Showing papers in "Tribology Transactions in 1986"

An Adsorption Model for Friction in Boundary Lubrication

Abstract: A model describing the relationship between friction coefficient and surface adsorption of a two-component lubricant (i.e. base lubricant and one additive) is developed for boundary lubrication. This model relates the coefficient of friction to the extent of metal-metal contact or, inversely, the fractional surface coverage of the lubricant. Using a suitable adsorption isotherm, an expression for friction coefficient is obtained, relating the friction coefficient to the additive concentration and the adsorption energy. The proposed model and the selected adsorption isotherm are experimentally verified by testing several model additives in a petroleum-base lubricant. An excellent correlation is obtained between the adsorption-free energies calculated from the friction coefficient data and the adsorption-free energies determined from adsorption studies. The ranking order of additives based on friction coefficient is explained in terms of the molecular structure and the structural effects on the adsorption-f...

Thermohydrodynamic Analysis of Laminar Incompressible Journal Bearings

Abstract: Thermohydrodynamic effects in journal bearings operating under steady-state loading are investigated. An analytical model for the finite journal bearings was formulated. The model includes correction factors for the cavitation effects and the mixing of the recirculating oil and supply oil at the inlet. A finite difference computer program was developed to numerically solve the Reynolds, energy, Laplace heat-conduction equations, and a viscosity-temperature relation simultaneously. The program includes a numerical solution technique for obtaining an isothermal shaft temperature and handles the mixing of the recirculating oil and supply oil. Good agreement with published experimental tests was obtained. It was also found that conservative solutions for the film temperature and load-carrying capacity can be obtained by imposing an adiabatic boundary condition on the bush inner surface. Such simple boundary conditions result in significant computational savings and, therefore, may be attractive to the practic...

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

Abstract: 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.

Performance Characteristics of Worn Journal Bearings in Both Laminar and Turbulent Regimes. Part I: Steady-State Characteristics

Abstract: The effects of geometric change due to wear on the dynamic characteristics of journal bearings are determined theoretically in both laminar and turbulent regimes. The dynamic characteristics such as spring and damping coefficients and whirl onset speed of a rigid rotor supported by two identical symmetrically aligned bearings are analyzed by a semianalytical finite element method, and the numerical results for various wear depth parameters are indicated in graphical form. The geometric change due to wear has significant effects on the principal spring coefficients and on the cross-coupled damping coefficients. The whirl onset speed for a worn journal bearing whose wear depth parameter is larger than 0.3 becomes higher than the speed for a nonworn bearing.

Friction-Induced Amorphization with ZDDP—An EXAFS Study

Abstract: Many experimental works are concerned with the microinvestigation of interface bodies created during the wear process (especially wear particles) Previous works wing STEM have shown that ZDDP wear protection is associated with the structureless nature of wear debris, so that friction-induced amorphization was a key factor to obtain the antiwear conditions In this paper, the synchrotron radiation, EXAFS (Extended X-ray Absorption Fine Structure) is used to probe the local environment of atoms (Fe and Zn) in interface ZDDP films Results clearly show the fully amorphous character of these materials Consequences on wear models are then discussed Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Tribology Conference in Atlanta, Georgia, October 8–10, 1985

Lubrication from the Vapor Phase at High Temperatures

Abstract: Vapor phase lubrication has been demonstrated using a conventional four-ball wear Lester run at 370°C using M50 hot hardness steel ball bearings. The four-ball wear tester was modified to control both the temperature and the environment surrounding the ball pot and spindle. The lubricant in all cases comprised a homogeneous gas phase mixture of nitrogen and phosphate ester vapors. The two phosphate esters used were tributyl phosphate (TBP) and tricresyl phosphate (TCP). The volume percent phosphate ester vapor in nitrogen ranged from 0.10 to 6.0 percent. The minimum temperature of the four balls was maintained at 370°C which was always at least 50°C higher than the temperature of the homogeneous vapors delivered so that there was no possibility of vapor condensation on the wear contacts. Vapor phase lubrication by phosphate esters appears to be as effective at a bulk temperature of 370°C as typical nonadditive mineral oils are in the liquid state at room temperature to 75°C. For example, TCP delivered fro...

