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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Parachute Swivel Mechanism for planetary entry

A parachute swivel mechanism (PSM) for planetary entry missions such as a Mars probe (MARSNET) or return of cometary material samples (ROSETTA mission) has been developed. The purpose of the PSM is to decouple the spin of the probe from the parachute, with low friction torque, during both the deployment and descent phases. Critical requirements are high shock loads, low friction, low temperatures, and several years of storage in the deep space environment (during the cruise phase of the probe, prior to operation). The design uses a main thrust ball bearing to cope with the load requirement and a smaller thrust ball bearing for guiding of the shaft. Except for use on the Viking and Galileo swivels, it appears that this type of bearing has very rarely been employed in space mechanisms, so that little is known of its friction behavior with dry lubrication. A slip ring assembly allows the transfer of electrical power for post-reefing of the parachute. A test program has been conducted covering the environmental conditions of Mars entry and Earth reentry. This paper describes requirement constraints, model missions of planetary entries, a bearing trade-off, analyses performed, design details, the lubrication system, and test results (friction torque versus load/spin rate). In addition, the design of the test rig is addressed.

Birner, R.↗
NASA NTRS1994

Development and Preliminary Evaluation of Aryl Ester Boundary Additives for Perfluoropolyethers

A series of additives were developed for evaluation as boundary lubrication enhancers for perfluoropolyethers. They are composed of a hydrocarbon aryl component (for lubrication improvement) and a fluorinated side chain (for solubility enhancement). The two moieties are joined by an ester linkage. Five boundary additives were evaluated in a perfluoropolyether basestock (Fomblin Z25) using a specially designed four-ball apparatus. Additives were evaluated at a one wt percent concentration. Conditions included: an atmosphere of dry air at atmospheric pressure, a 200N load, a speed of 100 rpm, room temperature, and 440C stainless steel specimens. Two monoesters, 2,4,6-trimethyl E2 and H5PDFO yielded wear rate reductions of approximately 60 and 35 percent, respectively. One diester, H4(E2)(sub 2), had no activity, while two other diesters (a diester of bisphenol A and H4(E4)(sub 2)) were pro-wear.

Williams, John R.↗
NASA NTRS1994

Development of long-life, low-noise linear bearings for atmospheric interferometry

This paper describes the development of dry-lubricated linear bearings for use on the Michelson interferometer for passive atmospheric sounding (MIPAS). Two candidate bearing systems were developed and tested. In the first, use was made of linear roller (needle) bearings equipped with a pulley-and-cable arrangement to prevent cage drift and to minimize roller slip. The second design was of a roller-guided bearing system in which guidance was provided by all bearings rolling along guide rods. The paper focuses on the development of these linear bearings systems and describes the approach taken in terms of bearing design, lubrication methods, screening programs, and thermal-vacuum testing. Development difficulties are highlighted and the solutions ultimately adopted are described.

Roberts, E. W.↗
NASA NTRS1994

Design of a high-speed reliable ball bearing

A high-speed, reliable ball bearing has been designed for at least fifteen years of operation in space effectors, MWA's, and RWA's. Advanced bearing concepts have been used in this design, such as: no ball retainer, which eliminates all retainer-related problems; an external lubricazing system that will supply the lubricant at a specified flow rate; and a cartridge assembly that will allow the instrument user to purchase a ready to use bearing assembly, with lubricator. Currently, two assemblies are on life test at 12,000 RPM and have accumulated over 20,000 hours each, with consistent low-torque losses. This paper will describe each of the salient features.

Singer, Herbert B.↗
NASA NTRS1994

A waved journal bearing concept with improved steady-state and dynamic performance

