In automatic transmissions a smooth clutch operation is related to the velocity dependence of the coefficient of friction (COF). Although JASO M348 and other full-scale clutch tests, or other OEM-specific test protocol performed on the SAE No. 2 Friction Test Machine, provide friction data, they are expensive and time intensive. Testing a new friction material recipe or a process change involves the assembly and fabrication of full-scale clutch components. Considerable developmental cost and time savings can be achieved if friction materials are tested in laboratories before being subject to full-scale clutch tests. This article discusses the advantages of the laboratory testing process using the Bruker UMT TriboLab™ benchtop tester.
Proving Clutch Performance
There is a desire to reduce the size and to gain a higher specific torque capacity for automatic transmission clutches (also known as wet clutches). To satisfy the demands of a competitive business environment, clutch design engineers and material designers can shorten the time product development takes with analytical models. Several material and design changes can be viewed and their impact on system performance can be assessed. An important performance factor in brakes and clutches is their harshness, vibration and noise (HVN) response.
HVN relates to the sliding velocity dependence of the COF. It is desirable that the COF gradually increases with the increase in sliding velocity and contact pressures. Friction is a system property instead of a material property, so analytical models are convenient in design work allows easy validation of the models, especially the friction behavior of new clutch materials. The benchtop tests should also satisfy the proof of performance validation, compared with the full-scale device or system.
Assessment of the COF Behavior
As mentioned before, to minimize HVN and ensure a smooth clutch performance, the COF should show a steady increase corresponding to the increase in sliding velocity. COF reduction with velocity increase results in clutch judder or shudder. There are analytical models that incorporate clutch-system stiffness, rotor/stator contact pressure, material pairs, COF, temperature, and sliding velocity, experimental data as an input for COF is still necessary, especially in relation to other variables. Experimental data can be obtained by full-scale tests such as; the JASO M348-2012 or other OEM-specific test protocol run on the SAE No. 2 Friction Test Machine (Figure 1). Such tests are time-consuming and expensive, and they need assembly and fabrication of full-scale clutch components.
Figure 1. Full-Scale clutch tests are conducted on test systems, such as the SAE #2 Friction Test Rig.
Time and cost savings are expected during the developmental period, using a simple benchtop screening test to rank materials, before their fabrication into components and their implementation in a full-scale test. This includes the clutch material, the reaction plate and the fluid for clutch operation, referred to as the automated transmission fluid (ATF). The aim of this developmental effort was to develop a benchtop screening protocol and test, which can be utilized for pre-screening materials before full-scale tests. Success in the effort was measured by the capacity of the benchtop test to rank materials in the same order as the full-scale tests. The objective was not to replicate the full-scale tests, consisting of 275 conditions for contact pressure, speed and temperature, but to aid the development of simple benchtop test sequences on the benchtop tester, where identical material rankings could be developed, including the velocity dependence spread over a range of velocities.
Benchtop Test Philosophy
Appropriate simulation of the vital full-scale tribosystem parameters is the key to a relevant and valid tribotest. In a wet-clutch system the parameters for testing, as per the SAE #2 Friction Material test, were:
- contact area
- contact pressure
- sliding speed
- static or breakaway conditions.
More parameters, such as fade and recovery, and environmental conditions including road debris, salt- water, and corrosion products on the rotor, are included for more complex testing of brake friction materials. However, the parameters are sufficient for the tribosystem environment of wet clutches.
Size Effects in Simulation
A minimum contact size is important for benchtop friction material testing, as well as the common choices of temperature, sliding speed and contact pressure. The importance of contact size is due to; the non-homogeneity of clutch materials, the necessity of including all constituents, and the reservoir and channeling effect of porosity and surface roughness. Figure 2 shows the variation in surface texture through 3D topographic images acquired by a Bruker white light interferometer (WLI). The effect of surface roughness on friction and inhomogenous composition may be lost if contact is very small, with the result that the benchtop test will be unable to perform a proper simulation of the actual tribosystem.
Figure 2. WLI topographic images of three clutch materials evaluated in this study.
Completing the Simulation
For the benchtop test, the following additional parameters are also important:
- duration of clutch material engagement at a particular speed,
- rate of application of the full contact pressure,
- dwell time between test conditions
- recirculation and filtering of the ATF between clutch material changes.
Figure 3 shows the benchtop test system, UMT TriboLab includes a heating chamber, rotary drive and liquid pumping system, and also a close-up example of a sub-scale clutch test sample.
Figure 3. 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).
Results of Screening Tests
Figure 4 presents the COF results of step-speed clutch material tests performed at 120°C on the full-scale test rig and the UMT TriboLab benchtop system. The results compare favorably for relative rankings, curve shape in relation to velocity, and the amount of COF for the materials.
Figure 4. Comparison of sub-scale clutch material tests and full-scale clutch tests. (Data from SAE 2 test courtesy of LuK USA, LLC.)
For Material B, a positive slope to the COF vs. velocity curve is seen, this suggests that the material is an ideal candidate for clutch design modeling. Other test sequences such as speed ramp and breakaway COF also exhibited a good correlation with data from the full-scale test.
The UMT TriboLab serves as an ideal benchtop screening tool through proper implementation of equivalent contact stress, sliding speeds, fluid conditions, acceleration, and temperature as used in the full-scale clutch test rig. The UMT TriboLab tool accelerates the decision time, provides input to modeling efforts, and reduces the down-selecting material costs for final proof during full-scale clutch rig testing.
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.