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At least 343 records · Page 19

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NASA NTRS1985

Constitutive modelling of lubricants in concentrated contacts at high slide to roll ratios

A constitutive lubricant friction model for rolling/sliding concentrated contacts such as gears and cams was developed, based upon the Johnson and Tevaarwerk fluid rheology model developed earlier. The friction model reported herein differs from the earlier rheological models in that very large slide to roll ratios can now be accommodated by modifying the thermal response of the model. Also the elastic response of the fluid has been omitted from the model, thereby making it much simpler for use in the high slide to roll contacts. The effects of this simplification are very minimal on the outcome of the predicted friction losses (less than 1%). In essence then the lubricant friction model developed for the high slide to roll ratios treats the fluid in the concentrated contact as consisting of a nonlinear viscous element that is pressure, temperature, and strain rate dependent in its shear response. The fluid rheological constants required for the prediction of the friction losses at different contact conditions are obtained by traction measurements on several of the currently used gear lubricants. An example calculation, using this model and the fluid parameters obtained from the experiments, shows that it correctly predicts trends and magnitude of gear mesh losses measured elsewhere for the same fluids tested here.

Tevaarwerk, J. L.↗

NASA NTRS1985

New directions in lubrication, materials, wear, and surface interactions - Tribology in the 80's

New directions in tribology are described. A range of topics is addressed, extending from fundamental research on tribological materials of all kinds and their surface effects, to final technological applications in mechanical components such as bearings, gears, and seals. The general topics addressed include: importance and definition of materials in tribology; future directions of research in adhesion and friction, wear and wear-resistant materials, and liquid lubricants and additives; status and new directions in elastohydrodynamic lubrication and solid lubricants; and tribological materials for mechanical components of the future.

Loomis, W. R.↗

NASA NTRS1985

Lubrication and performance of high-speed rolling-element bearings

Trends in aircraft engine operating speeds have dictated the need for rolling-element bearings capable of speeds to 3 million DN. A review of high-speed rolling-element bearing state-of-the-art performance and lubrication is presented. Through the use of under-race lubrication and bearing thermal management bearing operation can be obtained to speeds of 3 million DN. Jet lubricated ball bearings are limited to 2.5 million DN for large bore sizes and to 3 million DN for small bore sizes. Current computer programs are able to predict bearing thermal performance.

Zaretsky, E. V.↗

NASA NTRS1986

Optical and other properties changes of M-50 bearing steel surfaces for different lubricants and additives prior to scuffing

An ester lubricant base oil containing one or more standard additives to protect against wear, corrosion, and oxidation was used in an experimental ball/plate elastohydrodynamic contact under load and speed conditions such as to induce scuffing failure in short times. Both the ball and the plate were of identically treated M-50 steel. After various periods of operating time the wear track on the plate was examined with an interference microscope of plus or minus 30 A depth resolution and sometimes also with a scanning ellipsometer and an Auger spectrometer. The optically deduced surface profiles varied with wavelength, indicating the presence of surface coatings, which were confirmed by the other instruments. As scuffing was approached, a thin (approximately A) oxide layer and a carbide layer formed in the wear track in particular when tricresylphosphate antiwear additive was present in the lubricant. The rates of the formation of these layers and their reactivity toward dilute alcoholic HCl depended strongly on the lubricant and additives. Based on these results suggestions for improved formulations and a test method for bearing reliability could be proposed.

Lauer, J. L.↗

NASA NTRS1985

Long term evaluation of grease type lubricants in selected environments

The operating life of grease-type lubricants in various environments is evaluated. The test equipment and procedures are described. The lubricants were placed in the rolling element bearings of hysteresis synchronous motors and exposed to environments of a vacuum at 60 C, a vacuum at 93.3 C, 10 psia oxygen at 90 percent relative humidity at 60 C, 14.7 psia air at 90 percent relative humidity at 60 C, start-stop operation, and low temperature start capability for one or five years. The weights of the bearings were measured before and after lubrication; the weight data for each environment are analyzed. The data reveal that the high viscosity index perfluoroalkypolyether grease performs the best in a vacuum at moderate and high temperature, in low temperature start capability, and in start-stop steps; however, the grease does not work well in oxidizing and high relative humidity environments.

