Tightening methods Eco2TOUCH

Torque-controlled method

• Tighten bolts based on a set tightening torque
• Fully-automatic process sequence until desired tightening torque is reached
• Documented process parameters: Date, time, operator, torque set, torque realized, OK/Not OK display.

The torque-controlled tightening process is suitable for all screw connections up to torques of approximately 200,000 Nm. In torque-controlled tightening, a certain tightening torque leads to a preload. Studies show that approx. 90% of the torque/energy is used to overcome friction and approx. 10% is used to realize bolt load. This relationship is largely determined by the friction between the connection components and the application. The prerequisite for an accurate result is a known and stable friction. Factors affecting friction include: Lubricant, surface condition of bearing/turning surfaces, shear and bending forces during assembly, dimensioning and material specifications. The VDI 2230 Guideline* shows that under conventional conditions, an accuracy in bolt load of ±17% to ±23% can be expected. At least, if the surfaces of components to be joined are of good quality and the threads and bearing surface of the nut are square.

A HYTORC hydraulic torque wrench works with a piston that moves up and down, combined with a reaction pawl. The reaction pawl is an extra provision that ensures that in non-ideal bolted constructions, the set moment actually reaches the nut. This pawl is necessary for long bolts, high torques, or a springy support point. In such cases, the torque angle can become equal to the tool’s branch angle so that the nut moves back and forth but does not rotate with respect to the bolt. The reaction pawl prevents recoil of the insert during the return stroke and thus ensures net transfer of torque to preload.

Mechanical optimization of the joint that improves accuracy

In torque-controlled connections, torque is often taken as the reference value for accuracy. However, what matters in all connections is accuracy in bolt load. Therefore, in addition to accuracy of our tools, HYTORC also focuses on accuracy of the bolt load by optionally adding two thread systems:

• With the HYTORC washer system, bolted connections are tightened free of reaction forces and thus shear forces with a bolt load accuracy of ±10%.
• With the HYTORC Nut System, bolted connections are tightened not only shear-free but also torsion-free with an accuracy in bolt load of ± 5%. (A calibrated bolt load).

VDI2230 Conventional tightening factor : αA 1,4 -1,6 (±17% tot ±23%)
VDI2230 Tightening factor with optimized friction: αA 1,1 -1,2 (±5% tot ±9%)
VDI2230 Tightening factor with controlled friction: αA 1,0 -1,1 (±2,5% tot ±5%)

https://youtu.be/Gj-9QJhQuks

Stretch limit controlled method

• Maximum bolt load: The bolt reaches its individual yield point during tension and torsion. Due to the elimination of the torsional load after the end of the tightening process, a bolt load of approx. 90% of the Rp0.2 yield strength is achieved. The friction coefficient is an influencing factor: the lower the friction, the higher the remaining prestressing force.
• Friction-independent: Independent of friction, each bolt is loaded to its own unique yield point. Bolts and nuts of different make and batches can be used interchangeably without any problem.
• No special bolts required: low direct costs.
• Re-use bolts: Because the system stops before the Rp0.2 yield strength, bolts and nuts can be re-used.
• Low settlement losses: all connection components are under high preload in the main force line. Therefore, this tightening method has low settlement losses.
• Fatigue: higher prestressing bolts have a higher fatigue coefficient. As the preload increases, so does the fatigue resistance..
• Graphical representation of relevant parameters and process sequence.
• Method complies with VDI2862-2 risk class A. This guideline requires that the tightening process works with two independent parameters.

Elasticity-controlled tightening is a very well-proven method which has been used in the automotive industry for several decades. In this method, bolts are tightened to the yield strength/elasticity limit of a unique joint during the tightening process. Thereby, the yield strength is used as a control parameter for determining the bolt load. The objective here is to make maximum use of the strength potential of each unique bolt, as this differs from bolt to bolt. A material strength class says something about the minimum tensile strength of the material, however, manufacturers apply a safety margin and the bolt has latent reserves.

Regardless of the coefficient of friction under bolt head or on the thread, the bolt is tightened to its yield point. During assembly, the quotient is calculated by dividing the torque and the angle made on each other. At the point where the quotient changes significantly, the elastic limit of the joint has been reached and the system will stop the assembly process. As with the torque-angle method, the joint must first be tightened to a starting or joint torque before the angle of rotation can be measured. Often the cut-off value of the gradient can be set. Because the bolt is only very marginally plastically deformed (<0.2Rp), even bolts with a short clamped length can be tightened using the yield strength controlled method. Bolted connections cannot break under strain controlled tightening. See note VDI-2230 – Nov. 2015 Table A8. Bolts can also be reused.

The question is often asked whether there is then sufficient reserve for dynamic load during operation of the plant or application. The answer is yes. For this purpose, it is used the fact that when bolted connections are tightened, not only an axial stress, but also a torsional stress is generated due to the thread friction. The reference of the yield strength is determined by the resultant of both loads. Immediately after the torque is removed, the torsional component springs back by about 50%. As a result, the bolt load decreases, and the joint regains an elastic reserve that is sufficient for any dynamic loads during operation. The remaining torsional stress largely dissipates over time and under the influence of vibration. This is usually a process of days or weeks.

This method is particularly suitable for steel-on-steel connections with short to normal clamped lengths, as for example in structural engineering.
The Eco2Touch control system controls, monitors and documents all process parameters such as: date and time, operator, end moment, angle and any set control bandwidths. The hardware is comprehensive but the implementation (read: operation) is simple because the process is fully automatic. The operator only has to start the pump system. The system will switch itself off when the connection is completed within the set parameters and confirm this by means of an acoustic and visual signal. The method is therefore also suitable on construction sites and can be used for ‘one-off’ applications without the need for calculations and preparation..

VDI2230 aanhaalfactor: not applicable

https://youtu.be/d-iq4XdnU5g

Moment controlled-angle monitored

The control parameter in this tightening method is the desired torque, where the achieved rotation angle is the reference/monitoring parameter at the end of the process. The rotation angle serves as a second control parameter. With this, for example, the gouging of a bolt or, on the contrary, the unintentional flow of a bolt can be recognized. The method fits in well with the most commonly used method (moment-controlled) and therefore does not require modification of the protocol. It does, however, offer extra security in case of deviations, as the rotation angle is monitored as a second parameter.

https://youtu.be/XgBwRDuSlJc

The control parameter in this tightening method is the reaching of the yield strength, and the rotation angle made is the control/monitoring parameter at the end of the assembly process. If the rotation angle is exceeded before reaching the yield strength of the (bolted) joint, the process will be stopped.

Suitable for VDI2862-2 risk class A, B, C.

This analysis module determines the throughput angle of a bolt from a defined starting torque (usually 50% of end torque) to reaching the defined end torque. In this assembly method, the end torque is the control parameter. The clearance angle made serves as an indication of the preload before re-tightening the bolt. This method is often used for retightening/checking bolted connections.

Statistical evaluation of measured process parameters. Can also be used as a measuring device for process capability analysis (PCA).