We discuss (i) the measurement, (ii) interpretation and (iii) technical impact of temperature-dependent wall slip and its activation energy for polar and non-polar hydrocarbon Newtonian fluids moving relative to machined metal surfaces. A newly developed apparatus, the slip length tribometer (SLT), overcomes the drawbacks of existing measurement devices in terms of characterising relevant rough surfaces by measuring the slip length at different temperatures over a sufficiently large wetted area. The experimental data show that the bulk viscosity and slip length at the fluid–metal interface is independent of the shear rate up to
$10^5\,\mathrm {s}^{-1}$
, being consistent with recent results from molecular dynamics simulations by Mehrnia & Pelz (J. Mol. Liq., vol. 336, 2021, 116589). Furthermore, the activation energies for wall slip and bulk shear determined by means of the SLT differ by a factor of two, i.e.
$E_{\lambda }\approx 0.5 E_\mu$
for non-polar hydrocarbon molecules sliding relative to metal walls. This difference is explained by a generalised Eyring model applied to wall slip. The paper closes with the impact of wall slip on Sommerfeld's similarity theory of tribology and the resulting Stribeck curve (Sommerfeld, Z. Math. Phys., vol. 50, 1904, pp. 97–155; Z. Tech. Phys., vol. 2. Jahrg., 1921; Mechanik der deformierbaren Medien, 1944, Akademische Verlagsgesellschaft Becker & Erler). For this purpose, Reynolds’ equation generalised for wall slip is solved in combination with the experimentally determined constitutive relations for bulk shear and wall slip to predict typical characteristics of journal bearings. The results show that, for a typical journal bearing, where the ratio of slip length to average bearing clearance is of the order of
$10^{-2}$
, the influence of wall slip on both load-carrying capacity and dissipation is not negligible. This work combines nanofluidics and tribology in order to provide methods and knowledge for e.g. tailor-made fluids and interfaces.
High power density, high reliability and good controllability for varying load requirements are typical characteristics of hydraulic drive systems used for power transmission in stationary and mobile applications. Furthermore, hydraulic systems are usually safety related systems. Consequently, the handling of uncertainties in hydraulic systems is essential. A major source of uncertainty is, in particular, the wear induced change of the system behavior. Nowadays, the most common way to face uncertainty is the oversized system design to ensure reliable system operation. However, the uncertainty remains. In the first part of the paper we present a general approach to face uncertainty by means of a soft sensor network. Soft sensor networks make it possible to gather system information redundantly. In this way the occurrence of data conflicts are allowed which serve as an indicator of uncertainty. The resolution of these conflicts either lead to increased confidence for the model-based system information or allows the detection of changing component characteristics. The application of a soft sensor network to a hydraulic drive system is illustrated and discussed in the second part of this paper.
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