Tightening methods

A connection with know-how

Bolted connections are the most important connections in the industry. Modern calculation programs make it possible to design machines and installations ever lighter and with smaller safety margins. Materials are hereby subjected to higher loads, 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 brought to preload by applying torque. Usually, the criterion of a desired end-torque is used. However, other methods such as torque and angle and yield controlled methods, are increasingly being used to preload bolts at higher levels and to exploit more potential from the strength of the material. To meet these demands, the bolting industry is developing tools that are more sophisticated than ever.

In addition to good methodology, the demand for documentation of the bolting process as proof to clients and insurance companies. Fully automated processing of a tightening protocol (entered in advance) minimizes the risk of operator errors. To better detect the event of material- and construction errors, there is a trend in critical applications to use 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 frequently used bolting methods below.

Torque-controlled tightening method

In the case of the torque-controlled tightening procedure, the torque tool stops when pre-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 connection components and their surfaces have an influence this friction. Studies show that about 90% of the torque is used to overcome friction, only about 10% is used for elongation of the bolt. Since elongation is directly proportional to bolt load* (Hooke’s Law), it is important that the coefficient of friction is known and stable for accurate results in terms of bolt load. The VDI Guideline** shows that under conventional conditions, an accuracy in bolt load of ±17% to ±23% can be expected. Provided that the surfaces of components to be joined are of good quality and the thread/bearing surface of the nut is right-angled. Suitable lubricants should be used, corresponding to the coefficient of friction assumed in the torque calculations. The torque-controlled tightening method is still the most widely 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 connections, torque is often taken as the reference value for accuracy. However, the key factor in all connections is the accuracy of the bolt load. Therefore, by means of mechanical optimization a large gain can usually be achieved in terms of accuracy and stability of the bolt load. Some examples of mechanical optimization are:

Bolts and nuts with high degree of finish: correct surface roughness, correct tolerances, flattened nuts for so that the hole and mirror face are right-angled.
Coatings for friction optimization.
Through hardened washers to guarantee the correct surface over which the nut or bolt head rotates.
Axial support of the torque tool so that (de)assembly is free of bending forces / side load.

*The direct-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%)

Torque and angle-controlled tightening method

With the torque and angle method, the extension of the bolt is indirectly measured. Using 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 if a rotation angle is completely converted into an elongation of the bolt. Thus, it must be ensured that before starting the angle measurement all clamped parts are flush (no gaps) with each other, and that the application can withstand the required surface pressure. This is achieved by first applying a so-called rotation angle starting torque. The balance between starting torque and angle is entirely dependent on the nature of the application and requires precise calculation or determination by practical tests.

This method is particularly suitable for bolted connections with a short clamped length and is often seen in steel-on-steel connections such as in construction and mechanical engineering. This method is well suited to ISO 4014, ISO 4017 and ISO 4762 bolt-blind-hole connections, as the counter-nut cannot rotate with the bolt. If the counter nut were to turn, 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 bolts from galling. The accuracy in terms of bolt load and durability of a connection can be further improved by tightening to the yield strength-/elastic limit.

*applies only in the elastic region of the load, up to the elastic limit.

VDI2230 tightening factor: αA 1,2 -1,4

Yield point-controlled tightening method

Yield point controlled bolting is a very well established method which has been used in the automotive industry for several decades. With yield point controlled tightening of bolted connections, bolts are tightened to the yield strength-/elasticity limit of a unique connection during the tightening process. The yield point 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 for each bolt. A material strength class says something about the minimum tensile strength of the material, but manufacturers apply a safety margin and the bolt therefore has latent reserves.

Regardless of the friction coefficient under the 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 being made. 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 and angle method, the joint must first be tightened to a starting 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 point controlled method. Bolted joints cannot break under this method, according to VDI-2230 – Nov. 2015 Table A8. Bolts can also be reused.

Often, the question is asked whether there is sufficient reserves for dynamic load during operation of the installation or application. The answer is yes. This can be explained as follows: 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 sum of both forces. 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.
VDI2230 tightening factor: not applicable.

Over-elastic tightening method

For over-elastic tightening, either the torque-angle method is used as a basis, or the yield point controlled method in combination with a defined rotation angle. In the case of the torque-angle method, preload is brought in with a starting torque, after which a turning angle provides the final bolt load. In the case of yield point controlled tightening, pre-tensioning is done first at the yield point and then a rotation angle is executed. In both cases, the bolt is pre-tensioned in the plastic region, which means that bolts and nuts cannot be reused. It ensures the absolute maximum utilization of material capacity and bolt strength. The durability of the connection is thereby greatly increased:

As a result of the partial dissipation of torsional stress immediately after torque removal, stress reserves come available for operational forces at a later stage.
Through plastic deformation of the thread of the bolt and nut during tightening, a better distribution of the bolt load over the various threads is achieved, which also remains better distributed after relaxation back into the elastic range. Should further plastic deformation occur under the influence of operating forces, the related settlement losses will bring the joint back into the elastic range.
If further plasticization of the assembly occurs trough the operating forces (FA), then the associated relaxation and settlement losses (FZ) will return the joints to the elastic region.

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 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 the galling of a bolt or, on the contrary, the unintentional plastic deformation of a bolt can be recognized. The method fits in well with the most commonly used method (torque controlled) and does not require modification of the protocol. It does, however, offer additional security in case of deviations, as the rotation angle is monitored as a second parameter.

Suitable for VDI 2862-2 risk class A, B, C.

Torque-controlled - strain gauge

For applications with large variation in friction and high preload forces (close to yield strength) that threaten overloading. The control parameter in this tightening method is the desired torque, and the yield strength of the bolt is the control/monitoring parameter. Also this method does not require any modification of the standard protocol, because the yield strength is monitored only as an additional parameter. The yield strength is recognized by the change in ratio between moment and angle (initial non-linearity). 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 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.

Tightening by means of an externally controlled signal

In this tightening method, both the target final torque and a value from other external measuring equipment are used, for example, a load cell/bolt force meter. Once the external equipment indicates the target value, the assembly process is stopped. This requires a continuous measurement and is usually in the form of a current signal.

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

Analysis of the rotation angle

This analysis module determines the clearance angle of a bolt from a defined starting torque to reaching the defined end torque. In this assembly method, the end torque is the control parameter. The clearance angle created serves as an indication of the preload before re-tightening 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.