The issues of leakage with respect to the clearance between the pump plunger outer diameter and the pump barrel inner diameter and other operation conditions have been revisited in this paper. Both Poiseuille flow rate due to the pressure difference and Couette flow rate due to the plunger motion have been considered. The purpose of this study is to better understand the nature of the leakage with respect to pressure difference, eccentricity, and motion related to the plunger of typical sucker rod pump systems. More specifically, based on the newly derived relaxation time scales for transient solutions of the governing Navier–Stokes equations, the quasi-static nature of relevant measurement techniques is confirmed for the current production systems. This key observation is also demonstrated with a computational model using the experimentally measured pressure difference and the plunger movement.

This work investigates the role of water and oxygen on the shear-induced structural modifications of molybdenum disulfide (MoS 2 ) coatings for space applications and the impact on friction due to oxidation from aging. We observed from transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) that sliding in both an inert environment (i.e., dry N 2 ) or humid lab air forms basally oriented (002) running films of varying thickness and structure. Tribological testing of the basally oriented surfaces created in dry N 2 and air showed lower initial friction than a coating with an amorphous or nanocrystalline microstructure. Aging of coatings with basally oriented surfaces was performed by heating samples at 250 °C for 24 h. Post aging tribological testing of the as-deposited coating showed increased initial friction and a longer transition from higher friction to lower friction (i.e., run-in) due to oxidation of the surface. Tribological testing of raster patches formed in dry N 2 and air both showed an improved resistance to oxidation and reduced initial friction after aging. The results from this study have implications for the use of MoS 2 -coated mechanisms in aerospace and space applications and highlight the importance of preflight testing. Preflight cycling of components in inert or air environments provides an oriented surface microstructure with fewer interaction sites for oxidation and a lower shear strength, reducing the initial friction coefficient and oxidation due to aging or exposure to reactive species (i.e., atomic oxygen).

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