With this product, rather than turning the nut and tightening the large main bolt thread, the force is distributed to many smaller pressure bolts. As these pressure bolts are smaller, a conventional torque wrench is all that is needed to achieve the required pretension force.

The Heico-Tec design consists of the nut body that is screwed onto the main thread just like a conventional nut – but it is not tightened. Several pressure bolts with associated pressure pins are arranged around the main thread inside the nut body. When the pressure bolts are tightened one by one, they push the pressure pins against the clamped parts while simultaneously elongating the bolt. The pre-tension force created in this way is purely axial and therefore free from harmful torsion or bending.  An incorporated hardened load bearing washer protects the tensioned parts from high pressure loads caused by the pressure pins.

As the stress on the bolt with Heico-Tec is purely axial, not torsional, thread friction and deformation is greatly reduced, ensuring that the full load-bearing capacity of the bolt can be used. Also, the loss of elongation of the bolt that occurs from joint settlement as a result of using hydraulic tensioning is not present and full capacity of the joint is achieved.

With the use of common tension nuts with pressure bolts, the pressure bolts are arranged closer to the bolt thread and have a smaller hexagon head. This results in less space to manoeuvre and the use of such tension systems results in higher costs due to the need for a special thin-walled socket and wrench. Typically, this tension system is not as strong as standard hex nuts and cannot replace commercial hex nuts, unlike Heico-Tec, which can be used as a direct replacement.

The inherent mechanical flexibility of the design means the bolted joint performs as if it has a greater clamping length. This makes it highly resistant to loosening forces and, as the assembly
is not damaged during tightening and loosening, is totally reusable with its greater flexibility reducing the bolted joint’s dynamic stress and increasing its service life.