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RESEARCH PAPERS

J. Tribol. 1986;108(3):301-312. doi:10.1115/1.3261181.

Experimental evidence for plowing under dry and lubricated sliding conditions is presented and analytical expressions for the coefficient of friction due to plowing are obtained. The theoretical friction coefficient was found to be a function of the sharpness of the hard asperities, the interfacial “friction” conditions and the shape of the plastic zone. The agreement between theoretical and experimental friction coefficients from lubricated sliding and cutting experiments was remarkably good. The discrepancy between theory and experiment in the case of dry sliding between like metals was shown to be due to plastic deformation of the asperities. Consequently, a different model for plowing was proposed for the case of dry sliding between like metals which produced estimates for the coefficient of friction in fair agreement with the experimental results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):314-320. doi:10.1115/1.3261185.

A computer model for the dry, frictionless contact of real rough surfaces is presented. The model uses data directly recorded from a stylus measuring instrument and as a confirmation of the model it is shown to reproduce “smooth case” results with a high level of accuracy. Results are given for two important applications of the technique. The first considers the analysis of the contact pressure and displacements for a bearing surface including a debris induced dent in the contact zone. The results go someway to providing an explanation of early life failure often associated with debris contaminated oil. Secondly the relationship between load and real contact area is studied for a sample set of surface profiles. The results obtained are compared with random process theory. It is proposed that the numerical model represents a different approach to rough surface contact allowing certain assumptions about the nature of surface roughness to be relaxed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):321-325. doi:10.1115/1.3261186.

In order to explain the effect of normal stiffness in the loading system on wear, the mean dynamic normal load and the effective sliding distance were determined from the record of dynamic normal load. The mean dynamic normal load was approximately constant at normal stiffnesses smaller than 10 N/mm and increased markedly at normal stiffnesses larger than 103 N/mm. The effective sliding distance was approximately equal to the apparent sliding distance at normal stiffnesses smaller than 10 N/mm and decreased markedly at normal stiffnesses larger than 103 N/mm. The effective wear rate was newly defined in terms of the effective sliding distance. The change of effective wear rate with the change of sliding velocity was linearly correlated to the change of mean dynamic normal load under each normal load and normal stiffness.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):326-333. doi:10.1115/1.3261187.

The influence of laser surface transformation hardening on the sliding wear characteristics and mechanisms of ASTM class-40 gray and 80-55-06 ductile cast irons was investigated. A 1.2 kw, continuous wave, CO2 gas laser was employed to scan the beam successively across the surfaces of cast irons to generate hardened and tempered layers with various case depths. A pin-on-disk wear test system was then used to study the wear behavior as functions of case depth, microstructure, hardness, and surface roughness. As expected, a dramatic improvement in resistance to scuffing and sliding wear was obtained. However, the most significant result was the occurrence of negligible oxidational wear for a load range that increased with an increase in case depth. Resistance to mild and severe wear, mild-to-severe wear transition load, and frictional heating were increased with an increase in case depth. Analysis of worn surfaces and wear debris revealed that negligible oxidational wear in laser-hardened irons is due to two mechanisms: oxidation and adhesion of oxide to the substrate. In contrast, the mild oxidational wear of untreated irons occurs through the formation of loose oxide debris. The mechanisms of severe wear were plastic deformation, delamination, and adhesion; the rate process was controlled by adhesion for laser hardened irons and delamination for untreated irons.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):334-339. doi:10.1115/1.3261188.

Thermochemically produced stainless steels with varying nitrogen content were slid dry on high carbon martensitic steel counterfaces using a pin bush machine. The running-in wear was high but the steady state wear decreased with increased nitrogen contents of the steels. A work hardened layer formed on the pins, the degree of hardening increasing with the nitrogen content of the steels. The hard pins caused a considerable amount of wear of the bushes, possibly, by ploughing. The pins wore by transfer and oxidation and, by interfacial shear and, probably, brittle fracture of the work hardened layer at a heavy load.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):340-344. doi:10.1115/1.3261189.

