Thread whirling wins in comparison to thread rolling
Tribological contacts account for around 12% of global energy consumption. Bornemann Gewindetechnik is researching the optimization of trapezoidal threads together with the Institute of Production Engineering and Machine Tools at Leibniz Universität Hannover.
Tribological contacts account for around 12% of the world’s total energy consumption. Of this, 20 % is used to overcome friction and 3 % for the repair of worn components or replacement equipment for wear-related failures [1]. One example of this is heavy-duty lifting equipment for the maintenance of trains or trucks. Bornemann Gewindetechnik specializes in the manufacture of such complex or highly stressed threaded components. Together with the Institute for Production Engineering and Machine Tools (IFW) at Leibniz University Hanover, the family-run company has researched the tribological optimization of trapezoidal threads using the thread whirling process. This process is particu-larly suitable for long threaded components. Using state-of-the-art production techniques, Bornemann Gewindetechnik manufactures screw profiles up to 12 m long.
Whirled threaded spindles have machining-related surface microstructures that provide a lubricant retention volume. This reduces the friction of the whirled threaded spindles and increases the service life compared to conventionally rolled spindles. Researchers are investigating the exact relationship between the machining process and the friction-reducing effect as part of the “TopGewinde” project.
The influence of the microstructures generated by thread whirling on the tribological behavior of highly loaded trapezoidal threaded spindles in heavy-duty lifting systems is being investigated. For this purpose, a whirling process specially developed for process-integrated microstructuring is being examined.
This article will illustrate how the surface topography can be specifically adjusted using the whirling process, which represents a significant advantage over the thread rolling process. Figure 2 shows the surface topographies of the thread flank of a rolled trapezoidal threaded spindle in comparison to a whirled thread flank.
By specifically adapting the whirling process, it was possible to produce more pronounced structures compared to rolled threaded spindles. To characterize the surface structures, the structure parameters structure height “yf” and the structure distance “sf” were introduced (Figure 1). For tribological optimization, the whirling process was used to create structures with a smaller proportion of peaks, which minimize the direct solid contact between the surfaces of the friction partners. The greater distance between the structure peaks makes it possible to store the lubricant in the valleys of the roughness profile.
Characterization of tribological properties
The test rig shown in Figure 3 is used to investigate the influence of the surface topography of the thread flanks on the tribological behavior. Test spindles with thread Tr 80 x 10 mm were oscillated at a frequency of f = 0.81 Hz through an angle of v = 15°. The rotation of a load change corresponds to a translational stroke of s = 0.42 mm of the thread. With this setup, the threaded spindle was loaded with a weight force of FG = 91.3 kN during lifting and lowering. This corresponds to a surface pressure of p = 5.0 N/mm2, which is within the maximum load range for trapezoidal screw drives (TGT) in heavy-duty lifting systems. For this application, the threaded nut material of G-CuSn 7 ZnPb and the lubricant DGM HTF 940 were selected. The lubrication in the test procedure corresponds to a maintenance interval of one month. The interval for lubricating the spindle-nut assembly is therefore every 167 cycles. The test rig was set up by SincoTec. SincoTec is also the manufacturer of the force sensor Interface 125 kN and the torque sensor SincoTec 1200 Nm.
The wear condition of the flank surfaces was analysed to characterize the friction properties of the different surface topographies and surface structures. For this purpose, the project participants then cut the spindles with a cut-off grinder. With 20,000 load cycles, the service life of ten years was mapped experimentally for the threaded spindles examined. For the whirled threaded spindles, the process manipulated variables were selected in such a way that defined gradations between the heights of the microstructures on the load-support flanks of the threads were achieved. The process parameters are based on the experience of the Bornemann company. Furthermore, rolled threaded spindles were examined that were rolled using the continuous process.
Influence of surface structures on the coefficient of friction
Due to the oscillating movement over a small angular distance, lifting and lowering can be investigat-ed in a high number of load cycles. In this way, the entire service life of the lead screws can be mapped experimentally. Figure 4 shows the course of the coefficient of friction for the lifting move-ment with the trapezoidal screw drive. The friction coefficients of a rolled and a whirled lead screw are compared here. The resulting speed profile within a load change consists of an acceleration phase, a phase with constant speed and a deceleration phase – in each case for lifting and lowering. The stick-slip effect, which occurs at low speeds, is particularly favored at the point of direction reversal. This effect manifests itself in a brief sticking of the surfaces, followed by a sudden sliding of the friction partners. This movement can lead to vibrations, which are reflected in a greater fluctuation in the coefficient of friction [2]. The mean coefficient of friction rises sharply at the beginning of the test, reaches a maximum after around 2000 load cycles and then falls to a constant level after around 8000 load cycles. This behavior is referred to as the running-in phase and describes the general change in the coefficient of friction over the course of the test, depending on the respective tribological system [3].
The whirled trapezoidal thread spindle with the introduced surface structure shows a significant im-provement in tribological properties. This leads to a shortening of the running-in behavior by approx. 44 % as well as to a permanent reduction of the coefficient of friction to a lower level of μm = 0.085. This corresponds to a reduction of 25.5 % compared to the rolled threaded spindle. A complete reduction of the running-in phase was not possible with this thread in the tribological system described.
