TENSIONING TOOLS

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Technical field

The present invention refers to a structural flanged bolt or nut designed to be installed by tensioning tools, used when high clamping force and tightening precision is required for example in Wind Industry, Civil Construction or Cranes application

Background art

Most of current bolt and/or nut manufacturing methods are based on cold or hot forging processes. Although these techniques greatly expanded previous production capabilities, they are similar in principle to the original processes established in the 1800s. The twentieth century saw the development of our present-day bolt manufacturing methods, particularly through the advances engendered by the two world wars. Specifically the existing standard high load fasteners have been adopted in Europe for preloaded joints application since the '50.

At present bolts and nuts are most commonly obtained either by hot forming (upsetting the stock to form a head, and rolling the thread) or by cold forming.

In general, a cold formed high strength, bolt having a tensile strength higher than 1000 N/mm2 is manufactured by a method in which an alloyed carbon steel wire rod is subjected to annealing and wire drawing. Thereafter the wire is fed into a multistation forming machine that automatically cuts the wire at a designated length and die cuts the head of the screw blank into a predetermined shape.

The heading machine utilizes either an open or closed die that requires two or more punches to create the screw head and the different shaft diameter ready for the following threading operation. The bolt is then subjected to hardening and tempering in order to obtain the requested mechanical properties. Such a process is described, for example, in document US-A-3532560.

High load fasteners are mainly used to join flanged connections, and they undergo a considerable amount of loads and stresses. Joints are expected to hold a minimum preload of 70% of the yield point as a prerequisite for a safe connection after releasing the tightening force.

The most common tightening methods provide applying a torque on the nuts in order to generate a tension in the bolt to obtain the requested preload between flanges.

The relationship between the torque and the resultant preload is quite uncertain and unstable due to the difficulty of keeping the friction coefficients on the different interfaces stable and consistent time after time .

The known solutions, which foresee the presence of a bolt, a nut and two washers for each assemblying point involve additional problems and disadvantages.

Assembly precision is always problematic since there are invariably irregularities between the surface of the washer and the bolt/nut. This may compromise the possibility of precisely pre-loading forces of each assemblying point. Due to this problem and the above mentioned uncertain and unstable friction coefficient it is necessary to carry out careful annual checks on the clamping forces of each assembled kit in order to limit as far as possible the dangerous losses of preload.

These checks obviously increase the costs of service and maintenance of plants in which this type of fasteners are used.

It is well known that there are bolts and nuts with an integrated flange. This flange has the same purpose as the washer, and these bolts and nuts are in fact used without washers. See for instance document WO 2008/149386 Al.

Joints are expected to hold a minimum preload of 70% of the yield point as a prerequisite for a safe connection, but after releasing the torque the real preload can vary in such a high range that a 100% periodical check is unavoidable.

On the other hand, tensioning is recognized to be the most reliable method due to the application of the force directly on the bolt. Joints tightened using tensioning devices are subject to tension up to 95% of yield point, and are expected to hold a minimum preload of 70% of the yield point after releasing the stretching tension.

Adopting the existing standard high load fasteners, the load loss is never predictable in an accurate way.

Description of the invention

The present invention aims to provide a fixing system, in particular a high load flanged fastener for heavy metalwork elements that ensure constant and repeatable pre-loading forces. The problem underlying the present invention therefore lies in providing a high load fastener, which is effective in enabling the joints tightened using tensioning devices to reach a minimum preload of 70% of the yield point with a considerable safety margin by minimizing the preload losses.

Said problem is solved by a high load fastener for flanged connections having the features disclosed in claim 1. The dependent claims outline advantageous forms of embodiment of the present invention.

Illustration of drawings

Other features and advantages of the present invention will be more readily understood from the description that is given below by way of a non-limiting example with reference to the accompanying drawings, in which:

Figure 1 shows a diagram of applied preload forces (M36 joints) after traditional tightening by torque wrench (preload variability 50%);

Figure 2 shows a diagram of applied preload forces (M36 joints) after tightening by tensioning device (preload variability 5%) in accordance with the present invention; and

- Figure 3 shows a partially sectioned, side view of a traditional structural bolt (HV bolt) compared with a flanged fastener used in accordance to the present invention. Description of a form of embodiment of the invention

The basic idea, underlying the present invention lies on providing a fastener with dimensional and mechanical tolerances, processable by cold or hot forging in a multistep sequence and in the same time meeting the strict requirements required by the tensioning method to minimize preload losses.