Friction and Wear of Silicon Nitride at 150°C to 800°C

Abstract: The friction coefficient of hot-pressed Si3N4 on itself was measured in the temperature range of 150 to 800°C. Sliding occurred in dry argon, dry and humid air and the sliding velocity was varied from 0.1 to 10 cm/s. In dry argon and dry air, the friction coefficient is f = 0.8 ± 0.05 over the whole temperature range and wear occurs by fracture as it does at room temperature. In humid air, the friction coefficient is a complex function of sliding velocity, load, and temperature. Values of as low as f = 0.2 are obtained in a narrow velocity range at temperatures between 150 and 650°C. They are associated with the formation of a soft, amorphous, surface layer of silicon dioxide. At 800° C, the friction coefficient is f = 0.8 in all atmospheres. At every temperature and load, there is a velocity above which the friction coefficient increases rapidly and wear by microfracture is predominant. We speculate that the observed behavior is due to a balance between tribochemical reaction rate and the rate of mechani...

Infrared Spectroscopic Methods for the Study of Lubricant Oxidation Products

Abstract: Data processing techniques have been developed for the manipulation and enhancement of spectroscopic data They have been applied to the infrared spectra of degraded base oils and lubricants to provide information on oxidation processes and additive depletion Samples have been acquired from oils that have been subjected to laboratory scale oxidation and from lubricants during standard engine tests Results are evaluated in terms of chemical species that are lost and formed during oxidative degradation Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

An Experimental and Theoretical Study of Pressure and Thermal Distortions in a Mechanical Seal

Abstract: Distortions of mechanical face seal components have a significant effect on the seal performance. The finite element method is often employed to analyze complex seal sections; however, experimental verification is needed. In this paper, a technique for measuring pressure and thermal distortion of seal components is demonstrated for the first time in a production-type seal under normal running conditions. The technique uses capacitance probes which measure the fluid film geometry directly. The seal components have also been numerically analyzed for pressure and thermal distortion by the boundary integral element (BIE) method and the results are correlated with experimental measurements. While seals may be designed to be insensitive to the sealant pressure, the thermal distortion is shown to be very significant. The mechanisms and the effect of boundary conditions on thermal rotation are discussed. The BIE method is a rapid technique, which from an integral part of a modular seal design system and has been ...

The role of hydroperoxides in engine wear and the effect of zinc dialkyldithiophosphates

Abstract: Value train wear was examined in a motored 2.3 l engine similar to that wed in the Sequence V-D test. A procedure was developed in which model hydroperoxides were added to the crankcase oil to simulate used oil. Cam lobe and follower wear occurred, apparently as a result of direct attack by the hydroperoxides on rubbing surfaces rather than through oil oxidation. The extent of wear was proportional to the hydroperoxide concentration. The same type of hydro-peroxide-induced wear subsequently was identified in a fired bench engine and in a field test. The study also showed that zinc dialkyldithiophosphates (ZnDDP) control wear by decomposing hydroperoxides in addition to modifying surfaces in boundary contact. Secondary ZnDDPs' were more effective than primary ZnDDPs'. Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987

Study of Lyotropic Liquid Crystals in Viscometric Flow and Elastohydrodynamic Contact

Abstract: Lyotropic liquid crystals of triethanolammonium oleate in several nonaqueous solvents were prepared and characterized. Their rheology and elastohydrodynamic contact behavior were investigated. Under the conditions studied, they are non-Newtonian, shear thinning fluids whose behavior up to shear rates of 2500 s−1 can be described by a rheological equation. In elastohydrodynamic contact, at high pressure/high shear, they had effective viscosities larger than would be predicted from their low shear rate behavior, perhaps due to a limiting shear stress condition. They behaved as homogeneous liquid crystalline fluids and, like greases, certain formulations exhibited a tendency to starve the contact. Presented at the 42nd Annual Meeting in Anaheim, California, May 11–14, 1987

A Study of Piston Friction Force in an Internal Combustion Engine

Abstract: Quantitative estimation of friction losses in the initial design stage of an automobile engine is a crucial issue that determines the fuel economy and performance of the automobile. However, measurements and mathematical treatments of friction losses were difficult, and practical estimation methods were scarce. This study was carried out due to such circumstances, with emphasis placed on the analysis of mechanism of friction losses of the piston system, and the introduction of practical equations for calculation of those friction losses. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Tribology Conference in Pittsburgh, Pennsylvania, October 20–22, 1986

Density and Viscosity Models for Two-Phase Homogeneous Hydrodynamic Damper Fluids

Abstract: Theoretical cavitation models previously adopted in analyzing dynamically loaded hydrodynamic bearings or squeeze-film dampers are inconsistent with experimental observations in that bubbles, once formed, in many instances do not completely redissolve upon the reappearance of high pressure. A more representative approach could be to model the lubricant as a homogeneous two-phase gas-liquid mixture. The density and viscosity of such a homogeneous mixture are developed as functions of the pressure and the known mole fraction of the noncondensable gas to the liquid at supply entry. Omitting the possible effects of surface tension, it is shown that if the vapor pressure of the lubricant is of the order of 10−3 atm or less, it has only a minimal effect on the density of the mixture and may be ignored. On the other hand, solubility effects may be significant, though it is unknown to what extent gases, once drawn out of solution, will redissolve in the short time of the cycle period of the dynamic loading. Hence...

