Cornelis H. Venner

Numerical Simulation of a Transverse Ridge in a Circular EHL Contact Under Rolling/Sliding

This paper investigates the influence of a transverse ridge on the film thickness in a circular EHL contact under rolling/sliding conditions. It is a numerical simulation of the optical EHL work of Kaneta et al. (1992). One of the purposes of this investigation is to check the validity of the algorithm and the Newtonian, isothermal lubricant assumption for film thickness predictions under these conditions (ph = 0.54 GPa). It will be shown that, both quantitatively, the film thickness on the central axis Y = 0, and qualitatively, the film thickness profile through “pseudo interference graphs”, the agreement between experiment and Newtonian isothermal theory is good. This supports the argument that the rheological and the thermal behavior of the fluid only slightly influence the film thickness and pressure distribution of the lightly loaded non-smooth contact case.

Film thickness in elastohydrodynamically lubricated elliptic contacts

A multilevel solver for the circular contact was extended to elliptical contact problems. After verification of its predictions by comparison with results presented in literature, it was used to study the variations of film thickness with varying operating conditions and aspect ratio of the contact ellipse. Detailed computational results are presented and observed tendencies are traced back to the modelling equations. Subsequently it is demonstrated how and when, for contacts with the entrainment directed perpendicular to the major principal axis of the contact ellipse, the pressure and film thickness on the centre-line of the contact can be predicted accurately from an equivalent line contact analysis. Finally, survey graphs of the minimum and the central film thickness are presented and a formula is given that predicts the central film thickness as a function of load and lubricant parameters, and the ratio of reduced radii of curvature of the surfaces. This formula incorporates asymptotic behaviour and as a result it can be applied for all conditions. In particular, its accuracy for contacts with the major principal axis of the contact ellipse perpendicular to the entraining direction is demonstrated in this paper.

Transient Analysis of Surface Features in an EHL Line Contact in the Case of Sliding

This paper investigates in detail the influence of two different surface topographies on the pressure distribution and film thickness profile of a highly loaded (maximum Hertzian pressure 2 GPa) line contact as a function of the slide to roll ratio. To accomplish this the transient Reynolds equation is solved both in space and time. The first feature under investigation is localized, a so-called indentation, the second one is global: waviness. The observed lack of synchronism in the extremes of pressure and film thickness is explained theoretically by analyzing the Reynolds equation

Surface roughness attenuation in line and point contacts

Abstract Surface roughness effects in line and point elastohydrodynamically lubricated contacts are compared and it is shown that the underlying behaviours in both types of contact are identical. Long wavelength components of roughness are attenuated; short wavelength components pass through the conjunction unaltered. It is also shown that the roughness attenuation, plotted against a non-dimensional wavelength, follows almost identical curves in both cases.

Numerical analysis of the influence of waviness on the film thickness of a circular EHL contact

Surface roughness and/or surface imperfections are well known to significantly affect the performance of concentrated contacts. Any deviation from the smooth surface will act as a stress raiser for itself (bump) or of its neighborhood (dent), and will therefore reduce the fatigue life of the component it is part of. These imperfections can also act as initiation sites of other types of contact failure such as scuffing, when contact conditions such as load, speed and film thickness become more and more severe. With the help of increasing computer speeds and more efficient numerical techniques, a theoretical analysis of the failure of concentrated contacts becomes possible. The full answer will involve many aspects of the contact, including the generation of heat, thermal response of the lubricant and solids, non-Newtonian as well as surface chemistry effects. This paper concentrates on the way the lubricant film thickness is affected by waviness and tries to identify the locations and the conditions where the film thickness is minimal. The lubrication ofnonsmooth surfaces is a transient two-dimensional problem, which will be treated without any geometrical simplification. More precisely, this paper focuses on the influence of rolling speed and the slide-to-roll ratio on the film thickness separating a smooth surface and one with transverse waviness.

