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At least 163 records · Page 9

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NASA NTRS1996

Measurement of Gear Tooth Dynamic Friction

Measurements of dynamic friction forces at the gear tooth contact were undertaken using strain gages at the root fillets of two successive teeth. Results are presented from two gear sets over a range of speeds and loads. The results demonstrate that the friction coefficient does not appear to be significantly influenced by the sliding reversal at the pitch point, and that the friction coefficient values found are in accord with those in general use. The friction coefficient was found to increase at low sliding speeds. This agrees with the results of disc machine testing.

GEAR TEETH↗

NASA NTRS2024

Protocol Dependent Frictional Granular Jamming Simulations: Cyclical, Compression and Expansion

Granular matter takes many paths to pack in natural and industrial processes. The path influences the packing microstructure, particularly for frictional grains. We perform discrete element modeling simulations of different paths to construct packings of frictional spheres. Specifically, we explore four stress-controlled protocols implementing packing expansions and compressions in various combinations thereof. We characterize the eventual packed states through their dependence of the packing fraction and coordination number on packing pressure, identifying non-monotonicities with pressure that correlate with the fraction of frictional contacts. These stress-controlled, bulk-like particle simulations access very low-pressure packings, namely, the marginally stable limit, and demonstrate the strong protocol dependence of frictional granular matter.

DEM↗

NASA NTRS1962

Influence of Tire Tread Pattern and Runway Surface Condition on Braking Friction and Rolling Resistance of a Modern Aircraft Tire

A series of taxiing tests was conducted at the Langley landing loads track with both braked and unbraked (freely rolling) single and tandem wheels equipped with 32x8.8 type VII aircraft tires of different tread designs to obtain data on tire and braking characteristics during operation on dry and on contaminated concrete and asphalt run ways. Contaminants used were water, slush, JP-4 jet fuel, and organic and detergent fire-extinguishing foams. Forward velocities for the tests ranged from approximately 13 to 104 knots. Vertical loads of approximately 9,000 to 22,000 pounds and tire inflation pressures of 85 to 350 pounds per square inch were used. Results indicated that the unbraked tire rolling resistance increased with increasing forward velocity on dry and on contaminated runway surfaces. Peak tire-ground friction coefficients developed during wheel braking decreased rapidly with increasing velocity on contaminated runways but remained relatively unchanged on dry runways as the forward velocity was increased. Dry-runway friction coefficients were found to be relatively insensitive to tire tread pattern. However, the magnitude of the friction coefficients developed by tires on contaminated runways was extremely sensitive to the tire tread pat tern used, with circumferential-groove treads developing the highest values of friction coefficient, and smooth and dimple treads the lowest values for the tread patterns and runway conditions investigated.

Horne, Walter B.↗

NASA NTRS1972

Influence of silicon on friction and wear of iron-cobalt alloys

Sliding friction and wear experiments were conducted with ternary ordered alloys of iron and cobalt containing various amounts of silicon to 5 weight percent. The friction and wear of these alloys were compared to those for binary iron-cobalt alloys in the ordered and disordered states and to those for the conventionally used bearing material, 440-C. Environments in which experiments were conducted included air, argon, and 0.25percent stearic acid in hexadecane. Results indicate that a ternary iron - cobalt - 5-percent-silicon alloy exhibits lower friction and wear than the simple binary iron-cobalt alloy. It exhibits lower wear than 440-C in all three environments. Friction was lower for the alloy in argon than in air. Auger analysis of the surface of the ternary alloy indicated segregation of silicon at the surface as a result of sliding.

Buckley, D. H.↗

NASA NTRS1972

A fundamental review of the friction and wear behavior of ceramics

The basic concepts associated with the friction and wear of materials are discussed as they relate to ceramics. Properties of ceramics such as crystal structure, crystallographic orientation, mechanical deformation, and surface chemistry are reviewed as they influence friction and wear. Both adhesive and abrasive wear of ceramics are discussed. The friction and wear of ceramics are examined in contact with themselves and when in contact with metals. The influences of environmental constituents such as water and hydrocarbons on friction and wear are reviewed. Materials discussed, by way of example, include aluminum oxide, rutile, calcium fluoride, and lithium fluoride.

Buckley, D. H.↗

NASA NTRS1972

The effect of wall friction on magnetohydrodynamic generator performance

The effect of wall friction on magnetohydrodynamic generator performance is determined by introduction of a wall friction factor into the one-dimensional generator equations. This addition should be useful in improving generator analysis and determining optimum generator geometry. The curves presented can be used to determine the effects of changes in wall friction and generator performance. Wall friction has an increasing effect on the Mach number increases and a decreasing effect as the pressure drop across the generator increase.