The Effect of Bearing Support Flexibility on Critical Speed Prediction

Abstract: Some vendors and many users of rotating machinery are arbitrarily reducing the bearing damping by as much as 75 percent in order to accurately predict the location of the first and second lateral critical speeds. This paper shows that if the bearing support flexibility is considered, accurate critical speed predictions are possible without reducing the bearing damping. The equations for the equivalent support stiffness and damping are given for a flexible support spring, mass, and damper in series with the fluid film stiffness and damping properties of a tilting-pad bearing. These equations can easily be incorporated with any synchronous response and/or stability computer program. Examples of test stand results for four production rotors are shown illustrating the accuracy of this method. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

Effect of Load on Dry Sliding Wear of Aluminum-Silicon Alloys

Abstract: Aluminum-silicon alloy pins were slid against steel disks under nominally dry condition at a speed of 0.6 m s-1. Each pin was slid at a constant load for 5 min, the load being increased in suitable steps from 2 to 65 N. The results show the wear to increase almost monotonically with load, to be sensitive to the presence of silicon in the alloy, and to be insensitive to actual silicon content. The monotonic nature of wear rate-load characteristic suggests that one dominant wear mechanism prevails over the load range studied. Morphological studies of the pin surface and the debris support this contention and point to delamination as being the dominant mode of wear.

Angular particle impingement studies of thermoplastic materials at normal incidence

Abstract: Scanning electron microscope studies were conducted to characterize the erosion resistance of polymethyl methacrylate (PMMA), polycarbonate (PC), polytetrafluorethylene (PTFE), and ultra-high-molecular-weight polyethylene (UHMWPE). Erosion was caused by a jet of angular microparticles of crushed glass at normal incidence. Material built up above the original surface on all of the materials. As erosion progressed, this buildup disappeared. UHMWPE was the most resistant material and PMMA the least. The most favorable properties for high erosion resistance were high values of ultimate elongation, maximum service temperature, and strain energy and a low value of the modulus of elasticity. Erosion-rate-versus-time curves of PC and PTFE exhibited incubation, acceleration, and steady-state periods. PMMA also exhibited a deceleration period, and an incubation period with deposition was observed for UHMWPE.

Static Stresses in an Unpressurized, Rounded, Rectangular, Elastomeric Seal

Abstract: The Static Stress field in and unpressurized, rounded, rectangular, elastomeric seal is examined using both experimental and numerical techniques. A particular stress separation method is successfully employed in the photoelastic study. The salient features of the stress field and their relevance to the seal performances are outlined. An approximate theoretical model is proposed which permits the determination of the contact pressure profile.

An Evaluation of Finite Difference and Finite Element Methods for the Solution of the Reynolds Equation

Abstract: A comparative study is made of various finite difference and finite element solutions of the Reynolds equation for the steady, isoviscous, incompressible hydrodynamic lubrication problem. Numerical solutions are obtained for both one- and two-dimensional slider bearings, and the predicted load capacity is evaluated against that of the corresponding exact analytical solution. Two finite difference schemes of nominal second-and fourth-order accuracy are employed. Linear, quadratic, and cubic elements are used in the finite element solutions. The different solution methods are evaluated in terms of accuracy, computational complexities and cost. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

Some Chemical Reactions of Organic Disulfides in Boundary Lubrication

Abstract: The mechanism of boundary lubrication of organic disulfides in while oil, using four-ball test data, has been studied by examining the influence of load, wear test time, concentration and structure. The amounts of iron sulfide, thiol and elemental sulfur fanned from disulfides have been quantitatively determined in oil solution and wear debris. Monosulfide was identified in oil solution after wear test. Temperature in wear junction and additive concentration in oil control the rate and magnitude of sulfur products formation. The formation of iron sulfide occurs by decomposition of disulfide to elemental sulfur and thiol, which than read with the metal surface giving sulfide. The separate chemical reaction paths of disulfides with different rates and amounts of sulfur products formation depended on the lubrication conditions present. The chemical structure of disulfides influences the formation of determined sulfur compounds. Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987

