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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 utilized.

The HEICO-TEC® Tension Nut, from the company that produces the HEICO-LOCK ® wedge locking washer system, has a unique 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 such specialised tooling. And as this innovative new 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 innovative product, rather than turning the nut and tightening the large main bolt thread, the force is distributed to many smaller thrust bolts. As these thrust 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 thrust bolts with associated pressure pins are arranged around the main thread inside the nut body. When the thrust 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 disk 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 utilized. 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 screws, the pressure screws 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 HEICO 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.

The unique design of the HEICO-TEC® Tension Nut means it can be tightened and un-tightened by hand making it ideal for use in the field but for serial production HEICO have developed the HEICO-TEC® Multi-Tool which allows the simultaneous tightening of all the pressure bolts, simply by the press of a button. The HEICO-TEC® Multi-Tool is driven by a hydraulic unit incorporating a pump, controls and an oil container that can be used with your existing hydraulic supply.

When using HEICO-TEC® for active pre-tensioning with through-bolted connections an equivalent reactive nut is required on the opposite end. By replacing a hex nut in such applications with the new HEICO-TEC® Reaction Nut the concave shape of the contact surface means the Reaction Nut can flex elastically in the direction of the bolted joint. A short bolted joint is susceptible to fatigue failure and selfloosening but the unique design of the HEICO-TEC® Reaction Nut makes such a joint perform like a bolted joint which is 2-3 thread diameters longer and thus more durable and safer.

With the HEICO-TEC® Tensioning System working with large bolted joints is now more simple, fast and reliable.

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