Forming manufacturing processes at room temperature are suitable for the introduction of micro lubricant pokkets. They lead to pronounced work hardening and high residual compressive stresses near the surface [BLEI12]. In addition to shot peening and its derivatives [SCHU06, Science and Research 12 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 DOI 10.24053/ TuS-2024-0019 Introduction, Motiavation and Objective Spur gears are widely used as functional components in, for example, wind power gearboxes and industrial gearboxes. Involute gearing is the most common type for spur gears. During torque transmission, the involute kinematics and the resulting forces cause mechanical stress on the tooth root and tooth flank. If the stress exceeds the load carrying capacity of the tooth, the types of damage specified in DIN 3979 [NORM79] - tooth root fracture and pitting - occur, see Figure 1 [KLOC17]. Manufacturing processes can be used to increase the mechanical and tribological load carrying capacity of cylindrical gears. Load carrying capacity-increasing edge zone properties such as residual compressive stresses [BRIN82, REIM14, REGO16, GRES19] and surface structures that influence the wear resistance and lubrication condition of the tooth flanks [KLOC17, STAH17] are introduced. Particular mention should be made of micro lubricant pockets, which to date have mainly been produced by laser structuring [MAYE13]. These influence the hydrostatic and dynamic fluid pressures in the lubricating film, which improves the lubricating film load carrying capacity. Micro lubricant pokkets also keep wear particles away from the rolling sliding contact [HOLM12, HSU14]. In highly loaded lubricated elasto hydrodynamic contact of cylindrical gears, the case of mixed and boundary friction often occurs. The edge zone integrity with the properties of indentation geometry and surface density [WANG08], residual stress state and work hardening [TRAU15] play an important role here. Experimental investigation of the load-carrying capacity of machinehammered surfaces with variation of the process parameters Sebastian Sklenak, Mohammad Dadgar, Dieter Mevissen, Christian Westphal, Tim Herrig, Christian Brecher, Thomas Bergs* Presented at GfT Conference 2024 If the load exceeds the load carrying capacity of the tooth on the gear, tooth flanks or tooth root damage occurs. The resilience of gears can be increased with various manufacturing and finishing processes. Machine Hammer Peening (MHP) is currently being researched as an alternative to shot peening to increase the load carrying capacity of gears. Due to adjacent teeth, the machining of the tooth root and tooth flank is only possible with an impact angle β > 0 ° between the hammer head and the normal of the machining surface, depending on the gear geometry. The aim of this work is to gain knowledge about the rolling sliding resistance of hammered contact surfaces under the influence of different impact angles and machining directions. For this purpose, three impact angles and two machining directions are varied during the machining of the analog specimens by the MHP. In fatigue strength tests, the hammered test specimens are compared with a ground and a shot-peened variant. The fatigue strength tests show that the hammered test specimens with an impact angle β ≤ 30 ° achieve a higher number of load cycles on average than the ground reference variant (increase of up to 100 %). For the transfer of the results to the process for hammered tooth flanks, it can be hypothesized that for impact angles β ≤ 30 °, the machining direction locally on the flank in the opposite direction to the sliding speed leads to a higher rolling sliding strength in the range of short time fatigue strength. Keywords Machine hammer peening, contact fatigue strength, rolling-sliding contact, surface optimization, surface topography, shot peening, roughness, wear Abstract * Sebastian Sklenak M.Eng. 1 Mohammad Dadgar M.Sc. 2 Dr.-Ing. Dieter Mevissen 1 Christian Westphal M.Sc. 1 Dr.-Ing Tim Herrig 2 Prof. Dr.-Ing. Christian Brecher 1 Prof. Dr.-Ing. Thomas Bergs 2 1 WZL Werkzeugmaschinenlabor der RWTH Aachen, Camus-Boulevard 30, 52074 Aachen 2 MTI Manufacturing Technology Institute der RWTH Aachen, Camus-Boulevard 30, 52074 Aachen SCHU16] and cold rolling [RÖTT02], machine hammer peening (MHP) has been researched for several years to influence the edge zone integrity (Figure 1, right) [TRAU16]. Due to its process kinematics, MHP combines the positive effects of shot peening and cold rolling on the surface structure and edge zone integrity. In contrast to shot peening, however, it is not yet state of the art to use the MHP process kinematics with its usually orthogonal approach for machining involute cylindrical gear geometries. In the first funding period of the DFG project OptiGear (390969378), it was shown with disk on disk contact fatigue tests that an increase in load carrying capacity can be achieved compared to ground surfaces depending on the stroke and the indentation distance [MEVI22]. Due to adjacent teeth, it is only possible to machine the tooth flank and tooth root on gears with machine hammer peening with an impact angle β i > 0 °, depending on the gear geometry. The influence of impact angle and machining direction on the load carrying capacity in rolling sliding contact has not yet been researched. The aim of the work is to gain knowledge about the influence of the impact angle between the hammer head and the machining surface on the rolling sliding strength in the range of short time fatigue strength. Furthermore, the machining direction in connection with the impact angle is the subject of the experimental investigation. In the first step, the disk on disk test specimens are