4 research outputs found
In contact observation of model synovial fluid lubricating mechanisms
This paper examines the fundamental mechanisms of synovial fluid lubrication in artificial joints. Film thickness measurements were made for bovine serum solutions in a model test device. In contact imaging was also carried out to aid interpretation of these results. The results indicated that two types of film are formed; a boundary layer of adsorbed protein molecules, which are augmented by a high-viscosity fluid film generated by hydrodynamic effects. The high-viscosity film is due to inlet aggregation of protein molecules forming a gel which is entrained into the contact preferentially at low speeds. As the speed increases this gel appears to shear thin, giving much lower lubricant film thickness. Results suggest that protein-containing fluids do not obey classical Newtonian EHL models. © 2012 Elsevier Ltd. All rights reserved
Friction measurements with molten chocolate
A novel test is reported which allows the measurement of the friction of molten chocolate in a model tongue–palate rubbing contact. Friction was measured over a rubbing period of 150 s for a range of commercial samples with different cocoa content (85–5% w/w). Most of the friction curves had a characteristic pattern: initially a rapid increase occurs as the high-viscosity chocolate melt is sheared in the contact region followed by friction drop as the film breaks down. The exceptions were the very high (85%) and very low (~ 5%) cocoa content samples which gave fairly constant friction traces over the test time. Differences were observed in the initial maximum and final friction coefficients depending on chocolate composition. Generally, the initial maximum friction increased with increasing cocoa content. At the end of the test, the rubbed films on the lower slide were examined by optical microscopy and infrared micro-reflection spectroscopy. In the rubbed track, the chocolate structure was severely degraded and predominately composed of lipid droplets, which was confirmed by the IR spectra. The new test provides a method to distinguish between the friction behaviour of different chocolate formulations in a rubbing low-pressure contact. It also allows us to identify changes in the degraded chocolate film that can be linked to the friction profile. Further development of the test method is required to improve simulation of the tongue–palate contact including the effect of saliva and this will be the next stage of the research
Friction measurement indicating the transition between fully flooded and starved regimes in elasto-hydrodynamic lubrication
International audienceLubricant starvation in rolling element bearings can have different origins, such as high speed, large size or grease lubrication. However, the overall result is a performance reduction, involving a friction increase, a bearing life decrease or both. As such it is desirable to be able to predict when starvation will occur for a certain combination of geometry, operating conditions and lubricant. Previous work used optical interferometry to predict the onset of starvation and the ensuing oil film thickness reduction. A starvation parameter was derived linking lubrication parameters (viscosity), amount of oil and operating conditions (load, speed) to predict the onset of starvation. The current paper attempts to predict the onset of starvation through friction measurements on a ball-on-disc test device. The starvation parameter values obtained from friction measurements are similar to the ones found through optical interferometry