Optimization of Pocket Geometry for Friction Reduction in Piston-Liner Contacts.

It has recently been shown that rectangular surface pockets are effective in reducing friction in a piston-liner type contact, providing that they are oriented with their long axis transverse to the sliding direction so that entrained features fit completely within the contact area (Vlădescu, et al., Tribology International, 82, 28-42, 2015; Vlădescu, et al., Tribology International, 115, 140-153, 2017). The aim of the current study was to identify the optimal geometric parameters of theses rectangular features. To do this, a friction rig that simulated a piston-liner contact under highly controlled conditions was used to test a series of textured specimens with pockets of different depth, breadth and density. Each of these geometric parameters was varied and tested independently, while keeping the other two constant. Experimental conditions were set in order to place the contact in different lubrication regimes. Results were analyzed to determine a set of criteria for the optimum pocket geometry; however, this was shown to change depending on the test conditions and should therefore be adjusted depending on the position along the stroke. Specifically, at low speed when the contact is operating under boundary lubrication, pockets should be deep, wide, and densely spaced. This confirms recent findings, which suggested that, in this regime, pocket volume is often a more critical parameter than depth, width, or spacing individually. Conversely, under mixed lubrication toward the transition to the full film regime, pockets should be narrow and sparsely spaced. These results also explain the difficulties encountered in several previous studies that attempted to define a single optimum pocket geometry. Finally, the impact of pocket position relative to reversal was assessed for various lubrication conditions. This revealed how pockets should be placed close to, but not directly at, top and bottom dead center to provide a beneficial squeeze film, which is present at reversal. [ABSTRACT FROM AUTHOR]