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Conformability of a piston ring is a measure of the ring ability to conform to the ever-present cylinder bore deviations from a perfect circle by elastic deformation of the ring. Conformability is usually defined as the limit of bore distortion at which the clearance between the piston ring and bore can be kept at zero by elastic deformation of the piston ring. Retention of good conformability is important for ring/bore sealing and reduction of oil passing and blow-by. The most known technique for conformability analysis was developed almost sixty years ago and considers only two-dimensional deformation of the ring in its plane by the force of piston ring elasticity. It is known, however, that the piston rings with asymmetrical cross-section, a prevalent type of the modern compression piston rings, undergo a complicated three-dimensional installation displacements (twist) which affect many functions of the rings.

The film thickness-roughness ratio Λ has been used since the mid-1960s as a simplified criterion for the lubrication conditions in rolling bearings. However, due to an assumed Gaussian distribution of the roughness height amplitudes of the functional surfaces of rolling bearings and other unsubstantiated assumptions regarding the mechanisms of contact interaction, concerns have arisen about the representability of Λ in terms of lubrication. In this study, a more objective Λ-type ratio that does not depend on the law of roughness height distribution and the peculiarities of contact mechanics is introduced and defined as Λz. The relevant range of the new Λ ratio is lower than that of its conventional counterpart, and it overlaps with contemporary theoretical and experimental results. The study exhibits unique profilometric data of the functional surfaces of the roller bearings, produced by the major bearing manufacturers.

The paper is a step in an ongoing work targeted at the development of an engineering methodology to estimate sealing properties of a piston ring, and for the development of ring design guidelines. This work is necessary because the sealing properties of a ring significantly affect oil passing and blow by in the reciprocating air compressors and engines. It is known that the sealing function of a piston ring depends on the ring's ability to elastically deform in its plane under its own tension by conforming to the deviations of the cylinder bore. At the same time, if it has an asymmetrical cross section, the ring fitted in the cylinder bore is subjected to the additional, three-dimensional, distortions called twist. This type of cross section is usually imparted to the ring to further improve its sealing functions by interlocking (because of twist) between the piston groove and the cylinder wall.