Transient elastohydrodynamic lubrication film thickness in sliding and rolling line contacts

The contact behavior between cam and follower is greatly influenced by the kinematics and dynamics of the whole valve train system. This is the reason that both shape and thickness of the fluid film in the contact gap are mainly determined by applied loads and relative contact speeds as well as the curvatures of contacting elements. Most of the studies about lubricant film behavior between cam and follower have been performed without a consideration of transient effects in the contact gap. For the computational difficulties of transient effects, most contact conditions such as relative contacting speeds have been regarded as quasi-steady state during the whole operating cycle. In this work, in order to obtain stable convergence, a multigrid multi-level method is used for the computation of load capacity in the lubricant film. Nonlinear valve spring dynamics are also considered in the same way as Hanachi’s. From the computational results, transient EHL film thicknesses under the conditions of different contact geometries are computed for a pushrod type valve train system during an engine cycle. Several results show the squeeze film effect, which is generally not found with conventional EHL computations of the cam and follower contact. The results are also compared with those by the Dowson-Hamrock (D-H) formula, which does not consider the dynamic film effect. Without the dynamic film effect as in D-H’s formula, the minimum film thickness is highly dependent on the entraining lubricant velocity, whereas the minimum film thickness including the squeeze film effect is dependent on the applied load.     

An elastohydrodynamic study on two-axial-groove journal bearings

An elastohydrodynamic study of two-axial-groove journal bearings operating in laminar and superlaminar flow regimes is presented. The effect of transition is included in turbulence analysis. Static and dynamic performance characteristics are presented for several values of deformation coefficient and Reynolds number. Illustrative examples are cited to demonstrate the significance of the present investigation.     

The oil film in elastohydrodynamic elliptical contacts

The effect of speed, load and geometry on the oil film thickness and shape, in a roller bearing with barrelled rollers, is demonstrated experimentally by means of optical interferometry. At fairly high dimensionless roller speeds, film thickness increase becomes inhibited. This observation is attributed to a truncated inlet oil meniscus, a similar condition having been observed elsewhere in a ball and plate machine.     

Investigation of the temperature distribution in the elastohydrodynamic oil film

Results of investigations of the temperature distribution in elastohydrodynamic (EHD) contacts carried out on a two-disc machine for various contact parameters (rolling and sliding speeds and maximum Hertz pressures) are reported in this paper.The experiments were carried out with the aid of thin layer temperature transducers made of titanium that were evaporated onto the disc's surface in vacuum.Analysis of the errors affecting the temperature measurements in EHD contact was also carried out. It was shown that contact pressure and transducer insulation had a substantial influence on the precision of the temperature measurements made by the thin layer transducer. Error analysis has permitted the determination of the real value of the temperature rise in the contact zone by the application of suitable corrections.It was found that the relative sliding velocity and the contact loading force have substantial and similar influences on the maximum temperature rise in the EHD contact; however, the influence of the rolling velocity is relatively small.A simple empirical formula which permits the calculation of the maximum temperature rise in the contact zone as a function of load and velocity for the investigated oil is presented in this paper. The general formula as a function of non-dimensional parameters is also given. It was found that theoretical formulae which include the exact dependences of the viscosity variation on pressure and temperature (e.g. the Kannel equation) provide the best fit to the experimental results.Good agreement of the results obtained with other researchers' experimental results was also found.     

Abrasive wear of rolling bearings

Various mechanisms of microslip are discussed; abrasive wear is considered the most important. An equation to predict the wear rate of rolling bearings is presented, based on previous work, and illustrated by an example     

Low friction rolling element bearings

In this paper some unusual designs of rolling element bearings are discussed, with a view to reducing the friction to a minimum by optimizing their shape. Such bearings may have use in certain applications where low friction is important, some applications envisaged being radar antennae and gyroscope gimbals.     

基于弹流知识的球轴承设计

基于点接触弹流知识与赤兹接触理论,对球轴承设计参数沟道半径系数和滚动体直径与节圆直径的比进行了分析,结果表明,沟道半径系数对微型轴承其取值范围较大;对小的沟道半径系数,滚动体直径与节圆直径的比的大小对形成油膜的能力影响较小,并且最大接触压力随沟道半径系数的变化大,而随滚动体直径与节圆直径的比的变化不大。     

Pure rolling elastohydrodynamic lubrication of short stroke reciprocating motion

The behavior of point contact elastohydrodynamic lubrication (EHL) films under pure rolling short stroke reciprocating motion is investigated using both optical interferometry technique and theoretical analysis. The EHL films are recorded with a high-speed color camera and simulated with multigrid techniques. General variation of oil film under point contact reciprocating motion is explained by a comparison between a fully flooded simulation and an experiment with a frequency of 7.78 Hz. Influence of starvation is considered by another simulation under a higher working frequency of 14.4 Hz and the simulated results are compared with experimental results. Both simulations show good agreements quantitatively with experiments. In addition, general tribo-characteristics of EHL under fully flooded and starved conditions are discussed.     

