Effect of Air and Helium on the Head–Disk Interface During Load–Unload

The tribological characteristics of the head–disk interface are investigated during load–unload for air and helium-filled drives as a function of the pitch static angle and the roll static angle between slider and disk. A custom-made experimental tester inside a sealed environmental chamber was used to determine the regions of “safe” pitch static angle and “safe” roll static angle in air and helium environment during the load–unload process. The presence of head–disk contacts during load–unload were evaluated by measuring the acoustic emission signal and the decrease in rotational speed of the spindle. Scanning electron microscopy and optical surface analysis were used to investigate wear of the slider and the redistribution of lubricant on the disk surface after 10,000 load–unload cycles. The results indicate that the tribological performance of the head–disk interface is improved in helium environment compared to air environment.     

Clearance Regulation of Mechanical Gas Face Seals: Part II—Analysis and Control

This article presents the analysis and control of noncontacting mechanical gas face seals based on the state space model developed in Part I. Methods to analyze the controllability and observability of axial and tilt modes are described. The controllability analysis determines to what extent the dynamic response of the seal system modes can be shaped in a closed-loop feedback system, and the observability analysis determines if the seal system modes can be reconstructed from specific state measurements of the axial clearance and stator tilts. The error state-space method is employed to design a tracking controller to regulate the seal at a prescribed axial clearance. The control law is a function of all axial states; therefore, reduced order linear observers are designed to observe the unmeasured axial and tilt seal states. The axial clearance and tilt state estimates are used to reconstruct the gas film axial force and moments, which cannot be directly measured, for design and analysis. The analysis and control techniques are applied to the illustrative example presented in Part I. The results demonstrate that the gas film forces and moments can be estimated well and the seal system can be satisfactorily regulated with a sufficiently damped response that is within the bandwidth of today's electropneumatic actuators.     

Rheological Behavior of Greases: Part I—Effects of Composition and Structure

The aim of this work is to show the correlation between grease rheological behavior and its composition and structure (base oil viscosity, soap nature, and concentration).

A rheological investigation was carried out under the controlled stress mode. The results, obtained over different time intervals, show that grease behaves like a non-Newtonian viscoelastic material, with a narrow linear domain.

The soap nature and concentration are the dominant parameters when grease is submitted to low shearing. This was confirmed by TEM observations: the closer networks gave the highest rheological properties. Base oil viscosity influenced grease behavior only under high strain rates.

Some properties are discussed with respect to their influence on the contact replenishment, which is a critical point for mechanism lifetime prediction.

Finally, the authors suggest using rheometry as an alternative technique to standardized methods. Yield stress varies in the same way as penetration does, but is further influenced by the sample macrostructure.     

Coke Formation from Aircraft Engine Oils: Part II—Effects of Oil Formulation and Surface Composition

This is the second of two papers giving the results of a study undertaken to determine how aircraft engine oil degrades to form coke on oil-wetted surfaces. In part 2 the authors address the impacts of additives (which is related to oil types) and surface materials upon the oil coking process.

In Part 1 of this study, the authors showed that simple laboratory tests involving thin films of oil heated for specified timesltemperatures in open glass vials produced polymer and coke similar to the deposits seen on failed face seals taken from the operating aircraft engines and polymers isolated from used engine oils. Antioxidants inhibit and delay the coke producing reactions. After the antioxidant package is depleted to approximately 10 percent of the original level, the ester basestock undergoes accelerated oxidation to form oil-soluble polymers. As the oil spends additional time at elevated temperature, these polymers increase in molecular weight and undergo minor compositional changes becoming insoluble in the oil, producing deposits. If the time that a thin oil layer spends on a hot surface at elevated temperature can be limited so that the antioxidant does not deplete completely, coking can be prevented. This shifts the focus from coke minimization to coke prevention.

The authors results in Part 2 indicate that oil choice—which is largely a choice of antioxidant package—makes a large difference in how long oil can remain on a hot surface without forming polymer/coke deposits. For the oils studied in Part 2, the capabilities of the oils' antioxidant packages to inhibit coke formation varied 15 fold.

Surface material choice has minimal effect upon the rate of antioxidant depletion. However, once the antioxidant in the oil in a thin layer has become ineffective in inhibiting accelerated oxidation (approximately 10 percent of original concentration), the material the oil resides upon strongly effects the rate of the polymer/coke formation processes. Stainless steel speeds the process compared to a glass substrate.     

The Analysis of the Lubrication Condition and Friction Losses of a Single Acting Cross Head Guide Shoe of a Low Speed Cross Head Diesel Engine: Part II — A Practical Model for the Determination of the Oil Film Thickness

The cross head guide shoe plate of a large diesel engine operates under a large normal force which presses it against the rails between which it executes both axial and transverse motion. In order to ensure the safe operation of an unconventional bearing such as the guide shoe system of the cross head engine, an algorithm that can be used to predict accurately the oil film thickness that can be achieved for all the engine's operating regimes, is invaluable. A solution of the three dimensional form of Reynolds differential equation can provide the requisite oil film thickness. However, with even the availability of very fast computers and large computer storage capacity this solution for a large axial bearing such as the guide shoe becomes impractical in terms of computation time and computer core requirements. This paper describes alternative simplified algorithm for accurate prediction of the oil film thickness developed in the guide shoe bearing at different operating regimes of the engine.     

