ning options to reduce friction in the forming section of a paper machine. Conducting tests on newly developed or optimised forming fabrics on industrial paper machines, however, presents a challenge, due to the high cost of the forming fabric product with widths in the range of 10 m and lengths of 70 m, and the machine production losses. Therefore, the development of a new test method to test model forming fabrics on a smaller scale is desirable for the Science and Research 20 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 1 Introduction Paper and its affiliated products are produced by dewatering a paper stock suspension using a paper machine (see Figure 1) in an energy intensive process. In the first section of the paper machine, this suspension is applied onto a revolving water-permeable multilayer technical fabric known as the forming fabric. During this process, the paper stock suspension is continuously being dewatered using varying kinds of dewatering elements positioned underneath the forming fabric as the paper stock is retained on top and conveyed through the paper machine. The relative motion between the moving forming fabric and the stationary dewatering elements is known to be a major cause of friction and wear and therefore contributes significantly to the energy consumption of the paper production process [1]. A reduction of the occurring friction is thus desirable for the paper machine operator due to potential savings in energy costs and the accompanying reduction of CO 2 emissions [2]. As paper machine operating parameters and stationary dewatering elements are often pre-set to produce a particular paper, optimising the forming fabric remains as one of the remai- Analysing the friction behaviour of paper machine forming fabrics under water lubricated hydrodynamic conditions and systematical test parameter variation in three defined wear depths using a pin-on-disc test assembly Justus Rüthing, Frank Haupert, Regine Schmitz, Mirek Göbel* submitted: 27.09.2024 accepted: 7.01.2025 (peer review) Presented at GfT Conference 2024 The production of paper on modern paper machines is associated with significant electrical energy consumption, a considerable amount of which is required to overcome friction between paper machine dewatering elements and the paper conveying forming fabric. In this study, an optimised pin-on-disc test method is used to investigate the coefficient of friction (COF) of two paper machine forming fabrics in three different wear depths and varying water-lubricated conditions. The results show a dependence of COF on lubrication rate, with a maximum COF under lubricant-starved conditions, a minimum at intermediate lubrication, and increasing COF at higher lubrication rates. It is shown that the two tested forming fabrics can be differentiated based on their lubricationand wear-depth-dependent frictional behaviour by applying the devised test metrics. Keywords pin-on-disc, water lubrication, hydrodynamic, forming fabrics, wear, friction Abstract * M. Sc. Justus Rüthing Orcid-ID: https: / / orcid.org/ 0000-0001-7615-4979 Prof. Dr. -Ing. Frank Haupert Orcid-ID: https: / / orcid.org/ 0000-0002-3312-6844 Prof. Dr. -Ing. Mirek Göbel Orcid-ID: https: / / orcid.org/ 0009-0007-6203-6585 Dr.-Ing. Regine Schmitz Orcid-ID: https: / / orcid.org/ 0000-0002-4510-2559 Hamm-Lippstadt University of Applied Sciences, Marker Allee 76, 78, 59063 Hamm producers of paper machine forming fabrics and the paper machine operators as the primary beneficiaries. As of today, there are many known tribological test methods to test the wear of paper machine forming fabrics [3-8]. However, a method to reliably investigate the frictional behaviour of these forming fabrics is still limited. Specifically, the influence of the progressive paper suspension dewatering on the coefficient of friction (COF) has not been addressed in existing component scale tests. To address this gap, this model study applies an optimised pin-on-disc test rig to investigate the frictional behaviour of two forming fabrics under varying and controlled lubrication rates to model the progressive paper suspension dewatering in the forming section. The coefficient of friction is measured under defined lubrication ranging from fully lubricated to lubricant starved conditions and in three defined wear depths representing the life cycle of a forming fabric during machine operation to assess the influence of lubrication and wear-depth on the frictional, and therefore energetic, performance of paper machine forming fabrics. 