C. M. Ettles

Some Parameters Affecting Tactile Friction

The friction of a sliding tactile contact was measured in an apparatus which simulated a keyboard. Results were taken for several materials. The friction coefficient was found to decrease with increasing load and with increasing speed. Experiments at varying humidity and surface roughness helped to define the friction mechanisms. It is concluded that tactile friction is predominantly adhesive, but modified by liquid bridging between the ridges of the skin and the counterface. Increased bridging due to higher humidity causes increased friction from viscous shearing effects, while increased roughness allows fewer bridges to form, decreasing the friction.

The Analysis of Pivoted Pad Journal Bearing Assemblies Considering Thermoelastic Deformation and Heat Transfer Effects

The pivoted pad journal bearing is a relatively complex assembly since each pad operates under different conditions and deforms by different amounts. A global analysis method is described that includes thermoelastic deformation of the pads and allows the temperature to vary radially and circumferentially in each of the pads and oil films. The analysis includes the calculation of the entry temperature to each pad and convective cooling from the pad surfaces, and is sufficiently compact to fit on a personal computer. The output of the method is compared with five sets of published results for bearings over a range of size and configuration. Presented at the 46th Annual Meeting in Montreal, Quebec, Canada April 29–May 2, 1991

The Measurement of Surface Temperature in Dry or Lubricated Sliding

This paper describes the development and calibration of an original test apparatus to study the effect of frictional heating on lubricant effectiveness. The configuration used the Seebeck effect between a stainless steel ball and a disk of tool steel. Good agreement was found between the average temperature rise indicated by the thermoelectric voltage and Archards models. Surface temperature rises approaching 200 o C were obtained in lubricated contact and an upper bound temperature of 830 o C was found in dry contact. The likely effect of track heating was assessed by analysis and,,for this configuration, was found to be negligible

Calculation of a Safety Margin for Hydrogenerator Thrust Bearings

During the design or refurbishment of a thrust bearing a question that occurs frequently is, “How safe is this bearing?” Factors of safety are widely used in engineering design, and some applications require compliance with safety factors that are specifically stated in codes of practice. Most manufacturers have limits on the maximum allowable bearing temperature. These can be useful as a guide but offer no graded answer as to how much safer (or less safe) a bearing is made by applying a particular design decision. A numerical factor of safety would give a quantitative measure of the safety of a bearing and would be useful in judging whether a proposed design change is worthwhile in terms of improved safety. Experience has shown that failure of a babbitt bearing often develops from a local yielding (and subsequent flow) at a discrete point on the surface. This develops into rippling and folding of the material and escalates rapidly to a bearing “wipe.” This article describes the application of the Tresca and von Mises criteria of yielding to give a numerical factor of safety against plastic flow of the babbitt. The method uses data for the yield point of the babbitt as a function of temperature and assumes that the material on the bearing surface is constrained in both lateral coordinate directions. In practice, any data on the failure of hydrothrust bearings are invariably regarded as proprietary by manufacturers and there is apparently no published work available that can be used to correlate the model. However, the method developed in this paper is quite general and applicable to any fully hydrodynamic bearing loaded to the yield point of the surfaces. Results for large babbitt-faced journal bearings that were deliberately taken to failure are used for a correlation. Reasonable agreement is shown between predicted and actual failure. The calculation of a factor of safety can be a useful tool for evaluating initial designs or modifications during refurbishment. Some factors concerning the assessment of an acceptable minimum film thickness are also discussed. Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2004 Manuscript approved June 1, 2005 Review led by Itzhak Green

The Effect of Frictionally Generated Heat on Lubricant Transition

A study was performed to establish the transition temperature of two additives in cetane under severe conditions of pressure × velocity, which gave a surface temperature rise within the contact of up to 150°C. The interface temperature was measured using the slider as one leg of a thermocouple and the disk as the other; hence, an accurate interface temperature could be determined as the velocity and load varied. The test specimens were M50 tool steel, AISI 52100 steel and type 440C stainless steel balls against a fully hardened tool steel disc. The results complemented previous work which showed the identical trends but at slightly different absolute temperature levels.

The Use of Higher Order Finite Element Methods for the Solution of Reynolds' Equation

Assessments are made of the accuracy and speed of execution of two quadratic element formulations applied to the thrust bearing problem. Methods of reducing the execution time are discussed and tested. Reduction of the order of integration is recommended. The nine noded (Lagrangian) quadratic element was found to give reduced but acceptable accuracy when applied to films where cavitation occurred. The higher order formulations were found to be superior in run time if better accuracy is required than normally obtainable with linear elements or with a finite difference scheme. Presented at the 44th Annual Meeting in Atlanta, Georgia May 1–4, 1989

The Analysis of Thermoelectric Signals in Metallic Sliding

The thermoelectric signal from an Alumel pin sliding over a steel surface was recorded and analyzed. The load, speed and surface topography were varied and correlations were attempted of the voltage signal against several parameters. The average temperature of the whole contact was found to agree fairly consistently with the surface temperature rise models of Blok and Archard. Of the available methods of signal analysis, the power spectrum of the voltage signal was found to give the best understanding of the micro-mechanisms of sliding, particularly when compared against the power spectra of profilometer signals. The largest component of the temperature signal was found to be caused by wavelength components equal to the extent of the contact. The correlation distance β* of the test surfaces was much less than the contact extent and contributed negligibly to the voltage signal rise.

Contrarotating Journal Bearings for Naval Propulsion Systems

Contrarotating propeller systems offer well-recognized advantages in propulsion efficiency. Unfortunately, circular bore journal bearings have no load capacity when used in contrarotation, but a quasi-squeeze effect may be induced by offset or preloaded sub arcs as these pass through the loaded region of the film. With this type of bearing, even when under steady load, the journal must orbit within the clearance space. The propagation of such orbits is described. The response is quantified by a Fourier analysis of the motion. Contrarotating journal bearings may be designed to support loads of the same general magnitude as conventional circular (non-rotating) bearings. Presented at the 46th Annual Meeting in Montreal, Quebec, Canada April 29–May 2, 1991