Knowledge

The remarkable lubrication provided by saliva in the oral cavity is vital to human health and wellbeing. Yet, molecular mechanisms for saliva lubrication remain unclear. In this work we report a possible mechanism of synergistic interaction between salivary proteins. By isolating a number of salivary protein fractions, we identify major protein candidates that contribute to saliva lubrication. We discover that a key driver for low friction is a hydrated brush-like layer formed by glycosylated species, with an essential synergistic contribution coming from the low molecular weight components that facilitate spreading, adsorption and strengthening of the salivary film on hydrophobic substrates. Lessons may be learned from saliva for understanding other natural bio-aqueous lubrication systems and for the development of saliva mimics.

The effects of glucose and glycerol on the lubrication properties of agar fluid gels have been studied using soft tribology. A novel approach using the sediment and supernatant of centrifuged fluid gels has allowed investigation of the distinct contributions of both the gelled particulate phase and the continuous phase on fluid gel tribology. The friction coefficient of both the particulate phase and fluid gels was significantly lower than that of the continuous phase across the three lubrication regimes. This indicates that particle entrainment occurs at all entrainment speeds, enhancing lubrication by prevention of surface contact.

Softer fluid gel particles produced with intermediate levels of glycerol (up to 30%) show increased friction as would be expected for an increased contact area between the tribological surfaces. At high levels of glycerol, the friction does not increase. It is proposed that soft particles are produced but the increasing friction is overcome with the increased lubrication from the more highly viscous continuous phase. In contrast, the presence of intermediate levels of glucose (up to 30%) increases the friction of the aqueous continuous phase but does not affect the particle properties. Texture analysis, rheology and light scattering techniques were used to elucidate the structural changes of the fluid gels induced by the addition of co-solutes and the influence this has upon lubrication.

Semi-solid and liquid food thickeners typically take the form of either polymeric or particulate structures. These structures are known to control flow properties and mixing efficiency which can influence performance, texture and the perception of tastants and aromas. However, their structural influence on thin-film rheology (tribology), which is also relevant for texture perception, is not so well understood. In this investigation, the tribology in a boundary regime of lubrication is tested using kappa carrageenan lubricants formulated both in solution and as gelled particles in suspension (fluid gels) to provide new insights into the structural influence of thickener type on tribology. Polymeric lubricated systems were shown to be dominated by elastic deformation of the tribo surfaces and particulate suspensions were dominated by particles acting as contacting asperities of the mating surfaces. The tribology of gelled particles was shown to depend strongly on particle elasticity where less deformable (stiffer) particles reduce surface–surface contact and therefore reduce friction coefficients. The effect of particle volume fraction on friction coefficient is related to the number of particles entraining the contact and not particle–particle interactions or bulk rheological behaviour.

The attributes of a traction fluid are fundamental to the successful operation of a traction drive transmission. The fluid must lubricate and protect the components against wear and corrosion, whilst simultaneously providing high traction to transmit power efficiently. A selection of commercial and candidate fluids have been assessed with both a bench-test and a novel traction rig. The principal objective has been to achieve a balance between the conflicting requirements of low temperature viscometrics and high temperature traction. Fluid performance is found to vary according to the rig employed underlining the need to test under prevailing conditions. Data from the traction rig is validated against a variator module.

Many phosphorus-based antiwear films, including those formed by zinc dialkyl dithiphosphates (ZDDP), cause a significant increase in friction in thin film, high-pressure, lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown that friction is increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films.

New infinitely variable transmission (IVT) systems are under development for the automotive industry as a means to achieving significant fuel economy benefits. These systems rely on the lubricating fluid to transmit the drive train loads across the interface of the transmission components. This requires the development of new fluids that exhibit high traction properties under elastohydrodynamic lubrication (EHL) conditions. However, it has been reported recently that the traction performance of some fluids can reduce dramatically as temperature is reduced. This may place severe operational limits on IVT systems and suggests that the low-temperature traction properties of fluids for these systems should be studied in order to understand the mechanism for the observed reduction in traction. The work reported here is an experimental study aimed at identifying whether low temperature traction reduction is related to a fundamental change in rheological behavior specific to the fluids tested or to more generic changes in the EHL contact conditions. A series of model experiments were performed using a mini traction machine (MTM) on three high-viscosity polybutene samples. The results have been mapped against previously reported non-dimensional parameters used to identify different EHL regimes. The results show that dramatic reductions in traction occur when the contact transitions from the rigid piezo-viscous (RP) toward the rigid iso-viscous (RI) region. Similar results were also found for two other high-viscosity fluids of different molecular structure and lower traction properties. The results support the hypothesis that the reduction in traction observed at low temperature is due to a change in EHL contact conditions rather than being solely due to a change in the rheological performance of the test fluids.

