Chapter 15-Refrigeration Lubricants-Properties and Applications
Pages: 18 Published: Jun 2003
THE PRIMARY FUNCTION OF A LUBRICANT IS TO REDUCE FRICTIONAL LOSSES. This is achieved by interposing a film between the moving solid surfaces that reduces or eliminates direct solid to solid contact . In hydrodynamic lubrication (HD), where contact surfaces conform geometrically, only the lubricant viscosity governs friction and lubricant film thickness . In this region, which is typical of many bearing and piston surfaces, the contact surfaces are completely separated by the lubricant film and nominal contact pressures are low (<50 MPa). The lubricant film thickness varies with teh same dependence on operating parameters, such as speed, shear rate, and applied force, as the viscosity for this region. This makes the correct choice of lubricant viscosity become an important issues in the HD region. Low viscosity may lead to solid contact, while too high viscosity leads to an increase in frictional losses resulting in increase motor loading and higher power consumption. In regions of elastrohydrodynamic lubrication (EHD), such as bearings or gears with high-pressures loadings, additional lubricant properties such as pressure-viscosity coefficient and limiting shear stress come into play [3,4]. Temperature plays a critical role in both the HD and EHD regions because of its strong effect on the viscosity of a lubricant. In typical air conditioning (A/C) or refrigeration systems, both of these regions of lubrication may be encountered. The maximum pressure attained in the refrigeration system can be as high as 2.5 GPa, indicationg operation well into full developed elastohydrodynamic or boundary lubrication regions. Here, the thermophysical properties of the lubricant that become important are molecular conformability and resistance to physical degradation .
Paper ID: MNL10730M
ASTM International is a member of CrossRef.