Tightening methods
Bolted connections are the most important joints in industry. Modern calculation programs make it possible to design machines and plants ever lighter and with smaller safety margins. Materials are hereby loaded higher, closer to the physical limits of the material. This requires a more accurate and stable bolt load in bolted connections.
More than 99% of all industrial bolted connections are preloaded while rotating, by means of a torque. Usually, the criterion of a desired end torque is used. However, other methods such as torque-angle and strain-gauge controlled methods are increasingly being used that allow bolts to be preloaded higher and exploit more potential from the strength of the material. To continue to meet these needs, tools are being developed in the bolting industry that are more advanced than ever.
In addition to good methodology, the demand for documentation of the tightening process as proof to clients and insurers is growing. Fully automated processing of a (pre-entered) tightening protocol minimize the chance of operator errors. To reduce the chance of material and construction errors, there is a trend in critical applications to employ a second parameter as a check on the first (VDI 2862-2). This is commonly referred to as the control parameter.
We explain the most important and commonly used bolting methods below.
Torque-controlled
In the case of the torque-controlled tightening procedure, the torque wrench stops when a set final torque is reached. In this process, the knowledge of the coefficient of friction is very important. Both the lubricant and the quality of the connecting components and their surfaces influence this friction. Studies show that about 90% of the torque is used to overcome friction, about 10% is used for elongation of the bolt. Since elongation is directly proportional to bolt force* (Hooke’s law), it is important that the coefficient of friction is known and stable for accurate results in terms of bolt force. The VDI Guideline* shows that under conventional conditions, an accuracy in bolt force of ±17% to ±23% can be expected. Provided the surfaces of components to be joined are of good quality and the thread/bearing surface of the nut is square. Suitable lubricants should be applied in accordance with the coefficient of friction assumed in the moment calculations. The torque-controlled tightening method is still the most commonly used method. However, more sophisticated methods have been developed over the years for a better grip on the desired bolt load.
Mechanical optimization
In torque-controlled joints, tightening torque is often taken as the reference value for accuracy. However, what matters for all joints is accuracy in bolt load. Thus, through mechanical optimization, large gains in bolt load accuracy and stability can usually be achieved. Some examples of mechanical optimization are:
- Bolts and nuts with high finish: correct surface roughness, correct tolerances, flattened nuts for so that the hole and mirror plane are square.
- Coatings for friction optimization.
- Through and through hardened washers to guarantee the correct surface over which the nut or bolt head rotates.
Axial support of the torque tool so that bending forces can be (de)mounted free.*The straight-proportionality applies only in the elastic region of the load, up to the yield strength/elasticity limit.
*VDI2230 Tightening factor conventional: αA 1.4 -1.6 (±17% to ±23%)
VDI2230 Tightening factor with optimized friction: αA 1.1 -1.2 (±5% to ±9%)
VDI2230 Tightening factor with controlled friction: αA 1.0 -1.1 (±2.5% to ±5%)
Rotational torque + rotation angle-controlled
The moment angle method indirectly measures the elongation of the bolt. Through the pitch of the thread, a rotation angle can be linked to the elongation of the bolt, which in turn is directly proportional to bolt load* (Hooke’s law). However, this only applies when a turning angle is fully converted into a lengthening of the bolt. Thus it must be ensured that before starting the angle measurement all clamped parts are flush with each other, and that the application can withstand the required surface tension. This is achieved by first applying a so-called joint moment. This is entirely application-dependent and requires precise calculation or determination through practical testing.
This method is particularly suitable for bolted connections with a short clamped length and is commonly seen in steel-to-steel connections such as in structural engineering and machine building. This method lends itself well to ISO 4014, ISO 4017 and ISO 4762 bolt-blind-hole connections, as the lock nut cannot rotate with the bolt. If the lock nut were to rotate, the net rotation angle would be lower and would not result in the calculated bolt load. A suitable lubricant should also be used to prevent fretting of bolts. The accuracy in terms of bolt load and durability of a joint can be made even higher by tightening to the yield strength / elastic limit.
*apply only in the elastic region of the load, up to the elastic limit.
