Ken Mao

Measurement and Prediction of the Surface Temperature in Polymer Gears and Its Relationship to Gear Wear

A design of a four square gear test rig that allows the wear of polymer and composite gears to be monitored continuously during operation is described. The wear behavior of three typical gear materials is examined and it is shown that the wear characteristics differ greatly. For Acetal there is a sharp rise in wear as the transmitted torque is increased, effectively limiting the torque that can be transmitted by an Acetal gear pair. This wear transition is shown to be associated with the maximum surface temperature of the gear reaching the melting point of Acetal

Effect of Sliding Friction on Contact Stresses for Multi-Layered Elastic Bodies With Rough Surfaces

The stress distributions associated with frictionless and smooth surfaces in contact are rarely experienced in practice. Factors such as layers, friction, surface roughness, lubricant films, and third body particulate are known to influence the state of stress and the resulting rolling contact fatigue life. A numerical technique for evaluating the subsurface stresses arising from the two-dimensional sliding contact of two elastic bodies with real rough surfaces has been developed, where an elastic body contacts with a multi-layer surface under both normal and tangential forces. The presence of friction and asperities within the contact region causes a large, highly stress region exposed to the surface. The significance of these near-surface stresses is related to modes of surface distress leading to surface eventual failure (Mao et al., 1997).

Mixed Lubricated Line Contact Analysis for Spur Gears using a Deterministic Model

The unified approach based upon the Reduced Reynolds technique is applied to develop a deterministic transient mixed lubrication line contact model. This model is used in spur gear applications to comprehensively show effects of roughness, working conditions, i.e., rotational speeds and loads on pressure ripples and severity of asperity contacts. Results show effects of the speed, the load, as well as the RMS value are coupled which makes it difficult to evaluate lubrication states by only considering one variable. Considering the Ree-Eyring non-Newtonian behavior could alleviate pressure ripples significantly, compared with the Newtonian fluid assumption. Small RMS values of surfaces, which could be achieved by superfinish techniques, would be desirable when evaluating gear tooth surface contact performances.

Spur Gear Lubrication Analysis with Dynamic Loads

A thermal elastohydrodynamic lubrication line contact model, which could handle ultra-thin-film conditions, was developed to study effects of speed on the lubrication performance of a spur gear pair. Dynamic loads were calculated using a classic mass-spring model. They effect of speed on lubrication performance was studied comprehensively through its direct influence on lubrication and indirect influence by affecting dynamic loads of the gear pair. The effect of dynamic loads on film thickness, pressure distribution, and temperature field were studied. It was concluded that a comprehensive model combining a lubricated contact analysis and a dynamic analysis for a gear system is required for a reasonable performance evaluation.

Parametric studies of spur gear lubrication performance considering dynamic loads

Effects of friction excitation and gear backlash on spur gear lubrication performance, i.e. minimum film thickness, maximum pressure, etc. are investigated by considering the dynamic loads due to excitations of mesh stiffness, friction between gear tooth, and backlash. Dynamic loads are obtained using a semi-definite two-degree-of-freedom lumped-parameter model. Reduced Reynolds technique is applied to develop a lubrication model capable of dealing with any potential ‘asperity contacts.’ Varying spur gear geometry parameters and the Ree–Eyring fluid behavior are taken into account. Results show that friction excitation has a very limited influence on the dynamic load and hence on the lubrication performance. Backlash affects the dynamic load significantly but has a limited influence on the minimum film thickness. Maximum pressure increases with the value of backlash due to the direct relationship of pressure and load.

Surface coating effects on contact stress and wear: an approach of surface engineering design and modelling.

Abstract The present paper is a dedication to Professor Tom Bell, a distinguished scientist and engineer, for his great contributions and achievements in surface engineering design and modelling. An initial attempt has been made in surface system design through combining the authors’ previous achievements in understanding contact mechanics of surface engineering system. The approach is taken mainly from modelling the contact behaviour of multilayer surface system with real rough surface profiles in terms of contact stress and deformation, and both boundary and finite element methods have been employed to understand the system contact behaviour. Experimental investigations have also been carried out on the performance of surface engineered gears (steel and titanium). Tests were carried out on untreated, TiN coated, plasma nitrided and duplex treated steel gears under dry running conditions, and it has been shown that the duplex system gives the best wear resistance. For titanium gears, experimental tests were carried out on untreated and thermal oxidised titanium against steel gears and the thermal oxidised treated has showed a significant wear improvement compared to that for the untreated specimens. Finally, possible further investigations have been proposed.

