23rd International Colloquium Tribology - January 2022 363 Investigation of Rolling and Lateral Slip on the MopeD Qs2STg500 (Modellprüfstand erster Designentwurf Querschlupf 2 Scheiben Testgerät 500 N) K.-O. Karlson Hochschule Mannheim, Kompetenzzentrum Tribologie Mannheim (KTM), Paul-Wittsack-Straße 10, 68163 Mannheim Corresponding author: k.karlson@hs-mannheim.de H. Buse Hochschule Mannheim, Kompetenzzentrum Tribologie Mannheim (KTM), Paul-Wittsack-Straße 10, 68163 Mannheim J. Molter Hochschule Mannheim, Kompetenzzentrum Tribologie Mannheim (KTM), Paul-Wittsack-Straße 10, 68163 Mannheim 1. Introduction As part of a publicly funded project (PFP) [1], the KTM developed a test bench for rolling slip tests. The primary use of the MopeD test bench is to record frictional forces with variable slip amount and direction settings. The theme of the PFP was to investigate the frictional forces and the cross slip of worm gears. 2. MopeD Mechanics, Drives and Applications To make the slip conditions in the test setup as variable as possible, the tribometer has five axes. Four of them are electromechanically controlled and one is operated manually. The sample geometry consists of two disks, which are in rolling contact with each other. There is one electric direct servo-drive for each sample disk. One linear axis moves the samples into contact and applies a normal force. To investigate the cross slip, there is an axis to set an angle for the vertical sample. Figure 1 shows the MopeD test bench. Figure 1: MopeD Qs2STg 500 test bench [KTM] The MopeD is able to log data for the applied load, the circumferential frictional force at each sample disk and the axial force at the vertical sample disk as shown in figure 2. The MopeD offers high sliding speeds, high normal force application, high sample-rates, an automatic angle adjustment and the capability to test large specimens. Table 1 lists some machine data of the MopeD. 364 23rd International Colloquium Tribology - January 2022 Investigation of Rolling and Lateral Slip on the MopeD Qs2STg500 Figure 2: Measurable forces at the samples Due to its design the MopeD can be utilised to analyse a multitude of systems, some of which are listed below: • For friction, wear and traction properties among others: Tires, drills, screws, planetary roller screw drives and fluids • General 2-Disk material and coating tests with pure to variable rolling slip together with superimposed cross slip Table 1: Operating data of the MopeD test bench Value Unit Maximum normal force F N 500 N Typical pressure with spherical disk with R = ∞ and r = 26.5 mm; pmax 1240 Maximum sliding speed at the vertical sample disk v vsd 7 Maximum sliding speed at the horizontal sample disk v hsd 19 Adjustable swivel range for the vertical sample disk α 180 ° 3. Traction - and wear curves of Tires Two different kinds of rubber as tires are tested to determine the wear amount. In addition, the influence of the wheel load is measured to show variations in wear behavior due to weight differences in combustion engine and electrical engine cars. The tests have shown that the difference of the wear mass between the model tire and the PU tire amounts to 94 %. In addition, experiments with 10 N normal force have shown that there is a variance in the type of wear between the model and the PU tire. The PU tire has much finer particles than the model tire. Figure 3 shows both tires after their test runs. Figure 3: Model and PU tire after the tests Testing different wheel loads (15/ 20 N) has shown that the torque at the tire is increasing and a larger amount of wear particles is detectable. In this case, it makes sense to think about, how we can reduce the mass of a vehicle and its batteries. Figure 4: Tests with zero slip, various forces, and various materials In order to assess the toxicology of the wear particles, it is planned to count the particles with a scattered light sensor and to collect them, fractioned according to size, in a cascade impactor. After the fractionated collection of the particles, there could be a toxicological study to investigate the environmental impact of different rubber compounds. 4. Lubricated 2-Disk Setup For testing lubricants, the 2-Disk test bench by Optimol-Instruments has been the method of choice. The samples for this test, two disks with a typical diameter of 45 mm, are pressed against each other, forming a linear contact under rotation. Additionally spherical discs can be utilized in order to create a point contact if higher contact pressure is desired during testing. The contact is on the shell surface of the two disks [2]. Similar to the classic 2-Disk test we conducted a feasibility study on the MopeD. 23rd International Colloquium Tribology - January 2022 365 Investigation of Rolling and Lateral Slip on the MopeD Qs2STg500 During this study a spherical, vertically arranged disk is pressed on a horizontally fixed disk as shown in figure 5. Figure 5: Lubricated 2-Disk-Setup Figure 6: Angular COF-Data of a 2-Disk test During the test the normal force is 230 N (911 MPa) and the slip is changing linearly from 0 to 20% and then to -20 %. The system remains at the positive and negative slip extremes for 8 minutes each before alternating back for a test total runtime of 40 hours. Furthermore, the oil (Klübersynth GEM 4320N) temperature is set to 60 °C. The contact surfaces of the samples are sanded until an average roughness depth of 3 ± 10 μm is achieved. Figure 6 shows the amount and the direction of the friction coefficient of the vertically arranged disk in relation to the circumferentialand the axial-force as a distribution histogram. This visualization shows that the most commonly measured COF lies around 0.05. Future studys will also use an additional lateral slip to investigate different materials and lubricants. References [1] Panther GmbH, Hochschule Mannheim Kompetenzzentrum Tribologie, Förderzeichen: ZF4008703, Mannheim, 2020. [1] M. Grebe, Tribometrie, Tübingen: expert verlag GmbH, 2021. 