The elastohydrodynamic (EHD) lubrication regime occurs in many machine elements where a combination of hydrodynamic effect, elastic deformation of the loaded surfaces and increase in the viscosity of the lubricant with pressure ensures the formation of a very thin, but continuous film of lubricant separating the contacting surfaces. Electrical methods to determine this film's thickness have preceded optical methods, which are widely used today. Although they generally give more qualitative thickness information, electrical methods have the main advantage that they can be applied to metallic contacts in machines, which makes them useful tools in the study of elastohydrodynamically lubricated contacts. This paper is part of a larger study on the application of electrical capacitance for the evaluation of film formation in EHD contacts. The main focus is on the quantitative measurements of film thickness using electrical capacitance. A new approach allowing the lubricant film thickness to be extracted from the measured capacitance is developed using a chromium-coated glass disc and subsequently applied to a steel-on-steel contact. The results show good agreement with optical measurements and theoretical models over a range of film thickness.

With increasing pressure on engine oil manufacturers to extend oil drain intervals and reduce fuel consumption, whilst changing the composition of fully formulated oils to meet the new CEC, ILSAC and OEM specifications, there is an ever increasing need to understand the effect of oil degradation on the operating conditions and tribological performance of engines [1]. This work samples oil from the rear of the top piston ring of an engine during the first 15 minutes from cold start and operating at steady state under three different loads. These samples, used 40 hour sump oil and fresh oil have been subjected to tribological tests and chemical analysis.

The high frequency reciprocating rig (HFRR) was developed in the early 1990s as a test method to assess diesel fuel lubricity in order to provide wear protection for fuel injection pumps. This was necessary in response to the many field failures that occurred following the introduction of ultra-low sulphur diesel in Sweden. The prevalent fuel injection equipment (FIE) technology at this time utilised rotary pumps capable of reaching maximum fuel pressures of ∼650 bar in systems for direct injection engines. The continued drive for efficiency led to many changes in FIE technologies, materials and pressures. Modern high pressure common rail pumps reach significantly higher pressures, with 2200 bar available today and pressures up to 3000 bar discussed in the industry. Alongside these hardware changes there have been significant changes in the diesel fuel, with a continued move towards more highly refined fuels, and the introduction of highly paraffinic sources from Fischer-Tropsch processes and hydrogenated vegetable oils. Despite these changes there have been no widespread field issues since the introduction of HFRR specifications. The objective of this work was to understand the flexibility of the HFRR test by investigating the impact of increased test severity on wear. The main variable investigated was the effect of load, which was varied from 2-10N. Other variables investigated were frequency, test duration and stroke length. Additionally, preliminary studies were conducted using phosphate coated specimens more in line with those expected in current and future FIE systems. HFRR tests utilising a more highly loaded contact exhibit an increased mean wear scar. Trends observed are similar to those under standard conditions (2N), with differentiation between untreated fuels with poor lubricity and those that are additised. Certain fuels exhibit unusual friction coefficient behaviour, with temporary periods of significantly elevated friction. This is attributed to micro-seizure; however under additised conditions it was not observed. One interesting observation was a correlation between wear scar pattern and additive technology. Ester based lubricity additives give a striated wear scar, whereas acid technologies produce a stippled pattern, likely due to low level corrosion, a known possibility with this type of chemistry. The relatively soft nature of the phosphate coatings used meant that under the highly loaded contact conditions of the HFRR they quickly wore through; however the presence of additives minimised this wear and prolonged coating lifetimes. Further work indicated that other fuel additives such as cetane improver can impact lubricity performance, and may require different lubricity improver treat rates in order to meet specifications. In summary, increasing contact severity within the HFRR increases the level of wear in the system, with some evidence of micro-seizure. However, the use of additives under these conditions can reduce the wear to acceptable levels and prevents micro-seizure events. The trends and discrimination between conventional and higher load conditions remains similar. This indicates that modifications to the HFRR test method are unlikely to provide a means of defining how to achieve additional protection for future more severe conditions and the current test method remains suitable.