Development of Specialised Tribometer for Advanced Tribosystem Evaluation

Abstract

This thesis presents a comprehensive study on the development and validation of force-controlled tribometers to accurately evaluate tribo-contacts under realistic test conditions. The research addresses the gap in conventional tribological measurement systems. The research is divided into two principal sections: first, the design and advancement of a Force-Controlled Pendulum Tribometer (FCPeT-3), and second, the development of a Lateral Force-Controlled Impact Tribometer (LFIT) for transient tribo-contact analysis. In conventional tribometers, energy dissipation occurs not only at the intended contact but also within auxiliary components such as bearings, seals, and unintended sliding members. These additional sources of dissipation alter the energy transfer at the tribo-contact and compromise measurement accuracy. Furthermore, contact-type sensors such as load cells and torque transducers can alter system stiffness, thereby influencing the measured response, while motors and actuators often introduce noise, vibrations, and additional damping. Since friction is inherently system-dependent, even small disturbances from supporting elements, including drive motors, sensors, or misalignments, can obscure the intrinsic behaviour of the tribo-contact. For accurate and reproducible measurements, it is therefore essential to isolate the tribo-contact from such parasitic effects. To overcome these limitations, the FCPeT was developed in three successive versions. The first version, FCPeT-1, established the feasibility of localising energy dissipation at the tribo-contact while minimising system damping, making it suitable for assessing lubricant friction efficiency. However, FCPeT-1 lacked the capability to capture friction-induced noise and vibration. Building on this, FCPeT-2, primarily developed by Adarsh et al. [1], introduced design and control improvements that enabled the assessment of noise and vibration responses of lubricants under varying temperature conditions. Despite these advances, the reliance on ball bearings in the system introduced significant damping, limiting the accuracy of frictional characterisation. Moreover, neither FCPeT-1 nor FCPeT-2 incorporated the sliding-to-rolling ratio (SRR), a critical parameter in realistic tribological contacts. These limitations motivated the development of FCPeT-3. This version employs precision air bearings to eliminate bearing-induced friction, integrates a controlled SRR mechanism at the tribo-contact, and establishes a systematic framework for evaluating tribofilms formed during running-in. FCPeT-3 eliminates all unintended sliding members and optimises pendulum dynamics to ensure that energy dissipation is confined entirely to the tribo-contact. The instrument further provides independent control of normal load, SRR (0-200% and beyond), sliding velocity, and lubricant temperature (ambient to 300 °C). A high-resolution data acquisition system enables the simultaneous measurement of pendulum decay response, vibration, and acoustic radiation. Comparative evaluation of FCPeT-1, FCPeT-2, and FCPeT-3 demonstrates the superior fidelity of FCPeT-3, which exhibits minimal system-induced damping and enables an accurate characterisation of tribo-contact dynamics. Advanced analyses, including wavelet-based signal processing, reveal direct correlations between frictional energy dissipation, vibration, and acoustic radiation, providing new insights into asperity interactions for tribofilm evolution. In addition, Part 2 of this thesis presents the study of tribo-contact under high sliding acceleration (in the order of g) using a custom-designed LFIT. LFIT investigates the influence of varying normal loads on frictional behaviour during high acceleration sliding events. The findings are particularly relevant to automotive crash scenarios, where the friction between the human body and vehicle interior surfaces can shift abruptly from high to low within ~150 ms. Such transitions critically affect injury mechanisms during collisions. By replicating these dynamic conditions, the tribometer provides quantitative data for safety analysis and supports the design of advanced interior materials for enhanced crashworthiness. In summary, this thesis focuses on the evolution of the FCPeT tribometer family and establishes FCPeT-3 as a robust platform for isolating tribo-contacts and enabling advanced test conditions. By enhancing accuracy and realism, FCPeT-3 deepens understanding of friction, wear, and lubrication while improving the reliability of laboratory simulations. Complemented by the LFIT, this work extends tribological investigation from steady to highly dynamic contact conditions.