23rd International Colloquium Tribology - January 2022 367 The use of the MTM rig for wear testing Matthew Smeeth PCS Instruments, London, UK Corresponding author: matt.smeeth@pcs-instruments.com Clive Hamer PCS Instruments, London, UK 1. Introduction The wear rate of any lubricated contact is dependent on many factors, including, surface roughness, lubricant composition, environment, operating conditions, temperatures, etc. There are also many different wear mechanisms, often operating in parallel. Since wear in itself is not an intrinsic property of a system, different wear tests can give very different results. Despite their popularity and widespread use, all wear bench test methods all have some shortcomings when used to investigate complex lubricant additive combinations. However sophisticated the test method, they are inevitably unable to directly mimic real lubricated contacts conditions of machine elements. Interpretation of different bench test results can be difficult and misleading conclusions can sometimes be drawn. A new pure sliding wear test is described which can produce measurable wear withing a reasonable period of time. The repeatability and merits of the test method is discussed. 1.1 Background The thickness and distribution of reaction layers formed by ZDDP and other antiwear additives is controlled by several factors including load, temperature, sliding speed and additive concentration. Invariably, the thickness formed will increase at higher temperatures and higher pressures The chemical antiwear film formed between running surfaces is complex in composition and has been examined using a large number of different analytical techniques; however the exact mechanism by which they form is still not clearly understood. Spikes et al (1,2) and others has showed that antiwear additives form a film which is initially an amorphous glassy structure, which then converts to a nanocrystalline structure during prolonged rubbing. The nanocrystalline structure is relatively weak in comparison to the more crystalline structures, since they inherently possess greater obstruction to dislocation movement For this reason, short wear tests must be very carefully controlled, since the running in period at the start of the test (when the antiwear film is initiated) is critical. Cleanliness and complete lack of contamination is also critical, since this can adversely affect the repeatability of the test. Conversely, longer tests are less susceptible to these variations but obviously suffer the (considerable) disadvantage that the duration may be excessively long, to the point where gathering and meaningful amount of reliable test data is impractical. 1.2 Test procedure A series of tests were carried out using the MTM test rig under the following conditions Configuration Pure sliding, reciprocating Lower specimen 52100 steel disc, 760Hv Upper specimen 52100 steel ball, 3mm diameter, 800Hv Load 20N (2.74 GPa max Hertzian pressure) Temperature 100°C Frequency 20 Hz Stroke length 4mm Duration 16 hours Table 1: Test conditions A pure sliding test was chosen since the wear rate was relatively high. This allowed the wear scar formed on the ball to be measured as the primary wear indicator. Although mixed sliding and rolling wear tests have been carried out in previous studies the wear rates are considerably lower and have therefore required additional instrumentation to fully evaluate the wear. Using the reciprocating mechanism allowed a measurable wear track to form on the disc. Over the 16 hour period, 1.15 million contact cycles were run, which produced an easily measurable and clearly defined wear scar for all the fluids tested. 368 23rd International Colloquium Tribology - January 2022 The use of the MTM rig for wear testing Figure 1: Typical wear scar showing clearly defined boundary 1.3 Test fluids A group 2 mineral base oil with various concentrations of antiwear additive was tested. The viscometric properties of the base oil are shown below KV 40V 67.3 cSt KV 100C 8.8 cSt VI 103 Table 2: Base oil properties Different concentrations of the same additive was used in the base oil. 1.4 Results The graph in figure 2 shows the average wear scar dimension, taken as the average of the sliding the orthogonal direction. In all cases the wear scar boundary was clearly defined and repeat measurements were always within 20 microns of each other. The results are presented as the wear scar diameter as a function of P content. Figure 2: Variation of wear scar with blend P content Measurements were also taken of the disc using a while light interferometer, which can be used to calculate the wear volume lost during the test. The discs from 3 tests at different concentrations were measured and showed excellent correlation with the wear scar measured on the ball. As expected, relatively low concentrations of additive showed lower wear with a relatively smooth transition between the high and very low additive concentration regions The initial results indicated that the wear test was highly repeatable. To investigate this further, a series of tests were carried out on a commercial 0w-20 engine oil with the following properties Moly 60 Zinc 710 Phosphorous 633 Calcium 1344 Boron 250 Table 3: Formulated oil composition The test was very repeatable with a mean wear scar of 325 microns and standard deviation of 7 microns, shown in figure 3. 23rd International Colloquium Tribology - January 2022 369 The use of the MTM rig for wear testing Figure 3: Repeat wear scar measurements on formulated oil sample 2. Conclusion The test showed that a highly repeatable wear test could be carried out over a reasonable time scale. The test was proven to be robust using both base oil/ individual additive mixtures and fully formulated blends. The tests were carried out on Ph and Zinc containing oil,s but can be used as base line tests for ashless and other novel antiwear chemistries as a compassion. References [1] Luiz, J.F., Spikes, H. Tribofilm Formation, Friction and Wear-Reducing Properties of Some Phosphorus-Containing Antiwear Additives. Tribology Letters 68, 75 (2020) [2] Spikes, H. The History and Mechanisms of ZDDP. Tribology Letters 17, 469-489 (2004)