VDI2230 tightening factor: αA 1.2 -1.4
Stretch limit controlled
Strain-driven tightening is a well-proven method that has been used in the automotive industry for several decades. With strain-guided tightening of bolted connections, bolts are tightened during tightening to the yield strength/elasticity limit of a unique connection. Thereby, the yield strength is used as a control parameter for determining the bolt load. The goal here is to maximize the strength potential of each unique bolt, as it is different for each bolt. A material strength class says something about the minimum tensile strength of the material, however, manufacturers use a safety margin and the bolt has latent reserves. Regardless of the coefficient of friction under the bolt head or on the thread, the bolt is tightened up to its yield point. During the assembly process, the quotient is calculated by dividing the torque by the rotation angle. At the point where this quotient changes significantly, the elastic limit of the joint is reached, and the system will stop the tightening process.Just like with the torque-angle method, the joint must first be tightened to a starting or snug torque before the rotation angle can be measured. Often, the cut-off value of the gradient can be set. Since the bolt is only minimally plastically deformed (< 0.2 Rp), even bolts with a short clamped length can be tightened using the yield-controlled method.
Bolted joints cannot fail when tightened to the yield point, according to VDI-2230 – Nov. 2015 Table A8. Bolts can also be reused. The question is often asked whether there is then sufficient reserve for dynamic loading during operation of the plant or application. The answer is yes. This takes advantage of the fact that when bolted connections are tightened, not only an axial stress, but also a torsional stress is created due to thread friction. The reference of the yield strength is determined by the resultant of both loads. Immediately after the moment is removed, the torsional component springs back by about 50%. As a result, the bolt force decreases, and the joint again acquires an elastic reserve 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-to-steel joints with short to normal clamped length, such as in structural engineering. VDI2230 tightening factor: not applicable.
Over-elastic
In over-elastic tightening, either the moment-angle method is used as a basis, or the strain-controlled method combined with a twist angle. In the case of the moment-angle method, preload is first applied to a joint moment, after which a rotation angle provides the final preload. In the case of yield strength controlled tightening, preload is first applied at the yield strength and then a rotation angle is created. In both cases, the bolt is preloaded in the plastic zone, which means bolts and nuts cannot be reused. It ensures maximum utilization of material capacity and bolt strength. The durability of the joint increases greatly in the process:
- As a result of the partial dissipation of torsional stress immediately after moment removal, stress reserves are released for operational loading at a later stage.
- The plastic deformation of the threads of the nut and bolt during the tightening process results in a better distribution of the bolt load over the various threads, which also remains better distributed in the elastic range after relaxation. Should further plastic deformation occur afterwards under the influence of operating forces, then the associated settlement losses will bring the connection back into the elastic range.
- If further plasticization occurs in the bolt after assembly by operating forces (FA), the associated relaxation and settlement losses (FZ) will cause the joints to return to the elastic range.
Suitable for VDI 2862-2 risk class A, B, C
Torque-controlled – rotation angle monitored
The control parameter in this tightening method is the desired torque, where the rotation angle achieved is the reference/monitoring parameter at the end of the process. The angle of rotation serves as a second control parameter. This allows, for example, the fretting of a bolt or, on the contrary, the unintentional flow of a bolt to be recognized. The method fits well with the most commonly used method (torque-controlled) and then requires no adaptation of the protocol. However, it does offer additional assurances in case of deviations, because the angle of rotation is monitored as a second parameter.
Suitable for VDI 2862-2 risk class A, B, C.
Torque-controlled – strain limit monitored
For applications with large variation in friction and high preload forces (close to yield strength) that threaten overrunning. The control parameter in this tightening method is the desired torque, and the yield strength of the bolt is the control/monitoring parameter. This method also does not require modification of the standard protocol because the yield stress is only monitored as an additional parameter. The yield strength is recognized by the change of relationship between torque and angle (initial nonlinearity). If the yield strength is reached before the desired moment is reached, the assembly process is stopped. Thus, bolts can never break again.
Suitable for VDI2862-2 risk class: A,B,C.
Stretch limit controlled – angle controlled tightening method
The control parameter in this tightening method is reaching the yield strength, and the angle of rotation 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.
Tightening via an externally controlled signal
This tightening method uses both the target end torque and a value from other external measuring equipment, such as a loadcell/bolt force gauge. Once the external equipment indicates the target value, the assembly process is stopped. This requires continuous measurement and is usually in the form of a current signal.
Suitable for VDI2862-2 risk class A (limited), B, C.
Analysis of deflection angle
This analysis module determines the deflection angle of a bolt from a defined starting torque to reaching the defined end torque. In this assembly method, the final torque is the control parameter. The created spin angle serves as an indication of the preload before retightening the bolt. This method is often used for retightening/checking bolted connections.
Combination of tightening methods
Some systems in the market are capable of combining various assembly methods in any order.
