Showing papers in "Tribology Transactions in 2004"

The Effect of Laser Texturing of Steel Surfaces and Speed-Load Parameters on the Transition of Lubrication Regime from Boundary to Hydrodynamic

Abstract: Laser surface texturing (LST) is an emerging, effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were carried out on pin-on-disk test apparatus at sliding speeds that ranged from 0.15 to 0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two types of oil with different viscosities (54.8 cSt and 124.7 cSt at 40°C) were evaluated as lubricants. Electrical resistance between flat-pin and laser-textured disks was used to determine the operating lubrication regime. The test results showed that laser texturing expanded the range of speed-load parameters for hydrodynamic lubrication. LST also reduced the m...

Numerical Analysis of a Slider Bearing with a Heterogeneous Slip/No-Slip Surface

Abstract: The behavior of a fluid-film bearing depends on the boundary conditions at the interfaces between the liquid and the solid bearing surfaces. For almost all solid surfaces, the no-slip boundary condition applies. However, a number of researchers have recently found that slip can occur with specially engineered surfaces. These include molecularly smooth surfaces and surfaces with micron-scale patterns. By constructing an engineered heterogeneous surface on which slip occurs in certain regions and is absent in others, the flow in the liquid film of a bearing can be altered, and such characteristics as load support and friction can be improved. In the present study, a numerical analysis of a slider bearing with such an engineered slip/no-slip surface is analyzed. Slip is assumed to occur when a critical shear stress is exceeded and follows the Navier relation. The results show that with a critical shear stress of zero, a significant increase in load support and decrease in friction can be achieved with an app...

Tribometrology of skin

Abstract: The quantitative assessment of both skin health and skin care products is suggested based on skin tribological properties. Simultaneous multi-sensor measurements of both coefficient of friction and contact electrical impedance allow for fast and quantitative evaluation of skin conditions such as dryness and moisturization, and early diagnosis of skin diseases or of the deterioration in skin functions at a stage that may not be easily discernable visibly. It may be instrumental in developing and testing skin cosmetics and medicine.

Thermohydrodynamic Behavior of Misaligned Plain Journal Bearings: Theoretical and Experimental Approaches

Abstract: Hydrodynamic journal bearings are essential components for supporting and guiding the rotating shafts of high-speed machinery. Manufacturing defects in assembly or thermal distortions may introduce problems during running, such as misalignment. The destructive effects of this kind of running problem have justified the development of a numerical model to predict the bearing operating characteristics under steady-state conditions. The present work presents in detail the three-dimensional thermohydrodynamic approach adopted in this study in order to consider the thermal field variations. This model also includes lubricant film rupture and reformation phenomena by conserving the mass flow rate. In addition, an experimental validation is made by comparison with measurements carried out on our test device for various operating conditions and misalignment torques. The influence of misalignment direction is also investigated by considering numerical and experimental approaches used in the study of bearing behavio...

Journal Design Considerations for Turbomachine Shafts Supported on Foil Air Bearings

Abstract: Foil air bearings can offer substantial improvements over traditional rolling element bearings in many applications and are attractive as a replacement to enable the development of advanced oil-free turbomachinery. In the course of rigorous testing of foil journal bearings at NASA Glenn Research Center, shaft failure was repeatedly encountered at high ambient temperature and rotational speed, with moderate radial load. The cause of failure is determined to be excessive non-uniform shaft growth, which increases localized viscous heating in the gas film and eventually leads to a high-speed rub and destruction of the bearing and journal. Centrifugal loading of imbalance correction weights and axial temperature gradients within the journal due to the hydrodynamic nature of the foil bearings, determined by experiment and finite element analysis, are shown to be responsible for the non-uniform growth. Qualitative journal design guidance is given to aid in failure prevention.

Oxidation Stability and Tribological Behavior of Vegetable Oil Hydraulic Fluids

Abstract: The new generation of vegetable oil hydraulic fluids has a price advantage and provides field longevity comparable to synthetic esters at moderate temperature. This article investigates the oxidation stability and tribological behavior of fully formulated vegetable oil hydraulic fluids. Differences in origin (canola, soybean, and sunflower oil) and the degree of unsaturation are studied. Modified ASTM D943 (Dry TOST) was found reliable to evaluate the impact on oxidative stability of various additive packages in vegetable oils. Tribological testing utilized laboratory hydraulic pumps such as the Vickers V 104C, Vickers 20VQ, and Vickers PFB-5 under a variety of temperatures, pressures, pump outputs, and speeds. The conditions of aging fluids were monitored by evaluation of TAN, viscosity, wear rate, and depletion of antioxidants. Voltammetric technique was applied to determine the concentration of antioxidant in systems. Based on analysis of antioxidant consumption, the main degenerative process in vegeta...