Analysis of the waved journal bearing concept featuring a waved inner bearing diameter for use with a compressible lubricant (gas) is presented. A three wave, waved journal bearing geometry is used to show the geometry of this concept. The performance of generic waved bearings having either three, four, six, or eight waves is predicted for air lubricated bearings. Steady-state performance is discussed in terms of bearing load capacity, while the dynamic performance is discussed in terms of dynamic coefficients and fluid film stability. It was found that the bearing wave amplitude has an important influence on both steady-state and dynamic performance of the waved journal bearing. For a fixed eccentricity ratio, the bearing steady-state load capacity and direct dynamic stiffness coefficient increase as the wave amplitude increases. Also, the waved bearing becomes more stable as the wave amplitude increases. In addition, increasing the number of waves reduces the waved bearing's sensitivity to the direction of the applied load relative to the wave. However, the range in which the bearing performance can be varied decreases as the number of waves increases. Therefore, both the number and the amplitude of the waves must be properly selected to optimize the waved bearing design for a specific application. It is concluded that the stiffness of an air bearing, due to the hydrodynamic effect, could be doubled and made to run stably by using a six or eight wave geometry with a wave amplitude approximately half of the bearing radial clearance.

Dimofte, Florin↗
NASA NTRS1994

Properties of Perfluoropolyethers for Space Applications

The perfluoropolyether (PFPE) class of liquid lubricants has been used for space applications for over two decades. At first, these fluids performed satisfactorily as early spacecraft placed few demands on their performance. However, as other spacecraft components have become more reliable and lifetimes have been extended, PFPE lubricant deficiencies have been exposed. Therefore, the objective of this paper is to review the PFPE properties that are important for successful long term operation in space.

Jones, William R., Jr.↗
NASA NTRS1994

Hybrid bearings for turbopumps and the like

In rocket engines power is usually obtained by burning fuel and oxidizer which are mixed, pressurized, and directed to a combustion chamber by means of turbopumps. Roller bearings are generally used in these turbopumps, but because of bearing demands hydrostatic bearings were proposed. The use of such bearings is quite feasible because during flight hydrostatic lubrication can reduce roller bearing wear. A disadvantage of such proposals is that during startup, acceleration, and shutdown high pressure fluids are not available for hydrostatic bearings. The fluid lubrication film is not always present in bearings of turbopumps. During these periods a second bearing is required to carry the load. This requirement suggests the use of hybrid bearings in rocket engine turbopumps. Such duplex bearings were provided, but when their inner races are keyed to the shaft or journal two of them are required. And such duplex bearings do not wear evenly. A hybrid hydrostatic-rolling element bearing was provided wherein the rolling element bearing is locked on the stationary housing rather than on the rotating journal.

Justak, John F.↗
NASA NTRS1994

Experimental analysis of thread movement in bolted connections due to vibrations

The objective of this study was to identify the main design parameters contributing to loosening of bolts due to vibration and to identify their relative importance and degree of contribution to bolt loosening. Vibration testing was conducted on a shaketable with a controlled-random input in the dynamic testing laboratory of the Structural Test Division of MSFC. Test specimens which contained one test bolt were vibrated for a fixed amount of time and percentage of pre-load loss was measured. Each specimen tested implemented some combination of eleven design parameters as dictated by the design of experiment methodology employed. The eleven design parameters were: bolt size (diameter), lubrication on bolt, hole tolerance, initial pre-load, nut locking device, grip length, thread pitch, lubrication between mating materials, class of fit, joint configuration and mass of configuration. These parameters were chosen for this experiment because they are believed to be the design parameters having the greatest impact on bolt loosening. Two values of each design parameter were used and each combination of parameters tested was subjected to two different directions of vibration and two different g-levels of vibration. One replication was made for each test to gain some indication of experimental error and repeatability and to give some degree of statistical credibility to the data, resulting in a total of 96 tests being performed. The results of the investigation indicated that nut locking devices, joint configuration, fastener size, and mass of configuration were significant in bolt loosening due to vibration. The results of this test can be utilized to further research the complex problem of bolt loosening due to vibration.

Ramey, G. ED↗
NASA NTRS1994

Tribology and Microstructure of PS212 with a Cr2O3 Seal Coat

PS212 is a plasma sprayed metal bonding chrome carbide coating with solid lubricant additives which has lubricating properties at temperatures up to about 900 deg C. The coating is diamond ground to achieve an acceptable tribological surface. But, as with many plasma spray coatings, PS212 is not fully-dense. In this study, a chromium oxide base seal coating is used in an attempt to seal any porosity that is open to the surface of the PS212 coating, and to study the effect of the sealant on the tribological properties of PS212. The results indicate that the seal coating reduces friction and wear when it is applied and then diamond ground leaving a thin layer of seal coating which fills in the surface pits of the PS212 coating.

