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 Lubrication of Bearings 2 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. Lubrication of Bearings 3 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. 1 MAKING PRECISION GAGE BALLS - https://youtu.be/fEoonCLTCbE?t=1500, user: ROBRENS, accessed: 23/06/2021. Lubrication of Bearings 4 Assignment 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. Lubrication of Bearings 5 Assignment 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”. Lubrication of Bearings 6