3rd International Symposium on Instrumentation Science and Technology Aug. 18~22, 2004, Xi’an. China
Optical Interferometric EHL Oil Film Measurement
in Finite Line Contacts
SHEN Xue-jin, CHEN Xiao- yang
Department of Mechanical Automation, Shanghai University, Shanghai, 200072, China
ABSTRACT
A test apparatus was developed to measure the elastohydrodynamic lubrication (EHL) oil film thickness and
film shape for finite line contacts based on the principle of optical interferometry, which could be used to study
the oil film forming mechanism of roller bearings on pure rolling working conditions. The key problem of the
experiment is that the high quality interferograms could be caught continuously, that is depended on the
dynamic optical interferometric method, the smooth properties of driven and loading system of the test rig. A
local very flexible hydrostatic thrust bearing loading system and the coating layers techniques was used
successfully to reduce the tensile stress in glass disk, and overcome the unstable load caused by driving system
due to the inevitable error in the manufacture and assemble of the apparatus, and increase the refractive index
between the glass and oil interface. The results of actually measurement shown the variation of oil film shape
and film thickness in the contact area could be observed intuitively and recorded accurately by this test
apparatus. By comparing results of calculation and experiment, it was found the agreements between them are
quite coincident with each other.
Keywords: optical interferometry test, EHL, oil film measurement, finite line contacts, dynamic characteristic
1. INTRODUCTION
The profiled roller contacts are widely used in heavy radial load and high rotational speed conditions for
overcoming the problems of edge effects caused by their finite length and by misalignment. Extensive
mathematical analyses
[1]
and computer calculations
[2]
have shown that the most favorable effect in respect of
edge stress reduction is obtained when the generatrix of roller contacts corresponds to a certain logarithmic
function. But the theoretical basis of these calculation methods is elastostatic contact mechanics.
However, it is
well known that the roller contacts is separated by a viscous oil film during rolling, which results in that the
EHL state and the pressure distribution of oil film is different from that in elastostatic contact state. And the
load-bearing capacity, working velocity and service life of roller contacts is deeply under the influence of the
film thickness, film shape and pressure distribution of the film.
A few works have been done for studying EHL of finite line contacts during the past four decades. The first
testing investigation of EHL performance for both blended and unblended roller was made by Gohar, et al
[3]
who measured oil film thickness using optical interferometry in sliding contact, which gave several important
pictures of the effect of roller blending. A great improvement in fringe quality was obtained by Foord, et al
[4]
who, instead of relying on differences in refractive index, used a 20 per cent reflectivity chromium layer which
allowed the transparent material to be selected for its mechanical properties and enabled them to use pure
rolling and high speeds. Wymer, et al
[5]
developed a very flexible loading system which enabled the optical
interferometry usable to study EHL of finite line contacts under pure rolling and high contact pressure
conditions. They showed clearly that the shape and thickness of the oil film of blended and unblended rollers
were quite different, especially near the edge region of the roller. Parkings, et al
[6]
measured the film thickness
in thrust cone bearings using the interference method. The results revealed a very load-sensitive film thinning
effect at the edges of such contacts. The other publications on this topic were mostly under high speed and
lightly loaded conditions
[7~11]
.
In order to study the effects of crowning amount of logarithmic profile rollers on the elastohydrodynamic
lubrication, an optical interferometry method was applied to measure the oil film thickness between a profile
taper roller and a glass plate under pure rolling, generous oil supply, not too high rolling speed, moderate load
and material parameters conditions in this paper.
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