Wear behavior of cold pressed and sintered Al2O3/TiC/CaF2-Al2O3/TiC laminated ceramic composite

A novel laminated Al2O3/TiC/CaF2-Al2O3/TiC sandwich ceramic composite was fabricated through cold pressing and sintering to achieve better anti-wear performance, such as low friction coefficient and low wear rate. Al2O3/TiC/CaF2 and Al2O3/TiC composites were alternatively built layer-by-layer to obtain a sandwich structure. Solid lubricant CaF2 was added evenly into the Al2O3/TiC/CaF2 layer to reduce the friction and wear. Al2O3/TiC ceramic was also cold pressed and sintered for comparison. Friction analysis of the two ceramics was then conducted via a wear-and-tear machine. Worn surface and surface compositions were examined by scanning electron microscopy and energy dispersion spectrum, respectively. Results showed that the laminated Al2O3/TiC/CaF2-Al2O3/TiC sandwich ceramic composite has lower friction coefficient and lower wear rate than those of Al2O3/TiC ceramic alone because of the addition of CaF2 into the laminated Al2O3/TiC/CaF2-Al2O3/TiC sandwich ceramic composite. Under the friction load, the tiny CaF2 particles were scraped from the Al2O3/TiC/CaF2 layer and spread on friction pairs before falling off into micropits. This process formed a smooth, self-lubricating film, which led to better anti-wear properties. Adhesive wear is the main wear mechanism of Al2O3/TiC/CaF2 layer and abrasive wear is the main wear mechanism of Al2O3/TiC layer.

Computational simulation analysis for torus radius of edge contact in hip prostheses.

Stripe wear occurs when the components of hip prostheses move a sufficient distance laterally to contact the edge of the acetabular cup, causing abnormally high contact stresses. In this research, edge loading contact of prosthetic hip is analyzed in the most commonly used material pairs. The contact dimensions and maximal contact pressure are investigated in mutative normal edge loading with 3 different inclination angles (15°, 20°, 25°) and in alterable edge torus radius with edge loading of 2500 N and inclination of 20°. A computational case was conducted for a 14 mm radius alumina ceramic bearing with a radial clearance of 0.1 mm using a normal edge loading ranged from 0 N to 3000 N. Additionally, the Hertzian theory successfully captures principal curvature trends of the edge torus on the influence of maximal contact pressure and obtains the appropriate edge radius range for lower maximal contact pressure. This work elucidates the methods of applying classical contact theory to design the edge radius of hip bearings to better resist severe edge loading contact stresses and reduce the stripe wear.

An Initial Investigation of Hip Joint Contact Behaviour Using Advanced Non-Linear Finite Element Methods

The hip implant is a very successful treatment for serious osteoarthritis, especially in older patients, but less desirbale for earlier interventions. There is a growing consensus that most hip arthritis is due to shape abnormalities that cause impingement at the ball and socket, collectively called femoroacetabular impingement (FAI). The ball does not fit accurately into the socket, leading to premature wear, and then destructive arthritis. It is not then necessary to replace the whole hip joint; newly developed surgical techniques that accurately reshape the bones to relieve impingement and reduce wear have been shown to be effective. This surgery can be performed with a conventional open approach, or be arthroscopic (keyhole) surgery. It would be better to reshape bones to suit each individual patient. Finite element methods (FEM) have been widely used for biomechanical studies of hip implants and periacetabular osteotomy, but hardly at all in hip reshaping. Non-linear FEM is employed in the current study to perform biomechanical evaluations of differences in contact pressure between normal and arthritic hip joints to help basic understanding and lead to more accurate surgery. The hip joint bone structure is obtained through a medical CT scan and then the CT images have been converted into a format readable by FEM solvers. A sophisticated non-linear contact model of the hip joint bringing together the interactions of true geometry, natural movement and contact forces has been established using this advanced FEM.

Computational contact modelling of hip resurfacing devices

A combination of computational models and theoretical methods have been used and developed to study the contact of hip resurfacing devices under normal and edge loading conditions. Techniques were developed and the solutions based on using the finite element method. It was found that the study of hip joint modelling, numerical methodologies of mechanical wear simulations and shakedown analysis can be developed to study the contact mechanics and biotribology of hip resurfacing devices under central and edge loading conditions. Each method developed in this study provides a unique platform to study these problems.