Sliney, Harold E.↗
NASA NTRS1994

Friction and Wear Properties of As-deposited and Carbon Ion-implanted Diamond Films

Recent work on the friction and wear properties of as-deposited and carbon ion-implanted diamond films was reviewed. Diamond films were produced by the microwave plasma chemical vapor deposition (CVD) technique. Diamond films with various grain sizes and surface roughnesses were implanted with carbon ions at 60 ke V ion energy, resulting in a dose of 1.2310(exp 17) carbon ions/cm(exp 2). Various analytical techniques, including Raman spectroscopy, proton recoil analysis, Rutherford backscattering, transmission and scanning electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction, were utilized to characterize the diamond films. Sliding friction experiments were conducted with a polished natural diamond pin in contact with diamond films in the three environments: humid air (40 percent relative humidity), dry nitrogen (less than 1 percent relative humidity), and ultrahigh vacuum (10(exp -7) Pa). The CVD diamond films indeed have friction and were properties similar to those of natural diamond in the three environments. The as-deposited, fine-grain diamond films can be effectively used as self-lubricating, wear-resistant coatings that have low coefficients of friction (0.02 to 0.04) and low wear rates (10(exp -7) to 10(exp -8)mm(exp 3)/N-m) in both humid air and dry nitrogen. However, they have high coefficients of friction (1.5 to 1.7) and a high wear rate (10(exp -4)mm(exp 3/N-m) in ultrahigh vacuum. The carbon ion implanation produced a thin surficial layer (less than 0.1 micron thick) of amorphous, nondiamond carbon on the diamond films. In humid air and dry nitrogen, the ion-implanted, fine- and coarse-grain diamond films have a low coefficient of friction (around 0.1) and a low wear rate (10(exp -7)mm(exp 3/N-m). Even in ultrahigh vacuum, the presence of the nondiamond carbon layer reduced the coefficient of friction of fine-grain diamond films to 0.1 or lower and the wear rate to 10(exp -6)mm(exp 3)/N-m. Thus, the carbon ion-implanted, fine-grain diamond films can be effectively used as wear-resistant, self-lubricating coatings not only in air and dry nitrogen, but also in ultrahigh vacuum. The wear mechanism of diamond films is that of small fragments chipping off the surface. The size of wear particles is related to the extent of wear rates.

Miyoshi, Kazuhisa↗
NASA NTRS1995

Experimental analysis of thread movement in bolted connections due to vibrations

This is the final report of research project NAS8-39131 #33 sponsored by NASA's George C. Marshall Space Flight Center (MSFC) and carried out by the Civil Engineering Department of Auburn University (Auburn, Alabama) and personnel of MSFC. The objective of this study was to identify the main design parameters contributing to the loosening of bolts due to vibration and to identify their relative importance and degree of contribution to bolt loosening. Vibration testing was conducted on a shaketable with a controlled-random input in the dynamic testing laboratory of the Structural Test Division of MSFC. Test specimens which contained one test bolt were vibrated for a fixed amount of time and a percentage of pre-load loss was measured. Each specimen tested implemented some combination of eleven design parameters as dictated by the design of experiment methodology employed. The eleven design parameters were: bolt size (diameter), lubrication on bolt, hole tolerance, initial pre-load, nut locking device, grip length, thread pitch, lubrication between mating materials, class of fit, joint configuration, and mass of configuration. These parameters were chosen for this experiment because they are believed to be the design parameters having the greatest impact on bolt loosening. Two values of each design parameter were used and each combination of parameters tested was subjected to two different directions of vibration and two different g-levels of vibration. One replication was made for each test to gain some indication of experimental error and repeatability and to give some degree of statistical credibility to the data, resulting in a total of 96 tests being performed. The results of the investigation indicated that nut locking devices, joint configuration, fastener size, and mass of configuration were significant in bolt loosening due to vibration. The results of this test can be utilized to further research the complex problem of bolt loosening due to vibration.

