Analysis of the Accuracy of Mass Difference-Based Measurement of Dry Clutch Friction Material Wear

Abstract

The paper demonstrates that the dry clutch friction plate wear rate, measured based on the plate mass difference method, exhibits a transient behavior after each change of friction interface temperature level. The effect is hypothesized to be caused by a temperature-dependent change in the moisture content/mass level in the friction material. To test this hypothesis, a series of synchronized characterization experiments have been conducted by using two friction plates, one for wear tests and the other for drying in an oven under the same temperature conditions. Based on the analysis of test results, a moisture content compensation procedure, which reduces the transient wear rate from being 100% to being 50% higher compared to stabilized wear rate, is proposed and verified. The gained insights are used to set recommendations on the organization of routine wear characterization experiments aimed at avoiding the effect of moisture content influence on the accuracy of wear measurement. The main recommendations are to minimize the number of temperature target level changes through proper design of the experiment, insert a run-in test after every long test pause, and execute a pre-heat, blind wear test at the beginning of each test day.

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“…To suppress the moisture effect, the examined friction plate was being stored overnight in a sealed container with a silica gel and dried in an electric oven for 1 h at 100 • C at the start of each workday. Furthermore, based on the recommendations gained in [24], the following additional measures were applied: (i) all experiments conducted on a single friction plate correspond to a single temperature level; (ii) prior to recording the first characterization experiment (i.e., after the oven drying), a one-hour preparatory experiment is carried out at the target temperature level to heat up the machine and thermally precondition the friction plate (other inputs are set to their run-in values); and (iii) the first weight difference in the workday is determined from the weight measurements recorded after the preparatory experiment and the subsequent wear characterization experiment (empty magenta circles in Figure 17).…”

“…The significant drop in COF and the corresponding substantial increase in wear rate typically occur during fading conditions, which are linked to high temperatures (300 • C in [22] or even above 450 • C according to [23], depending on friction material). The temperature level used in the characterization experiment in Figure 13 equals 0.7 T d,high , where T d,high denotes the maximum temperature expected during regular clutch exploitation and is used in [24] as a basis for test-result normalization. It is worth mentioning that the fading effect for the given friction material was detected at the temperature level of 1.8 T d,high [24].…”

“…The temperature level used in the characterization experiment in Figure 13 equals 0.7 T d,high , where T d,high denotes the maximum temperature expected during regular clutch exploitation and is used in [24] as a basis for test-result normalization. It is worth mentioning that the fading effect for the given friction material was detected at the temperature level of 1.8 T d,high [24]. Therefore, it was reasoned, based on the connection between COF and the leaf spring stiffness observed in Figure 11a, that the remaining friction plate-pressure plate non-parallelism under the rush conditions of numerous consecutive high-power closing cycles may result in the local high-pressure regions, i.e., local high temperatures and corresponding local fading-like effects, which are non-detectable by the corresponding measurement equipment.…”

“…ature level used in the characterization experiment in Figure 13 equals 0.7 Td,high, where Td,high denotes the maximum temperature expected during regular clutch exploitation and is used in [24] as a basis for test-result normalization. It is worth mentioning that the fading effect for the given friction material was detected at the temperature level of 1,8 Td,high [24].…”

“…However, as the effect is predominantly connected with the initial heating (warm-up) phase, the overall duration of wear tests was found to be increased by only 5 to 10% compared to the original tests from [17]. Based on the above hypothesis, the control routine was adjusted to impose a minimum cooling delay larger than zero, where the value of the lower cooling delay limit is set dependent on the operating point parameters (higher value for higher power; [24]). The second part of the response shown in Figure 14 (n ≥ 200), which corresponds to the application of a lower cooling delay of 7 s, points to a significantly more stable (stiffer) COF response compared to that of the original, unconstrained case (n < 200).…”