2 Materials 2.1 Pin-on-disc test rig utilised for forming fabric characterisation For the characterisation of the paper machine forming fabrics an optimised, in-house designed and built test rig in the pin-on-disc configuration described in [10, 11], is used (see Figure 2). Normal force is applied by a mechatronic controlled load and test unit. The load and test unit are able to apply precise force to the specimen in the range of 1 to 200 N. During measurement a precise normal force application (± 0.3 N) is realised using a feedback control system and spindle drive. Utilising a load cell, the friction force is measured. To measure the weardepth of the forming fabric specimen, a µm-accurate laser deflection sensor (Micro-Epsilon optoNCDT ILD1700-10) is used to measure the wear of the forming Science and Research 21 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 Figure 1: The VOITH Paper Huatai PM12 Paper Machine in China (paper machine forming fabric in blue) [9]. Figure 2: Optimised pin-on-disc test assembly for the tribological characterisation of paper machine forming fabrics with peristaltic pump (right) and close up of the contact area (left). by a defined wet grinding process consisting of a two times two-minute grinding procedure with the SiC wet abrasive paper grain sizes of 360 and 600. To ensure the quality of the conducted experiments the counterpart surface roughness metrics of Ra of 0.11 ± 0.02 µm, Rpk of 0.38 ± 0.15 µm, Rvk of 0.43 ± 0.16 µm, Rk of 0.23 ± 0.03 µm are measured and controlled in defined intervals using a white light interferometer (FRT Mirco-Prof ® ). 2.3 Specimen - polymer forming fabrics Paper machine forming fabrics are woven multi-layer technical fabrics made from varying polymer fibre materials, fibre thicknesses and weave patterns with an overall thickness of approx. 1000 µm (see Figure 3). A modern Self Support Binding (SSB) forming fabric is made up of two interwoven parts: the upper paper side and the bottom machine side (see Figure 4). The upper side of the forming fabric is designed to define the surface properties of the paper product. The machine side is designed to resist the tribological stresses, good wear resistance and minimal power consumption, during paper machine operation [12]. To test the general applicability of the optimised test rig, two forming fabrics with two differing kinds of machine side materials where selected, investigated and compared in this study with the methods described in the following chapter. 3 Methods 3.1 Parameters for the COF measurement of forming fabrics To model the paper machine forming section on a model test rig, an analysis of a paper machine forming section Science and Research 22 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 fabric specimen by detecting changes in specimen thickness that occur as a function of test time during measurement. Counterpart rotation is realised by a servomotor, enabling stable test rig operation at sliding speeds of 5.0 m/ s. Temperature sensors are located underneath the counterpart and are used to monitor the counterpart’s overall temperature. To conduct measurements under lubricated conditions, a programmable peristaltic pump (Ismatec ® Reglo ICC) is used. With this set up, lubricants can be applied directly onto the counterpart’s surface with lubrication rates ranging from 0.01 to 5.70 ml/ min over the water injection attachment. Surrounding the counterpart, there is a liquid enclosure protecting the tribometers electronics. All of the features of the described test rig are controlled by a MATLAB ® (The MathWorks, Inc.) based in-house designed operating system which is operated using a Graphical User Interface (GUI). During the conducted tests, applied normal force F N in [N], friction force F X in [N], speed v in [m/ s], specimen wear distance s in [µm] and temperature T in [°C] are measured and displayed as a function over test time t in [min]. Hereby F N and F X are used, to calculate the steady-state coefficient of friction µ [-]. 