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PROFILES

J. of Lubrication Tech. 1979;101(3):245-250. doi:10.1115/1.3453338.
FREE TO VIEW
Abstract
Topics: Tribology
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. of Lubrication Tech. 1979;101(3):251-257. doi:10.1115/1.3453339.

Measurements of lubricant shear rheological behavior in the amorphous solid region and near the liquid-solid transition are reported on three lubricants under pressure. Elastic, plastic and viscous behavior was observed. The maximum yield shear stress (limiting shear stress) is a function of temperature and pressure and is believed to be the property which determines the maximum traction in elastohydrodynamic contacts such as traction drives.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):258-264. doi:10.1115/1.3453342.

A shear rheological model based on primary laboratory data is proposed for concentrated contact lubrication. The model is a Maxwell model modified with a limiting shear stress. Three material properties are required: Low shear stress viscosity, limiting elastic shear modulus, and the limiting shear stress the material can withstand. All three are functions of temperature and pressure. In applying the model to EHD contacts the predicted response possesses the characteristics expected from several experiments reported in the literature and, in one specific case where direct comparison could be made, good numerical agreement is shown.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):266-273. doi:10.1115/1.3453346.

The performance of traction drives depends to a large extent on the rheological properties of the fluid in the EHL contact. Through the use of the recently proposed J + T constitutive equation, the influence of elastic effects in the fluids is examined. The results were compared with those obtained from conventional analysis. It is shown that essentially three regions of influence exist. For small values of spin and side slip the elastic effects in the fluid dominate and no consideration of the spin and side slip is required. At higher values of spin and side slip the elastic effect still exists but slip is influenced by the spin and slide slip. At still higher values of side slip and spin, the elastic effects in the fluid may be neglected. The various boundaries of the regions of influence depend on the aspect ratio of the contact.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):275-281. doi:10.1115/1.3453349.

Experiments are reported where face-seat specimens were operated hydrodynamically at conditions close to thermal equilibrium. Changes in surface waviness and mean film thickness were monitored, and compared with theoretical predictions of thermal growth of waviness. Discrepancies were attributed to imperfect response of the gimbals to axial run-out. This run-out was increased by thermal effects at high sliding speeds and led to interactions which influenced the growth of two-lobed waves on the contact face. Operation proved stable, as predicted, under the carefully controlled operating conditions of fixed axial load and self aligning contact.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):283-290. doi:10.1115/1.3453352.

Hydrodynamic effects in a flat seal having an angular misalignment are analyzed, taking into account the radial variation in seal clearance. An analytical solution for axial force, restoring moment, and transverse moment is presented that covers the whole range from zero to full angular misalignment. Both low pressure seals with cavitating flow and high pressure seals with full fluid film are considered. Strong coupling is demonstrated between angular misalignment and transverse moment which leads the misalignment vector by 90 degrees. This transverse moment, which is entirely due to hydrodynamic effects, may be a significant factor in seal operating mechanism.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):293-302. doi:10.1115/1.3453357.

An analytical formulation for the roller motion in a cylindrical roller bearing is presented in terms of the classical differential equations of motion. Roller-race interaction is analyzed in detail and the resulting normal force and moment vectors are determined. Elastohydrodynamic traction models are considered in determining the roller-race tractive forces and moments. Formulation for the roller end and race flange interaction during skewing of the roller is also considered. Roller-cage interactions are assumed to be either hydrodynamic or fully metallic. Simple relationships are used to determine the churning and drag losses.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):305-311. doi:10.1115/1.3453360.

Cylindrical roller bearing performance simulations are expressed in terms of the general motion of the bearing elements as derived by integrating the differential equations of motion. Roller skew as induced by relative race misalignment is simulated. It is shown that skidding can be reduced by using a lubricant providing relatively high traction. However, such a fluid results in increased bearing torque and power loss. The influence of geometrical parameters, such as roller/cage, or race/cage clearance and radial preload, on the roller and cage motion is also investigated.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):312-318. doi:10.1115/1.3453363.

An analytical formulation for the generalized ball, cage, and race motion in a ball bearing is presented in terms of the classical differential equations of motion. Ball-race interaction is analyzed in detail and the resulting force and moment vectors are determined. The ball-cage and race-cage interactions are considered to be either hydrodynamic or metallic and a critical film thickness defines the transition between the two regimes. Simplified treatments for the drag and churning losses are also included to complete a rigorous analytical development for the real-time simulation of the dynamic performance of ball bearings.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):319-326. doi:10.1115/1.3453364.

Dynamic simulations of the performance of a ball bearing are presented in terms of the general motion as obtained by integrating the differential equations of motion of the various bearing elements. It is shown that bearing misalignment significantly influences the ball/cage and race/cage interaction and, hence, the stability of cage motion. The increased radial to axial load ratios promote skidding which couples with the lubricant behavior to impose accelerations on the ball which ultimately influence the ball/cage interactions. Hence, the lubricant behavior and the large load variation on the balls play dominant roles not only in determining the extent of skidding but also in establishing the overall stability of the cage motion.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):327-337. doi:10.1115/1.3453365.

