TORQUE TIGHTENING

The friction points should always be lubricated when using the torque tightening method.

 

Torque tightening and preload

The amount of preload created when tightening is largely dependent on the effects of friction.

 

There are three main “torque components”:

• the torque to tighten the bolt;
• the torque to overcome friction in the bolt and nut threads;
• the torque to overcome the friction on the outer side of the nut (contact surface).

 

** Preload (residual load) = applied torque minus friction losses **

 

Lubrication reduces friction

Lubrication reduces friction during tightening and reduces the risk of breakage during installation, increasing the lifespan of the bolt. The variations in the friction coefficients influence the preload obtained at a specific torque. High friction results in reduced torque to preload conversion.

The value of the friction coefficient supplied by the lubricant manufacturer must be known in order to be able to accurately establish the required torque value. Lubricants and anti-seize agents must be applied both on the sealing surface of the nut and on the thread.

This example illustrates how lubricant can reduce the effect of friction and convert more preload tension on the bolt.

 

The nominal values of pressure and torque, indicated by the manufacturer, correspond to the maximum safety limits. The best practice is to limit yourself to 80% of these values.

 

Bolting procedure

During tightening, it is common practice to only tighten one bolt at a time; this can lead to situations of load on a point or a distributed load. To avoid such situations, the bolting is carried out in stages following a predetermined order:

Step 1: Tighten with a wrench making sure 2-3 threads remain above the surface in addition to the nut.
Step 2: Tighten each bolt with a third of the required final torque following the sequence shown above.
Step 3: Increase the tightening of each bolt up to two-thirds of the final torque required following the sequence shown above.
Step 4: Increase the tightening up to the required final torque following the sequence shown above.
Step 5: Execute the final control on each bolt working clockwise starting from bolt 1 to the final torque completed.

Loosening torque
The torque used when bolts are loosened at a higher torque than the torque normally required. It is mainly used in case of corroded and deformed nut and bolt threads.

The loosening torque cannot be calculated precisely, however, depending on the situation, it is possible to calculate up to 2½ times the input torque.

The use of penetration oils or anti-seize lubricants is always recommended when performing loosening operations.

Tightening at height under secure conditions with hydraulic aluminium torque spanners

Introduction of Enerpac DSX aluminium torque spanners with square adaptor for increased clamping security at height

Risks during clamping at height

Falling objects, tool clamping, sparks and twisted hydraulic hoses. In many countries, falling objects remain one of the main causes of workplace injuries and fatalities.

For clamping projects in dangerous places, choosing the right tool makes all the difference in preventing injuries, near misses and project interruptions.

Safer clamping tools

A recent addition to our extensive portfolio of clamping tools, the innovative Enerpac DSX series of hydraulic torque spanners with square drive adapter has patented, first-class features to make operations at height easier and safer.

Learn about the security features and why you should add them to your inventory now.