Screwing technology is still a key technology in assembly, particularly due to its high load-bearing capacity, reusability and the possibility of non-destructive loosening of connections. Screws are the most frequently used machine elements and offer a wide selection of standardized designs for a wide range of applications.
The most important goal of screw joints in assembly technology is to achieve a defined and constant preload force. This must be set in such a way that, on the one hand, the function of the threaded fastener joint is guaranteed at every possible operating force and, on the other hand, the permissible load is not exceeded. The often unknown settling phenomena of the connection and the assembly-related fluctuations in the preload force achieved are problematic.
In batch production, the preload force achieved is usually difficult to determine. In practice, various tightening methods are therefore used to indirectly control the preload force. The tightening torque is often used as an indirect measured variable. Other parameters such as rotation angle, screw-in time and Friction values also serve as important control variables in the assembly process. Innovative methods for detecting the screw head contact also help to improve the consistency of the preload force.
A central challenge of torque-controlled tightening is the fluctuating Friction values which, together with the torque dispersion of the tightening tool, influence the resulting preload force. The distinction between head friction and thread friction is particularly crucial here. As these frictional influences add up, fluctuations in the preload force of 50 % or more can occur even with high repeat accuracy of the torque.
To ensure a safe connection, the threaded fastener joint must therefore be designed to function reliably despite these deviations: it must not be overloaded with a higher preload force and must still provide the required holding force with a lower preload force. Despite these weaknesses, torque-controlled tightening is by far the most commonly used tightening method - mainly due to its simple technical implementation.
One way to increase precision is to combine it with rotation angle detection. This method is particularly useful when fluctuating material properties of the parts need to be recorded. The actual assembly process remains unchanged, but the retensioning angle that occurs above a certain Threshold torque is also monitored. This must lie within defined limit values, the so-called green window.
By evaluating the retensioning angle, conclusions can be drawn about possible assembly errors:
- If the retensioning angle is too shortthis often indicates the absence of a sealing element.
- Re-clamping angles that are too long indicate insufficiently hardened parts.
The Threshold torque, from which the angle of rotation detection starts, is usually between 20 % of the final torque (for hard Boltet joint cases) and 80 % of the final torque (for soft Boltet joint cases).
The angle-controlled tightening process
In the rotation angle method, both the torque and the rotation angle of the threaded fastener joint are used to controller the tightening process. In final tightening, however, the rotation angle and not the torque is used as the control variable. This means that the screw is first tightened to a starting torque and then tightened further by a predefined retightening angle. The torque can be used as an additional control variable.
With this tightening method, the screw can be tightened either in the elastic or in the plastic range. In the plastic range, further turning of the screw only leads to a minimal increase in torque, which is why the torque can no longer be used reliably as a control variable. Certain parameters must be precisely adhered to for tightening in this range, as the screw is permanently deformed and loses its reusability.
The terms plastic and elastic can be illustrated using Hook's stress-strain diagram. In elastic deformation, the preload force at constant Friction values is proportional to the applied torque. As the torque increases, the elongation of the screw also increases. When the load (preload force / torque) is removed again, the deformation decreases. However, as soon as the yield point of the screwdriver is reached, the increase in torque flattens out and the screw enters the plastic range. The deformation remains in this range even after the load is removed. If the maximum load is exceeded, the screwdriver constricts, which ultimately leads to its destruction.
The plastic range of a screwdriver varies depending on the design: It can be short and steep or long and flat. However, a wide plastic range is necessary in order to successfully apply the angle-controlled tightening method. This method makes it possible to largely eliminate the disturbance variables of the influence of friction and at the same time utilize the maximum load capacity of the screw.
The yield point-controlled tightening process
In order to avoid the limitations of the expansion screw and at the same time not have the dependence on fluctuating Friction values as a disadvantage, yield point-controlled tightening was developed. In this method, both torque and rotation angle are recorded as control variables. The principle is based on the use of the decreasing slope in the stress-strain diagram, which serves as a cut-off criterion when the yield point is reached.
In the stress-strain diagram, it can be seen that the increase is linear at the beginning and increasingly flattens out when the yield point is reached. During this process, the axial force is proportional to the torque and the elongation is proportional to the rotation angle. In mathematical terms, the rise of a curve is the derivative of the function. If the derivative of the torque is reduced to about 50 % of the initial value after the rotation angle has been reached, the yield point is reached and the tightening process is terminated. For safety reasons, limit angles and limit torques can also be introduced as monitoring variables.
Tightening controlled by yield point avoids the disadvantages of fluctuating Friction values or the limitations of screwdriver selection. In many cases, the screws can be dimensioned smaller due to the increased safety when reaching the required preload force, which enables a reduction in costs.
The method can only be used for connections in which the screwdriver is the weakest component. The screw head digging into the mating surface, for example, could otherwise be misinterpreted by the Screwdriving System as a stress limit.
DEPRAG Clamp Force Control (CFC)
The DEPRAG CFC adaptive fastening strategy achieves an improved and more constant preload force (clamping force) even with fluctuating insertion torques. The complete screw joint is made up of two main components: seating point detection and the screw joint based on either differential torque or Fasten to angle.
Typical applications for this method are direct screw connections in plastic or metal. The patented process for EC servo screwdrivers in conjunction with the AST12 or AST40 is used especially for widely varying Insertion torques.