A Rough Shear-Thinning Correction for EHD Film Thickness

Abstract: A film thickness correction for shear-thinning in elastohydrodynamic contacts has been available that requires parameters obtained from flow curves generated for the specific liquid. For instances for which reduced accuracy is acceptable, a new rough film thickness correction has been formulated for pure rolling and a moderate amount of sliding based on Van Krevelen's graphical technique of estimating shear-thinning from the molecular weight. It has been shown to be useful for high-molecular-weight base oils, when lubricant specific shear-thinning data is absent, although it should most often overestimate the film thinning.

Simulation and Control of an Active Tilting-Pad Journal Bearing

Abstract: This study developed an active tilting-pad journal bearing with a feedback control system to regulate the orbit of a rotating shaft. The control is implemented by means of linear actuators installe...

Operating Limits for Acoustic Measurement of Rolling Bearing Oil Film Thickness

Abstract: An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected. Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer...

Interaction of ZDDP with Borated Dispersant Using XANES and XPS

Abstract: The thermochemical reaction and tribochemical reaction of zinc dialkyldithiophosphate (ZDDP), a borated dispersant, and the mixture of ZDDP and borated dispersant on steel surfaces were investigated. Both pin-on-disk and ball-on-disk were used to generate tribofilms. The chemical state of nitrogen, boron, phosphorus, and sulfur in heated oil solutions, thermal films, and tribofilms were analyzed by X-ray absorption near edge structure (XANES) spectroscopy to obtain the chemical nature of species on the surface and in the bulk of the films. High-resolution X-ray photoelectron spectroscopy (XPS) has also been used to analyze boron (B) in tribofilms. The borated dispersant in base oil by itself yields good anti-wear behavior. This can be attributed to the presence of boron in the dispersant. The wear scar widths (WSW) for ZDDP alone, and in combination with the dispersant, yield similar results within the experimental error. It was found that the borated dispersant facilitates the decomposition of ZDDP and t...

Calculation of journal bearing dynamic characteristics including journal misalignment and bearing structural deformation

Abstract: A detailed journal bearing analysis for accurate evaluation of film dynamic characteristics is presented. The new formulation is based on a local perturbation of the oil film at each computational node that captures the important effects of journal misalignment and bearing structural deformation in rotor dynamics and engine NVH applications. The new algorithm is an extension to the classical approach of evaluating film dynamic characteristics based on journal eccentricity perturbation. The governing equations for the oil film pressure, stiffness, and damping are solved using a finite difference approach and their output is validated with numerical results from the literature.

Influence of Cage Clearance on Bearing Lubrication

Abstract: The effect of the cage clearance on the lubricant supply and elastohydrodynamic (EHL) film thickness has been studied in a ball-on-disc device. A single pocket from a standard nylon cage was mounted around the ball. The cage was instrumented so that the clearance between the cage and ball could be altered. Film thickness measurements were made with and without the cage present and for different clearances. Two lubricants were tested: a lithium hydroxystearate grease and its base oil. Film thickness was measured with increasing speed to determine the onset of lubricant starvation. Without a cage present the grease lubricated contact starved at a very low speed, typically 0.02 m/s and the film thickness dropped to a fraction of the fully flooded value. Starvation did not occur within the speed range for the base oil. The presence of the cage significantly changed the starvation response. For the base oil reducing the clearance induced starvation by locally removing the lubricant from the track. The grease g...

Wear Testing of Seals in Magneto-Rheological Fluids©

Abstract: Magneto-rheological (MR) fluid-based dampers can be used in suspensions to modify ride, handling, and suspension frequencies in autos. We have developed a unique seal wear test method to simulate the wear of rod seals in Magneto-rheological fluid-based dampers. In this test, seal material samples in the form of rectangular coupons are immersed in a bath of MR fluid, and a section of the rod is brought into contact with the seal coupon. The rod is loaded against the coupon with a known normal force, and a variable-frequency reciprocating machine is used to stroke the rod back and forth across the material coupon surface. Test conditions such as normal load, frequency and amplitude of oscillation, and temperature are adjusted to simulate desired conditions. This bench test method ranks various seal materials qualitatively in the correct order (as determined in field tests with these seal materials) in terms of wear and abrasion resistance and constitutes a valid test procedure for the screening of seal mate...