All measurements of EHD film thicknesses have been carried out in simulated test machines. This study uses an actual bearing. A test rig using a 65 mm bore radial cylindrical roller bearing has been constructed with a specially designed sapphire window in the outer track. Full loads, and speeds to 3000 rpm were applied. With specially polished rollers and chromic oxide coating on the window excellent interferometric film thickness measurements were found possible. A Xenon flash lamp was used and a Xenon laser of 0–50 pps, pulse half width of 150 ns and peak power of 100 watts was developed for this research. A microscope and 35 mm camera as well as video tape were used for recording results. Arrangements were made to study any chosen roller and the side of the bearing was also open to view. First the film measurements, when corrected for inlet zone viscous heating, agreed admirably with theoretical predictions for mid and exit film thickness. The effect of inlet boundary length on the film was then investigated in some depth. Studying the effect of the multiple roller system, a number of techniques were used to demonstrate that the inlet boundary length, which controls the lubricant film thickness, was itself controlled by the film thickness between the rollers and track in the unloaded zone. The ribs of oil, formed at either edge of the roller, are only secondary sources of oil for replenishment of the inlet film. It is in fact usual (as shown by the convex shape of the inlet zone) for oil to feed out of the inlet zone into the ribs. Oil globules were sometimes observed riding on an air cushion at the entry to the roller-track conjunction, though completely inoperative as providers of oil.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):338-347. doi:10.1115/1.3453368.

The effects of porosity on the performance of gas bearings are investigated. A thin-wall approximation is developed to simplify the coupling between the Reynolds equation in the fluid film and the three dimensional Laplace equation in the porous matrix. The Newton-Raphson method is used to linearize the modified Reynolds equation and the column method is employed to solve the system of linear equations in matrix forms. The solution of a solid-wall bearing is included as a special case. Bearing characteristics are presented in tables and charts for varied permeabilities, slip coefficients, porous layer thicknesses, eccentricity ratios, length-to-diameter ratios, and for a wide range of compressibility number.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):349-354. doi:10.1115/1.3453371.

Radial stiffness of annular (ring-type) gas path seals is calculated for both constant-clearance designs and tapered designs for which the inlet clearance is larger than the outlet clearance. Under some conditions a constant-clearance seal can have a negative stiffness; this undesirable property can be completely eliminated by use of tapered seals. Leakage rates are only moderately higher in tapered seals.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):356-363. doi:10.1115/1.3453375.

Microscopic effects, generated by micromotions of particles in suspension in a viscous fluid, drastically change the character of the flow between solid walls. To the modified momentum and continuity equations, an equation of angular (spin) particle momentum is added. A vectorial system of equations is presented, for variable material coefficients. General properties of this system are discussed and differential equations for pressure and velocity field are derived. For constant viscosity and micropolar coefficients across the lubricating film, important simplifications lead to easier workable expressions. Under this assumption, the short bearing performance has been analyzed. The fluid pressure increase (compared to Newtonian flow) is represented by a surface depending on two groups of micropolar parameters; the overall bearing characteristics also exhibit larger values with respect to simple Newtonian lubricants. However, for similar gap geometries, the friction coefficient has lower values. Some formulas, regarding the velocity field, friction stresses and side flow, are general and may be applied to any bearing length-diameter ratio.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):364-375. doi:10.1115/1.3453376.

The new principle of flow self-regulation is applied to hydrostatic screws and nuts. These self-regulating hydrostatic screws and nuts permit a continuous and automatic distribution of the total flow in two half flows in the two recesses merely due to the special shape of the double thread. Formulas are presented and compared with those of ordinary hydrostatic screws and nuts supplied by two pumps. The comparison shows that the self-regulating screws and nuts have a stiffness equal to that of ordinary ones supplied by two pumps, and also that they have pumping and friction dissipated power and therefore a total dissipated power generally equal to those of ordinary ones supplied by two pumps. Their global efficiency is comparable to that of ordinary hydrostatic screws and nuts supplied by two pumps and is consequently better than that of the ordinary ones supplied by only one pump but through two flow regulators. A calculation example is presented, of a self-regulating hydrostatic lead-screw and nut, dissipating minimum pumping power. Finally a prototype of a screw and nut of this kind is shown.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):376-380. doi:10.1115/1.3453377.

Seizure resistance of the cast graphite particle-aluminum composite alloys, containing graphite particles of varying sizes has been studied using a Hohman wear tester. The size of the spheroidal graphite particles was varied from 30 μm to 400 μm, and in one case 80 μm size flake graphite was used to observe the effect of shape of graphite. When the graphite content of graphitic aluminum alloys is more than 2 percent, these alloys can be self-mated under condition of boundary lubrication without seizing. The size and shape of the graphite particles had no significant effect on the seizure resistance of these alloys, in the range of conditions investigated in this study. This is attributed to the extensive deformation and fragmentation of graphite due to the low yield strength of the aluminum matrix and the low flow stress of the graphite particles. During wear, the deforming aluminum matrix accentuates the deformation and fragmentation of subsurface graphite particles and causes them to come to the mating surface, thus providing continuous lubrication and preventing seizure. Even after a short run-in period, a continuous layer of graphite is observed on the mating surfaces of graphite particle-aluminum composite alloys. This layer persists even after extensive wear deformation.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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

TECHNICAL BRIEFS

J. of Lubrication Tech. 1979;101(3):381-385. doi:10.1115/1.3453378.

In this analysis the customary neglected centrifugal effects on the performance of hydrostatic porous thrust bearing with incompressible lubricant has been studied and the effects of their interaction of pressure distribution and load capacity illustrate the possibility of replacement of the nonporous bearing material by porous one.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):385-386. doi:10.1115/1.3453379.
Abstract
Topics: Equations
Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1979;101(3):386-389. doi:10.1115/1.3453380.

Using a numerical method the film thickness and the pressure distribution in hydrostatic extrusion of a work-hardening material under hydrodynamic conditions are determined. A minimum or critical speed for full fluid lubrication to develop is predicted. The effect of the length of die-land on the critical speed, and the effect of speeds above the critical speed on the extrusion pressure are also presented.

Commentary by Dr. Valentin Fuster

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