Friction force models play a fundamental\ud role for simulation ofmechanical systems. Their choice\ud affects the matching of numerical results with physically\ud observed behavior. Friction is a complex phenomenon\ud depending on many physical parameters and\ud working conditions, and none of the available models\ud can claim general validity. This paper focuses the\ud attention on well-known friction models and offers a\ud review and comparison based on numerical efficiency.\ud However, it should be acknowledged that each model\ud has its own distinctive pros and cons. Suitability of\ud the model depends on physical and operating conditions.\ud Features such as the capability to replicate stiction,\ud Stribeck effect, and pre-sliding displacement are\ud taken into account when selecting a friction formulation.\ud For mechanical systems, the computational efficiency\ud of the algorithm is a critical issue when a fast\ud and responsive dynamic computation is required. This\ud paper reports and compares eight widespread engineering\ud friction force models. These are divided into two\ud main categories: those based on the Coulomb approach and those established on the bristle analogy.The numerical\ud performances and differences of each model have\ud been monitored and compared. Three test cases are discussed:\ud theRabinowicz test and other two test problems\ud casted for this occurrence

The analysis of mechanical efficiency constitutes an important phase in the design analysis of gear drives. The objective of this investigation has been the development of a general algorithm for the determination of efficiency in split-power spur-gear trains. The model includes meshing losses only; for a more realistic estimation other sources can be considered separately. The systematic nature of the formulation, based on the dual correspondence between the kinematic structure of the gear drive and a labelled graph, allows a ready coding of the efficiency analysis in a general computer program. The numerical results are in line with those given by other authors using different methodologies.

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