Challenges and solutions
Fluctuations in the Insertion torque can be caused by various factors, such as changes in the screw or bore geometry, the structure of the component material, changing surface conditions of the screwdriver thread or by springy elements and settling phenomena. In such cases, reliable detection of the seating point ensures a uniform initial state and enables constant final tightening, resulting in a uniform preload force.
Advantages:
- Improved consistency of the preload force: The preload force remains constant, even with varying Insertion torques.
- Robustness against random fluctuations: The method is insensitive to random torque increases that are not caused by the seating point.
- Low parameterization effort: The set-up of the process is quick and easy.
Calculation method:
The main feature of the process is the seating point detection. The torque curve is continuously monitored and converted into a mathematical evaluation function. The seating point is considered to be detected if this function exceeds a defined limit value.
As soon as the seating point is detected, both the torque and the rotation angle are calculated retroactively at this Timestamp. The upper torque limit serves as the abort criterion for the Fastening stage. Optionally, the OK window for seating point detection can be monitored by lower and upper limits for torque and Angle.
The final values of the Fastening stage (torque and rotation angle) at the Timestamp of the seating point or at the end of the Fastening stage are used as reference values for the following program section.
Fastening to differential torque can also be replaced by fastening to angle.
The DEPRAG friction torque value procedure
The main aim of screw joints for parts is to achieve a constant preload force. The common tightening strategy of "fastening to torque" works well if the properties required for the torque-controlled fastening process are of consistent quality.
However, special challenges arise when processing self-tapping and self-tapping screws. Here, fluctuations in part quality, such as changes in the screw or hole geometry, material structure of the parts, changing surface properties of the screw threads or the core hole bore, as well as springy elements and settling phenomena, can lead to irregular Insertion torque during the forming or cutting process.
When threaded fastener joints are tightened to predetermined final tightening values, these fluctuating Insertion torques can lead to varying preload forces, which can result in the following problems:
- Damage to the screwdriver or parts (e.g. breakage)
- Failure of the threaded fastener joint (loss of preload force)
- Failure to reach the screw head support
The solution: Precise control of the preload force despite fluctuating Friction values
The DEPRAG friction torque value procedure offers a solution here. During the screw-in process, the Insertion torque is recorded in a parameterizable angle range, which is applied for the forming or cutting process. An average value is calculated from these measured values, which is referred to as the Friction value. The Friction value determines the further tightening process via a threshold torque, whereby the differential torque is used for the next step. The sum of the Friction value and differential torque finally results in the shut-off torque.
Advantages:
- The required preload force is applied reliably even if the torque values are constantly changing during the process.
Disadvantages:
- The final tightening values are not constant due to the fluctuating Friction values. It is therefore not possible to evaluate the quality of the individual screw connections using the final tightening values (e.g. using the Cmk index).
- The available measured variables for quality assessment are instead the differential torque value or the angle of rotation value, which is measured from the threshold torque until the shut-off torque is reached.
The linear measurement
The elongation of a screw and the resulting preload force are mathematically linked more precisely than the torque and the preload force. Therefore, a direct strain measurement enables a very precise determination of the preload force. One way to measure this is to mechanically record the elongation via the hole in the screw. This hole must be deeper than the clamping length of the screwdriver used. However, this method is only suitable for special cases with larger screws and is rarely used in practice.
Ultrasonic linear measurement
A more precise method of length measurement is ultrasonic measurement of bolt elongation. An ultrasonic pulse is introduced into the screw head. The pulse propagates through the screw, is reflected at the end of the shaft at the steel/air interface and returns to the screw head. The time difference between the echoes of the pulse is used to calculate the length of the screwdriver.
This measurement can be carried out at a very high frequency so that several thousand measurements per second can be performed and a high resolution is achieved. However, the stress states of the screw material and the temperature of the screw must be compensated for as disturbance variables. The method is now ready for series production and is successfully used in the automotive industry for highly sensitive safety screw connections. However, additional monitoring of torque and rotation angle is still required.
Another disadvantage of this process is that it requires screws with a vapor-deposited sensor, which means that an expensive sensor element remains on the part for every processed screwdriver.
Special procedures
Most of the methods described are tailored to metric threaded fastener joints made of steel. In practice, however, there are numerous other types of threaded fastener joints, such as self-tapping screws, self-drilling or self-forming screws and connections with metallic screws in thermoplastics or thermosets. These variants require special attention.
In principle, the relationship between torque, Friction values and the generated preload force remains. In the case of plastic screwdrivers, however, the relationship cannot be based solely on the material properties of the screw, as the material properties of the parts also have an influence. In the case of self-drilling or self-forming screws, additional disturbance variables also occur - the so-called Insertion torques.
The challenge of direct screw connections
In cases where a thread has to be formed, thread forming torques are required in addition to the friction components in order to achieve the final torque. Due to the strong fluctuations in these Insertion torques, the inaccuracies in the preload force achieved are significantly higher than in the standard cases described. The speed of the screwdriver plays a decisive role, especially for direct screw joints in thermoplastics, as it can significantly influence the quality of the screw joint.
Conclusion: Precise threaded fastener joints require suitable tightening methods and their parameters
Precise determination of the preload force in threaded fastener joints requires the use of suitable measuring methods or calculation tools. Every threaded fastener joint has its own challenges, and design requirements and Insertion torques must be taken into account, especially for special materials and screw types. In order to achieve optimum results, the screw, the parts, the tightening method and the fstening tool must be designed for the respective application.