2.2 Counterpart - dewatering element alumina To model the paper machine dewatering elements on the pin-on-disc test rig, an Alumina (Al 2 O 3 ) that is used on the stationary dewatering elements is employed as the rotating counterpart material (see Figure 2). Before the start of each experiment, the counterpart disc is prepared Figure 4: View of the bottom machine side (left) and top paper side (right) of a paper machine forming fabric [9]. Figure 3: Side view of a polymer forming fabric with the upper paper side (in blue) and the bottom machine side (in white) [9]. has been conducted. To derive the tribosystem parameters from that analysis the following paper machine operating parameters were considered: dewatering element vacuum (normal force), paper machine operating speed (sliding speed) and paper suspension dewatering (lubrication rate). As other parameters of influence like temperature and humidity could not be controlled using the current test set up, described in 2.1, they were not included and could not be considered. Therefore, all experiments were conducted in between the temperature range of 24 -25 °C, of which no adjustments were made during the experiments. Further wear occurs on the machine side of the paper machine forming section with an increase in its run-time. As the contact area of the individual machine side fibres increases with wear depth, it is also being considered and tested as a parameter of influence on the COF. Normal Force: The selected normal force was derived from the average load induced by the vacuum aided dewatering elements in addition to the load applied by the paper suspension. Using the sample size of 400 mm 2 to test on the test rig currently, the normal force was calculated to be 8 N with every conducted test. Sliding Speed: Modern paper machines are operating under high speeds of up to 33 m/ s. Due to limitations in the test rig drive-unit, a sliding speed of 5.0 m/ s was chosen to ensure good test rig run ability and repeatability of the generated test results. Lubrication Rate: As the paper is constantly being dewatered while being conveyed through the forming section, several lubrication rates ranging from 2.50 to 0.00 ml/ min (see Figure 5) were chosen to model the dewatering on the test rig scale. Wear Depth: Wear, increasing with paper machine run time, can be observed on the paper machine forming fabric machine side contact area. As the contact area of the fibres used to create the forming fabric machine side change with added wear, three defined wear depths are being tested to investigate the influence on the COF. 3.2 Experimental procedure To investigate the COF of forming fabrics in the paper machine forming section, an experimental procedure has been devised. Using the testing parameters described in 3.1, the first test is conducted using a new forming fabric sample (wear depth 1) and a lubrication rate of 2.50 ml/ min. After a test time of 50 min, the same sample is being tested with a lubrication rate of 1.00 ml/ min for a further 50 min continuing this progression as displayed in Figure 7, until the lubrication rate of 0.00 ml/ min (dry conditions) is reached. With the lubrication rate of 0.00 ml/ min, this test is being run until the next wear depth (wear depth 2) is reached. With the targeted wear depth reached, the test procedure is repeated starting with a lubrication rate of 2.50 ml/ min and the last wear depth (wear depth 3) has been tested. To ensure repeatability of the conducted experiments, this test procedure is repeated three times using three new samples from the same batch in total to give the mean and standard deviation of the COF. Science and Research 23 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 Figure 5: Display of the lubrication rates used to model the dewatering of the paper machine forming fabric. Figure 6: Display of the three defined wear depths tested on the measured wear curve (left) and its depth equivalent in the paper machine forming fabric machine side. Figure 7: Representation of the devised test procedure to measure the COF of paper machine forming fabrics by 1. Measuring the COF using six defined lubrication rates; 2. Wearing down the sample by conducting dry measurements until the next wear depth is reached. This procedure is repeated two times until the all three wear depths have been measured. To ensure repeatability, this procedure is repeated three times. attributed to the onset of mixed lubrication. The introduction of lubricant reduces the fabric-counterbody contact, and thereby lowers friction. The minimum COF observed at approximately 0.50 ml/ min corresponds to a balance between boundary friction and hydrodynamic effects, where sufficient lubrication is present to reduce surface interaction without yet introducing significant viscous drag. This minimum is observed consistently across all investigated wear depths, with the exemption of fabric B wear depth 2. At lubrication rates exceeding 0.50 ml/ min, the COF increases for both fabrics, indicating an increasing contribution of hydrodynamic friction. In this regime, the added lubricant can no longer fully enter into the contact, leading to increased drag effect. This