Hydraulic friction- <span>and torsion-free tightening</span>
Hydraulic friction- <span>and torsion-free tightening</span>

Hydraulic friction- and torsion-free tightening

TECHNOLOGY

Hydraulic friction- and torsion-free tightening
ITH Stretch Method

ITH is the world market leader for bolt tensioning cylinders (hydraulic tensioning devices) based on the tightening method of hydraulic friction- and torsion-free tightening (ITH Stretch Method). As early as 1979, ITH company founder Hans Hohmann successfully introduced the ITH Stretch Method on gas and steam turbines. Since then, ITH Bolt Tensioning Cylinders have been used in all branches of industry.

Continuous innovations, numerous patented solutions and successfully implemented customer and industry solutions within the last decades prove the innovation and market leadership in the field of hydraulic friction- and torsion-free tightening.

In hydraulic friction- and torsion-free pretensioning, a force-controlled process, the assembly pretensioning force FM is generated by axial elongation of the bolt by means of a bolt tensioning cylinder (SSZ). The method is particularly suitable for pretensioning bolts larger than M20, but can also be used for smaller nominal diameters. Since the assembly tensioning force FVM is introduced into the bolted joint only by tensile loading, no thread torques MG and no head and nut friction torques MK occur. The frictional moments at the contact torque MVM have no significant effect on the assembly tension force FVM.

Schritt 1
Schritt 1

Step 1

In the first step, the bolt tensioning cylinder is screwed onto the projecting thread by means of a hand ratchet or an automatic unscrewing device. During this process, the tool encloses the nut and is supported on the component.

Schritt 2
Schritt 2

Step 2

By applying the hydraulic pressure p (control variable) in the second step, a purely axial elongation I (control variable) of the bolt takes place. No torsional or bending stresses are generated in the bolt. The bolt tensioning cylinder generates the bolt force FVH. The nut, which is not under load, lifts off the bearing surface. 

Schritt 3
Schritt 3

Step 3

Then, in the third step, the unloaded nut is tightened until it rests on the component. This is done either via a toothed drive by means of a manual torque wrench or with an automatic (motor-driven) adjusting device. After the After contact, the bolt is further loaded with the contact torque of the nut MVM under torque control. MVM depends on the bolt dimension (usually > 30 Nm). The assembly clamping force FVM is set.

Schritt 4
Schritt 4

Step 4

Finally, after the nut has been applied, the bolt is relieved of the hydraulic pressure p. The applied assembly tension is removed. During this process, the applied assembly tension force FVM is transferred to the target assembly pretension force FM in the bolted joint due to springback losses FRV.

ITH Stretch Method

In hydraulic friction- and torsion-free tightening, the required assembly preload force (FM) is applied by purely axial elongation of the bolt by means of a bolt tensioning cylinder.

This results in a very small scatter of the assembly tensioning force (FVM) of ± 1 %. This results in precise and uniform assembly pretensioning forces (FM).

Vorteile auf einen Blick
Vorteile auf einen Blick

Advantages of the IHF Bolting System at a glance

  • No bending and torsional stresses in the bolt
  • Higher utilization of yield strength
  • Exact application of preload forces
  • The system works frictionless
  • Repeatable tightening: high reproducibility of preload forces in series applications
  • Simple and reliable assembly - proven to be faster than conventional torque methods
  • Smaller bolt dimensions with the same preload force
  • Reusability of IHF Fasteners
  • Manage entries