Dolan, F. J.↗

NASA NTRS1986

The use of silver in self-lubricating coatings for extreme temperatures

The advantages and disadvantages of elemental silver as a tribological material are discussed. It is demonstrated that the relatively high melting point of 961 deg C, softness, marked plasticity, and thermochemical stability of silver combine to make this metal useful in thin film solid lubricant coatings over a wide temperature range. Disadvantages of silver during sliding, except when used as a thin film, are shown to be gross ploughing due to plastic deformation under load with associated high friction and excessive transfer to counterface surfaces. This transfer generates an irregular surface topography with consequent undesirable changes in bearing clearance distribution. Research to overcome these disadvantages of element silver is described. A comparison is made of the tribological behavior of pure silver with that of silver formulated with other metals and high-temperature solid lubricants. The composite materials are prepared by co-depositing the powdered components with an airbrush followed by furnace heat treatment or by plasma-spraying. Composite coatings were formulated which are shown to be self-lubricating over repeated, temperature cycles from low temperature to about 900 deg C.

Sliney, H. E.↗

NASA NTRS1986

Lubricant effects on bearing life

Lubricant considerations for rolling-element bearings have within the last two decades taken on added importance in the design and operation of mechanical systems. The phenomenon which limits the useful life of bearings is rolling-element or surface pitting fatigue. The elastohydrodynamic (EHD) film thickness which separates the ball or roller surface from those of the raceways of the bearing directly affects bearing life. Chemical additives added to the lubricant can also significantly affect bearings life and reliability. The interaction of these physical and chemical effects is important to the design engineer and user of these systems. Design methods and lubricant selection for rolling-element bearings are presented and discussed.

Zaretsky, Erwin V.↗

NASA NTRS1986

Towards an optimised sputtered MoS2 lubricant film

It is shown that the tribological quality of MoS2 lubricant films formed by magnetron sputtering is determined by the choice of sputtering conditions. By selecting the appropriate conditions, films of extremely high lubricity and endurance (in vacuum), which are well suited to many space applications, are obtained. Such MoS2 films, when applied to precision ball hearings, give rise to the lowest torques (for the given test conditions) yet seen in our laboratory. While a remarkably good performance is obtained in vacuum, tests in air show a marked deterioration in lubricating qualities. It is demonstrated that this is attributable to the adsorption of water vapor on MoS2 surfaces and that the degree of deterioration is related to the partial pressure of water vapor present. Analysis of results indicates that the factors relevant to obtaining optimum films are deposition rate and film composition.

Roberts, E. W.↗

NASA NTRS1987

Hardness of CaF2 and BaF2 solid lubricants at 25 to 670 deg C

Plastic deformation is a prominent factor in determining the lubricating value of solid lubricants. Little information is available and its direct measurement is difficult so hardness, which is an indirect measure of this property was determined for fluoride solid lubricant compositions. The Vickers hardness of BaF2 and CaF2 single crystals was measured up to 670 C in a vacuum. The orientation of the BaF2 was near the (013) plane and the CaF2 was about 16 degrees from the degrees from the (1'11) plane. The BaF2 has a hardness of 83 kg/sq mm at the 25 C and 9 at the 600 C. The CaF2 is 170 at 25 C and 13 at 670 C. The decrease in hardness in the temperature range of 25 to 100 C is very rapid and amounts to 40% for both materials. Melts of BaF2 and CaF2 were made in a platinum crucible in ambient air with compositions of 50 to 100 wt% BaF2. The Vickers hardness of these polycrystalline binary compositions at 25 C increased with increasing CaF2 reaching a maximum of 150 kn/sq mm near the eutectic. The polycrystalline CaF2 was 14% softer than that of the single crystal surface and BsF2 was 30% harder than the single crystal surface. It is estimated that the brittle to ductile transition temperature for CaF2 and BaF2 is less than 100 C for the conditions present in the hardness tester.

Deadmore, Daniel L.↗

NASA NTRS1986

Effects of silver and group II fluoride solid lubricant additions to plasma-sprayed chromium carbide coatings for foil gas bearings to 650 C

A new self-lubricating coating composition of nickel aluminide-bonded chromium carbide formulated with silver and Group II fluorides was developed in a research program on high temperature solid lubricants. One of the proposed applications for this new coating composition is as a wide temperature spectrum solid lubricant for complaint foil gas bearings. Friction and wear properties were obtained using a foil gas bearing start-stop apparatus at temperatures from 25 to 650 C. The journals were Inconel 748. Some were coated with the plasma sprayed experimental coating, others with unmodified nickel aluminide/chromium carbide as a baseline for comparison. The additional components were provided to assist in achieving low friction over the temperature range of interest. Uncoated, preoxidized Inconel X-750 foil bearings were operated against these surfaces. The foils were subjected to repeated start/stop cycles under a 14-kPa (2-Psi) bearing unit loading. Sliding contact occurred during lift-off and coastdown at surface velocities less than 6 m/s (3000 rPm). Testing continued until 9000 start/stop cycles were accumulated or until a rise in starting torque indicated the journal/bearing had failed. Comparison in coating performance as well as discussions of their properties and methods of application are given.

