Tests and practical experience have shown time and again that the calculated torques for bolts > M 30 or 1¼'' are not coinciding with the actual values with adequate precision.

The main influential factor for these differences is thread friction in the bolt and nut contact area, for which to a large extent only empirical or estimated values are available. The effective friction force is determined by the friction coefficient. In addition, a bolted connection will undergo settling which is predominantly caused by the smoothing out of surface irregularities.

As these factors are of considerable importance in calculating the tightening torque, they can lead to substantial bolt stress variations.

The following lists of factors influencing friction coefficient variations are to illustrate this uncertainty:

1) Thread friction is a function of:

• the roughness of the thread surface i.e. the way how the thread is produced, whether cut or rolled
• surface roughness, i.e. bright, phosphated or blackened;
• type of lubrication: dry, lightly oiled, heavily oiled;
• surface treatment of the mother thread;
• inserted thread length;
• possibly repeated tightening and loosening of the bolts.

2) Friction variations between head or nut contact area are a function of:

• roughness of the contact surfaces;
• surface condition (dry, lubricated, painted);
• hardness differences between the contact surfaces or material pairing;
• dimensional and angular deviations between contact surfaces.

The factors influencing the bolt stress can most effectively be reduced by using hydraulic tension cylinders, especially in the case of larger-diameter bolts. Compared with the conventional torque method, the tension cylinder offers the advantage of eliminating the additional torsional and bending stresses over the bolt cross section. Even more decisive is the lack of any type of friction which allows to precisely determine the remaining bolt prestress by previous tests, taking into account respective design parameters.

It is possible to calculate with a tightening factor of α_A of 1.2 to 1.6, depending on the diameter/length ratio, and to use the yield point of the bolt up to 90%. The prestress of the bolt tightened first is influenced by the tightening of the other bolts so that a minimum of two passes is required.

This will at the same time compensate for the settling produced by the smoothing out of the unloaded mating surface during prestressing (thread and nut contact area).

The table "Bolt tension forces including tolerances for large-clearance metric thread" shows the theoretical tension forces for a selected bolt series.

Due to the non-parallelism between nut and contact area and the thread tolerance, settling phenomena after the nut has been thightened cannot be included by this method either. (It is recommended to request the bolts and nuts manufacturer to observe strict squareness tolerances.)

As the tension force applied in this method will not only cause elongation in the shaft but also in the thread, it is important to choose the correct thread series or thread tolerances acc. to DIN 2510. An inadequate thread clearance may cause jamming of the nut, when the bolt is elongated. Taking into aaccount the nut height consultation with the bolts manufacturer is absolutely necessary.

The bolts should be long enough to leave at least 1 · d above the nuts free for positioning the tension cylinder.

The exact minimum lenght will depend on the strength class of the bolts and the tensioning tool used. Washers should be large enough to be pressed onto contact surface by the tension cylinder during bolt thightening. Enlarged washers should be preferred over standardised washers. Consultation with the tension cylinder supplier is necessary.

Hydraulic tension cylinders often require more space than torque spanners, because the entire device must be positioned in the bolt axis.

We recommend to use bolt tension cylinders by GmbH, Auf'm Brinke 18, D-59872 Meschede, Germany.

The following tables show the tension forces and dimensions for single and multistage bolt tension cylinders.

Single-stage bolt tension cylinders
Multi-stage bolt tension cylinders

Torque spanners for bolts requiring torque-type prestressing can also be obtained from .

Information available upon request.