Study On Mechanism And Experiment Of Elastohydrodynamic Grease Lubrication

Lubrication is the most popular method to reduce friction and avoid wear. About80%rolling bearings and sliding bearings are lubricated by grease. The proposed grease is mainlyto provide a layer of lubricating film between the friction interfaces of bearing so as to avoiddirect contact from friction interfaces. The friction coefficent becomes higher while thelubricating film becomes thinner or broken, which will evently cause failure of mechnismparts and devices probably. And because the thickness of lubricating film reflects thelubrication performance directly, many people study the film thickness and performance ofgrease lubrication by many means, including the theoretical and experimental methods.However, the rheological properties of grease are extremely complex and grease showsstrong non-newtonian fluid properties. These result in the complexity calculation of greaseelastohydrodynamic lubrication in engineering rather than oil lubrication. Lubricationphenomena often occurr in the millimeter of tiny area and the thickness of the lubricating filmare often micron-scale even nanoscale. Therefore, it is difficult to carry out measurement ofthe complex phenomena of lubrication in such a small area. The main purpose of this researchproject is to improve and perfect some deficiencies in the theory of grease lubrication and itsmeasurement techniques.The detailed work accomplished in this dissertation is as follows.Firstly, a grease EHL tester is developed. By the design and improvement of themechanical transmission, image acquisition device, fuel feeding and loading device, thegrease EHL tester is developed. The transmission stability is improved, the control becomesconvenient, and the captured images are clear and stable. At last, the stationary test resultsshow good consistency with the theoretical results. The design and improvement results meetthe purposed requiement.Secondly, a pervasive formula for relative optical interference intensity technique (ROII)is proposed when the interference order beyonds zero. The conditions which can apply ROIIare discussed. And based on the actual intensity-thickness curve, the measurement accuracyof ROII is improved. Through dynamic measuremnt technique, the interference order can bedetermined. The lubricating film thickness of base oil is measured, and the whole shape offilm thickness is calcalated by the pervasive formula.Thirdly, a measuement method of film thickness called multi-wavelength interferometryis proposed. In this method, it is not necessary to determine the interference order, while theonly requirements are the images which captured in stationary and poposed conditions. This method can utilize the feature that monochromatic light with different wavelengths havedifferent intensity resolution at the same fim thickness, so as to achive higher measurementaccuracy. The key problem of this technique is to the caliberation and fitting ofintensity-thickness curve.Fourthly, the comparision between the film thickness in experiments and theory isaccomplished. In experiments, the film thickness is obvious getting thicker when theentrainment speed becomes higher. At the same time. the position of the minium thickness isgetting closer to the contact center. These phenomna show consistency with theory. Thecomparisions show the measuring methods have good accuray and stability. The improvedROII and dynamic measurent techque is feasible. When comparing experimental results withthose of calculated by Hamrock-Dowson film thickness formula, there are also the samechange rule. Good consistency exists in low speed area for central film thickness and wholespeed area for the minimum film thickness, while application limitations are proved under thelow load and high speed conditions for the Hamrock-Dowson formula. When camparingthose results in three-wavelength interferometry and dynamic measurement, they have goodconsistency. The experimental results show that the three-wavelength interferometry isspecial and feasible.Finally, numerical analysis of grease lubrication under specific conditions is performed.Due to the high initial viscosity, obvious non-Newtonian property and small equivalent radiusin the grease lubrication of automotive wheel hub bearing, great difficulties arise in thecorresponding EHL calculation. By approximating the line contact type of wheel hub bearingin ball bearing form, the Reynolds equation, film thickness equation and viscosity-pressureequation are established and solved dimessionlessly and differential numerically. Finnally thedistribution of pressure and the shape of film thickness are obtained. The results show that1)in greased and lubricated wheel hub bearings, there exist a second pressure peak andthickness necking similar to those under oil lubricated condition;2) light load, high velocityand large rheological exponent all help to form thicker lubricating film with a more obvioussecond pressure peak; and3) the lubrication between the rolling ball and the rolling cave isactually in an alternate state of EHL and boundary lubrication. The experiments aboutrheological properties show obvious shear thinning and non-newtonian fluid properties. What’more, different soaps result in different high tempreture performance, this is because of thedifferent behaviour of soap fibers at high tempture and the lubricating mechanism.