Bishop, A. R.↗

NASA NTRS1972

Friction and wear life properties of polyimide thin films

A transition in the friction coefficient and wear life properties of Pyralin polyimide (PI) thin films was found to exist at a temperature between 25 deg and 100 deg C. Above this transition, PI thin films gave long wear lives and low friction coefficients. The presence of H2O in air improved the friction and wear life properties at 25 deg C; but at 100 deg C, H2O had a detrimental effect. At 100 deg C and above, a dry argon atmosphere gave lower friction coefficients and longer wear lives than did a dry air atmosphere.

Fusaro, R. L.↗

NASA NTRS1972

Influence of alloying elements on friction and wear of copper

The friction and wear characteristics were determined for copper binary alloys containing 10 atomic percent aluminum, silicon, indium, and tin. A ternary alloy containing 10 atomic percent aluminum and 5 atomic percent silicon was also examined. The effectiveness of each of the alloying elements aluminum and silicon were very effective in reducing friction. Silicon, however, also reduced wear appreciably. With lubrication, silicon, indium, and tin were all effective alloying elements in reducing friction and wear from values obtained for copper. Silicon was the most effective single element in reducing friction and wear in dry sliding and with lubrication.

Buckley, D. H.↗

NASA NTRS1972

A fundamental review of the friction and wear behavior of ceramics.

The basic concepts associated with the friction and wear of materials are discussed as they relate to ceramics. Properties of ceramics such as crystal structure, crystallographic orientation, mechanical deformation and surface chemistry are reviewed as they influence friction and wear. Both adhesive and abrasive wear of ceramics are discussed. The friction and wear of ceramics are examined in contact with themselves and when in contact with metals. The influences of environmental constituents such as water and hydrocarbons on friction and wear are reviewed. Materials discussed, by way of example, include aluminum oxide, rutile, calcium fluoride, and lithium fluoride.

Buckley, D. H.↗

NASA NTRS1973

Friction, wear, and transfer of carbon and graphite to copper, chromium, and aluminum metal surfaces in vacuum

Sliding friction experiments were conducted with amorphous and fully graphitized carbons sliding on copper and on films of chromium and aluminum on copper. Auger emission spectroscopy analysis was used to monitor carbon transfer to the metal surfaces. Friction and wear were also measured. Metal surfaces were examined both in the clean state and with normal oxides present. Results indicate that different metals have an important effect on friction, wear, and transfer characteristics. With amorphous carbon, the least chemically active metal gave the highest wear and amount of carbon transfer. Both forms of carbon gave lower friction and wear and lower transfer rates when in contact with clean, as opposed to oxide-covered, chromium surfaces. With copper, the reverse was true; cleaning was detrimental.

Buckley, D. H.↗

NASA NTRS1973

Friction, wear, and thermal stability studies of some organotin and organosilicon compounds

Thermal decomposition temperatures were determined for a number of organotin and organosilicon compounds. A ball-on-disk sliding friction apparatus was used to determine the friction and wear characteristics of two representative compounds, (1) 3-tri-n-butylstannyl (diphenyl) and (2) 3-tri-n-butylsilyl (diphenyl). Friction and wear test conditions included a 1-kg load, 25 to 225 C disk temperatures, and a dry air atmosphere. The tin and silicon compounds yielded friction and wear results either lower than or similar to those obtained with a polyphenyl ether and a C-ether. The maximum thermal decomposition temperatures obtained in the silicon and tin series were 358 and 297 C, respectively. Increasing the steric hindrance around the silicon or tin atoms increased the thermal stability. Future work with these compounds will emphasize their use as antiwear additives rather than base fluids.

Jones, W. R., Jr.↗

NASA NTRS1973

Adhesion, friction, and wear of a copper bicrystal with (111) and (210) grains

Sliding friction experiments were conducted in air with polycrystalline copper and ruby riders sliding against a copper bicrystal. Friction coefficient was measured across the bicrystal surface, and the initiation of adhesive wear was examined with scanning electron microscopy. Results indicate a marked increase in friction coefficient as the copper rider crossed the grain boundary from the (111) plane to the (210) plane of the bicrystal. Adhesion, friction, and initiation of adhesive wear was notably different in the adjacent grains of differing orientation. A slip-band adhesion-generated fracture mechanism for wear particle formation is proposed.

