Screw connections are probably the most frequently used connecting elements in mechanical engineering. They are designed to join components simply and securely. Due to the shape of the thread and the resulting clamping force, the internal and external threads are clamped together to form a fixed connection. Screw connections are particularly critical for dynamically loaded assemblies. 

The screw has to withstand different loads. On the one hand, of course, the external load acts on the screw; this can be tensile, compressive or shear load. In addition, the tensioning of the threads in relation to each other causes an internal stress that puts the screw under tensile load. 

The shape of the thread means that the component has a large number of large notches, which are critical for dynamic loading and can lead to crack initiation in the base of the thread. For this reason, especially with safety-critical components, care must be taken to ensure that the thread is not additionally weakened by the manufacturing process.

Traditionally, there are two alternative routes for producing threads: Thread cutting and thread rolling. In thread cutting, the shape of the thread is produced using a machining process. Overall, this process is very flexible and comparatively inexpensive to manufacture; the latter is particularly important for small quantities or special thread forms. In addition, the process is actually possible on any machining process. 

In thread rolling, the shape of the thread is formed into the component. It is therefore a forming process. As with all forming processes, this process is very productive and cost-effective, especially in series production. In single-part production, however, the tools required are very expensive and therefore not economical. A major known advantage of thread rolling is that the forming process increases the strength of the thread. 

However, if increased strength is required in individual part or small series production, both processes reach their technical and economic limits. A third, alternative production variant can then be selected. When roller burnishing, also known as deep rolling, a thread, a machined thread is subsequently deep rolled into the thread root, thus increasing the dynamic strength of the thread. Deep rolling therefore offers the possibility of highly flexible and economical production of high-strength threads.

In this context, the questions immediately arise: How are the dynamic strengths of the different thread variants to be assessed? And can deep rolling achieve a service life comparable to that of thread rolling? 

We now want to answer these questions using the example of an M12 threaded rod made from 42CrMo4 steel. For this purpose, the threaded rods were tested in accordance with DIN 969 on a POWER SWING MAC resonance testing machine from SincoTec. In order to record complete Wöhler curves, the low cycle fatigue strength (LCF) and high cycle fatigue strength (HCF) ranges were tested using the pearl string method and the staircase method. 

The threaded rods were manufactured by the various project partners in the project. ECOROLL AG itself produced both the cut and the subsequently roller burnished threads. The rolled threads were kindly provided by LMT Fette, one of the leading manufacturers of thread rolling tools. 

As can be seen in the picture, deep rolling was carried out with a single roller mechanical deep rolling tool of type EF90. With this tool, the roller is mounted freely so that it is located exactly in the thread root itself when the rolling force of F_w = 2 kN is applied. This facilitates set-up on the machine and ensures a reproducible deep rolling process across a large number of threads. 

In addition to the service life analysis, the near surface area of the thread root was examined and hardness depth curves, residual stress measurements and full width of half maximum (FWHM) measurements were carried out in order to evaluate and interpret the results. The measurements of the subsurface area and the determination of the Wöhler curves were carried out as part of a student project at the Esslingen University of Applied Sciences - Faculty of Machines and Systems.

If you compare the different thread variants using the recorded Wöhler diagrams, you can clearly see that thread rolling increases the service life of a thread compared to a cutted thread. The bending point of the Wöhler straight line and thus the transition to fatigue strength is 84% higher for the rolled thread than for the cutted thread. Expressed in load stresses, this means an increase from 65.7 MPa to 121.0 MPa. 

The reason for this can clearly be found in the microstructure analyses. The micrographs alone show how the microstructure of the rolled threads has a continuous grain flow and the grains are deformed. The hardness in the thread base is higher than in the cut case down to a depth of 1 mm. The residual compressive stresses are particularly noticeable. Up to a depth of 1 mm, there are constant residual compressive stresses of approx.-650 MPa are present. With the cutted thread, tensile stresses are already present after 50 µm. 

If you compare this with the deep rolled thread, massive residual compressive stresses can also be measured in the thread root. Although these are not constant over such a great depth, they do extend to a depth of 700 µm. This also results in the expected increase in service life. Compared to the cutted thread, the fatigue strength can be increased by 30 % by deep rolling. 

Although deep rolling is not as effective as thread rolling, the results show that deep rolling is a very good alternative to rolling and an excellent complement to thread cutting, especially in the area of single part or small series production. 

Acknowledgments
We would like to thank the student team at Esslingen University of Applied Sciences for producing these results. We are also grateful that Esslingen University of Applied Sciences allowed us to investigate this topic as part of a student project. And we would like to thank the company LMT Fette for providing the rolled threaded rods.