Friction in starved hydrodynamically lubricated line contacts
Résumé
The hydrodynamic friction coefficient, which determines the friction coefficient at high Sommerfeld numbers (viscositynvelocity/load) has a unique solution for the fully flooded case. As such the friction coefficient can be predicted as a function of the above number. As shown by Cameron (1976) [1] the friction coefficient in the fully flooded regime increases as the square root of the Sommerfeld number. For very low Sommerfeld numbers, the asperity interaction causes the friction coefficient to increase when entering the mixed lubrication regime. Even though a unique (and low) friction coefficient exists in the fully flooded regime, the situation is more complex in the starved regime. First of all the friction coefficient is higher in the starved regime, and secondly the coefficient depends on the degree of starvation. This paper analyses the load carrying capacity, the Poiseuille flow based friction and the Couette flow based friction, as a function of the degree of starvation. It is shown that the Poiseuille friction force diminishes fastest with starvation, followed by the load carrying capacity, and finally the Couette term diminishes slowest of all three terms. As a consequence the friction coefficient for sliding starved conditions is dominated by the Couette term. The current paper analyses the friction coefficient evolution as a function of starvation for line contact conditions, using analytical and numerical tools. Finally, curve fitted equations are given for the friction as a function of the starvation level.