hydrodynamic contribution becomes particularly pronounced for forming fabric B, where wear depths 2 and 3 show a steep increase in COF with increasing lubrication rate. The stronger sensitivity of fabric B to lubrication rate is most likely related to differences in fibre material compared to fabric A. The influence of wear depth at higher lubrication rates further supports this interpretation. For forming fabric B, increasing wear depth leads to a widening of the fibres and an associated increase in contact area, which enhances the hydrodynamic contribution to friction. In contrast, forming fabric A exhibits a more stable COF response at higher lubrication rates, with overlapping COF values across all wear depths. This suggests a lower sensitivity of fabric A to hydrodynamic drag. At low lubrication rates, wear depth differentiation remains weak for both fabrics due to the dominance of boundary lubrication and the associated data scatter. However, for forming fabric A, wear depth 3 exhibits a consistently higher COF compared to wear depths 1 and 2. This behaviour can be attributed to an increased effective surface area and reduced local surface pressure, which promotes higher friction under boundary-dominated conditions. Science and Research 24 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 4 Findings The results of the experimental procedure are presented as the COF plotted against lubrication rate for forming fabric A (Figure 8) and forming fabric B (Figure 9). For both fabrics, the data are grouped according to the three investigated wear depths, referred to as wear depth 1 (green), wear depth 2 (yellow), and wear depth 3 (red). The following analysis of the research findings presents a scientific interpretation of observed trends and is grounded in the measured COF data. Across all tested conditions, both forming fabrics exhibit a similar qualitative dependence of COF on lubrication rate. Independent of fabric type and wear depth, the COF decreases with the introduction of lubrication, reaches a minimum at a lubrication rate of approximately 0.50 ml/ min, and subsequently increases as the lubrication rate is further increased to the maximum tested value of 2.50 ml/ min. This behaviour resembles a Stribeck-like curve, characterised by a transition from boundary-dominated friction to mixed lubrication and an increasing contribution of hydrodynamic friction at higher lubrication rates. At a lubrication rate of 0.00 ml/ min, the contact can be considered starved of lubricant following the interruption of lubricant supply. Under these conditions, boundary lubrication dominates the frictional behaviour due to direct contact between the forming fabric fibres and the counterbody. In this regime, both forming fabrics A and B exhibit overlapping COF values across all wear depths, with relatively high scatter (see Figure 8 and 9) in the measured data. This overlap indicates that weardepth-dependent effects are largely masked by boundary-dominated friction, limiting the ability to differentiate wear states based on COF alone. The observed scatter further highlights a limitation of the measurement procedure under dry conditions. With increasing lubrication rate from 0.00 to 0.50 ml/ min, the COF decreases for both fabrics, which can be Overall, the results demonstrate that the measured COF arises from a combination of boundary and hydrodynamic friction components, with their relative contributions varying as a function of lubrication rate, fabric structure, and wear depth. While the absence of direct film thickness measurements limits a fully quantitative hydrodynamic analysis, the observed trends are consistent with established tribological concepts. The findings emphasise that both lubrication regime and fabric geometry must be considered when evaluating the frictional performance of forming fabrics. 5 Summary In this study, two different forming fabrics were characterised using the COF obtained with the devised test method, defined test parameters, and an optimised pin-ondisc test rig. Analysis of the experimental results identified the lubrication rate as a parameter of significant influence on the COF compared to the additionally investigated wear depth. The highest COF values were measured under dry conditions at a lubrication rate of 0.00 ml/ min while the introduction of lubrication led to a reduction in COF, followed by an increase at higher lubrication rates. This behaviour can be explained by a transition from boundary-dominated friction under dry conditions to mixed lubrication at intermediate lubrication rates, and an