Fast and Inexpensive Methods for Screening New Clutch Friction Materials

The system-wide properties of vehicles using wet clutches can be affected by the frictional behavior of the clutch; fuel economy, harshness, vibration, noise, and comfort can all be influenced by the frictional properties of the clutch.

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The functional lifetime of a wet clutch can be characterized using wear characteristics such as velocity dependence of the coefficient of friction (COF) and friction curves. Current market trends demand the development of smaller clutches with higher torque capacities. However, such developments require new frictional materials and lubricants.

Intensive, rapid, and cost-effective methods for testing new materials could help to reduce the time and funds needed to develop new materials [1,2]. Furthermore, both suppliers and original equipment manufacturers (OEMs) recognize that standardized testing procedures for new clutch friction materials are required.

Developing new testing methods

Friction data are typically acquired using full-scale clutch tests, such as the JASO M348 test carried out on the SAE No. 2 friction test machine (Figure 1). However, such tests require a specialist rig and are often expensive, inflexible, and time-consuming.

SAE tests are run to failure and therefore combine friction and durability data, rendering them unable to provide specific data about the frictional properties of the clutch. Furthermore, to run SAE tests, it is necessary to assemble full-scale clutch components from each new friction material recipe or process.

Benchtop tests that can rank friction materials in the laboratory without requiring full-scale clutch tests could reduce the time and money required to develop new materials [3,4]. Benchtop testing, therefore, represents a simple and effective solution for testing the frictional properties of new clutch materials.

Figure 1. An SAE #2 friction test rig, used for full-scale clutch tests.

Testing all of the material parameters that can affect system-wide friction represents a large number of tests, so it is economical to make small batches of single sheets of new clutch materials for benchtop testing (Figure 2).

Benchtop testing does not replace full-scale component level testing but allows for fast and easy selection of materials to be taken to the next stage of testing, yielding a more efficient selection process [5]. As friction is not a material property, but a property of the entire system, the results from the analytical models used during developmental testing must be validated by full-scale tests [4,5].

Learn more about the UMT Tribolab benchtop friction tester

The Mechanical Engineering, Gear Research Center in Munich conducted a studying on friction screening and concluded that “the friction behavior of wet clutches cannot be predicted and requires intense testing especially when new friction materials and lubricants are to be developed.”

Figure 2. The UMT TriboLab benchtop tribotest system used in the simulation of clutch material testing, and detail of sub-scale clutch material sample (ruler in cm).

Developing benchtop testing

To develop a scaled-down tribotest system suitable for mechanical testing in the laboratory, important parameters for tuning a tribosystem, including the clutch friction material, the reaction steel plate, and the transmission fluid lubricant, must be considered.

Important factors to consider include the material test sample, the contact geometry, the contact pressure, the motion applied (sliding speed or linear velocity), and the testing environment (lubrication and temperature).

To conduct a valid tribotest, the critical full-scale tribosystem parameters should be simulated as closely as possible by the mechanical tester using an appropriate rotary table and heat chamber. As the full-scale test consists of more than 275 separate conditions of speed, contact pressure, and temperature, this cannot be fully replicated by benchtop testing; however, a simplified subset of the test sequences can be conducted by a benchtop tester, which can be used to rank materials for further development and testing [7].

Modeling and testing advances

The preparation of benchtop test rigs and development of methodologies to stimulate and support full-scale testing has been investigated in many studies.

Historically, a clutch material model has involved a “pin-on-disc” setup, combined with comparison studies. However, recent developments in test rig technology and design now allow test rigs to simulate full-scale geometry and methodology more closely.

Therefore, it is now possible to effectively simulate clutch behavior in a benchtop test. However, it is still important to distinguish between whether the clutch is working in a full film or boundary lubrication torque regime.

Advanced test rigs now allow script generation and the mechanical components to closely simulate historical methodologies (such as JASO M348), dyno systems, and real life conditions.

Conclusion

Easy, flexible, fast and cost-effective testing procedures are required for the development of new friction materials, lubricants, and additives for wet clutch systems. Large numbers of new materials can be tested rapidly and efficiently using benchtop testing, providing a short list of materials to be taken forward for full, in-depth friction and durability testing. Benchtop testing has shown excellent agreement with full SAE testing (Figure 3).

References

1. P. Marklund and R. Larsson, “Wet clutch friction characteristics obtained from simplified pin on disc test,” Tribology International 41(9-10), (September–October 2008), pp. 824–30.
2. W. Ost, P. De Baets, and J. Degrieck, “The tribological behaviour of paper friction plates for wet clutch application investigated on SAE#II and pin-on-disk test rigs,” Wear 249 (2001) pp. 361–71.
3. R. Acuner, H. Pflaum, and K. Stahl, “Friction Screening Test for Wet Multiple Disc Clutches With Paper Type Friction Material,” 2014 STLE Annual Meeting, (May 18-22, 2014), Lake Buena Vista, Florida.
4. S. Shaffer and S. Papanicolaou, “Clutch Friction Material Screening Using Universal Mechanical Testers” Bruker Application Note #1009 (2015).
5. A. Senatore, V. D’Agostino, R. Di Giuda, and V. Petrone, “Experimental Investigation and Neural Network Prediction of Brakes and Clutch Friction Material Behaviour Considering the Sliding Acceleration Influence,” Tribology International 44(10), (2011), pp. 1199-207.
6. W. Ost, P. De Baets, and J. Degrieck, “The Tribological Behaviour of Paper Friction Plates for Wet Clutch Application Investigated on SAE#II and Pin-on-Disk Test Rigs,” Wear 249(5-6), (2001), pp.361-71.
7. W. Scott and P. Suntiwattana, “Effect of Oil Additives on the Performance of a Wet Friction Clutch Material,” Wear 181-183(pt. 2), (1995), pp. 8850-55.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.