manufactured and characterized in different variants (see Figure 2). The experimental tests are carried out on a Disk on Disk test rig. The rolling sliding resistance is then evaluated in the range of short time fatigue strength. The influence of the impact angle and machining direction is Science and Research 13 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 DOI 10.24053/ TuS-2024-0019 Figure 2: Motivation and objective Figure 1: Gear damage and Machine Hammer Peening [KLOC17] tion of the test shaft, pulling (+) and pushing (-), were processed. The indentations on the test shoulder of the shafts were produced in a spiral shape during the hammering process, see Figure 3. After mechanical surface hammering, the highest average surface hardness of ~850 HV30 was obtained for the variant β = 15 ° [DADG24]. The machining direction (pushing and pulling) shows no significant influence on the surface hardness for the three different impact angles. With an average surface hardness of ~700 HV30, the shot peened variant lies between the hammered variant β = 45 ° with ~770 HV30 and the ground variant with ~650 HV30. Overall, the analysis of the surface hardness shows that the hardness of the hammered variants decreases as the impact angle β increases, as expected, but is still significantly higher than that of the shot-peened and ground variants. To compare different manufacturing processes for surface hardening, one variant was shot peened after grinding. The shot peening process was provided by Metal Improvement Company in Unna, Germany. To characterize the test shafts, the roughness of the test shafts was measured using a stylus instrument. Five individual measuring sections with l r = 0.8 mm and a total measurement length of l t = 4.8 mm were measured. The probe has a tip radius of r tip = 2 µm and a probe tip angle of α tip = 90 °. Figure 3 shows the initial roughness of the test shafts before the experimental tests. All hammered variants have a lower roughness compared to the ground reference variant. It can be seen that the roughness increases slightly as the impact angle β increases. As confirmed in the literature, the roughness of the shot peening variant is significantly higher than the ground reference [WECK95, REGO16]. The average roughness of the ground test shafts was Ra mean = 0.28 µm before the tests. In addition to evaluating the roughness on the basis of profile measurements, the topographies of the variants were also measured. Figure 3 compares the optically Science and Research 14 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 DOI 10.24053/ TuS-2024-0019 evaluated in relation to a ground reference variant and a shot-peened variant. For a holistic view of the test results, the residual stress state and the change in surface roughness, as well as the wear of the test shaft and mating shaft, are also taken into account in the evaluation. Manufacturing and characterization of the specimens A classic production process chain for case hardened components was used to manufacture the test specimens (soft machining, case hardening, hard machining). A grinding wheel of the specification 89A 802 J 5A V217 with the dimensions 500x60x203.2 from Tyrolit was used for machining the ground test surfaces by the process of external circumferential grinding between centers. The peripheral speed of the workpiece and the grinding wheel was v w = 0.4 m/ s and v c = 40 m/ s respectively. In total, a radial stock allowance of q = 0.1 mm was machined in the roughing and finishing process. The spark-out time was t = 2 s. After grinding, the cylindrical test specimens were machined using the accurapuls V.2002 electrodynamic hammering system as the final production step, see right in Figure 1. The operating principle of the hammering system describes the movement of an oscillating mass with a copper coil, which oscillates in a changing magnetic field of a permanent magnet. The hammer system was guided by an ABB IRB6660 industrial robot (Figure 1, right). The hammer head diameter d = 8 mm and the impact frequency f = 120 Hz were kept constant. Furthermore, based on the findings from the first project period, the ram stroke was also kept constant at h = 0.3 mm and the distance between the indentations on the functional surface of the shafts at s = 0.07 mm [MEVI22]. For the experimental tests, six test variants with three impact angles β = 15/ 30 / 45 ° and two different directions of rota- Figure 3: Manufacturing and characterization of the test specimens measured topographies of three variants. The grinding grooves are still clearly visible after hammering and shot peening. In contrast to the shot-peened variant, the indentations are not visible in the topography of the hammered variant. This also explains the increased roughness of the shot-peened version. Rolling-sliding strength of hammered surfaces in the gear analog contact To determine the rolling sliding strength of the hammered surfaces, tests were carried out on the Disk on Disk test rig, Figure 4. The Disk on Disk contact is an analogy tooth flank contact and transfers the tribological stress condition in the tooth flank contact to an equivalent geometry in the form of two disks [KLOC17]. The test shaft is cylindrical (d 1 = 42.05 mm), while the counter shaft has a crowning radius (r crow. = 166 mm) in the axial direction in order to be able to apply damage-relevant pressures under the technical test rig conditions [GOHR82, STRE97]. The test system is driven by an electric motor and a belt drive. The slip of s 1 = -28 % is realized via a slip gear box. The load is applied via a hydraulic pressure cylinder in combination with a lever system. Lubricating oil is supplied by means of temperature-controlled injection lubrication (T oil = 90 °C, Renolin CLP 100). A constant load level with a Hertzian pressure of p max = 2900 MPa was set for the comparison of the variants in the fatigue strength range. Figure 5 compares the average number of load cycles from three tests for all eight variants. The ground variant serves as a reference and achieves an average number of load cycles of Science and Research 15 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 DOI 10.24053/ TuS-2024-0019 Figure 5: Rolling-sliding strength in Disk-on-Disk contact Figure 4: Disk-on-Disk test rig roughness compared to the initial state. Due to smoothing effects in the running in, a reduction in roughness is to be expected as a result of the pressure distribution in the rolling contact [MEVI21]. In the case of the hammered variants, it can be seen that the variant (pushing or pulling) with a shorter running time has a higher roughness after the test with the same impact angle. This means that the increase in roughness in the hammered variants could be a cause of earlier damage occurrence. In the next step, the increase in roughness was analyzed in greater depth using contour measurements. For a more in depth analysis of the test results, a contour measurement of the test and counter shafts was carried out before and after the test. Figure 7 shows the contour change in the cross-section based on the aligned contour measurements for one test per variant. Overall, an indentation of a few micrometers can be seen on the test and counter shaft in the contact surface in all tests. The indentation can result either from plastic deformation, wear or a combination of these two effects [LÖPE15, MEVI21]. The ground and shot peened variants show smaller indentations compared to the hammered variants. The microscope images in Figure 6 show intact surfaces next to the pits for the ground and shot peened variants. Therefore, the indentation due to the compressive stress can primarily be attributed to plastic deformation in both variants. In the case of the hammered variants, a run in or run-out of the hammering process in the form of an elevation or indentation is still visible on the test shaft to the right and left of the contact area. In the case of the hammered test shafts, the increased roughness after the test and the greater deepening of the running surface in the cross-section mean that wear of the contact surface can be assumed. This assumption is confirmed by the matching contours of test and counter shaft, eg. variant (-)45 in Figure 7. The wear may result from the significantly higher surface hardness. Overall, Science and Research 16 Tribologie + Schmierungstechnik · volume 71 · issue 4/ 2024 DOI 10.24053/ TuS-2024-0019 N L = 1.23 106 LC. The average number of load cycles of the shot-peened variant shows a significant increase in rolling resistance compared to the reference [WECK95, REGO16]. For the hammered variants, the β = -15 ° variant shows a lower rolling sliding strength compared to the reference variant. All other hammered variants have a higher number of load cycles on average. However, the scatter of the hammered variants is greater than for the reference and the shot-peened variant. It is noticeable that the rolling sliding strength of the hammered variants decreases as the impact angle increases and the rolling sliding strength of the β = +45 ° variant is even lower than the variant with pushing machining direction. For the transfer of the results to the process for hammered tooth flanks, it can be hypothesized that for impact angles β ≤ 30 °, the machining direction locally on the flank in the opposite direction to the sliding speed leads to a higher rolling sliding strength in the short time fatigue range. However, this thesis still needs to be verified with gear running tests. Figure 6 shows representative damage patterns in the form of microscope images for 5 variants. The typical characteristics of pitting can be seen in all variants. A shell shape with a fatigue area at the tip and a forced fracture in the rest of the fracture surface [BREC17]. The damage patterns of the variants with pushing machining direction (-) do not differ in their damage characteristics from the variants with pulling machining direction (+). The images show that the pittings of variants (+)15 and (+)30 exhibit further surface cracks starting from the forced fracture area of the pitting. The cracks can be explained in connection with a higher surface hardness compared to the shot-peened and ground variants [DADG24]. When evaluating the roughness after the test, all variants except for the shot peened variant show an increased Figure 6: Damage patterns and roughness the wear of the hammered test shafts may be a cause of the greater scatter in the running time. In summary, the short time fatigue strength tests show that the hammered variants with an impact angle β ≤ 30 ° achieve a higher number of load cycles on average than the ground reference variant. The roughness analysis shows that although the initial roughness is reduced by the hammering process, the roughness after the test is higher for the hammered variants compared to the reference variant due to wear. Based on the findings from the tooth flank analogy contact, tooth flanks should be machined with a small impact angle and pulling machining direction in relation to the direction of the sliding speed if possible. The planned evaluation of residual stresses and hardening depth curves may provide further explanations for the results from the rolling sliding strength tests in the future. Acknowledgement The authors gratefully acknowledge financial support by the German Research Foundation (DFG) [390969378] for the achievement of the project results. 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