Adsorption of zinc di-n-octyldithiophosphate synthesized with radioactive 65 Zn has been studied on iron, steel, silica, and carbon. Adsorption on iron powder in an inert atmosphere is described by Elovich kinetics and Langmuir type isotherms. Adsorption equilibrium was not complete even after ten days. Results indicate activated chemisorption with an apparent activation energy of 15 kJ/mol (3.6 kcal/mol) on relatively uniform adsorption sites. Adsorbent composition, oxygen concentration, and solution contaminants, such as methanol, strongly influence adsorption capacity. Monolayer coverage, derived from the Langmuir isotherm, is in reasonable agreement with differently derived literature values. Pronounced variations observed on different adsorbents are thought to be caused by catalytic decomposition or oxidation of ZDDP. Because of this sensitivity, it appears that laboratory studies with simplified systems cannot provide much information on the actual structure of an antiwear layer formed in practical multicomponent lubricants.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):346-349. doi:10.1115/1.3261193.

The three dimensional shape effect of asperity on abrasive wear was investigated with in-situ experiments in the scanning electron microscope. The geometry of model asperity was represented by attack angle and dihedral angle, where attack angle changed in 0∼90 deg and dihedral angle in 0∼180 deg. Wear modes of shearing, cutting, and wedge forming were observed and each mode was related to attack angle and dihedral angle by wear mode diagram. Wear rate in cutting mode increased with attack angle and was maximum at a certain dihedral angle.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):352-357. doi:10.1115/1.3261198.

The influence on the heat treatment response and rolling contact fatigue behavior of thermomechanical processing (TMP) to refine grain and soluble carbide size was studied for VIM-VAR AISI M-50 steel. Material was processed by warm rolling and then given a final, hardening heat treatment. As-received material was also given the same final heat treatments to provide a comparison. Rolling contact fatigue testing indicates equivalence of the as-received (spheroidize-annealed) and TMP specimens evaluated. This is believed to result from inability of this TMP to refine insoluble carbides and also to the presence of porosity in both as-received and also the processed material. The porosity consists of voids at the ends of the insoluble carbides and does not appear to have been reported heretofore.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):359-362. doi:10.1115/1.3261201.

The fretting behavior of the nitrogen-bearing austenitic stainless steel, Nitronic 60, has been studied at 20, 400, 500, and 600°C in air. A spherical ended rider was vibrated against a flat specimen, both machined from the steel. Normal load was 3N, frequency 50Hz and peak-to-peak amplitude 40 μm. Coefficient of friction after 106 cycles showed a decrease as the temperature was increased, falling from 0.62 at 20°C to 0.10 at 600°C. Wear volume also dropped dramatically at 400° and above, with no measurable wear at 600°C. A smooth protective oxide was formed at 400, 500, and 600°C, typical of the glaze oxide previously found on nickel-based alloys. The oxide layer was approximately 10 μm thick and had the spinel structure. The electron diffraction pattern most closely resembled that of the spinel NiO, Cr2 O3 but chemical tests showed that it contained iron. It is thought to be a mixed spinel containing all three metal ions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):364-367. doi:10.1115/1.3261205.

Scanning Auger analysis of fracture surfaces of chips from cutting 4340 steel bars and 4130 steel tubing using CCl4 as a model cutting fluid provide evidence that it is possible for cutting fluid or vapor to penetrate into the chip along fissures created during chip formation. Similar analysis of the rake face on the tools provide evidence of partial penetration between the tool and chip as well. The effect of the penetration is to reduce the energy required for the cutting process by facilitating the chip formation and reducing the adhesion forces between the tool and chip. The penetration can be explained by the capillary action of fissures that provide reactive surfaces and fast propagation paths for the cutting fluid and vapor.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):368-371. doi:10.1115/1.3261206.