The high load fastener according to the present invention may either be manufactured by applying a cold forming technique, or through a hot forming technique.

When applying a cold forming technique, a process for obtaining a high load fastener according to the present invention normally comprises the following subsequent steps :

1 ) Wire rod annealing;

2 ) Wire rod coil pickling and phosphating;

3 ) Wire rod drawing ;

4 ) Wire rod cutting;

5) Cold forming on multiple stations.

On the other hand, when applying a hot forming technique, a process for obtaining a high load fastener according to the present invention normaly comprises the following subsequent steps:

1 ) Bar cutting;

2) Induction heating;

3) Hot forming by upsetting;

4) Stem machining.

Through either of these methods it is possible to obtain a 10.9 or higher class steel, i.e. a steel workpiece having a minimum resistance to traction (Rm) of 1040 N/mm ² .

The applicant has carried out extensive experimental test runs and found that, surprisingly, a joint tightened using tensioning devices to reach a minimum preload of 70% of the yield point with a considerable safety margin by minimizing the preload losses can only be obtained by using a high load fastener having the following combination of features:

a) a underhead surface area with a planarity not higher than 0,3 mm, as defined and measured according to ISO 4759-1 and ISO 1101;

b) a underhead squareness between the underhead and the thread axis of the bolt lower or equal to 0,2 mm as defined and measured according to ISO 4759-1 and ISO 1101;

c) a shank diameter tolerance lower than +/- 0,05 mm; d) a hardness comprised between 34 and 37 HRC.

Through the feature a) it is possible to maintain adequate pressure stress level on the flange material without any need of interfaces such as washers or similar; based on this feature and removing the washer under the head of the bolt or nut, preload losses after the release process with tensioners may reduced by 7% in respect of the prior art solutions .

In what relates feature b), it is useful to remind that a traditional method as the one for instance disclosed european patent No. 1214945 of cold or hot forming of high load fasteners having a diameter larger than 30mm results in tolerances which are 2-3 times higher than those applied according to the present invention. The reduction of the tolerances reduces the deformation of the bolt and allows to avoid the use of a washer under the nut. Based on the restricted tolerances and removing the washer under the nut, the preload loss after the release process with tensioners may be reduced by a further 5% if compared to a prior art solution such as a traditional fastening according to the standard DIN6914-15-16. Turning now to feature c), it may be noted that the reduction of the shank diameter tolerances allows a linear behaviour of the bolt deformation under tensioning stress to be mantained.

Feature d) is obtained by means of a heat treatment subsequent to the hot or cold forming process for manufacturing the flanged bolt or nut. This heat treatment consists in hardening and tempering of the workpiece and results in a restricted range of hardness (34-37 HRC) within the standard specification tolerances (32 HRC - 37 HRC as prescribed by the ISO 898). Based on the higher hardness characteristic it is possible to safely apply during the tensioning process, a second cycle at higher force (95% of preload instead 90%, an additional 5.5%) because of the higher yield point. After releasing the tensioning force, the preload of the bolt is 7.5% higher than through tensioning at 90% of standard yield point, because a higher force is applied during the tensioning operation.

The result of the use of a high load fastener according to the present invention may immediately be appreciated by comparing the diagrams shown in respective figures 1 and 2.

In fact, the diagram of fig. 1 clearly shows how the application of a traditional torquing method with traditional fasteners and washers results in highly unstable, unpredictable and different preloads.

As a matter of fact the residual preload FV is sometimes higher, sometimes lower than the prescribed value FN according to the DIN standard. On the other hand, fig. 2 shows that by using flanged bolts and nuts according to the present application the preload values FV which may be achieved are always very close to the prescribed values FN, thereby obtaining a more stable and safe connection.