Liquid lubrication mechanism

Liquid lubrication mechanism. There are four defined regimes of liquid lubrication hydrodynamic (thickness of lubricant film (h), h > 0.25 pm), elastohydrodynamic (h 0.025 to 2.5 pm), boundary (h 0.0025 pm), and mixed. These regimes are dependent on oil viscosity (Z) and relative velocity (V) and are inversely proportional to the load (L), (ZV/L). Fig. 5.1, known as the Stribeck-Hersey curve, depicts these regimes in terms of friction coefficient versus viscosity, velocity, and load (ZV/L) (Fusaro, 1995). 

Nascent surface Explain the difference in the concept of liquid lubrication mechanism in (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation ... 

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

Lubrication mechanism How do you expect each of the following regimes to change in going from thickness of lubricating film h to a coefficient of friction, p in the Stribeck-Hersey curve Fig.5.1 There are three defined regimes of liquid lubrication (a) hydrodynamic, h 25 pm, (b) elastohydrodynamic, h = 0.025 to... 

Haltner and Oliver found that several metallic sulphides brought about an improvement in the load-carrying capacity when mixed with molybdenum disulphide. The sulphides included stannic and stannous sulphides, lead sulphide, ferrous suiphide and cuprous and cupric sulphides, and in a standard test procedure there was up to a ten-fold increase in load-carrying capacity. They speculated that the action of the added sulphides was similar to that of extreme-pressure additives in liquid lubricants. This would imply the formation of some protective film on the substrate surface. Pardee later suggested that the effective mechanism was more likely to be oxidation inhibition. An alternative would seem to be the possibility that certain sulphides can act as an additional source of sulphur to form sulphide on the substrate surface, and thus improve adhesion of the molybdenum disulphide, as discussed in the previous chapter. 

While molybdenum disulphide lubrication is adversely affected by liquids, liquid lubrication can be improved by molybdenum disulphide. There is in fact a considerable industry based on the use of molybdenum disulphide in liquids, in dispersions, lubricating oils, greases, anti-seizes and pastes. There is an important contradiction in the fact that in many situations the lubricating action of molybdenum disulphide is either partly or completed destroyed by the presence of liquids, whereas in other situations it can provide useful lubrication benefits in a liquid medium. A great deal of effort has been applied in attempts to establish the mechanism and the conditions by which lubrication by molybdenum disulphide can occur in the presence of a liquid. 

Jones Jr WR, Shogrin BA, Jansen MJ (2000) Research on liquid lubricants for space mechanisms. J Synth Lubr 17 109-122... 

Liu XQ, Zhou F, Liang YM (2006) Tribological performance of phosphonium based ionic liquids for an aluminum-on-steel system and opinions on lubrication mechanism. Wear 261 1174-1179... 

Zhang L, Feng DP, Xu B et al (2009) The friction and wear characteristics and lubrication mechanism of imidazole phosphate ionic liquid. Sci China Ser E-Tech Sci 52 1191-1194... 

Grease varies in texture from soft to hard and in color from light amber to dark brown and, in contrast to liquid lubricants, will stay in place in a bearing assembly with comparatively elementary mechanical seals. Grease also assists in sealing against extraneous material and will lubricate without constant replenishment. 

Liquid lubricants have been used in spacecraft mechanisms since the earliest satellites were launched. The vast previous experience gained in terrestrial applications with liquid lubricants means that they were the obvious choice in the early applications and they remain in widespread use today. Some of the advantages of liquid lubrication for aerospace applications, whether by oil or grease, are as follows ... 

Jones, W. R., et al., Research on Liquid Lubricants for Space Mechanisms , Proc. 24th Aerospace Mechanisms Symposium, pp. 299-310, 1998. 

The wear rate on liquid lubrication by electrolytes is defined only by metal dissolution and intensified friction, and can be varied in response to the corrosion rate under static conditions to a value that characterizes the mechanical failure of the metal. 

Chemical Lubrication under Extreme Loads. So far the lubrication mechanisms discussed have involved ever thinner layers of lubricating liquid. The obvious limit to that progression is the complete absence of an external lubricant. When devices operate under extreme conditions of load, speed, temperature, and other parameters, conventional lubricants will usually begin to break down and drastic mechanisms must be employed to prevent complete seizure and failure of the machinery. One way to approach that problem has been the development of sacrificial lubricants, which, under extreme conditions, react with fresh metal surfaces formed by wear to produce a new inorganic chemical layer that can then be more easily sheared, thereby preventing seizure. 

Different friction coefficients measured for each film thickness. This latter point implies that the mechanism of friction across ionic liquids is qualitatively different to nonpolar liquids, where the friction coefficient is independent of film thickness. Put together, these findings point at new ways to control friction in microscopic systems and to develop ionic liquid lubricants for specialised applications... 

Liquid slip can be promoted by the adsorption of long chain surfactants from solution on to smooth solid surfaces. This suggests a possible new mechanism of boundary lubrication by organic friction modifiers - encouragement of local slip of the liquid lubricant against the bounding solids to produce a local reduction in shear stress. Such a mechanism would imply that friction modifiers might be able reduce friction in full film lubrication conditions. 

The two most often used fillers in thermoplastics are PTFE and PDMS or silicone. The respective functions of the two materials are quite different. PTFE is almost the wonder additive for tribological applications, whether in a solid polymer or liquid lubricant. The mechaiucal properties of PTFE hold over a range of -260°C to 260°C with a melt temperature of 321°C. It is chanicaUy inert and hydrophobic. The effect of PTFE as a solid lubricant can be understood from its performance as a homopolymer. As a homopolymer, PTFE has the lowest coefficient of friction of all polymeric materials sliding against metal surfaces. This is due to the lack of bulky side groups on the molecular chain and the formation of a low friction transfer film on the opposing counterface. However, due to the poor mechanical properties, PTFE alone also... 

Lubricity. In any mechanical seal design there is rubbing motion between the dynamic seal faces. This rubbing motion is most often lubricated by the fluid being pumped. Therefore, the lubricity of the pumped liquid at the given operating temperature must be considered to determine if the chosen seal design and face combination will perform satisfactorily. 

---