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RESEARCH PAPERS

J. of Lubrication Tech. 1981;103(4):473-485. doi:10.1115/1.3251713.
Abstract
Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):486-494. doi:10.1115/1.3251714.

Interactions between zinc dithiophosphate (ZDDP) antiwear additive and commercial calcium sulfonate detergent are studied using a plane/plane contact configuration friction machine lubricated under boundary conditions. Comparative tests have allowed us to envisage three types of interactions, respectively, due to, — first: chemical interaction between both additives in the oil phase leading to an effective ZDDP concentration decrease. — second: detergent effect due to the presence of the calcium sulfonate surfactant molecule, preventing basal materials from agglomerating during running. — third: specific role due to the overbasing agent, considered as a colloidal dispersion. When an overbased detergent is used, the two latter contributions are predominant. This study suggests that a new approach in boundary lubrication should be envisaged in relation to the evolution of the interface products which must be regarded as a highly concentrated colloidal system.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):497-501. doi:10.1115/1.3251718.

A method has been developed to characterize lubricants, starting from the failure of thin film lubrication in sliding concentrated steel contacts. For a number of lubricants, differing in viscosity and chemical composition, the collapse of the partial EHD film is taken as a criterion of the lubrication behavior. The contribution of viscosity and chemical composition of lubricants to the load carrying capacity of the partial EHD film at a speed of 1 m/s (P1 ) can be separated. This is achieved by plotting the value P1 as a function of the logarithm of the bulk viscosity (η). The following linear empirical relation P1 = β log η + α is found to be valid within the viscosity range of 2–200 • 10−3 Pa.s. Lubricants with the same chemical composition have an equal value of α.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):503-507. doi:10.1115/1.3251721.

A series of experiments are described where combined linear and rotating motions are imposed upon a polymer while it is in contact with a smooth and clean counterface. Friction data are presented for a series of polymers, which reflect the main tribological groups, for a range of linear and angular velocities. The main conclusion is that within certain ranges of contact conditions, which the paper defines in part, three polymers, PTFE, high density polythene and ultrahigh molecular weight polythene, exhibit a response which is markedly different from that of the other polymers investigated. For the PTFE group of polymers the measured force attains a pronounced maximum value as rotation is introduced. The other polymers show no such effect. The phenomena are associated with the natural tendency of PTFE type polymers to undergo reorientation of the matrix at the interface during sliding. The results suggest a method of sensing the nature of the energy dissipation process during sliding and also accounts for an artefact which may be observed with PTFE materials when their friction is monitored in a pin-on-disk configuration.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):509-516. doi:10.1115/1.3251724.

This paper facilitates practical calculation of rolling contact fatigue life, allowing for material, lubrication, and surface topography effects. Modifying factors for the predicted fatigue life are given for: material matrix strength; material defect severity; EHD film thickness/surface roughness ratio Λ; surface asperity slope σθ and boundary lubricated traction coefficient μa . Readily evaluated formulas are provided for the above factors. Calculated results show predicted life versus Λ with σθ and μa as parameters. Comparison with experimental data shows that the model covers most documented test results of life versus Λ. Scanty experimental data covering σθ and traction effects on life are compatible with the model predictions.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):521-525. doi:10.1115/1.3251730.

For pure rolling fatigue conditions, the effect of microstructural changes and internal stresses due to grinding and fatigue processes itself was studied. Using Sachs’ method, the field of internal stresses was determined and it has been shown that its evolution is correlated with microstructural changes. These microstructural changes lead to changes in mechanical properties, especially to the reversible strain limit stress (R.S.L.S.) which has been determined in each point of the subsurface contact. Assuming a given relation between R.S.L.S. and fatigue limit, and using fatigue criterion which includes R.S.L.S., the effect of internal stresses, microstructural evolution, and Hertz stresses was analyzed. It has been found that the grinding internal stresses have no significant effect. On the contrary, the R.S.L.S. value was strongly reduced by grinding thermal effect in subsurface region, and was also strongly changed by the fatigue process. Finally, this analysis method was used to determine the best field of internal stresses necessary to minimize the fatigue damage.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):526-532. doi:10.1115/1.3251731.

Traction in lubricated elliptical Hertzian contacts with colinear speeds is studied. A method is described to calculate the stresses, temperatures and shear strain rates distribution along and across the film. Visco elasto-plastic behavior of the lubricant, convection, and conduction effects and pressure distributions are taken into account in this method. Elastic energy release is discussed. Shear modulus and limiting shear stresses are determined from experiment by curve fitting. Correlation between theory and experiment is analyzed.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):533-538. doi:10.1115/1.3251732.

The effects of heat generation and the resulting temperature profiles on the traction coefficient for two rollers in EHD contact are determined. An exact solution for the velocity profile through the film thickness is obtained to simultaneously satisfy the momentum balance and the nonlinear Maxwell model where the viscous term follows the Eyring “sinh” law. These results are coupled with the heat diffusion equation to yield simultaneous solutions for temperature and shear stress distributions in the film. Several cases are investigated for LVI 260 lubricant and are compared with experimental observations. The thermal theory is in good agreement with the experimental data. It is shown that the thermal theory predicts a maximum traction while the isothermal theory predicts increasing traction with increasing sliding speed.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):539-546. doi:10.1115/1.3251733.

