Central film thickness prediction for line contacts under pure impact

International audienceThe lubricant film thickness in lubricated rolling contacts can be predicted quantitatively thanks to the work of Dowson, Higginson, Hamrock and others. However, most industrial applications are governed by time dependent operating conditions. For the most extreme time dependent case (impact), no film thickness predictions exist. The current work derives a dimensionless Reynolds line contact equation based on the Johnson dry contact impact analysis. It is shown that similar to the film thickness under rolling conditions, the Piezoviscous Elastic impact problem is governed by only 2 parameters. The numerical results are curve fitted to obtain a central film thickness predictive equation

Influence of surface topography on the tribo-characteristics of an eccentric-tappet pair

The cam-tappet pair is one of the most important components in an engine and determines the overall performance of the motor. Compared with modern car engines using rolling-sliding tappets for the transmission of the rapid up-down lifting motion of the cam to open and close the engine's inlet and exhaust valves, the flat-tappets are often subject to fatigue failure. Upon stripping down a damaged motor, the tappets are invariably found to be heavily pitted. This study performs a theoretical simulation of the surface topography influence on the tribo-characteristics of an eccentric-tappet pair. The asperities on the flat-tappet surface generate fluctuations of the pressure, film thickness and temperature distributions. The pressure fluctuations result in alternative stresses in the metal. The periodical variations of pressure, minimum film thickness and thermal rise are investigated by changing the eccentricity and roughness wavelength. The results suggest an explanation for the rapid occurrence of cracks or even complete fracture in the cam and flat-tappet pair

Experimental and numerical study of the effect of raceway waviness on the oil film in thrust ball bearings

International audienceUsing a custom-made optical interferometer, the waviness values on the raceway of the washer of two thrust ball bearings are obtained by applying the Hertzian contact theory. The experimental system can also simulate elastohydrodynamic lubrication (EHL) in the thrust ball bearings, so the variations of oil film over one revolution of the bearing are measured. Based on the experimental parameters, EHL calculations are carried out to reproduce the contacts between the balls and the glass, especially those between the balls and the raceway. The experimental and theoretical results show that the variation of the oil film in the thrust ball bearing is closely related to the raceway waviness

Central film thickness in EHL point contacts under pure impact revisited

Squeeze effects in elastohydrodynamic lubrication are extremely general, however, their contribution to film formation is difficult to quantify. Full numerical solutions allow one to assess the importance of the transient terms, but simple equations to predict the transient film thickness do not exist. The current paper analyses the film thickness generated by an impacting ball. The problem is first studied assuming isoviscous elastic lubrication. It is shown that the film thickness evolution depends on a single parameter. Similarly, the central film thickness in a piezoviscous elastic impact problem is shown to depend on two parameters. These results are presented in the form of a single graph and two curve fitted solutions are supplied

Amplitude reduction in EHL line contacts under rolling sliding conditions

Surface roughness plays an important role in the performance of highly loaded elastohydrodynamically lubricated contacts. As the pressures are very high, each of the surface roughness components deforms differently, and as a result the roughness inside the highly loaded contact is different from the measured roughness. Under pure rolling conditions the amplitude reduction theory describes the waviness deformation as a function of wavelength and operating conditions. The current work suggests that similar predictions are possible under rolling sliding conditions, provided that the wavy surface velocity u2 exceeds the smooth surface velocity u1. For u2<u1 the maximum value of Ad/Ai depends on the slide to roll ratio and may be significantly less than 1.0

Central film thickness prediction for line contacts under pure impact

The lubricant film thickness in lubricated rolling contacts can be predicted quantitatively, thanks to the work of Dowson, Higginson, Hamrock and others. However, most industrial applications are governed by time dependent operating conditions. For the most extreme time dependent case (impact), no film thickness predictions exist. The current work derives a dimensionless Reynolds line contact equation based on the Johnson dry contact impact analysis. It is shown that similar to the film thickness under rolling conditions, the Piezoviscous Elastic impact problem is governed by only 2 parameters. The numerical results are curve fitted to obtain a central film thickness predictive equation

Influence of Surface Waviness on the Thermal Elastohydrodynamic Lubrication of an Eccentric-Tappet Pair

The effect of single-sided and double-sided harmonic surface waviness on the film thickness, pressure, and temperature oscillations in an elastohydrodynamically lubricated eccentric-tappet pair has been investigated in relation to the eccentricity and the waviness wavelength. The results show that, during one working cycle, the waviness causes significant fluctuations of the oil film, pressure, and temperature, as well as a reduction in minimum film thickness. Smaller wavelength causes more dramatic variations in oil film. The fluctuations of the pressure, film thickness, temperature, and traction coefficient caused by double-sided waviness are nearly the same compared with the single-sided waviness, but the variations are less intense. </p