Brainard, W. A.↗

NASA NTRS1973

Rheological effects on friction in elastohydrodynamic lubrication

An analytical and experimental investigation is presented of the friction in a rolling and sliding elastohydrodynamic lubricated contact. The rheological behavior of the lubricant is described in terms of two viscoelastic models. These models represent the separate effects of non-Newtonian behavior and the transient response of the fluid. A unified description of the non-Newtonian shear rate dependence of the viscosity is presented as a new hyperbolic liquid model. The transient response of viscosity, following the rapid pressure rise encountered in the contact, is described by a compressional viscoelastic model of the volume response of a liquid to an applied pressure step. The resulting momentum and energy equations are solved by an iterative numerical technique, and a friction coefficient is calculated. The experimental study was performed, with two synthetic paraffinic lubricants, to verify the friction predictions of the analysis. The values of friction coefficient from theory and experiment are in close agreement.

Trachman, E. G.↗

NASA NTRS1971

Extended ion pumped vacuum friction test

Boundary layer friction data under ion pumped vacuum was taken for sixteen material couples. The test series was an extension of a previous study of the effects of modified ion pumped environments. Sliding distances imposed in the present effort greatly exceeded any studied in the previous contiguous, flight or ground tests. Wear out of specific couples, in particular, thin film lubricants was noted. The behavior of the test hardware including wear out of the mechanisms was noted. As a result, the impact of test interruption was observed for several test couples. Recovery of the friction upon re-establishing sliding in vacuum was generally rapid. The results of the extended sliding study reinforce the previous conclusion that sliding distance (mechanical history) is the primary factor in establishing the force limiting boundary layer friction. General friction value under the extended sliding confirm those observed in previous orbital and the related ground test studies.

Hammel, R. L.↗

NASA NTRS1973

Pulsatile Newtonian frictional losses in a rigid tube.

Analytic expressions for the instantaneous average velocity and friction factor have been obtained for pulsatile Newtonian laminar flow with mean flow. Experimental pulsatile pressure gradients were found to be complex wave forms whose characteristics strongly influenced the experimental pulsatile average velocity and friction factor. Experimental and theoretical reduced instantaneous friction factor histories varied in a non-sinusoidal manner throughout the pulse cycle and exhibited extremum points. At high pulsation pressure ratios, substantial deviation occurred in the portion of the pulse cycle where the instantaneous average velocity approached zero. At low pulsation pressure ratios, the experimental and theoretical time average friction factor equaled unity and was independent of pulsation modulus. One potential application of this result is in the study of blood flow through small blood vessels.

Phillips, E. M.↗

NASA NTRS1973

In-flight and laboratory vacuum-friction test results

Coefficient of friction measurements were made for six unlubricated metal couples exposed to the space environment aboard the OV-1-13 spacecraft and exposed to laboratory vacuum. Materials studied included mutually soluble, partially soluble, and insoluble metal combinations. Two samples of each material couple were tested in space and in the laboratory using the disk and rider technique. Linear velocity was 0.10 cm/s (2.5 in/min) and rider normal load was 4.45 N (1 lb) for the gold versus silver couples and 8.90 N (2lb) for the other combinations. Results showed that friction data obtained in a clean ion-pumped laboratory vacuum of 10 to the minus 10 power materials with low mutual solubility can be correlated to operation in the vicinity of a typical scientific spacecraft that is exposed to an ambient pressure as low as 10 to the minus 12 power torr. The expected increase in coefficient of friction with solubility was shown. Material couples with high mutual solubility present the hazard of unpredictable drastic friction increase in orbit which may not be evident in laboratory testing at levels down to 10 to the minus 10 power torr. It was also shown that gross cold welding of unlubricated metals exposed to a satellite environment does not occur.

Devine, E. J.↗

NASA NTRS1974

Effect of strain hardening on friction behavior of iron lubricated with benzyl structures

Sliding friction experiments were conducted with iron, copper, and aluminum in contact with iron in various states of strain. The surfaces were examined in dry sliding and with various benzyl compounds applied as lubricants. Friction experiments were conducted with a hemispherical rider contacting a flat disk at loads of from 50 to 600 grams with a sliding speed of 0.15 cm/min. Results indicate that straining increases friction for dry sliding and for surfaces lubricated with certain benzyl structures such as dibenzyl disulfide. With other benzyl compounds (e.g., benzyl formate), friction coefficients are lower for strained than for annealed iron.

Buckley, D. H.↗

NASA NTRS1974

Influence of low temperatures of friction and wear

A brief review of studies on low-temperature friction are briefly reviewed. A facility and technique for studying friction both in air and in vacuum at temperatures of +20 and -190 C is described. Results of a wear study of structural steels operating together with chromium steel are presented. It is shown that reduction of the test temperature in vacuum leads in certain cases to marked increase of the wear magnitude and friction coefficient. This is associated not only with the general change of the mechanical properties of the materials but also with the influence of temperature reduction on the hardening and structural formation taking place in the surface layers during friction.

Udovenko, V. F.↗