Cutting Tool Materials: Mechanical Properties—Wear-Resistance Relationships

Abstract: The abrasive wear resistance of cutting tool materials was found to be related to the properties of fracture toughness and hardness. This study has shown that the abrasive wear resistances of three classes of tool materials, brittle, brittle-ductile, and ductile, are controlled by the specific response of each material to the abrader. For brittle ceramics, the mechanical properties parameter KIC 3/4 H1/2 describes the abrasive wear resistance as determined in a grinding test and predicts performance in machining applications in which abrasion predominates. For cemented carbides and tool steels which can accommodate much higher degrees of plastic deformation and are less prone to fracture, fracture toughness has considerably less influence on the abrasive wear resistance. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

Nonlinear Dynamic Analysis of Noncontacting Coned-Face Mechanical Seals

Abstract: The complete nonlinear equations of motion of a flexibly mounted stator in a noncontacting coned-face mechanical seal are solved numerically. The solution utilizes a transient dynamic analysis and takes into account rotor axial runout and assembly tolerances in the form of initial stator misalignment. Cavitation of the fluid film is also accounted for. A parametric investigation is performed and the effect of various design parameters and operation conditions on the seal dynamics is presented and discussed. A critical shaft speed is found above which the seal becomes dynamically unstable. A critical rotor runout is found which, if exceeded, will cause seal failure due to local face rubbing contact. A comparison is made between the numerical results and those of a simpler analytical solution. It is found that the analytical solution is valid for most practical applications of mechanical seals. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

The Effects of Lubricant Rheology on the Performance of Journal Bearings

Abstract: Experiments have been carried out on a relatively simple journal bearing rig to test the theory that extra load-carrying capacity is generated the elasticity of viscoelastic lubricants. Model lubricants were used. The viscoelastic lubricants were solutions of high-molecular-weight polyisobutene in low-molecular-weight polyisobutene while the inelastic lubricant was low-molecular-weight polyisobutene. The extra load-carrying capacity has been established. An analysis of the detailed results strongly indicates that the extra benefit of elasticity is much greater than that predicted by existing theory. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in San Diego, California, October 22–24, 1984

Isolation and Chemical Characterization of a Zinc Dialkyldithiophosphate-Derived Antiwear Agent

Abstract: A secondary-alkyl zinc dialkyldithiophosphate (ZDP) was thermally decomposed in the presence of air. The resulting precipitate, when blended in a base grease, demonstrated better antiwear performance in a 4-ball wear machine than the parent additive. When extracted with acetone, a white powder with excellent antiwear properties was obtained from the precipitate. Chemical analysis indicated that this white powder was approximately 86 mass percent zinc pyrophosphate, 11 mass percent zinc orthophosphate, and 3 mass percent of an unknown sulfur compound. The antiwear effectiveness of a commercial zinc pyrophosphate and a zinc orthophosphate was also measured, but neither provided comparable antiwear protection. The white powder was amorphous/microcrystalline, whereas the commercial zinc pyrophosphate and zinc pyrophosphate were highly crystalline. These results indicate that zinc pyrophosphate is one of the thermal degradation products of ZDP. In addition, zinc pyrophosphate is suspected of being primarily re...

Possible Flash Temperatures in Slider and Recording Disk Transient Contact

Abstract: A numerical solution in two dimensions of thermal effects in steady-state sliding confirms Blok's original analysis. It is shown that for transient sliding (assuming sudden contact), surface temperatures of about 50 percent of the steady-state value are reached in one sliding width of a contact patch and 80 percent in ten widths. If voids or singularities exist close to the surface in the moving body, transient surface temperatures easily exceeding the Blok steady-state value can occur. These results are applied to asperity contact between sliders and magnetic recording disks. It is shown that temperatures exceeding several hundred degrees centigrade are quite possible, depending on asperity size and porosity of the bulk materials. Such high temperatures could lead to lubricant breakdown and head “crashes.” Suggestions are made on methods of reducing flash temperature, principally by the application of very thin coatings of high thermal capacity on the disk surface. Presented at the 40th Annual Meeting in...