Modeling of Abrasive Wear in a Piston Ring and Engine Cylinder Bore System

Abstract: Engine-related improvements such as more efficient engine components, improved engine oils, and high-performance coating materials, need to be verified in terms of their effects on the tribological performance of the piston ring/cylinder bore system. The main purpose of this research is to develop an abrasive wear model for the piston ring/cylinder bore system during steady-state operation by considering the effects of temperature, load, oil degradation, surface roughness, and material properties. The model can be used either in theoretical modeling or integrated with finite element analysis. Based on a laboratory simulator, a three-body abrasive wear model has been developed to model the wear progression of the piston ring/cylinder bore system during steady state operation. The proposed novel abrasive wear model addresses the effects of temperature, load, oil degradation, surface roughness, and material properties. The feasibility of the proposed model is illustrated by a numerical example.

Elastohydrodynamic Film Thickness and Tractions for Oil-in-Water Emulsions

Abstract: Emulsions, consisting of a small volume of oil dispersed in water in the form of small particles, are popular lubricants for metal rolling and some machine design applications. A number of mechanisms have been suggested for the lubricating behavior of emulsions, among which plate-out, starvation, and dynamic concentration are of particular interest here. At low speeds, the emulsion provides essentially the same lubricating ability as neat oil for a point contact, consistent with plate-out. At some critical speed, the emulsion behavior departs from the neat oil, associated with starvation of the inlet zone. At a second critical speed, dynamic concentration becomes the important mechanism. This article measures the film thickness and traction coefficients of oil-in-water emulsions in the different regimes of behavior and compares the results to existing theoretical understanding. The effect of droplet size is isolated as a causative element in fluid film formation.

Unbalance Response of a Super-Critical Rotor Supported by Foil Bearings—Comparison with Test Results©

Abstract: This article compares theoretical and experimental results on the unbalance response of a super-critical rotor supported by two kinds of gas foil bearings. Analysis of the super-critical rotor is performed with viscoelastic foil bearings (VEFB) and conventional bump foil bearings and compared with early published experimental data. The vibration orbits obtained by theoretical investigation on the viscoelastic foil bearings (VEFB) and the conventional bump foil bearings are compared with the experimental vibration orbits of the super-critical rotor supported by both foil bearings. A numerical analysis program can calculate the static and dynamic characteristics of the elastically supported gas foil bearing using the measured stiffness and damping of the elastic foundation. After the stiffness and damping of both foil bearings are calculated using the perturbation method, vibration orbits of the flexible rotor modeled by the finite element method are calculated. Vibration orbits calculated by numerical anal...

The Structural and Tribological Properties of MoS2-Ti Composite Solid Lubricants

Abstract: MoS 2 -Ti composite solid lubricant films were deposited on an AISI D2 tool steel and silicon wafer by CFUBMS (closed field unbalanced magnetron sputtering). The deposition process was performed fo...

Transmission Electron Microscopy of Boundary-Lubricated Bearing Surfaces. Part I: Mineral Oil Lubricant

Abstract: Transmission electron microscopy (TEM) was performed on the near-surface material (depth <500 nm) of tapered roller bearing inner rings (cones) that were tested at two levels of boundary-lubricated conditions in mineral oil with no additives. Site-specific thinning of cross section cone surface sections for TEM analyses was conducted using the focused ion beam (FIB) milling technique. High-resolution structural and compositional characterization of near-surface material and surface layers was performed on an untested cone as well as cones tested at Λ∼1.1 and 0.3. This approach revealed near-surface microstructural distortion and grain size gradients that were attributed to surface finishing operations during manufacture. The characteristics of oxide surface layers and micro-cracks on the tested bearing surfaces were evaluated and found to depend on lubrication conditions.

Study on Sludge Formation during the Oxidation Process of Turbine Oils

Abstract: Against the backdrop of increased needs for longer operating life of turbine oils, there is a tendency to use amine-type antioxidants for steam turbine oils as well as gas turbine oils. Amine-type ...

A Routine High-Pressure Viscometer for Accurate Measurements to 1 GPa

Abstract: Of the fields for which the piezoviscous property of liquids is of interest, elastohydrodynamics is unique in deriving its existence from this property. Even recent publications in the field, however, have based the description of the pressure variation of viscosity on myths rather than empirical data. The author offers a description of the construction and operation of an easy-to-use viscometer capable of accurate measurements to a pressure of 1 GPa. The classic measurements of Nobel laureate P. W. Bridgman are repeated and examples of measurements on lubricants are given.

