A Two-Stage Self-Aligning, Two-Sided Flat-on-Flat Friction-Measurement Device for Universal Testing Machines Under Elevated Contact Pressures

The precise determination of the coefficient of friction is of fundamental importance in many engineering applications, especially in cases where different material pairs, surface-treated systems, polymers or coated surfaces come into contact with each other. Classical tables of friction data can often only be used as approximate values, since friction behavior is considerably influenced by surface pressure, sliding velocity, contact geometry and the condition of the surfaces. In this article, we present the principle, structural design and application possibilities of a new friction-measuring device that can be adapted to a universal testing machine (UTM). Our device provides a symmetric, two-sided flat-on-flat contact configuration, a closed loading system and a two-stage self-aligning mechanism, with adjustable normal load. The relevant rotational axes are positioned in the contact planes to reduce parasitic moments and the coupling between the tensile measuring force and the applied normal load, thereby reducing a potential source of measurement variability. With the device, both static and dynamic friction coefficients can be determined (and the transition ranges can be explored in detail), at different nominal contact pressures (p, up to 50 MPa) and sliding velocities (v, mm/min). The demonstrative measurement results show that the system is suitable for the separate examination of the characteristic stages of the friction process, such as the initial force build-up, the static friction maximum and the steady-state dynamic range.