The paper describes a technique for solving the inverse lubrication problem under point contact elastohydrodynamic conditions, i.e. the calculation of a film thickness and shape corresponding to a given hydrodynamic pressure distribution by an inverse solution of Reynolds’ equation. The effect of compressibility and influence of pressure upon viscosity are included in the analysis. The technique will be of use in solving the point contact elastohydrodynamic lubrication problem at heavy loads.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):547-557. doi:10.1115/1.3251734.

The paper describes a numerical procedure for solving the point-contact elastohydrodynamic lubrication problem under isothermal conditions at moderate loads. Results are presented showing the shape of the film and variation of hydrodynamic pressure. Analysis of results for a range of operating conditions gives the following approximate formulas for minimum and central film thickness, repsectively: Hm = 1.9 M−0.17 L0.34 and Ho = 1.7 M−0.026 L0.40 where H, M, and L are the Moes and Bosma nondimensional groups. In common with earlier solutions based upon the forward-iterative method the solution breaks down under moderately heavily loaded conditions. Ways of extending the solution to heavier loads using the authors’ inverse solution of Reynolds’ equation under point-contact elastohydrodynamic conditions are discussed.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):558-565. doi:10.1115/1.3251737.

Oil-lubricated plain hybrid journal bearings have been investigated theoretically and experimentally to determine the hybrid (hydrostatic/hydrodynamic) performance. The paper consists of two parts: Part I deals with the theoretical treatment of results, and Part II describes the experimental investigation. It is demonstrated that when two rows of inlet lubricant sources are employed in a plain hybrid bearing, greater load-carrying capacity is obtained by positioning the entries near the ends of the bearing rather than at the center or at quarter stations. These results extend previous work by presenting data for a wide range of power ratio (K). The parameter K is defined as the ratio of friction power to pumping power. Increasing K has the same effect as increasing speed for a particular bearing system. A new basis for optimizing hybrid bearings is described. The bearings to be optimized are compared with a reference bearing, on a basis of load/total power; a technique which is not required in other bearings where load does not increase rapidly with power ratio. Plain hybrid bearings are compared with conventional recessed hydrostatic journal bearings and with axial groove hydrodynamic journal bearings. It is found that plain hybrid bearings are superior in performance to recessed journal bearings at low eccentricity ratio and low speed. Also plain hybrid bearings are comparable to axial groove hydrodynamic journal bearings at a high eccentricity ratio and high speed with advantages for variable directions of loading. Furhermore the hydrostatic effect tends to raise the whirl onset speed.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):566-572. doi:10.1115/1.3251738.

An extensive experimental program was carried out to test the theoretical predictions discussed in Part I of this paper. The design of the bearing test rig is described. Line-source plain hybrid journal bearings have been investigated and results are presented for bearings at the optimum and higher speeds. Such parameters as load, eccentricity, oil-film pressure, speed, inlet and outlet temperatures, friction torque, oil flow-rate, and attitude angle have been measured. A description of the appropriate instrumentation is also included. Whereas close agreement was found between theory and experiment, there was a tendency for measured loads to be slightly higher than predicted, particularly as the eccentricity ratio approached a value of unity. It was also found that at high values of power ratio corresponding to higher temperature rise conditions, frictional torque was lower than predicted.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):573-577. doi:10.1115/1.3251739.

A new type of gas lubricated externally pressurized thrust bearing was developed. The bearing consists of a central hole for feeding the gas and a row of 24 small holes located circumferencially at a constant radius for supplying a small amount of liquid which, by the combined wetting action and the gas flows, forms a thin liquid layer adhering onto one of the surfaces. The pattern of the liquid film was observed experimentally, and the bearing performance influenced by the thin liquid film was determined. The load capacity and film stiffness were found to increase approximately 2 and 6 times respectively.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):578-586. doi:10.1115/1.3251742.

The role of circumferential surface waviness in contacting mechanical face seals is examined. First, a two dimensional model is developed. The model considers waviness amplitude, surface roughness, asperity contact, hydrodynamic and hydrostatic pressures, and fluid cavitation. Given waviness, roughness, number of waves, geometry, and sealed fluid, the model predicts the fraction of load supported by fluid pressure, the relative wear rate, friction coefficient, and leakage rate. A numerical method for solving this problem is presented. Using this model, the effects of waviness amplitude, roughness, number of waves, face width, and sealed pressure on seal performance are examined. Based on these results, certain conclusions are reached concerning the role of waviness in commercial face seals. Finally, using the model, it is shown how waviness can be applied to advantage to reduce seal friction and wear rates.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):587-592. doi:10.1115/1.3251745.

The dynamic response of a flexibly-mounted ring to runout of the rotating seat in mechanical face seal is analyzed assuming small perturbations. It is found that tracking ability of the stator depends only on its dynamic characteristics and operating conditions and is not affected by the amount of runout. Three different modes of dynamic response are shown and the condition for parallel tracking is presented. The present analysis is limited to flat-faced seals with no secondary seal damping. Nevertheless it provides a good insight into the dynamic behavior of noncontacting face seals.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1981;103(4):593-600. doi:10.1115/1.3251746.

Pressure profiles measured across the film/pocket interface of an externally-pressurized air-lubricated circular thrust bearing are presented. One of the thrust pads was vibrated in an attempt to simulate a pneumatic hammer type motion. The results show that at the bearing surface the pressure perturbations in the pocket are generally much higher than those in the film. It is argued that these results tend to reconcile an earlier discrepancy between theory and experiment in the prediction of stability.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

Commentary by Dr. Valentin Fuster

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