Lubricant thermal conductivity and heat capacity under high pressure

Larsson, Roland; Andersson, Ove

doi:10.1243/1350650001543223

Cited by 85 publications

(71 citation statements)

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“…PB is commonly considered to be a Newtonian fluid, but at higher molecular weights, and 30 more importantly, at higher pressures, it is known to shear thin at the shear rates 31 encountered in this work [3] been observed [5,6]. Hence the behaviour observed with PB in this work is relevant to …”

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confidence: 70%

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Through-Thickness Velocity Profile Measurements in an Elastohydrodynamic Contact

2013

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7594 8991This work presents for the first time through-thickness velocity profiles obtained in an EHL contact by photobleached imaging. The velocity profile was inferred by following the evolution of the shape of a photobleached plug formed through the thickness of the fluorescently-doped lubricant, oligomer polybutene (PB), in the contact when shear was applied. The proposed methodology was validated by successfully obtaining the expected linear profile with PB experiencing Couette flow. The methodology was then applied to PB in an EHL contact. The variation of the profiles within the contact area was also investigated.The velocity profile of PB in an EHL contact severely deviates from the common linear assumption and exhibits inhomogeneous shear: three regions of varying shear rate have been observed. The phenomenon is shown to be neither due to thermal nor diffusion effects. PB also shows significant slip at the glass-liquid interface. The amount of slip varies with position in the contact. Possible causes, such as pressure induced viscosity enhancement, as well as the significance of the findings and the benefits of the technique are discussed.The linear velocity profile in an EHL contact is usually assumed for both the film thickness and friction predictions. The profile has however never been measured experimentally until now. This work enables the validation of conventional assumptions and the study of flow heterogeneity of lubricants in a contact. This facilitates an improved understanding of the rheology of confined lubricant and hence more accurate predictions of tribological properties.

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confidence: 70%

“…It can become viscoelastic, and in some cases it may 26 even display glassy behaviour [4]. It has been shown that lubricants can solidify [5,6] at 27 pressures commonly encountered in EHL applications. These observations suggest that 28 the commonly assumed linear velocity profile may be inappropriate.…”

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confidence: 99%

Through-Thickness Velocity Profile Measurements in an Elastohydrodynamic Contact

2013

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“…If the film is assumed to form a central shear plane, a value 4 is obtained [32]. If the fluid shears mainly close to the walls, as recently observed using fluorescence for polybutene [33], then, at the limit, the second term in Eq. 6 becomes negligible since heat is generated at the solid surfaces.…”

Section: Temperature Effectmentioning

confidence: 85%

Effect of Base Oil Structure on Elastohydrodynamic Friction

2016

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The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding-rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide-roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.

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“…The temperature and pressure dependency of the thermal conductivity ( ) and heat capacity per volume ( , • )( , ) are based on the models of Larsson and Andersson [20]. Thereby, different parameters are available for different types of lubricant.…”

Section: Temperature and Pressure Dependency Of Lubricant Propertiesmentioning

confidence: 99%