Friction is an important feature for engineering systems. The friction coefficient, a fundamental parameter for machine design, is unfortunately not easy to determine because of the large number of variables influencing its value. Different lubrication regimes, ranging from fluid-film to boundary and corresponding to different ways of evolution of the friction coefficient, are usually considered. This work reviews studies on friction, mainly experimental, carried out with several kinds of lubricated contacts working under various conditions. The influence of several variables on the evolution of the friction coefficient is shown by using typical Stribeck-like and lambda curves. Some comparisons between experimental and theoretical results are also reported. Relatively simple formulas for the evaluation of the friction coefficient, based on appropriate models of the lubricant behaviour, have been developed that provide results in quite good agreement with the experimental ones under fluid-film conditions when thermal effects can be neglected.
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