Smooth clutch operation in automatic transmissions is strongly tied to the velocity dependence of the coefficient of friction (COF). Full-scale clutch tests, such as the JASO M348 or any other OEM-specific test protocol run on the SAE No. 2 Friction Test Machine, are typically used to obtain friction data, but these tests are both expensive and time consuming. Further, the evaluation of each new friction material recipe or process change requires the fabrication and assembly of full-scale clutch components. Significant savings in development time and cost can be realized by screening friction materials in the laboratory prior to full scale clutch tests. This application note discusses the benefits of this process using the Bruker UMT TriboLab™ benchtop tester.

Automatic transmission clutches, also known as wet clutches, are under increasing demands for higher specific torque capacity at reduced sizes. To meet the demands in a competitive business environment, materials designers and clutch design engineers can reduce the product development time through the use of analytical models. Using such models, a large number of design and material changes can be explored, and their effect on system performance can be evaluated. A key performance characteristic in both clutches and brakes is referred to as their harshness, vibration and noise (HVN) response. HVN is strongly tied to the sliding velocity dependence of the COF. It is desired that the COF exhibit a steady or slightly increasing value as the sliding velocity increases, and also over a wide range of contact pressures. It is also well known that friction is a system property and not a material property. As such, the convenience of using analytical models in design work is tempered by the need for validation of the models, particularly the friction behavior of new clutch materials. Further, the benchtop tests must meet the subsequent burden of proof of performance compared with the full-scale device or system.

As described above, to minimize HVN and assure smooth clutch performance, it is desired that the COF exhibit a steady or slightly increasing value as the sliding velocity increases. A decrease in the COF with increasing velocity can lead to clutch shudder or judder. While there are analytical models that incorporate clutch-system stiffness, material pairs, rotor/stator contact pressure, temperature, sliding velocity, and COF, experimental data are still a required input for the COF, particularly as a function of the other variables. Full-scale tests, such as the JASO M348-2012 or any other OEM-specific test protocol run on the SAE No. 2 Friction Test Machine, (see Figure 1), are typically used to obtain such data. However, these tests can be expensive and time consuming, and they require the fabrication and assembly of full-scale clutch components.

While the contact pressure, sliding speed and temperature are obvious choices for proper simulation, a minimum contact size is also important in benchtop friction material testing. This is because of the non-homogeneous nature of clutch materials, and the need to include all constituents as well as the reservoir and channeling effect of the surface roughness and porosity. Such variation in surface texture is shown in Figure 2, via 3D topographic images obtained using a Bruker white ligh interferometer (WLI). If the contact is made too small, the effect of both the inhomogeneous composition and the surface roughness on the friction can be lost, and the benchtop test will not properly simulate the actual tribosystem.