Ramsey, G. ED↗
NASA NTRS1995

Some mechanical design aspects of the European Robotic Arm

The European Robotic Arm (ERA) is a contribution to the Russian Segment of the International Space Station Alpha. It will start operating on the Russian Segment during the assembly phase. ERA is designed and produced by a large industrial consortium spread over Europe with Fokker Space & Systems as prime contractor. In this paper, we will describe some of the overall design aspects and focus on the development of several mechanisms within ERA. The operation of ERA during the approach of its end effector towards the grapple interface and the grapple operation is discussed, with a focus on mechanisms. This includes the geometry of the end effector leading edge, which is carefully designed to provide the correct and complete tactile information to a torque-force sensor (TFS). The data from this TFS are used to steer the arm such that forces and moments are kept below 20 N and 20 N.m respectively during the grappling operation. Two hardware models of the end effector are built. The problems encountered are described as well as their solutions. The joints in the wrists and the elbow initially used a harmonic drive lubricated by MoS2. During development testing, this combination showed an insufficient lifetime in air to survive the acceptance test program. The switch-over to a system comprising planetary gearboxes with grease lubrication is described. From these development efforts, conclusions are drawn and recommendations are given for the design of complex space mechanisms.

Lambooy, Peter J.↗
NASA NTRS1995

Properties of Perfluoropolyethers for Space Applications

The perfluoropolyether (PEPE) class of liquid lubricants has been used for space applications for over two decades. At first, these fluids performed satisfactorily, as early spacecraft placed few demands on their performance. However, as other spacecraft components have become more reliable and lifetimes have been extended, PEPE lubricant deficiencies have been exposed. Therefore, the objective of this paper is to review the PEPE properties that are important for successful long term operation in space.

Jones, William R., Jr.↗
NASA NTRS1995

High-Performance Ball Bearing

High-performance bearing features strong, lightweight, self-lubricating cage with self-lubricating liners in ball apertures. Designed to operate at high speed (tens of thousands of revolutions per minute) in cryogenic environment like liquid-oxygen or liquid-hydrogen turbopump. Includes inner race, outer race, and cage keeping bearing balls equally spaced.

Bursey, Roger W., Jr.↗
NASA NTRS1995

Experiments On Vibration-Induced Loosening Of Bolts

Report describes experimental study of loosening of bolts in presence of vibrations. Eleven design parameters considered likely to affect tendency to loosen evaluated for relative importance. Parameters were bolt diameter, lubrication on bolt, lubrication on mating materials, bolt-hole tolerance, preload, nut-locking device, grip length, thread pitch, class of fit, joint configuration, and mass of configuration. Bolts tested in vibration for two values of each design parameter.

Ramey, G. ED↗
NASA NTRS1995

Space environmental effects on spacecraft: LEO materials selection guide, part 1

This document provides performance properties on major spacecraft materials and subsystems that have been exposed to the low-Earth orbit (LEO) space environment. Spacecraft materials include metals, polymers, composites, white and black paints, thermal-control blankets, adhesives, and lubricants. Spacecraft subsystems include optical components, solar cells, and electronics. Information has been compiled from LEO short-term spaceflight experiments (e.g., space shuttle) and from retrieved satellites of longer mission durations (e.g., Long Duration Exposure Facility). Major space environment effects include atomic oxygen (AO), ultraviolet radiation, micrometeoroids and debris, contamination, and particle radiation. The main objective of this document is to provide a decision tool to designers for designing spacecraft and structures. This document identifies the space environments that will affect the performance of materials and components, e.g., thermal-optical property changes of paints due to UV exposures, AO-induced surface erosion of composites, dimensional changes due to thermal cycling, vacuum-induced moisture outgassing, and surface optical changes due to AO/UV exposures. Where appropriate, relationships between the space environment and the attendant material/system effects are identified. Part 1 covers spacecraft design considerations for the space environment; advanced composites; polymers; adhesives; metals; ceramics; protective coatings; and lubricants, greases, and seals.