Reduced phosphorus concentration effects on tribological performance of passenger car engine oils

Abstract: Phosphorus is present in engine oils in the form of the antiwear and antioxidation additive zinc dialkyldithiophosphate (ZDDP). Its effects on wear and friction were studied at different temperatures using a high-frequency reciprocating rig (HFRR). The electrically insulating tribofilm formation was measured using an electrical contact resistance (ECR) technique. The wear and friction performance of a fully formulated fresh oil containing 0.05 wt% phosphorus was compared with the corresponding used oil drained from a vehicle. The results show that the wear performance of fresh oils having phosphorus concentration from 0.02 to 0.1 wt% is very similar. Further reduction of phosphorus concentration below 0.02 wt% leads to high wear. The coefficient of friction increases with increased phosphorus concentration at temperatures above 80°C but decreases with increased phosphorus concentration at temperatures below 80°C. The used oil and the fresh 0 wt% P oil running on the original fresh steel surface exhibit hi...

Theoretical Analyses and Design Guidelines of Oil-Film-Lubricated Mechanical Face Seals with Spiral Grooves

Abstract: The oil-film-lubricated mechanical face seal is a kind of pure hydrodynamic lubricated noncontacting mechanical face seal with zero leakage. On the basis of systematic theoretical analyses, the design calculation formulas under zero-leakage condition for mechanical face seals with different spiral groove patterns, including double-row spiral grooves such as splay pattern and single-row spiral grooves, are derived. The effects of groove geometry including number of grooves, balance ratio, spring force, rotating speed, and differential pressure on the seal performance are discussed in detail. Finally, the design guidelines of this kind of seal with zero leakage and no wear are put forward. The seals designed according to the guidelines can withstand the pressure disturbance and speed change by means of a slight change of oil-film thickness. Seals developed according to the design guidelines have been tested on a test rig in detail and applied successfully in high-speed turbocompressors of the oil refinery a...

Measured Performance of Tilting-Pad Journal Bearings over a Range of Preloads—Part I: Static Operating Conditions

Abstract: This article presents the measured static characteristics for five tilting-pad journal bearings with a range of preload factors. The bearings had five shoes with static forces applied both on the pad and between pads. The journal diameter was 70 mm, with a length-to-diameter ratio of 0.75, a pivot offset ratio of 0.50, and a nominal assembled radial clearance of 81.3 μm. The rocker-backed pads have nominal radial machined clearances of 176.8, 114.3, 91.4, 73.7, and 61.0 μm. These clearances constitute a range of preloads from −0.333 to 0.540. Most tilting pad bearings are intentionally designed with positive preloads, although with pivot wear and/or pad and pivot distortion or machining tolerances, bearings can operate with zero or negative preloads. Measured results for bearings with negative preload factors have not previously been available. Measurements reported here were made for a variety of speed-load conditions encompassing a Sommerfeld number of 0.1 to 4.5. The measured parameters are eccentricit...

Comparison of the Dynamic Behavior and Lubrication Characteristics of a Reciprocating Compressor Crankshaft in Both Finite and Short Bearing Models

Abstract: The compression force of refrigerant gas, the viscous and inertial force of the piston, and the centrifugal force of balancer weight induce rotating whirl of the crankshaft in a small reciprocating compressor. It is necessary to develop an analytical model for the accurate prediction of dynamic behavior of the compressor mechanism having coupled characteristics between the piston and crankshaft. The reciprocating compression mechanism is dynamically modeled by considering the viscous frictional force of a piston and the variation in the contact length of the piston-cylinder system, and then numerical analysis is performed for the coupled dynamic behavior of the piston and crankshaft. For the accurate predictions of the dynamic behavior and characteristics of lubrication of the crankshaft-journal bearing system, a finite bearing model is adopted. In addition, the dynamic trajectory and characteristics of lubrication of the crankshaft such as power consumption and oil leakage are compared between the finite...

Ultra-High-Speed Performance of Ball Bearings and Annular Seals in Liquid Hydrogen at Up to 3 Million DN (120,000 rpm)©

Abstract: The ultra-high-speed performance of a hybrid ceramic 25-mm-bore ball bearing, as well as that of floating-ring seals with a sealing diameter of 30 mm, was determined in liquid hydrogen at speeds to 120,000 rpm, 3 million DN, under thrust loads to 3,140 N and sealed pressures to 1.7 MPaG. The bearings had Si3N4 balls and a single outer land-guided retainer. The hybrid ceramic bearing exhibited excellent performance at high load, contrasting with the seized all-steel bearing; however, the ceramic balls developed superficial micro-cracks due to frictional heating. Effective cooling of the bearing was achieved by increasing the jet speed through the nozzles. The bearing power loss drastically increased with increasing speed and cooling flow rate. The critical load capacity without bearing damage was also evaluated. Furthermore, the sealing performance of the one-ring seal was superior to that of the two-ring seal in preventing the sealing ring seizure against the runner.

