High-Speed Friction Measurements Using a Modified Surface Forces Apparatus

Abstract

Methods of measuring friction forces in the surface forces apparatus (SFA) are presented for sliding velocities from \1 nm/s to [10 m/s. A feed-forward control (FFC) system for the piezoelectric bimorph slider attachment is introduced to allow experiments at velocities up to *4 mm/s. For still higher speeds, a motor-driven rotating mini-disk setup using a pin-on-disk geometry is presented, with modifications to enable sliding velocities in the ranges 1 cm/s-5 m/s and 1-25 m/s. Example data sets demonstrate the applicability of the approach to modeling important tribological systems including hard-disk drives. We find that mechanical system parameters such as the resonant frequencies and mutual alignments of different moving parts become increasingly important in determining the tribological response at sliding velocities above *1 cm/s (for SFA or bench top devices). Smooth or stick-slip sliding-common features of low-speed sliding-become replaced by large-amplitude oscillatory responses that depend on the load and especially the driving speed or rotational/reciprocating frequencies.Detailed recordings and modeling of these complex effects are necessary for fully understanding and controlling frictional behavior at high speeds.

Measurement and Characterization of “Resonance Friction” at High Sliding Speeds in a Model Automotive Wet Clutch

“…Friction forces at such high sliding speeds start to behave very differently from what is commonly observed at low speeds, below cm/s [2][3][4][5][6]. Large amplitude oscillatory (i.e., sinusoidal) friction forces F now appear that span both sides of the F = 0 axis, with an amplitude DF greater than the mean friction force F h i.…”

“…We used a recently developed high-speed tribometer attachment to the SFA 2000 [5,11] for measuring friction forces (and surface temperature changes) at high sliding speeds. This attachment uses the pin-on-disk geometry which is commonly used in tribology research.…”

“…due to dynamic effects or surface features. As described in [5], thermocouples were also placed within the friction material (see below). High-speed data sampling ([2.5 9 10 3 samples/s) was performed using a digital oscilloscope (Tektronik model DPO3000).…”

“…Engaging the clutch abruptly when the engine is turning at high speed is known to cause a jerky start, known as shudder, which is both uncomfortable as well as contributing to long term damage of the clutch itself [7]. In wet clutches, shudder has been associated with stick-slip instability [5]. Indeed, transmission fluids that exhibit shear thinning-a negative slope in the friction force-shearing velocity (F-V) curve-are known to facilitate the development of both stick-slip (characterized by small amplitude saw-tooth spikes) and shudder (characterized by large amplitude sinusoidal oscillations) [8][9][10] and see Fig.…”

“…Indeed, transmission fluids that exhibit shear thinning-a negative slope in the friction force-shearing velocity (F-V) curve-are known to facilitate the development of both stick-slip (characterized by small amplitude saw-tooth spikes) and shudder (characterized by large amplitude sinusoidal oscillations) [8][9][10] and see Fig. 12 of [5]. Research efforts focusing on new clutch fluid formulations have, therefore, aimed at improving anti-shudder performance [8].…”