Starved Lubrication of Elliptical EHD Contacts

This paper focuses on the lubrication behavior of starved elliptical Elasto-HydroDynamic (EHD) contacts. Starvation is governed by the amount of lubricant available in the inlet region and can result in much thinner films than occurring under fully flooded conditions. Therefore, it would be desirable to be able to predict the onset and severity of starvation and to be able to relate film reduction directly to the operating conditions and lubricant properties. The aim of this work is to explore the influence of these parameters on starvation. A combined modeling and experimental approach has been employed. The numerical model has been developed from an earlier circular contact study [1]. In this model, the amount and distribution of the lubricant in the inlet region determines the onset of starvation and predicts the film decay in the contact. Numerical simulations for a uniform layer on the surface show that a single parameter, characteristic of the inlet length of the contact in the fully flooded regime, determines the starved behavior. Film thickness measurements under starved conditions were performed to validate this theory. For a circular contact excellent agreement was found. In theory the same mechanism applies to elliptic contacts, however, the behavior is more complicated.

Elastohydrodynamic lubrication of rough surfaces

Abstract The detailed understanding of elastohydrodynamic lubrication (EHL) with rough surfaces has become an important problem as the ratio of film thickness to surface roughness is decreasing. Recently it has been recognized that the roughness inside an EHL contact might be different from the roughness outside the contact owing to elastic deformation. As will be explained in this paper, a first step in understanding the deformation of a real rough surface is the detailed knowledge of the amplitude reduction of harmonic features, which forms the topic of this paper. For the line contact problem it is shown that the deformed amplitude of a harmonic feature (waviness) depends on the original amplitude of the feature, its wavelength and the contact operating conditions, including the slide-roll ratio. The amplitude reduction can be completely described by a single dimensionless parameter. For the point contact problem it is shown that the deformed amplitude of isotropic waviness depends on the original amplitude of the feature, its wavelength and the contact operating conditions. Once again, the amplitude reduction can be completely described by a single dimensionless parameter. The presented relations for line and circular contact can be used as a basis for a modified Λ parameter predicting the relation between the real ratio of film thickness to amplitude inside the EHL contact. A numerical example of the calculation of the deformation is given.

Surface Roughness Effects in an EHL Line Contact

In this paper the influence of surface roughness on the pressure profile and film thickness in a steady state EHL line contact is investigated using input from an actually measured roughness profile in the calculations. Pressure profiles and film shapes for different load conditions are shown. The presented results strongly indicate that in the steady state situation considered here a significant deformation of the roughness profile occurs. As a result the often used λ parameter being the ratio of film thickness and standard deviation of the roughness (h/σ) with σ based on the undeformed roughness profile may give misleading information as far as the effect of the roughness on pressure and film shape is concerned.

Numerical Simulation of the Overrolling of a Surface Feature in an EHL Line Contact

In this paper a Multigrid extension of a stationary solver is outlined for the EHL solution of a line contact under transient conditions. The solver is applied to calculate pressure and film thickness profiles at each time step when an indentation is moving through the contact, which results in an asymmetric pressure profile. The time-dependent results are compared with the stationary solutions. The pressure as a function of time is presented as well as the integrated pressure (over time) as a function of the spatial coordinate. These time-dependent pressures are used to compute the sub-surface stress field, which shows higher stresses below the trailing edge of the indentation. Therefore the risk of fatigue is higher below the trailing edge of the indentation, as is experimentally observed. The transient pressures can be used for a fundamental study of the emitted frequency spectrum of rolling bearings, as used in condition monitoring.

Amplitude Reduction of Waviness in Transient EHL Line Contacts

The problem of surface waviness in ElastoHydrodynamic Lubrication is commonly considered as a first step towards understanding the lubrication of rough surfaces. Both problems are generally transient. The current paper investigates the deformation of sinusoidal waviness in the contact, more precisely it studies the amplitude reduction of one-sided waviness in a transient EHL line contact. The film thickness amplitude at X = 0 is called A d (deformed amplitude). It is shown that for waviness amplitudes smaller than the film thickness A i H c , the ratio A d / A i is independent of the undeformed amplitude A i . In other words: for small amplitudes the deformed amplitude depends linearly on the initial amplitude A i . Rather surprisingly, the same behaviour is found for A i > H c . The results show that qualitatively waviness with short wavelengths λ/ b A d ≃ A i . Long wavelengths on the contrary, disappear nearly completely: A d ≪ A i thus A d / A i ≃ 0. However, quantitatively the amplitude reduction was found to depend also on the contact operating conditions. With a generalized coordinate ∇, instead of λ/ b , the relative amplitude A d / A i can be described by a single function of ∇ for all operating conditions: A d / A i = F (∇),∀( M, L ). A possible way of expressing ∇ is:∇ = λ/ bM 3/4 / L 1/2 .