The thermal hydrodynamic lubrication theory is extended to hydrostatic extrusion of a strain hardening metal. The effects of viscous heating in both inlet and work zones, and surface temperature of work zones are included. The results are compared with previous theoretical models. It indicates that thermal strain hardening effects play an important role in determining hydrostatic extrusion for high reduction of area and high strength metals.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):372-376. doi:10.1115/1.3261207.

A steel ball is dropped onto a glass plate covered by an oil droplet where impact occurs. A small manganin pressure transducer deposited on the plate and in the oil, enables pressure-time traces to be recorded at different points within the region of pressure. Their shapes are similar to those obtained under a steady ehl (elastohydrodynamic) rolling contact. Dimples are formed in each surface during the latter stages of the approach. These, together with the radial pressure distributions, are similar in shape to those obtained theoretically elsewhere. The nature of the pressure-time response suggest that no surface to surface contact occurs during the impact.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):377-379. doi:10.1115/1.3261208.

Extreme-pressure (EP) additives containing sulphur have been tested using a fourball machine and the structure and thickness of the surface films which formed during wear tests have been studied using conventional methods of physical analysis. A nuclear technique based on a deuteron-proton stripping reaction, was employed to give complementary information (unobtainable by more usual techniques) which produced a better understanding of the structure and formation of such EP films.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):380-385. doi:10.1115/1.3261209.

The point of view is taken that for ceramics, cone cracking on the microscale assumes the same role as plastic asperity deformation in metal materials, namely, as the agent causing stress raising micropits which precipitate surface fatigue. Empirical fracture data are interpreted in the context of published fracture mechanics analyses of cone cracking in static and sliding contact and used within the Greenwood-Williamson stochastic microcontact model to predict the relative likelihood of cone cracking when a rough flat ceramic contacts a smooth ceramic flat of the same material. The Greenwood-Williamson model is reviewed and its predictions are shown, for the steel and ceramic surfaces considered, to compare favorably to the more general anisotropic microcontact model ASPERSIM. A microfracture index analogous to the Greenwood-Williamson plasticity index, is shown to be a determinant of the ability of a surface to resist cone cracking.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):387-393. doi:10.1115/1.3261212.

Using idealized assumptions, the stress condition in the semi-infinite body of the Hertzian contact region can be calculated for various geometries as a function of the surface pressure (normal force), the friction (tangential force at the surface), and the residual stresses in the material. Equivalent stresses can be formed from the coordinate stresses using various stress hypotheses (distortion energy hypothesis, shear stress hypothesis, and alternating shear stress hypothesis). The effects of friction, residual stresses, and contact geometry on the location and magnitude of the equivalent stresses appearing in the material have been investigated, the stress hypotheses being evaluated in terms of the extent to which they take account of these effects in an appropriate form. These investigations show clearly that the distortion energy hypothesis is the best representation of the extent of material stress in the case of dynamically loaded rolling elements. The shear stress hypothesis can be considered as a good approximation whereas the alternating shear stress hypothesis is only capable of providing useful conclusions to a limited extent.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):394-400. doi:10.1115/1.3261213.

The presence of contacting asperities in lubricated rolling bearings modifies the subsurface stress field strongly in the neighborhood of the surface and, to a lesser extent, at larger depths where the maxima of the shear or von Mises stress of a smooth Hertzian contact normally exist. The near surface stresses are of importance because they may result in micropitting, a mode of surface distress which leads to the eventual fatigue failure of the contacting surfaces. A mathematical method is presented in this paper which allows the statistical calculation of important parameters (maximum von Mises stress or maximum shear stress amplitude) of the stress fields generated under elastically deforming asperities during their passage through a lubricated contact. The asperities themselves are modelled using estimates of the surface spectral moments obtained from single-profile trace measurements. The method is applicable to both isotropic and anisotropic rough surfaces. Moreover, the important effect of the shear surface tractions, including tractions over the asperities, is contained in the analysis. Computed examples are presented for different surface textures and film thicknesses in the case of a deep groove ball bearing. Finally, a qualitative attempt is made to correlate features of these stress fields with the presence of surface pitting, and the limitations of the analysis are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):403-409. doi:10.1115/1.3261217.