Wagner, R. C.↗

NASA NTRS1987

Dynamic characterization of viscoelastic polymer solutions in a lubricated cylinder - Plate apparatus

The characterization of several viscoelastic lubricants which are oil or water based has been studied in an apparatus consisting of a lubricated cylinder-plate contact. The friction loads were measured as a function of speed. The experimental results show the influence of the molecular weight and of the concentration of the polymeric additive as well as the influence of the viscosity of the oil-base on the load and friction coefficient. Also a test for mechanical degradation was performed on the polymer solutions. Several additives can favor a viscoelastic lubrication.

Doremus, P.↗

NASA NTRS1989

Effects of lubrication on the performance of high speed spur gears

An experimental analysis was conducted to determine power loss and gear noise of high speed spur gears with long addendum under various conditions of load, speed, and oil jet pressure for into mesh lubrication. Power losses were calculated from temperature measurements of lubricating oil, gears, gear box, and oil flow rate. Furthermore, power loss was divided into windage loss, friction loss and churning loss. The results show that windage loss and churning loss were the main components of gear power loss of high gear speed. In addition, lubricating conditions had some influences on gear noise especially under low oil temperature or high viscosity.

Mizutani, Hachiro↗

NASA NTRS1988

Additives for high-temperature liquid lubricants

A preliminary research program was conducted to demonstrate a new concept for additives to liquid lubricants. It was demonstrated that suspensions of o-phthalonitrile and a substituted 1,2-maleonitrile in mineral oil and dilute solutions of o-phthalonitrile and tetrafluoro-o-phthalonitrile extended the lifetime of bearings under boundary lubricating conditions. The solutions exhibited coefficients of friction under high loads of 0.02-0.03. These results were consistent with the hypothesis that these compounds react with the hot metal surface to form a planar lubricating film by means of a metal or metal oxide template reaction. Also, the adherence was very strong due to the chelating action of the planar macrocycles postulated to form under the experimental conditions.

Lawton, Emil A.↗

NASA NTRS1990

Positive lubrication system

As part of the development of an autonomous lubrication system for spin bearings, a system was developed to deliver oil to grease-lubricated bearings upon demand. This positive oil delivery system (PLUS) consists of a pressurized reservoir with a built-in solenoid valve that delivers a predictable quantity of oil to the spin bearing through a system of stainless steel tubes. Considerable testing was performed on the PLUS to characterize its performance and verify its effectiveness, along with qualifying it for flight. Additional development is underway that will lead to the fully autonomous active lubrication system.

Smith, Dennis W.↗

NASA NTRS1989

Effects of lubrication on the performance of high speed spur gears

Mizutani, H.↗

NASA NTRS1991

Solid lubricants

The state of knowledge of solid lubricants is reviewed. The results of research on solid lubricants from the 1940's to the present are presented from a historical perspective. Emphasis is placed largely, but not exclusively, on work performed at NASA Lewis Research Center with a natural focus on aerospace applications. However, because of the generic nature of the research, the information presented in this review is applicable to most areas where solid lubricant technology is useful.

Sliney, Harold E.↗

NASA NTRS1991

The PM-200 lubrication system

Plasma sprayed composite coating of metal-bonded chromium carbide with additions of silver and thermochemically stable fluorides were previously reported to be lubricative in pin on desk bench tests from room temperature to 900 C. An early coating formulation of this type, designated as PS-200, was successfully tested as a cylinder coating in a Stirling engine at a TRRT of 760 C in a hydrogen atmosphere, and as a backup lubricant for gas bearings to 650 C. A subsequent optimization program has shown that tribological properties are further improved by increasing the solid lubricant content. The improved coating is designated as PS-212. The same powder formulation was used to make free-standing powder metallurgy (PM-212) parts by sintering or hot isostatic pressing. The process is very attractive for making parts that cannot be readily plasma sprayed such as bushings and cylinders that have small bore diameters and/or high length to diameter ratios. The properties of coatings and free-standing parts fabricated from these powders are reviewed.

Sliney, Harold E.↗

NASA NTRS1991

The rheology and hydrodynamics of dry powder lubrication

This paper conceptualizes a powder lubrication mechanisms which closely resembles that of a hydrodynamic fluid film. Based on the observations of past investigations and on the author's experiments, it is postulated that a layered shearing of the compacted powder generates velocity, density, and temperature profiles akin to fluid film bearings. Thus, a lubricant consisting of a fine powder unserted either deliberately or one generated by the water of the mating surfaces, constitutes a viable lubricant that generates the required flows and pressures to prevent contact between the surfaces.

Heshmat, Hooshang↗