23rd International Colloquium Tribology - January 2022 233 Wet Friction Material and Fluid Screening on Benchtop Rig Sanchez, Carlos J. Southwest Research Institute, San Antonio, Texas, USA Corresponding author: carlos.sanchez@swri.org Lee, Peter M. Southwest Research Institute, San Antonio, Texas, USA 1. Introduction This work discusses the use of a benchtop screening test for evaluating the performance of wet clutch materials and automatic transmission fluids (ATF). Tests were conducted with a Bruker TriboLab using small-scale friction discs and reaction plates to replicate the SAE #2 test. Specifically, the bench test modelled the GM 3-Day wear test; contact pressures, sliding speeds, temperatures, engagements, and a condensed test profile that runs over a few hours. Friction coefficient is calculated using a torque cell and measured throughout, and 3D wear profiles of the friction discs are measured ex situ. Tests conducted using the condensed test profile displayed correlation to the full-scale test in ranking different types of friction materials, and potential for ranking fluid formulations. Similar to the full-scale test, one of the primary indicators is the gradient of friction coefficient versus sliding speed at the start of tests compared to the end of test. Although the bench test varies in magnitude from the full-scale test, the trends show an indication of performance for clutch materials and fluids. 1.1 Background and Methods Wet friction materials are designed to operate in some form of fluid. A common example are the materials used in clutch packs and torque converters. These materials are typically made from a woven material, paper being prevalent. By design, friction materials are porous and compressible. The porosity allows for absorption of the fluid, and creates many points of contact for the counter surface to slide against more readily. In clutch packs, the friction material is bonded to one plate and engages with a smooth metal plate to transfer torque. The performance of a clutch system will depend on the material type, geometry, compressibility, surface finish, and fluids. In the automotive industry, there is great interest to improve the effectiveness of clutch engagement to increase fuel economy and overall performance. There are several test methods available for evaluating friction materials and transmission fluids (ATF) in full clutch systems. One such method is the SAE #2 test. This test uses full size clutch packs, and runs them in a similar manner as an actual engine. There are several test profiles that can be run, but the one presented herein is the General Motors (GM) 3-day wear test. This test runs at 3 pressures, 3 temperatures, and 14 speeds. There are several phases of the test that involve speed ramps both up and down, continuous slip sequences, and stepped speeds; which loop several times. As the name suggests this test takes 3 days to run. The goal of many tribological tests is to scale down to a bench setup to increase throughput and efficiency when evaluating materials. For wet friction materials, the aim is to investigate the relationship between coefficient of friction (COF) and speed. Clutches will engage at various speeds while transferring torque. This increase and decrease in speed under load will also result in stick-slip behaviour. Decreased COF with increasing speed will result in an effect called “shudder.” When designing a bench test, the contact geometry of the materials, the rotation of the instrument, the loading mechanism, and the test environment are considered. The bench test uses a 1 inch diameter disc, with a ring shaped friction material bonded to the top surface. The friction material is manufactured and bonded in the same manner as a full sized friction disc. The reaction disc was manufactured from mild steel, and given a specific surface finish. Based on the contact area of the ring shaped material, the contact pressures can be scaled in accordance with the SAE # 2 test procedure. Likewise, the rotational speeds are scaled according to the effective radius of the smaller disc. All tests were performed on a Bruker UMT TriboLab instrument. The instrument uses a rotational drive unit to rotate the reaction disc, and a six-axis load sensor to measure the forces acting on the friction material disc. The test setup also includes a heated oil bath, with a peristaltic pump that circulates the test fluid at all times. A thermocouple is placed above the reaction disc to control and measure the system temperature. The bench test consists of variable pressure engagements at different speeds and temperatures. Within the sequences are a series of stick-slip engagements, and continuous 234 23rd International Colloquium Tribology - January 2022 Wet Friction Material and Fluid Screening on Benchtop Rig slip cycles. The test is designed to replicate the GM 3-day wear test, which contains 10 phases. The first, fourth, and last phase are the measurement and inspection of the parts. The actual test sequences comprise running the materials through various speeds, contact pressures, and temperatures. There are numerous cycles, some of which repeat or loop. In this study, the 3-day wear test was reduced to run within one day. This was accomplised by running the individul sequences in a specific order that yielded comparable friction results and wear on the friction material. The data reduction produced coefficient of friction versus speed for the stick-slip, discrete speeds, and continuous slip cycles. Additianlly, coeffieicnt of friction versus time to analyze breakayway friction response. The friction materials were all inspeted in the same manner as the full scale test. A Keyence optical microscope was used to measure the profile height of the material at different phases of the test. Results of the bench test showed similar trends in performance for the friction materials tensted. The magnitudes of the friction response, and the overall wear were much lower than that seen on the full-scale test. However, the relative behavior from one material type to another was comparable. Therfore, the bench test has the potential to serve as a screening method for studying the behavior of new material pairs. The outsome of the bench test can then be used to determine suitable candidates for fullscale analysis. 2. Conclusion A benchtop screening method was developed for evaluating the performance of wet clutch materials and automatic transmission fluids. The test uses small-scale friction material discs, and reaction plates. Through careful modelling of the real system conditions, the bench results demonstrated similar friction curve behaviour to those seen in an SAE #2 test rig using the same parts and fluids. Through this method, relative comparisons can be made to rank the performance of friction materials. The bench test offers more flexibility and efficiency when investigating numerous material types. At present, the method presented herein has been shown to be most effective in screening friction material, but more work is needed to determine effectiveness for fluid comparisons. References [1] Fuji Y, Snyder T, Waldecker R, Tobler W, Davis L, Scherzer M, Zander, D, Dynamic Characterization of wet friction component under realistic transmission shift condition. SAE Technical Paper 2006- 001-0151. 2006 [2] Senatore A, D’Agostino V, Di Guida R, Petrone V. Experimental Investigation and neural network prediction of brakes and clutch friction material behaviour considering the sliding acceleration influence. Tribology International 2011; 44(10): 1199- 1207. [3] Shaffer S, Freshly T, Papanicolaou S. Benchtop screening of wet clutch materials. Tribology International 2018; 121(08): 161-166.