Experimental Study of a New Compliant Foil Air Bearing with Elastic Support

Abstract: A new air-lubricated compliant foil journal bearing with elastic support, which has uniform surface stiffness and is much simpler in structure than previous compliant foil bearings (CFBs), is introduced in this article. Experiments have been conducted on the application of this type of CFB to a high-speed test rig, and this CFB can operate stably at 151,000 rpm. From the tests it is clear that the radial clearance C has a direct impact on the performance of this CFB, so the numerical relationship of structural parameters is listed in this article. Experimental results indicate that the CFB presented here offers preferable system dynamic and stability performance and has adequate damping to effectively reduce the possibility of self-excited and fractional frequency whirl.

Tribo-Catalysis in the Synthesis Reaction of Carbon Dioxide

Abstract: Palladium was rubbed against aluminum oxide in an atmosphere of carbon monoxide and oxygen. The synthesis of carbon dioxide was enhanced during the rubbing. The activation energy of this reaction was 2.49 kJ/mol. As the sliding commenced, the desorption of carbon dioxide overshot its equilibrium value before stabilizing. The overshoot correlated with the time interval between the friction tests. The desorption mechanism of carbon dioxide is discussed in terms of the NIRAM (negative-ion-radical action mechanism) approach; exoelectron emission was enhanced by the friction of palladium. The electrons reacted with the adsorbed species to make negative-ion-radicals. These radicals further reacted on the surface to form carbon dioxide, which then desorbed.

Scuffing Behavior of Piston-Pin/Bore Bearing in Mixed Lubrication—Part II: Scuffing Mechanism and Failure Criterion

Abstract: Surfaces and subsurfaces of test specimens that failed in scuffing were examined. A scuffing criterion is developed stating that scuffing failure would occur when the maximum surface tangential traction is larger than the modified shear strength. A scuffing uncertainty factor is introduced to reflect the influence of factors that affect wear and scuffing and the inaccuracy in modeling.

X-Ray Photoelectron Spectroscopy Analysis of Antiwear Tribofilms Produced on Boundary-Lubricated Steel Surfaces from Sulfur- and Phosphorus-Containing Additives and Metal Deactivator Additive

Abstract: X-ray photoelectron spectroscopy (XPS) was performed on AISI 52100 steel surfaces subjected to sliding in the boundary lubrication regime at 32 and 100°C. The specimens were lubricated with base oil blended with individual additives containing sulfur (S), phosphorus (P), or metal deactivator, as well as base oil with all the previous additives in the same amounts as in the single blends. XPS spectra were analyzed to confirm the formation and determine the chemical composition of the antiwear tribofilms produced on the steel surfaces during sliding. The use of S- and P-containing additives on the tested disk surfaces revealed that tribochemical reactions resulted in the formation of antiwear tribofilms containing S- and P-rich components. Results for the multi-additive blend provided evidence for two components in the produced tribofilm, appearing to consist primarily of sulfide and phosphate. This investigation provides new insight into the competing roles of these compounds on the tribological properties...

Friction and Wear Characteristics of TiN Coated Vane for the Rotary Compressor in a R410A Refrigerant

Abstract: HFC is a potential alternative refrigerant for CFC, which depletes the ozone layer. The rotary compressor has been widely used for refrigeration and air-conditioning systems due to its compactness and high-speed operation. R410A, an HFC refrigerant, is used in a refrigerator compressor, but its frictional characteristic is not established. In this study, the influence of R410A refrigerant on the roller-vane surfaces was studied, and friction and wear characteristics of the TiN coated vane were investigated. The friction and wear data were obtained with a specially designed high-pressure wear tester. A testing environment charged with HFC refrigerant more closely simulates the operating conditions of a real rotary compressor. In pure oil without R410A, wear of the TiN coated vane was larger than that of the uncoated vane. But when the refrigerant was dissolved in oil, wear of the uncoated vane was larger than that of the TiN coated vane. This showed that a TiN coated vane is good relative to wear resistanc...