increasing contribution of hydrodynamic friction at higher lubrication rates. In this regime, viscous drag becomes increasingly significant and contributes to the observed rise in COF. Forming fabric B exhibits a pronounced increase in COF with increasing lubrication rate and wear depth, indicating a higher sensitivity to hydrodynamic friction effects as the test progresses. This suggests that forming Science and Research 25 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 Figure 8: Comparison of the COF of forming fabric Sample A over the lubrication rate in the three wear depths. Figure 9: Comparison of the COF of forming fabric Sample B over the lubrication rate in the three wear depths. Literature [1] J. H. Tylczak und F. J. Friedersdorf, “A new test for pulp and paper forming fabric materials”, Wear, Jg. 302, 1-2, S. 1082-1087, 2013, doi: 10.1016/ j.wear.2012.10.026. [2] K. Holmberg, R. Siilasto, T. Laitinen, P. Andersson und A. Jäsberg, “Global energy consumption due to friction in paper machines”, Tribology international, Jg. 62, S. 58- 77, 2013, doi: 10.1016/ j.triboint.2013.02.003. [3] G. A. Hemstock und H. B. Neubold, “Effect of pigment properties on the wear of plastic fourdrinier wire”, Tappi Journal, Jg. 71, Nr. 5, S. 127-132, 1988. [4] H. Einlehner, “Abrasion testing apparatus,” US4633701A. USA. [5] R. Pitt, “Fillers and fabric life in alkaline papermaking”, Tappi Journal, Jg. 67, Nr. 4, S. 96-98, 1984. [6] G. J. Gill, J. E. Holton und M. M. Kazanzian, “Using the Valley Abrasion Tester to determine the wearing effect of a paper filler on plastic forming fabric”, TAPPI, Jg. 65, Nr. 7, S. 54-56, 1982. [7] H. B. Neubold, P. Sennet und H. H. Morris, “Abrasiveness of pigments and extenders”, Tappi Journal, Jg. 65, Nr. 12, S. 90-93, 1982. [8] J. Vlossak, “Improving the accuracy and flexibility of the valley clay tester”, tappi, Jg. 57, Nr. 3, S. 123-125, 1974. [9] VOITH, Mediendatenbank. [10] J. Rüthing, F. Haupert, R. Schmitz, M. Sigrüner und N. Strübbe, “A new approach for the friction and wear characterisation of polymer fibres under dry, mixed and hydrodynamic sliding”, T+S, Jg. 69, eOnly Sonderausgabe 2, S. 18-25, 2022, doi: 10.24053/ TuS-2022-0034. [11] R. Schmitz, F. Haupert, J. Rüthing, M. Sigrüner und N. Strübbe, “Tribologische Charakterisierung von Polymerfasern unter Trockenreibung, Mischreibung und Hydrodynamik mittels einer optimierten Pin-on-Disc-Prüfmethode”, T+S, Jg. 68, 3-4, 2021, doi: 10.24053/ TuS-2021- 0015. [12] S. Adanur, Paper Machine Clothing: Key to the Paper Making Process. Boca Raton, FL: CRC Press, 2017. [Online]. Verfügbar unter: https: / / permalink.obvsg.at/ Science and Research 26 Tribologie + Schmierungstechnik · volume 72 · issue 5/ 2025 DOI 10.24053/ TuS-2025-0026 fabric B performs more favourably in earlier test stages, while its frictional performance deteriorates with increasing wear. In contrast, forming fabric A shows a higher initial COF across all lubrication rates but a comparatively stable frictional response with increasing wear, indicating a lower susceptibility to wear-induced changes in hydrodynamic friction. While no clear distinction between forming fabrics A and B could be established at low lubrication rates between 0.00 and 0.25 ml/ min, a clear differentiation becomes possible at lubrication rates of 0.50 ml/ min and above, with the strongest contrast observed at 2.50 ml/ min. Using the optimised pin-on-disc test rig and the applied test methodology, it was demonstrated that forming fabrics can be differentiated with respect to their lubrication-dependent and wear-dependent frictional behaviour based on COF measurements. It is noted that the test rig in its current development stage cannot fully replicate the industrial paper machine operating conditions being limited by its test speed of 5.0 m/ s and without temperature and humidity control. Therefore, this study emphasized the methodological validation with the development of next generation test rig capable of higher sliding speeds and controlled environmental conditions already in progress. Collectively, these findings show that the presented method enables a systematic, forming fabric specific comparison of the COF under varying operating conditions providing a foundation for optimising forming fabric selection and design to reduce the energy consumption in paper machines. 6 Acknowledgements The authors thank the German federal ministry of research, technology and space and research (BMFTR) and J.M. Voith SE & Co. KG for the funding of this study as part of the FH-Kooperativ project TriboMath26 - 13FH010KX1.