A theory for the shear stress between a rough elastic cylinder and a cylinder with a soft layer has been developed. The theory is based on a Fourier transform approach for the elasticity equations coupled with surface deflection equations for transient contacts. For thick layers (h > .001 in.) the shear stress on the surface approaches the shear of the layer alone. The elastic shear deflection (∼100 μin.) as a result of the tangential load is significant and increases if a surface layer such as a thin coating is added to one or both cylinders. The predicted interfacial shear stresses are considerably altered by surface roughness on uncoated surfaces and these effects are ameliorated by the addition of a thin soft surface coating.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):411-419. doi:10.1115/1.3261220.

The film thicknesses and pressures in elastohydrodynamically lubricated contacts have been calculated for a line contact by using an improved version of Okamura’s approach. The new approach allows for lubricant compressibility, the use of Roelands viscosity, a general mesh (nonconstant step), and accurate calculations of the elastic deformations. The new approach is described, and the effects on film thickness, pressure, and pressure spike of each of the improvements are discussed. Successful runs have been obtained at high pressure (to 4.8 GPa) with low CPU times.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):421-424. doi:10.1115/1.3261223.

The dynamic bearing coefficients are obtained from a solution to the variational equivalent of Reynolds equation. A perturbation method is applied to find the individual dynamic coefficients. The Finite Element Method is used in the numerical evaluation of the equations. The flow is assumed to be laminar, the lubricant is Newtonian. Allowance is made for viscosity-temperature dependency in circumferential and axial directions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):426-431. doi:10.1115/1.3261226.

A facility and apparatus are described for determining the rotordynamic coefficients and leakage characteristics of annular gas seals. The apparatus has a current top speed of 8000 cpm with a nominal seal diameter of 15.24 cm (6 in.). The air-supply unit yields a seal pressure ratio of approximately 7. The inlet tangential velocity can also be controlled. An external shaker is used to excite the test rotor. The apparatus has the capability to independently calculate all rotordynamic coefficients at a given operating condition with one excitation frequency.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):433-437. doi:10.1115/1.3261228.

An experimental test facility is used to measure the leakage and rotordynamic coefficients of constant-clearance and convergent-tapered annular gas seals. The results are presented along with the theoretically predicted values. Of particular interest is the prediction that optimally tapered seals will have significantly larger direct stiffness than straight seals. The experimental results verify this prediction. Generally the theory does quite well, but fails to predict the large increase in direct stiffness when the fluid is prerotated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):439-444. doi:10.1115/1.3261231.

A test procedure is described by which quick measurements of stiffness and damping coefficients of elastomer O-rings can be made for a wide range of the parameters affecting O-ring dynamics. Tests were performed to investigate the effects of squeeze and pressure on the dynamic characteristics of Nitrile (Buna N) and Fluorocarbon (Viton 75) O-rings. Results of these tests are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):446-453. doi:10.1115/1.3261234.

Three cases are studied as variations in the geometry and the thermal boundary conditions pertinent to the classical flash temperature theory. First, the case of multiple surface heat sources is considered. It is shown that the relative location of the sources is the critical factor in predicting the local temperatures at each contact. Second, the case of a short cylinder (or a disk) is analyzed. In addition to the lateral surface boundary conditions, convective cooling from the side faces is considered. It is shown that a considerable reduction in bulk temperature can be achieved by effective cooling from the side faces. Third, the case of a hollow cylinder is studied where, in addition to the outer boundary conditions, uniform internal heating (or cooling) is considered. It is demonstrated that the bulk temperature can be varied significantly by changing the magnitude (and direction) of the internal heat flux. In both the second and the third cases, over a wide range of the pertinent parameters, it is shown that the local temperature rise remains virtually unchanged.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):455-461. doi:10.1115/1.3261237.