Influence of surface structures on wear
The wear that occurs when the two friction partners come into contact is of decisive importance for the service life of the trapezoidal threaded spindle. In order to identify the wear mechanisms occurring on the threaded spindle, the surface of the support flank was examined over the 20000 load cycles with a regular supply of lubricant and the wear pattern of the support flank was then analyzed (Fig. 5). After the test, the investigation showed a clearly different wear pattern depending on the surface structures on the support flank. The rolled threaded spindles without surface structures on the support flank show pronounced adhesive deposits on the support flank of the threaded spindle, which are caused by significant removal or adhesive wear of the nut material. As the height of the structure increases, the area of adhesive wear also decreases significantly (Fig. 5).
Due to the low sliding speed and the high surface pressure in tribological contact, the friction present in the tribological system is classified between solid-state friction and mixed friction. Due to the pronounced surface structure (Figure 6, right), the number of micro-contacts can be reduced compared to the unstructured surface topography (Figure 6, left). This results in a lower proportion of solid-state friction due to the surface structures. A more pronounced surface structure on the thread flank is able to absorb a larger amount of lubricant. With the surface structures, the adhesive wear on the thread flank was reduced to 10.1 %, compared to 36.3 % with a rolled threaded spindle.
When friction partners made of different metals come into contact, a material transfer takes place from the cohesively weaker bonded friction body (in this case the nut) to the cohesively stronger bonded base body (in this case the threaded spindle) [4]. The additional lubricant in the friction con-tact results in a lower number of contacting roughness peaks. The progressive load on the contact surface of the threaded nut results in the detachment of particles that are deposited on the con-tacting roughness peaks of the support flank. These gradually form a layer on the contacting areas of the support flank and prevent direct contact between the spindle and nut material. This process con-tinues until a static condition is reached and no further deposits are deposited on the support flank. This process determines the running-in behavior, which changes to a static state after the contacting roughness peaks are covered with adhesion deposits. This state also influences the coefficient of friction. The initial surface does not undergo any abrasive wear during the lifting movement.
Outlook and utilization options
In competition with rolled threaded spindles, it has been shown that surface structures on the support flank of a threaded spindle for heavy-duty lifting systems offer considerable added value. The whirling process can be used to produce a greater variety of surface structures. Furthermore, it has been shown that the surface of the threaded spindle is subject to very little wear, which means that the surface structures on the support flank are largely retained after an experimentally simulated service life of 10 years. Figure 7 shows the three main potentials of microstructured load flanks for the design and construction of trapezoidal threaded spindles.
The defined adjustment of the surface structures on the thread flank can reduce the coefficient of friction by 25.5 %. For the example of a heavy-duty lifting system, this reduction in the coefficient of friction results in a proportional reduction in energy consumption, taking into account the respective support of the lifting system, as the frictional contact between the spindle and nut is decisive for the efficiency.
Another aspect is the reduction of wear on the threaded nut. In these investigations, the wear of the threaded nut could only be examined indirectly on the basis of the adhesive deposits on the support flank. Due to the reduction of the adhesive deposits to approx. 10 % of the support flank, it can be assumed that there was less micro-contact between the friction partners and therefore less wear on the threaded nut. From the reduced adhesion on the support flank, it can be concluded that the overall wear of the threaded nut is also reduced. This can reduce maintenance costs and increase the service life of the entire trapezoidal screw drive.
The third and decisive advantage of the micro-structured support flanks of a lead screw results from the design of the drive motor. By reducing the coefficient of friction by 25 %, a lower frictional torque can be assumed in the design.
This makes it possible to select a motor of a smaller size, which reduces the power consumption of the overall system and significantly lowers the investment costs for the entire lifting system. However, the running-in behavior must be reduced for this aspect and a constant coefficient of friction must be guaranteed over the entire service life.
Through further research, the surface structure can be optimized depending on the thread geometry. The process for generating these specifically adjusted surface structures is currently being patented by the project partners.
This allows the individual tribological system to be addressed even more specifically. Another aspect for future research and development is the modification of the surface topography of the threaded nut in order to specifically compensate for the running-in behavior. A constant reduction in the coefficient of friction would make it possible to further reduce the size of the drive motor design. These points will be further addressed in the future in a jointly planned research project with Bornemann Gewindetechnik and the IFW.
Acknowledgments
The authors would like to thank ZIM for funding the project “TopGewinde – Tribologically optimized surface topographies to increase the service life of screw drives using the whirling process”.
They would also like to thank Hans Bornemann and Moritz von Soden from the manufacturer Bornemann Gewindetechnik for their excellent cooperation in the research project.
Literature
- Holmberg K, Erdemir A (2017 Influence of tribology on global energy consumption, costs and emissions. Friction 5, 263-284 (2017)
- Haessing D A, Friedland B (1990) On the Modeling and Simulation of Friction. American Con-trol Conference, San Diego.
- Denkena B, Böß V, Nespor D, Gilge P, Hohenstein S, Seume J (2015): Prediction of the 3D Sur-face Topography after Ball End Milling and its Influence on Aerodynamics, 15th CIRP Confer-ence on Modelling of Machining Operations, Procedia CIRP 31, S. 221 227
- Buckley D H (1981) Surface Effects in Adhesion, Friction, Wear and Lubrication. S. 456. Else-vier, Amsterdam.
Contact
Christian Wege, M. Eng.
Institute of Production Engineering and Machine Tools, Leibniz University Hannover
Tel.: +49 (0) 511 762 4606
wege@ifw.uni-hannover.de