Thermal Resistance of a Tapered Roller Bearing

Abstract: An apparatus to simulate the thermal behavior of a tapered roller bearing was constructed and used to determine tapered roller bearing thermal behavior. Bearing specimens can be loaded axially and/or radially while rotating. Shaft heating is also provided and mechanical losses measured. An infrared scanner and thermocouples allow for temperature measurement. An analytical model of the bearing and bearing/housing system was also developed and is presented. The model is capable of considering various modes of bearing operation. Experimental and theoretical resistances are compared. Typical thermal resistances from shaft to housing were found to be of the order of 1 K/W. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Tribology Conference in Atlanta, Georgia, October 8–10, 1985

Thick Antiwear Films in Elastohydrodynamic Contacts. Part I: Film Growth in Rolling/Sliding EHD Contacts

Abstract: It has been previously reported in the literature that a particular, unnamed, phosphonate antiwear additive can enhance elastohydrodynamic film thicknesses on rolling metal surfaces at elevated temperatures. In this paper, several different pure phosphonate additives have been prepared and their performance in rolling contacts has been studied. All the phosphonates tested formed thick films from base oil solution in rolling EHD contacts. The rate of film formation varied with the alkyl or aryl groups present and with the temperature but, in some cases, films started to form at room temperature. These films grew steadily with running over several hours, to reach thicknesses of more than one-half micron, augmenting the normal elastohydrodynamic films by this amount. Further studies have demonstrated that these thick antiwear films can withstand up to 10 percent sliding and are formed in the presence of other additives. The implications of the phenomenon of thick antiwear films as a means of increasing speci...

Tribological Properties of PVD Silver Films

Abstract: This paper presents an experimental study made on physical-vapor-deposited (PVD) silver films. A conventional ion plating unit was used in depositing ion-plated, vacuum-deposited, and gas-deposited silver films. Experimental work on nucleation, growth, and interface formation is briefly presented. Some adhesion, microhardness, and x-ray test results are also included. Friction and wear test results are reported in details. The good lubrication properties are attributed initially to the low shear strength in the direction of sliding, and also to the transfer-back-transfer mechanism. The wear mechanism however, is initially a microcutting process, followed by wear debris abrasion process and, finally, a fatigue wear process. Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985

Stability of Finite Elliptical Pressure Dam Bearings with Rotor Flexibility Effects

Abstract: The stability of two-axial groove cylindrical journal bearing is improved by milling a step or pressure dam in the upper half surface and by cutting a deep groove or relief-track in the lower half surface. Dynamic analysis of elliptical bearings having similar construction of dam and relief-track, called elliptical pressure dam bearings, has revealed that these bearings are far more stable than ordinary cylindrical, cylindrical pressure dam, elliptical, three-lobe, etc., when supporting a rigid rotor. In this paper, the stability of finite elliptical pressure dam bearings supporting flexible rotors is discussed. The dimensionless rotor flexibilities selected are 0.5, 1.0, 2.0, and 4.0, which cover almost the entire range of practical interest. Effects of design parameters such as vertical preset, L/D ratio, Sommerfeld number, dam location and relief-track width ratio on the stability of these bearings, corresponding to various dimensionless rotor flexibilities are presented in the form of graphs. Also, th...

Evaluating Tilting-Pad Thrust Bearing Operating Temperatures

Abstract: An analysis of tilting-pad thrust bearing operating temperatures is described. The analysis includes an evaluation of various tem­ pera-ture locations based on their ability to reflect bearing operating conditions. Experimental results for a 267-mm (JO .5 -in) 8-pad thrust bearing indicates that while a babbitt location on the trailing edge of a pad is often the hottest, the 75/75 percent location provides temperature data on the high-pressure region of the pad. Babbitt temperatures also provide a convenient indicator of the transition from laminar to turbulent flow in the oil film.

AC impedance measurements of the resistance and capacitance of lubricants

Abstract: An AC impedance technique in combination with a thin-film (12 μm) cell was employed to separately meusure the electrical resistance and capacitance of electrode-lubricant interfaces and bulk lubricant layers. In this study, three different additives were investigated by measuring the AC impedance spectrum of each additive mixed separately with a commercially available mineral oil. These additives were oleic acid, N-oleyl-1,3 diaminopropane (fatty amine), and mixed alkyl acid orthophosphute (acid phosphate). For lubricants containing 2 mass percent of an acid phosphate or fatty amine, the capacitance of the electrode-lubricant interface, Ci, and the capacitance and resistance of the bulk lubricant layer, Co and Ro, respectively, were evaluated separately from the AC impedance spectra. The lower limit of the resistance of the electrode-lubricant interface, Ri, was aDo estivnated. For additives, such as oleic acid, which physically adsorbs on the electrodes, the resistance and capacitance of the electrode-lu...