Torque tightening is an indirect method of approaching the voltage limit, and therefore optimal fixation is not ensured. However, its ease, speed and reduced cost make it the most used method of controlling bolted joints.

The relationship between the applied torque and the axial stress achieved is affected by numerous factors, such as:

• Materials
• Friction between joining surfaces
• Thread
• Dimensions
• Temperature
• Lubrication

The accuracy of the applied torque depends on:

• The accuracy of the torque wrench used
• The system with which the key tells us that the pair has been reached
• Operator’s experience and skill

Today, most industries pay close attention to key accuracy and calibration and ignore the human factor and the key system itself.

However, the factors that introduce the most error in the actual applied torque are the latter two, since the accuracy of the keys usually hovers around 2-6% error, while the operator’s skill and drive system can exceed 10-15% error.

Therefore, and to achieve the closest to the design tightening, it is essential both the training and training of the operators, as well as the selection of wrench systems that facilitate the application of a correct effective pair.

Mechanical trigger or click wrenches

These are keys that, when reaching the selected torque, release the trigger head briefly by clicking, telling the operator to stop applying force. Operator’s unawareness at the exact time at which the torque will be reached, joined to the short shift that the click system introduces, it is difficult for the operator not to exceed the applied torque, even with experience.

Mechanical sliding wrenches

Once the fixed torque is reached, the system releases the wrench from the handle where the effort is applied, and therefore the overtightening. The drawbacks are the cost and robustness of the mechanical system. Therefore, only low torque range wrenches are manufactured with this system.

Digital wrenches

The wrench indicates the torque that the user is applying and informs both by means of lights and beeps that selected torque is being reached, so the operator is aware on when to stop applying force on the wrench. They also allow to record historical data for verification, issuance of certificates, etc.

In any of the versions, there are variants:

Prefixed wrenches: They have a prefixed torque, which the user cannot modify. It is useful in those applications when always the same torque needs to be applied, since it eliminates the possibility of human error in the selection of the correct torque.

Interchangeable heads: They allow to exchange the heads between square drive ratchet, open end, or ring spanners, depending on the application to be made. In operations where access is complicated or where the tightening varies frequently, it is the most flexible and versatile solution.