Application Design Criteria And Friction Characteristics Of Friction Pair Texture Parameters

Frictional wear between the contact surfaces of frictional subsets of relatively moving mechanical parts is the main factor limiting the service performance and working life of mechanical equipment;finding ways to improve the frictional characteristics between the contact surfaces of frictional subsets has thus been the focus of much research.Hydraulic end-face friction pairs experience high pressure over long periods of time,wide operating-speed ranges,and large fluctuations in pressure caused by alternating loads and other complex operating conditions;this means that there are highly demanding requirements for the friction performance of these contact surfaces.In recent years,with the development of surface-texture research,texture treatments of friction subcontact surfaces has become a popular area of research for improving their tribological performance.Previous research has failed to produce a system for designing appropriate weaving parameters for complex hydraulic working conditions.In this work,the results of simulations of the dynamic-pressure lubrication effect were integrated with the results of tests examining the tribological characteristics of materials.The design of surface-texture parameters was investigated to establish the influence law for the friction characteristics of frictional subsets.This paper presents texture-design guidelines for practical working conditions,and it lays a theoretical foundation for the design and application of textured surfaces in hydraulic end-face friction pairs.The results will play a key role in the future improvement of the friction characteristics of these systems.The main contributions of this paper are as follows.(1)Insufficient lubrication caused by high pressure and low speed in the start–stop processes of hydraulic systems is an important cause of oil-film failure.In this work,a dynamic-pressure lubrication model for an idealized textured surface was established under these working conditions based on the Reynolds equation and the Jakobsson-Floberg-Olsson(JFO)cavitation model.The distribution of the dynamic-pressure lubrication effect,the strength of action,and the dynamic-pressure coupling effect of the textured surface were analyzed.Using the evaluation index of the total and load-bearing capacity of the idealized textured surface,the influence weights of the texturing parameters on the dynamic-pressure lubrication effect were obtained using single-factor analysis and simulation calculations with orthogonal test groups.This revealed the influence order diameter > density > depth > shape,providing the influence law for the effect of the texturing parameters on the dynamic-pressure lubrication effect.(2)A micro-under-lubrication automatic oil-replenishment device,regulated by a programmable logic controller(PLC),was designed and assembled for the tests.This simulates the insufficient lubrication that occurs in high-pressure and low-speed working conditions and can realize the state of the contact surfaces after long-term wear within a shorter time than would otherwise be required.Using an orthogonal test method,two typical motion conditions-rotation and reciprocation-were tested,and the friction characteristics of textured surfaces were analyzed using the amount of wear and the relative wear rate as evaluation criteria.The optimum values of texturing parameters were obtained under these working conditions by using the extreme-difference analysis method.These optimal parameters were 700 μm diameter outer circle,triangular shape,300 μm depth,and 8% surface density.The influence weights of each texturing parameter were found to be diameter > depth > surface density > shape.(3)The abrasive debris generated between the contact surfaces and carried by the oil will have an effect on the friction characteristics of the contact surfaces of the friction pair,thus causing changes in the wear profile between them.Observing the wear morphologies of the specimens in the orthogonalized test groups revealed that the ability of the textured surfaces to collect debris in the presence of oil contamination reduces the possibility of three-body wear and effectively improves the tribological performance.The analysis results show that the texturing not only improves the lubrication conditions between the contact surfaces through the dynamic-pressure lubrication effect,but it also improves the friction performance of the contact surfaces by collecting impurities from the oil.(4)Hydraulic systems operate over a wide range of alternating load conditions.In this work,we analyzed and studied the combined deformation of the textured surface under pressure-change conditions due to the squeezing effect and the change of the textured structure after collecting debris.During operation,the structure is subjected to extrusion effects,producing periodic deformations and resulting in changes in the volume of the textures and the flow characteristics of the oil film.This is the fundamental mechanism of the secondary lubrication effect provided by the textures,and it effectively improves the friction characteristics of the textured surface under alternating load conditions.Combining the coupled effects of the integrated deformation of the texture on its surrounding fluid-flow characteristics and the weakening effect of the dynamicpressure lubrication,application design guidelines for different working conditions and different friction substrate materials were obtained.Specifically,it was found that the maximum upper limit of the surface density of the texture should be used to make the texture spacing larger than the diameter of the texture after the integrated deformation.(5)There are two typical forms of motion in a hydraulic end-face friction pair-rotary and reciprocating-and an asymmetric texture will have obvious directionality in both forms of motion.In this work,the effect of the angle between the direction of the asymmetric texture arrangement and the oil-flow direction(i.e.,the direction of contact-surface motion)on the dynamic-pressure lubrication and friction characteristics of the fabricated surface was investigated.Comprehensive analysis of the results of the dynamic-pressure lubrication simulations and tribological tests showed that the arrangement direction of the weave has obvious directionality,and optimal selection of the arrangement direction effectively improves the dynamic-pressure lubrication and friction characteristics of the asymmetric texture when the angle between the edge structure of the weave and the oil-flow direction is close to vertical(80 °-90 °)with respect to the acuteangle direction.