It is generally agreed that the friction between a tire and a wet pavement (skid resistance) is controlled by the surface texture characteristics. Therefore, by measuring the relevant parameters describing texture, or by measuring a physical process dependent on texture, regression techniques can be used to relate skid resistance to the chosen texture parameter or process. Two scales of texture are of particular importance: microtexture (small-scale asperities) and macrotexture (large-scale asperities). This paper describes work performed to: (1) review candidate macrotexture and microtexture measurement methods that can be made at highway speeds (at or about 64 km/h [40 mph]), which are presently used or have potential for use in pavement texture measurement; (2) design and build a prototype of the most promising method; and (3) evaluate the effects of pavement surface texture on skid resistance. A prototype noncontact vision system that makes texture measurements at highway speeds was developed, and several improvements were made to upgrade the system to provide an improved prototype. Both hardware and software enhancements have yielded a texture measurement system that can obtain pavement macrotexture data in a fast, efficient, and reliable way.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):462-466. doi:10.1115/1.3261240.

Motion of the cage in a high-speed angular contact ball bearing is experimentally investigated as a function of prescribed unbalance, up to operating speeds corresponding to three million DN. The predictions of cage motion made by the recently developed computer model, ADORE, are validated in the light of the experimental data. It is shown the cage whirl velocity is essentially equal to its angular velocity at all levels of unbalance and over a wide range of operating conditions. For the inner race guided turbine engine bearing, the cage/race interaction takes place directly opposite to the location of the unbalance and the severity of the interaction increases with the level of unbalance and the operating speed. ADORE predictions, over the entire range of unbalance and bearing operating conditions, are in very good agreement with the experimental observations.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):468-475. doi:10.1115/1.3261243.

Spur gear endurance tests were conducted with six lubricants using a single lot of consumable-electrode vacuum melted (CVM) AISI 9310 spur gears. The sixth lubricant was divided into four batches each of which had a different additive content. Lubricants tested with a phosphorous-type load carrying additive showed a statistically significant improvement in life over lubricants without this type additive. The presence of sulphur type antiwear additives in the lubricant did not appear to affect the surface fatigue life of the gears. No statistical difference in life was produced with those lubricants of different base stocks but with similar viscosity, pressure-viscosity coefficients and antiwear additives. Gears tested with a 0.1 wt percent sulfur and 0.1 wt percent phosphorus EP additives in the lubricant had reactive films that were 200 to 400 Å (0.8 to 1.6 μin) thick.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):478-486. doi:10.1115/1.3261246.

In order to thoroughly characterize the cam/tappet contact, a simulator was designed and constructed which presents a lifter which is stationary except for rotation. The cam instead moves through the lift curve so that the contact remains essentially in the horizontal plane. Three components of contact force on the lifter are measured. A series of eight nominally flat lifter foot materials were investigated for frictional loss as a function of speed. In addition, the effects of oil temperature, running in, lifter rotation and lifter foot spherical radius was studied. With one of the lifters (Al2 O3 ) the surface temperature of the cam was measured at peak lift employing an infrared scanning system. Peak temperatures in excess of 300° C were observed for some operating conditions. Preliminary measurements were made with a roller lifter. The loss per revolution ranged from 1.75 J to 3.9 J for the flat followers. The roller follower loss was 18 percent of that of the flat follower for similar conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(3):487-493. doi:10.1115/1.3261247.

The effect of support structure on the performance of a connecting rod bearing is analyzed by elastohydrodynamic lubrication theory. The support structure is modified iteratively until an optimum design is obtained. With the connecting rod of a typical gasoline engine as an example, it was found that a 150 percent improvement in film thickness and a 34 percent reduction in peak pressure can be obtained by changing the structural design. This design, however, increases the peak octahedral shear stress in the rod. Structural design changes considered were found to have surprisingly little effect on the power loss of the bearing.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster

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