Lubrication space application

Perfluoropolyethers emerged on the market in the early 1970s. The first perfluoropolyether was the homopolymer of hexafluoropropylene oxide produced by DuPont, which has the structure [—CF2CF(CF3)0—] and this new lubricant material was called Krytox.31,32 Krytox was and is used in most of the vacuum pumps and diffusion oil pumps for the microelectronics industry because it does not produce any hydrocarbon or fluorocarbon vapor contamination. It also has important applications in the lubrication of computer tapes and in other data processing as well as military and space applications. 

Space Applications. All known liquid lubricants and fatty acids evaporate, and they are, therefore, unsuitable for space conditions. 

A vast amount of information has been published on the testing of molybdenum disulphide materials for space use. It seems probable that most if not all American satellites and spacecraft have contained some application of molybdenum disulphide, and a number of space applications are listed in Table 1.1. A notable early example was its use on the extendible legs of the Apollo Lunar Module in 1969. Application of molybdenum disulphide in more conventional bearing systems is described in Chapters 9 to 13, but the wide variety of lubricant uses is shown in Table 1.2 by a list of applications not described in more detail elsewhere in the book. 

Anderson, M.J. and Roberts, E.W., An Assessment of Solid Lubricant Films for Use in High Temperature Space Applications, Proc. 5th European Space Mechanisms and Tribology Symp., ESTEC, Noordwijk, Holland, (28-30 Oct. 1992). (ESA SP-334, Apr. 1993, p. 379). 

There are no universal adhesives for the diverse applications seen in the aerospace business. Adhesives not only have to operate in space applications from about - 100 to - - 120°C, in civil and military aircraft under hot/wet climatic conditions, where relative humidities can approach 100% at temperatures up to 60-80°C, and close to aero engine assemblies where temperatures can reach in excess of 220°C but also have to resist the numerous fluids which are present in most modern aircraft de-icing fluid (alcohols and phosphate esters), fuels, bilge fluid, oils and lubricants, water and water-based electrolytes. 

There are several low vapor pressure solid (dry) lubricant and antistick (antiseize) compound materials that are vacuum-compatible. These include the sulfides (M0S2 and WS2 lubricants, usable to 10 Torr), silicides (WSi2 - antistick), and selenides (WS 2 - electrical conductors). Care should be taken to ensure that any binder materials used in the materials are also vacuum-compatible. Sputter-deposited M0S2 and M0S2 + Ni lubricants, in particular, have been shown to be acceptable in vacuums and are used by NASA for space applications. 

However, investigations up to now have mainly concentrated themselves on ambient environments even though it is known that ionic liquids have a very low vapor pressure, making them suitable for vacuum applications such as in space mechanisms, the disk drive industry, and microelec-tromechanical systems (MEMS). Due to the ultra-low vapor pressure of most ionic liquids, they have been expected to be good lubricants in vacuum. Further experimental works are required to evaluate lubrication behavior of ionic liquids under ultra-high vacuum conditions and in inert atmospheres. 

A number of additional applications of the ideas of this chapter could be profitably considered if space permitted. Included among these are adhesives, lubricants, waterproofing, and the recovery of oil from the pores of rocks. Like detergency and flotation, these topics involve a variety of surface and colloid phenomena. The interested reader will find an introduction to these fields in some of the references listed at the end of this chapter, especially Adamson (1990), Davies and Rideal (1961), and Osipow (1962). 

Graphitic BN (h-BN) is used as lubricant with low friction in numerous applications. Compared to graphite the h-BN can be used as lubricant in an oxidizing atmosphere up to 900 °C as well as at extremely low temperatures, e.g., in space because no water inclusions between the atomic sheet layers are present (graphite always contains small amounts of water between the layers). Due to its excellent resistance against oxidation, its extremely low friction coefficient, and its chemical inertness, h-BN can be inserted into alloys or ceramics [105]. It can be used as a solid surface lubricant [106] or added to a liquid to get dispersions with lubricating properties. 

Some applications of the synthetic fluids are motor oil, trucks, marine diesel, transmissions and industrial lubricants, aviation and aerospace lubricants, fire-resistant fluids, and greases. Specifications for several military lubricants can be met only by a synthetic product. All commercial and military jet aircraft engines use synthetic lubricants, in addition to the space shuttle, NASA, and nuclear submarines. 

The most important single development in the use of molybdenum disulphide as a lubricant was probably the initiation of studies by the US National Advisory Committee for Aeronautics (NACA) in 1946. Their first report was published in 1948. This work by NACA and its successor the National Aeronautics and Space Administration (NASA) laid the foundations for the great expansion in use during the past forty years. The overall increase in activity in this period was so rapid that by 1952 Climax published a list of 154 different applications. 

The fundamental theorems needed to make use of a molecular dynamics simulation have now been listed. Applications to other problems such as lubrication by a thin film or the related one of viscous flow between two closely spaced plates or down a narrow cylindrical tube will be discussed below. 

PRODUCT APPLICATIONS plastic parts (e.g., piston rings, cams, ball bearing retainers, space shuttle s, etc.), greases, lubricating aerosols, oil additives, metalworking compounds... 

The extensive use of hydrocarbon oils as lubricants in terrestrial applications has resulted in a good understanding of their properties. They are inherently good boundary lubricants and additives are readily soluble in them. In general, however, they do not have a low vapour pressure, and so their use in space is usually limited to sealed lubricant systems. The oils used in space tend to be super-refined and, as such, they have had all of the light fractions of oil and impurities removed. Table 12.2. 

The application of the Reynolds equation, even in its simplified two-dimensional version, to specific bearing problems generally requires detailed, laborious computational treatments to arrive at usable solutions. The foregoing presentation in this chapter is intended to give the uninitiated reader an idea of the basic concepts of hydrodynamic lubrication. The limited space allotted to the subject does not permit an extensive exposition. A large array of books and periodical literature is available to the student who wishes to develop skill in the fluid film aspect of lubrication engineering. A few of the texts and monographs which treat hydrodynamic lubrication and related problems in detail are listed below. 

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