Silverman, Edward M.↗
NASA NTRS1996

Cryogenic Brush Seal Test Results

Brush seals are compliant, contact seals that have long-life, low-leakage characteristics desirable for use in rocket engine turbopumps. 50.8-mm (2.0 inch) diameter brush seals with a nominal initial radial interference of 0.127-mm (0.005 inch) were tested in liquid nitrogen at shaft speeds up to 35,000 rpm and differential pressure loads up to 1.21 MPa (175 psi) per brush. The measured leakage rate of a single brush was 2-3 times less than that measured for a 12-tooth, 0.127-mm (0.005 inch) radial clearance labyrinth seal used as a baseline. Stage effects were studied and it was found that two brush seals with a large separation distance leaked less than two brushes tightly packed together. The maximum measured groove depth on the Inconel 718 rotor was 25.4 (mu)m (0.001 inch) after 4.31 hours of shaft rotation. The Haynes-25 bristles wore approximately 25.4-76.2 (mu)m (0.001-0.003 inch) under the same conditions. Three seal runner coatings, chromium carbide, Teflon impregnated chromium, and zirconium oxide, were tested in liquid hydrogen at 35,000 and 65,000 rpm with separate 50.8 mm diameter brush seals made of Haynes-25 bristles and having a nominal initial radial interference of 129 rpm. Two bare Inconel-718 rotors were also tested as a baseline. The test results revealed significant differences between the wear characteristics of the uncoated and coated seal runners. At both speeds the brush seal with the bare Inconel-718 seal runner exhibited significant bristle wear with excessive material transferring to the runner surface. In contrast, the coated seal runners inhibited the transfer and deposit of bristle material. The chromium carbide coating showed only small quantities of bristle material transferring to its surface. The Teflon impregnated chromium coating also inhibited material transfer and provided some lubrication. This coating, however, is self-sacrificing. The Teflon remained present on the low speed runner, but it was completely removed from the high speed brush seal, which was tested considerably longer. The tests of the Teflon coating revealed the importance of using a lubricating and low friction coating for brush seals to reduce bristle and seal runner wear. The zirconium oxide coating exhibited the greatest amount of coating wear, while the brushes incurred only slight wear. Further testing of ceramics is recommended before making a final judgement on the viability of ceramic coatings for brush seals because of the contrast between the results reported by Carlile and the results presented herein. Strictly based on the results presented hereinabove, the chromium carbide and Teflon impregnated chromium coatings were considered preferable to the uncoated Inconel-718 and zirconium oxide coatings because of their good wear resistance and characteristics to inhibit bristle material wear and transfer to the seal runner.

Proctor, Margaret P.↗
NASA NTRS1996

Friction and Wear of Ion-Beam-Deposited Diamondlike Carbon on Chemical-Vapor-Deposited, Fine-Grain Diamond

Friction and wear behavior of ion-beam-deposited diamondlike carbon (DLC) films coated on chemical-vapor-deposited (CVD), fine-grain diamond coatings were examined in ultrahigh vacuum, dry nitrogen, and humid air environments. The DLC films were produced by the direct impact of an ion beam (composed of a 3:17 mixture of Ar and CH4) at ion energies of 1500 and 700 eV and an RF power of 99 W. Sliding friction experiments were conducted with hemispherical CVD diamond pins sliding on four different carbon-base coating systems: DLC films on CVD diamond; DLC films on silicon; as-deposited, fine-grain CVD diamond; and carbon-ion-implanted, fine-grain CVD diamond on silicon. Results indicate that in ultrahigh vacuum the ion-beam-deposited DLC films on fine-grain CVD diamond (similar to the ion-implanted CVD diamond) greatly decrease both the friction and wear of fine-grain CVD diamond films and provide solid lubrication. In dry nitrogen and in humid air, ion-beam-deposited DLC films on fine-grain CVD diamond films also had a low steady-state coefficient of friction and a low wear rate. These tribological performance benefits, coupled with a wider range of coating thicknesses, led to longer endurance life and improved wear resistance for the DLC deposited on fine-grain CVD diamond in comparison to the ion-implanted diamond films. Thus, DLC deposited on fine-grain CVD diamond films can be an effective wear-resistant, lubricating coating regardless of environment.

Miyoshi, Kazuhisa↗