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FOREWORD

J. of Lubrication Tech. 1968;90(3):525. doi:10.1115/1.3601591.
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Abstract
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. of Lubrication Tech. 1968;90(3):526-530. doi:10.1115/1.3601592.

In thick-film lubrication, Reynolds’ equation is generally satisfactory. However, the assumptions made in deriving this equation cannot be justified for non-Newtonian, viscoelastic liquids. It is concluded that no satisfactory mathematical treatment is yet available for calculating the load-carrying capacity of such liquids. In thin-film lubrication, elastohydrodynamic calculations indicate that the lubricant film may be quite thick even under heavily loaded conditions, but discrepancies exist between calculation and experiment. These can be explained by assuming non-Newtonian behavior, or unusual viscoelastic effects, but the assumptions are largely unfounded. There is virtually a complete absence of data on the behavior of liquids under impact loading. Such data are needed to resolve whether thin-film lubrication is primarily chemical or primarily physical.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):531-539. doi:10.1115/1.3601593.

A substantial body of qualitative information concerning highly viscoelastic materials suggests they may be much superior to Newtonian fluids in some lubrication applications. Contrarily, most quantitative studies have predicted negligible or even adverse effects of the non-Newtonian behavior. This paper is accordingly addressed to a broad consideration of rheological properties and a critical analysis of the validity of common simplifying assumptions, in order to educe major effects which have not been well understood. Largely omitted are those rheological factors which have already been subjected to intensive scientific study. It is seen that the behavior of viscoelastic fluids in elongational deformations and in deformations of short duration are of primary interest; positive effects of two orders of magnitude have been shown in closely related problems. It is shown that the exploitation of these non-Newtonian rheological properties requires favorable geometric and kinematic conditions in a bearing which may differ appreciably from those employed with Newtonian fluids. The conclusions reached are speculative rather than definitive because of the primitive state of the development of this branch of rheology at the present time. They may be helpful, however, and do serve to direct attention to several kinds of lubricant-rheological studies requiring special emphasis.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):543-547. doi:10.1115/1.3601596.

Polymer solutions such as polymer-containing (VI-improved) oils show a variety of mechanical effects in flow that differ qualitatively from the ones observable under the same conditions in Newtonian liquids. This has, of course, been realized for a long time but the reason for their occurring has been traced to different sources. The point of view that the author has supported for a number of years, actually his idea for this happening, is that polymer solutions become elastic in steady flow. This thesis can be supported beginning with molecular theory through the mechanics of observable effects down to tests by a large variety of experimental methods. This point of view is by no means universally accepted in the rheological community. This elasticity causes “normal stresses” that may influence the load-carrying ability of journal bearings. However, the great problem is measuring their properties at the rates of shear occurring in practice (in the millions sec−1 ) and the existing eccentric geometry.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):549-552. doi:10.1115/1.3601601.

A general basis for discussing nonlinearity in the flow of molecular fluids consists in applying the continuum mechanics of Coleman and Noll to a flow process governed by an Arrhenius activation energy equation. The theory predicts the familiar exponential increase of viscosity with pressure and a decrease in viscosity with high shear stress, and also predicts the existence of “normal stresses” under high shear stress. Schematic calculations are presented for the behavior of a lubricant under extreme stress.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):555-559. doi:10.1115/1.3601607.

The dynamic response of polymer fluids to small sinusoidal shearing motions may be characterized by their relaxation spectra. Recent experiments show that great changes can be induced in the relaxation spectrum by steady shearing of the sample. This is the case of most relevance to bearing studies in, for example, a dynamically loaded journal bearing. It is shown that almost all relaxation processes longer than a small multiple of the (shear rate)−1 are removed by steady shearing. This result implies that in a bearing undergoing dynamic loading with Fourier components which are low harmonics of the shaft speed a polymer fluid is expected to behave very like a quasi-Newtonian fluid with variable viscosity. The action of a sheared squeeze film is also considered and the implications for synovial joint lubrication are briefly mentioned.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):561-569. doi:10.1115/1.3601616.

Based on an examination of the characteristics of shear viscosity versus shear rate, it was postulated that high tensile and compressive stresses might exist in certain liquids at very high shear rates. If obtainable, these stresses could be important as load-bearing mechanisms in high-speed machine elements, and as a sealing mechanism in radial face seals. Such stresses should be evident in a polymer fortified oil, or in a liquid comprised of molecules possessing an appreciable length to width ratio. Therefore, a jet reaction viscometer reaching 107 sec−1 shear rate was developed to explore this possibility. Tests with polyisobutylene dissolved in a kerosene showed that elastic stresses were dominant with respect to viscous stresses at high shear rates. Tensile stresses up to more thn 1000 psi were obtained. However, the life of the polyisobutylene molecule was short. Hence it is concluded that normal stresses of appreciable magnitude can exist in high-speed machine elements under favorable conditions to affect their operation.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):571-576. doi:10.1115/1.3601620.

