LOB Assignment

Assignment
Neil
June 24, 2021
Describe with neat sketches the various experimental methods of
measuring elastohydrodynamic film thickness.
1. The electrical resistance method involves measuring the electrical resistance of the lubricating
film.
It is useful for the detection of lubricating films, but there are some problems associated
with the assessment of the film thickness. The resistance is almost zero when metal-to-metal
contact is established between the asperities of opposite surfaces and then increases in
quite a complex manner with the thickness of lubricating film. The method is primarily
used in detecting the breakdown of lubricating films in contact. Many difficulties arise
in the evaluation of film thickness and the method is rather unreliable. Shown in Fig. 1
Figure 1: Resistive method
2. The electrical capacitance method involves the measurement of the electrical capacitance
of the lubricating film(which is usually non-conductive).
The film thickness can be estimated to reasonable accuracy by this method. A major
problem associated with this method is that the dielectric constant of the lubricating
oil varies with temperature and pressure. The constant must be determined before the
measurements of film thickness. Shown in Fig. 2
3. The X-ray method involves passing an X-ray beam through the lubricated contact between
two surfaces.
Contrast is produced by the fact that the contacting bodies are far more opaque to
X-rays than the fluid in the film. This can therefore result in a high contrast image of
the contact zone which can be used to measure film thickness. The X-ray beam shines
along the tangent plane between two lubricated rolling discs and the film thickness is
evaluated from the radiation intensity measurements of the emerging beam. The problems
in applying this technique are principally associated with maintaining the parallelism of
Assignment
Figure 2: Capacitive method
the beam to the common tangent of the contacting surfaces and with the calibration of
film thickness to the obtained image.
4. The mechanical method involves the measurements of differences in strain caused by
elastohydrodynamic films.
Here a strain gauge is used to obtain measurements.The main advantage of this method
is that it can be used for EHL film thickness evaluation in real operating machinery.
Other techniques require a specialised experimental setup.
5. The optical interferometry method.
The method utilises a steel ball which is driven in nominally pure rolling by a glass disc.
Figure 3: Optical interferometry technique.
The disc is coated on one side with an approximately 10 [nm] thick semi-reflecting layer of
chromium. When the disc is rotated in the presence of lubricant an elastohydrodynamic
film is formed between the ball and the disc. White light is shone through the contact
between the glass disc and the steel ball. The semi-reflecting chromium layer applied
to the surface of the disc reflects off some of the light while some light passes through
the lubricant and is reflected off the steel ball. The intensity of the two reflected beams
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Assignment
is similar and they will either constructively or destructively interfere to produce an
interference pattern, resulting in a graduation of colours depending on film thickness.
Since the elastohydrodynamic film thickness is of the same order as the wavelength of
visible light, it can be used to measure the generated elastohydrodynamic film thickness.
The interference pattern is reflected back through the objective to the viewing port of
the microscope. The corresponding optical film thickness is determined from the colours
of the optical interference pattern and the real film thickness found after dividing the
optical film thickness by the refractive index of the fluid. The schematic is shown in Fig.
3
Write a note on the Hertz contact stresses and its application.
Unusually high stresses may occur when a load is applied over a small area of contact. This
phenomenon typically arises on a microscopic scale when a force is transmitted through two
bodies in contact. Practical examples include contact of a wheel and a rail, valve cams and
tappets, in rolling bearings and mating gear teeth. High contact stresses may lead to failures
such as cracks, pits and flaking in the surface material.
The exact determination of contact stresses for complicated surfaces is a difficult process.
It depends strongly on the geometry of the contacting surfaces. The assumptions made by
Hendrick Hertz in order to simplify the formulation of equations of contact are:
1. The contact is frictionless.
2. The bodies are elastic, or the loads applied are such that Hooke’s law is obeyed locally.
3. The materials are isotropic and homogeneous.
4. There are no external shear stresses.
5. Surfaces are non-adhesive, meaning they require no force to separate
Applications:
Hardness testing: An understanding of Hertzian stresses is important in surface hardness
testing and the design of measuring equipment. The major point of importance being
that the Hertzian deflections are not a linear function of applied load, and it is also
dependant of the shape and form of the indentor used.
Kinematic mounts: A kinematic mount is one which exactly constrains the location and
motion of a body in space and restricts exactly the number of degrees of freedom required.
That is, it does not over constrain parts introducing unnecessary internal stresses. This
is very important in very precise, high repeatability opto-mechanical systems. This is
achieved via point mounts. The study of developed stresses in the mounting points is
essential to the systems repeatability.
Optical lens mounts: If lens mounts are such that they apply large contact stresses on the
lenses, then they result in often unacceptable optical distortion around the mounting
points for the lens, resulting in poor quality/distorted images in the vicinity.
Micrometers: since micrometers are such precise instruments, the clamping forces exerted on
the body being measured play a very important role in the measurement repeatability.
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Assignment
Engineering metrology: In some scenarios the deformations resulting from the probe pressure,
self weight of the object being measured, surface deformations of gauge block, etc are
also taken into account in order to compensate for them.1
Describe the regimes of elasto-hydrodynamic lubrication with a neat
sketch. What applications do these regimes manifest?
Th e lubrication of concentrated contact is normally influenced by two major physical effects
such as elastic deformation and the increase in fluid viscosity with pressure. There may be
several regimes of lubrication depending on the magnitude of the above two effects.
We have the following regimes...
Figure 4: Regimes of elasto-hydrodynamic lubrication
Isoviscous-rigid: the elastic deformation of the solids can safely be neglected because it is
very insignificant in magnitude. This type of lubrication is seen to occur in circular arc
thrust bearing pads and in industrial coating processes.
Piezoviscous-rigid: the pressure within the conjunction is quite high and it may be necessary
to consider the pressure-viscosity characteristics of the lubricant. However, the deformation
within the contact zone is neglected. This type of lubrication may be encountered in
moderately loaded cylindrical taper roller, and between the piston rings and cylinder
liners.
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MAKING PRECISION GAGE BALLS - https://youtu.be/fEoonCLTCbE?t=1500, user: ROBRENS,
accessed: 23/06/2021.
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Isoviscous-elastic: the elastic deformation of the surfaces is adequate to warrant inclusion
of elastic equation along with hydrodynamic equation. This form of lubrication can be
seen in the materials of low elastic modulus or seals and human joints.
Piezoviscous-elastic: the elastic deformation of solids as well as the variation of viscosity
with pressure must be considered. The lubrication regime which considers both these two
aspects is called piezoviscous regime. This type of lubrication is typically encountered in
ball and roller bearings, gears, and cams.
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References
1. M. C. Ghosh, B. C. Majumdar, ”Theory of Lubrication”, Tata McGraw Hill Education
Private Limited, 2013.
2. Gwidon W. Stachowiak, Andrew W. Batchelor, ”Engineering Tribology”.
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