In the world of assembly, threaded fasteners are essential components that hold structures and machines together. However, simply installing a fastener is not enough to ensure the integrity and safety of the final product. As we have seen in our article, the clamping force that is generated in the fastener when it is tightened is known as prestress or preload and it’s the force that holds the parts together. The fastener must be tightened to a specific level of clamping force, depending on the application and the manufacturer's recommendations.
Unfortunately, measuring the clamping force directly can be difficult and time-consuming, and requires advanced instrumentation which is impractical for use in high-volume assembly lines. 
Because there is a direct relationship between the amount of torque applied to a fastener and the resulting clamping force, it is generally possible to achieve a specific level of clamping force by measuring the torque applied to the fastener and following the manufacturer’s specifications. In reality, friction and other environmental factors make it impossible to ensure that the same torque will produce the same exact level of clamping torque in different joints, so different tightening methods are used depending on the application requirements. 

Torque-control tightening

The most common tightening method is torque-controlled tightening. This method is easy to implement, cost-effective and simple to train on. Installation torque specifications are widely available making it a recognized and standardized method for achieving the desired level of tightness. Generally speaking, a torque-controlled tool will shut off automatically after reaching a preset torque value. Depending on the precision and repeatability needed by the manufacturer, different types of technologies are available at different price points. For an overview of the three main types of screwdriving technology, please refer to our previous article on the topic.

If there were no variation between joints in the torque-tension relationship, the torque-control method would be the perfect way to achieve the desired level of clamping force every time. However, as mentioned before, variable factors such as friction add imprecision to the process. After all, less than 20% of the torque applied to the fastener produces fastener preload. The rest  is used to overcome friction in the threads and clamping surfaces.
Therefore, while torque-controlled tightening remains the most widely used fastening method in assembly despite its limitations, for more demanding and high-precision applications, manufacturers have to look at more advanced tightening strategies.

Torque-and-angle tightening

One such strategy is known as torque & angle monitoring and control. As the name suggests, this method uses both the torque and the rotation angle to achieve the desired level of preload in the screw. Because higher friction can cause the torque to rise more quickly (i.e. after fewer degrees of rotation of the screw), the idea is that by monitoring the angle we can account for the factors that affect the relationship between torque and clamping force to ensure a more consistent elongation in the fastener, resulting in a more repeatable amount of clamping force applied between joints.
This can be achieved in two ways.

The first, torque mode with angle monitoring, is similar to standard torque-controlled tightening, with the key addition of ensuring that the final angle is between the pre-set minimum and maximum values. In the case that the desired torque is reached but the angle is outside of the pre-set boundaries, the tightening operation will result in an error.

The other mode, angle mode, gives priority to reaching a pre-set angle, while monitoring the tightening torque. 

Starting from a pre-set threshold torque, ideally the torque at the seating point of the screw, the tool will begin measuring the rotational angle and stop tightening upon reaching the desired number of degrees. Additionally, with electric screwdrivers like Kolver’s PLUTO, MITO & NATO Series, the operator can also set a minimum and maximum torque; if the final torque is outside those boundaries when the desired angle is reached, the operation will result in an error.

Furthermore, torque&angle monitoring provides valuable data and graphs for quality control and process improvement. The measurements taken throughout the tightening process can be analyzed to identify trends, anomalies, and potential issues, allowing for adjustments to be made to the assembly process to improve performance and reduce the risk of defects.

Yield control and advanced tightening strategies

While torque&angle control provides a superior level of precision compared to simple torque control, it has two shortcomings: first, the specified angle of rotation must start from the seating point of the screw (i.e. when the screw head touches the surface). If we start measuring the angle too late or too soon, we will invariably alter the final clamping torque applied. Furthermore, as bolt length-to-diameter ratios get smaller, more preload is generated with each degree of rotation, so greater accuracy is required. 
Secondly, the final torque boundaries are typically treated as fixed parameters that don’t account for the varying amount of friction-induced running, or prevailing, torque encountered up until the seating point of the screw. In other words, the torque&angle method ensures consistency of angle and total torque between joints, but doesn’t necessarily ensure consistent clamping torque between joints.
In safety-critical applications, most common in industries like aerospace and automotive, achieving a consistent level of clamping force is crucial, so more advanced tightening strategies must be employed. In these applications, the installation torque specified is typically indicated as torque above running and that’s where smart torque tools that can handle advanced tightening techniques become essential. 

One such technique aims at dynamically determining the prevailing torque encountered and using it to determine the final installation torque so that a consistent amount of clamping force is applied consistently from joint to joint. Kolver’s K-DUCER line of transducer-based electric screwdrivers makes it easy to employ this strategy; check out our articles on prevailing torque locking and running torque strategy, if you’re interested in diving deeper into this topic.
This technique can be further enhanced by automatically detecting the seating point of the screw at which point the clamping torque starts to be applied. This is ideal when dealing with self-tapping screws, as explored in our article about multi-step strategies. 

Another way to provide more accuracy and repeatability is yield control. This technique, common in the automotive industry, aims to tighten a fastener to its elasticity limit, maximizing the strength potential that’s unique to each fastener. This is typically achieved by reaching a threshold torque and then monitoring the torque gradient (i.e. the rate of change of torque over angle) in real time and completing the tightening once a certain gradient is reached.

The main benefit of yield-controlled tightening is the higher level of clamping force generated along with the uniform preload that it provides with each operation, even when dealing with varying levels of friction; its main disadvantage is that extreme precision in the equipment used is necessary in order to avoid over-tightening and fastener failure. 

Conclusions

As we have seen, there are several methods of tightening fasteners in assembly processes, each with its own advantages and disadvantages. 
Torque-controlled tightening is the most cost-effective and widely used method and involves measuring the torque applied to the fastener during tightening to achieve a specified level of tightness. Torque&angle-controlled tightening involves measuring both the torque and the angle of rotation of the fastener to achieve a more precise and accurate measure of the clamping force generated. 
Running (or prevailing) torque compensation is a method that adjusts the torque applied during tightening to compensate for variations in friction and other factors that affect the relationship between torque and clamping force. 
And yield-controlled tightening involves tightening the fastener until it reaches a specified yield point, at which the fastener deforms plastically, indicating that it has reached the desired level of tightness, regardless of the amount of friction encountered during rundown.
Furthermore, additional tightening strategies can prevent specific issues that may arise in a production line environment, like cross-threading, bit-slip, stripped threads, and more.

Ultimately, the choice of technique should be based on a thorough understanding of the specific assembly requirements and constraints, and a careful evaluation of the cost, time, and performance trade-offs involved in each method.