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