Squeeze film characteristics of conical bearings with combined effects of piezo-viscous dependency and non-Newtonian couple stresses

In this paper, the analysis of squeeze film characteristics of conical bearings with combined effects of piezo-viscous dependency and couple stress fluid is presented. On the basis of the Stokes microcontinuum theory of couple stress fluid model and Barus experimental research, a modified Reynolds equation is derived, the standard perturbation technique is used to solve the highly non-linear Reynolds equation and approximate analytical solution is obtained for the squeeze film pressure, load carrying capacity and squeeze film time. According to the results obtained, the effect of viscosity pressure dependency on the squeeze film lubrication of conical bearings with couple stress fluids is to improve the load carrying capacity significantly and lengthen the squeeze film time as compared to iso-viscous Newtonian case.     

Newtonian quantitative elastohydrodynamic film thickness with linear piezoviscosity

Examples are provided for linear pressure-viscosity response at low pressures, which are said to exist in the concentrated contact inlet, including lubricants at high temperature, low viscosity liquids and water/glycol solutions. Full isothermal, Newtonian simulations of the EHL problem reveal film-forming, which is quite similar to that of exponential liquids with the exception of the reduced sensitivity to the Dowson material parameter, G, or the Moes parameter, L. Film thickness formulas are offered, which have been experimentally validated. In a significant departure from most investigations of this kind, the viscosity of the experimental liquids was not adjusted to provide successful comparisons with measurement.     

Simple equation for elastohydrodynamic film thickness under acceleration

A simple approximation of EHD film thickness under varying speed conditions is proposed. The equation is based on continuity of flow, by which the film formed at the contact inlet moves downstream within the contact with little subsequent change in its thickness even though the boundary velocities are changing. The approximation is supported by experimental results of non-steady state film thickness measurement using ultra-thin film interferometry. It is also shown by numerical simulation that the approximation holds for film thickness in the rigid piezoviscous regime under line contact so long as the squeeze film effect is insignificant.     

Molecular dynamics simulations of elastohydrodynamic lubrication oil film

This paper reviews recent research in molecular dynamics studies of the traction properties of hydrocarbon fluids under elastohydrodynamic lubrication, focusing on the technical problems that arise on making predictions of the traction properties of an oil film with a submicron thickness at the actual sliding contacts of the machine elements by at a nanoscale molecular simulation. The effect of the oil film thickness and shear rate are examined including the result of a submicron thickness simulation of the oil film using a tera‐flops computer. The mechanism of the phase transition of the fluids under high pressure, the boundary slip, and the momentum transfer related to the molecular structure of the fluids are also presented. Copyright © 2010 John Wiley & Sons, Ltd.     

Critical stresses in rolling contact fatigue

An analysis of the main hypotheses concerning the critical stresses in rolling contact fatigue is presented. It is considered that none of these hypotheses correlates adequately with all the experimental aspects of the phenomenon of rolling contact fatigue. A new hypothesis is proposed; this hypothesis states that the equivalent stress is the critical stress in rolling contact fatigue. By considering the influence of the residual stresses it is shown how the optimum fatigue life in rolling contact can be achieved.     

Quantitative elastohydrodynamic film thickness of mechanically degraded oil

The low-shear viscosities of new oil and used (degraded) oil were measured to moderately high pressure. Shear-thinning flow curves were generated over a range of shear stress important to EHL film forming. Mechanical shear degradation only affected the viscosity at low shear stress while the viscosity at high stress remained unchanged. A recently published film thickness equation for double-Newtonian shear-thinning, employing the measured rheology, predicts the surprising result that thickness of the EHL film was not changed by the degradation although the low-shear viscosities were reduced substantially. Colorimetric measurements of the film thicknesses validate the predictions. Concern over the permanent loss in viscosity due to degradation may be misplaced when addressing EHL films.     

Experimental observation of a dimple-wedge elastohydrodynamic lubricating film

One of the main features of typical elastohydrodynamic lubricating (EHL) contacts is the unique horseshoe film shape, which can be readily observed by using interferometry and quite accurately modelled by the well-established EHL theory. However, an anomalous EHL film, characterized by a wedge shape together with a tiny dimple at the inlet region, is observed under pure sliding conditions with ultra slow speeds of 3–800 μm/s in an optical EHL test rig. The variations of the wedge and the inlet dimple with different sliding speeds and loads are investigated using a series of polybutene oils of high viscosities. It is found that the inclination of the wedge is dependent on sliding speeds, loads and oil viscosities. The dimple always occurs at the inlet. The appearance of an inlet dimple together with a wedge film shape is reported for the first time. The phenomenon can be attributed to a non-Newtonian characteristic of the lubricant: the limiting shear strength. Additionally, the influence of starvation on the film shape is also examined.