TAILIEUCHUNG - Engineering Tribology 2011 Part 3

Tham khảo tài liệu 'engineering tribology 2011 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 66 Engineering Tribology showed that EHL film thickness does not vary with water concentration and maintains a value close to that of the constituent mineral oil. The pressure-viscosity coefficient of water is negligibly small 16 17 so that without forming an entrapment of oil around the EHL contact elastohydrodynamic lubrication would not be possible. Apart from a limited temperature range emulsions exhibit poor storage capability and they may not only be degraded by oil oxidation but also by bacterial contamination of water. Applications Emulsions and aqueous solutions are mostly used as cutting fluids in the metal working industry and as fire resistant lubricants in the mining industry. Aqueous solutions of polyglycols are often used as fire-resistant hydraulic oils with the added advantage of low viscosity and low pour points . -40 C. As a lubricant however polyglycol solutions offer only mediocre performance. The pressure-viscosity coefficient of a polyglycol solution is only X 10-8 Pa-1 compared to X 10-8 Pa-1 for a mineral oil 18 . Even small quantities of water can significantly reduce the pressure-viscosity coefficient. Thus the primary applications of these fluids are as fire resistant lubricants because even if all the water were evaporated from the lubricant the polyglycol would burn only with difficulty. GREASES Greases are not simply very viscous lubricating oils. They are in fact mixtures of lubricating oils and thickeners. The thickeners are dispersed in lubricating oils in order to produce a stable colloidal structure or gel. Thus a grease consists of oil constrained by minute thickener fibres. Since the oil is constrained and unable to flow it provides semi-permanent lubrication. For this reason greases are widely used in spite of certain limitations in performance. The most widespread application of greases is as low-maintenance semipermanent lubricants in rolling contact bearings and some gears. The grease may be packed into a .

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