Solutions of high molecular weight linear polymers in very viscous solvents exhibit two peculiar effects even at rather low concentrations. With increasing gradient the viscosity first drops to a minimum and then increases over a wide gradient range. Moreover, prolonged shearing at constant gradient induces an increase of viscosity which dependent on concentration and gradient either continues with steadily decreasing slope or after a while turns over into a gradual decrease with a finite limit not much different from the starting value. The first effect is partly due to an actual increase of intrinsic viscosity with increasing gradient and partly to the time effect. The former contribution is proportional to the concentration, the latter to the second or higher power of concentration. Both effects occur with the polymer improved lubricating oils.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):580-590. doi:10.1115/1.3601625.

The advancement of the fields of elastohydrodynamic lubrication and high pressure metal forming in the past few years has focused attention on the need for reliable data of the variation of viscosity with pressure, temperature, and shear stress in well-defined fluids. This paper describes an investigation in which these effects were observed. The equipment used was a high pressure capillary-type viscometer which made possible the continuous variation of shear stress over a wide range at pressures up to 80,000 psi. Well-defined paraffinic and naphthenic base oils and several polymer blends of these oils were investigated as well as a polybutene, a diester, and two silicone fluids.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):592-602. doi:10.1115/1.3601628.

The majority of non-Newtonian materials exhibit an out-of-phase stress-strain relationship (i.e., evidenced by the fundamental and associated harmonics) over a wide range of frequencies. This nonlinear response of the material when subjected to a periodic disturbance of amplitude γ, say, may be due in part to viscoelasticity, thixotropy, or a combination of the two. Other factors such as particle size and concentration can also contribute to this effect. Apparatus has been developed which enables harmonic analysis to be carried out with comparative ease in the low to mid-frequency range; the nonlinear response of a material offers no experimental problems. The experimental work reported here on a soap and synthetic base grease confirms the views that: Greases are nonlinear viscoelastic in response over the whole range of strain amplitude, frequencies, and temperatures investigated. Particle size and concentration are observed to influence the flow stability of each grease. At small strain amplitudes the synthetic base grease is mainly elastic in response, but the stress moves out-of-phase with strain as the strain amplitude is increased. The behavior of the stress fundamental component suggests yielding of the structure. The harmonic content appears to be more sensitive to changes in strain amplitude and temperature than the fundamentals. The method of reduced variable has been applied to the fundamental and associated harmonics with reasonable success, supporting the implications of this method that increasing the temperature is equivalent to decreasing the frequency. An approximate method of correlating dynamic and steady shearing motion results in terms of the function α (phase shift in dynamic response) is outlined, the resulting correlation being difficult to apply mainly because of the mathematical shortcomings.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):604-607. doi:10.1115/1.3601633.

Experiments were performed in a coaxial cylinder viscometer in which there were realized separately and simultaneously flows of grease and bright stock (Newtonian fluid) in the axial and peripheral direction. The latter was at uniform shear stress field. The effect of peripheral on axial-flows was a great decrease in resistance toward deformation in the axial direction. This is a very important peculiarity of greases, which are non-Newtonian media, when considering their flow in labyrinth boxes and some other devices. The effect of axial on peripheral flow showed somewhat increased resistance toward the latter. Such behavior was attributed to continuous inflow of fresh grease into the annular space where it did not remain long enough for structural destruction to reach a limiting value.

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):614-617. doi:10.1115/1.3601638.

Rolling element bearings are vibration generators, and in addition are stiff, so that they transmit rotor generated noise as well to the machine frame and casing. Self-acting (hydrodynamic) bearings are also very rigid, so that they are excellent transmitters of rotor generated vibration, e.g., front gear teeth, turbine blading, or magnetic hysteresis. A typical rotor weighing 1000 lb may be supported on bearings having a stiffness of 3 × 106 lb/in., and hence will be a good vibration transmitter up to a frequency of 172 cps. Hydrostatic bearings afford an opportunity to control the bearing frequency response so that attenuation of middle and high frequencies can be secured. Analysis of the hydrostatic bearing as a closed-loop servomechanism reveals methods of designing them for attenuation without serious consequences in other performance factors. They may be used as the primary bearing, or as separate isolator bearings in conjunction with rolling element or self-acting bearings. Some examples of possible applications are discussed.

Commentary by Dr. Valentin Fuster
J. of Lubrication Tech. 1968;90(3):618-629. doi:10.1115/1.3601639.

The conventional method of calculating static load allowables for rolling element bearings is based on a brinell which will not affect the rotational fatigue life of the bearing. For an oscillating bearing requiring only limited cycle life, this is a very conservative approach. Increased allowables are possible where static allowables are based on 2/3 of the fracture capacity of the bearing, provided performance of the bearing is not adversely affected by these loads. Results of tests performed on one in. dia balls and 3/8 in. dia rollers are reported. Fracture capacities were determined and permanent deformation as a function of load is listed for the five material combinations tested. Permanent deformation variation at temperatures to 1500 F is discussed. The effect of lubricants and the significance of brinell indentations on rolling friction and on endurance life is reported. Static allowables based on maximum Hertz stress values are listed for each material combination from data obtained. These allowables are much higher than those presently used. Maximum Hertz stress static allowables to 1,000,000 psi can be considered with high strength steel for both ball and roller specimens. Tests on six and one half in. dia ball and roller bearings confirmed that the bearings could withstand very high static loads and still operate at high dynamic loads for the number of cycles required in wing pivots and other similar applications.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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

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