Exploring Operation Mechanisms of the Flexible Metal-to-Metal Face Seal: Part II—Scoring and Leakage Analysis

The numerical model previously developed in Part I was applied to analyze the contact wetness, oil exchange rate, and surface temperature distributions within the sealing band of the flexible metal-to-metal face seal (FMMFS). The analysis showed that the above three parameters were essentially critical to the seal scoring and leakage. Both macro- and microgeometries of the FMMFS contribute to variations of seal performance.     

Antiwear Film Formation by ZnDTP,Detergent, and Dispersant Components of Passenger Car Motor Oils—Part II: Effects of Low-Molecular-Weight Dispersant on Film Formation

Passenger car motor oils (PCMOs) are undergoing a rapid evolution. Studies have found that some exhaust emission catalysts may be deactivated by phosphorus, largely derived from zinc dialkyldithiophosphate (ZnDTP), the mainstay antiwear and antioxidant agent in PCMO formulations for the past 50 years. Consequently, future engine oils will contain significantly reduced phosphorus levels. Since ZnDTP is the dominant antiwear and antioxidant in current PCMOs, lower phosphorus content will impact engine oil formulation strategies.

To better understand the effects of ZnDTP reduction on wear control, electrical contact resistance (ECR) studies have been carried out on blends containing ZnDTP, detergent, and low-molecular-weight (LMW) succinimide dispersant. In contrast to previous results obtained with high-molecular-weight (HMW) dispersant, the combination of ZnDTP and LMW dispersant gave an ECR trace closely resembling that of ZnDTP alone. Thus, the chemical structure of the succinimide dispersant can have a profound effect on ZnDTP antiwear film formation. ECR experiments on three-way combinations of ZnDTP + LMW succinimide dispersant + overbased phenate detergent provided a much better film than that from a similar formulation using an HMW succinimide dispersant. This study demonstrates that the ECR experiment is sensitive to the chemical structures of components controlling the function of modern PCMOs, making ECR a convenient tool to optimize the performance of the remaining ZnDTP in lower phosphorus PCMO formulations.     

A Mixed Lubrication Model for Journal Bearings with a Thin Soft Coating—Part II: Flash Temperature Analysis and its Application to Tin Coated Al-Si Bearings

Flash temperature has a profound influence on the tribological behavior of contact surfaces. An asperity flash temperature model is developed for journal bearings with a thin soft coating. The asperity flash temperature of tin coated 339 Al-Si alloy bearings in contact with case hardened steel journals is analyzed. Factors influencing the flash temperature are discussed. Correlations of the flash temperature with asperity roughness parameters, such as the RMS roughness and skewness values, are explored. It was found that the maximum rise in temperature increased as the roughness and skewness increased.     

Solid Lubrication of Silicon Nitride with Cesium-Based Compounds: Part II — Surface Analysis

Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) were used to characterize the wear surfaces of selected samples from Part 1 of the authors study. Results are presented for films generated on silicon nitride (Si₃N₄) originally coated with cesium oxytrithiotungstate (Cs₂WOS₃), cesium sulfate (Cs₂SO₄), and a hydrated cesium silicate (Cs₂O·3SiO₂·nH₂O), all applied in a sodium silicate binder (Na₂SiO₃). Results show the presence of mostly Si, O, and Cs within the wear tracks of post-tested specimens. In some cases, W and S were not detected on samples that originally contained these elements, suggesting that decomposition had taken place. To simulate the reactions that might occur in a tribo-contact, mixtures of Si₃N⁴ and Cs₂WOS₃ powders were heated in air to 700°C and analyzed using XPS and Bremsstrahlung-excited AES. It was found that Cs₂WOS₃ accelerates the formation ofSiO₂ on Si₃N₄ under static conditions. These results support our hypothesis that high temperature chemical reactions between the cesium-containing compounds and the Si₃N₄ surface form a lubricious cesium silicate film. A mechanism is proposed based on the glass-modifying tendency of alkali metals and the hot-corrosion of Si₃N₄     

Effects of micro-wedges formed between parallel surfaces on mixed lubrication – Part II: modeling

The hydrodynamic effects of micro-wedges formed by asperities on two parallel surfaces in parallel sliding have been proved to be significant on mixed lubrication in the Part I of the present research. In this part of study, a theoretical model for the concept of micro wedges is proposed and used for calculating the hydrodynamic load and friction. The results show that the main influential factor on hydrodynamic effects was the separation of the two rough surfaces. Such a separation governed by the heights of highest asperities determined the minimum gaps of the micro-wedges and so thus the hydrodynamic effects. The calculated results were consistent with experimental evidence, especially, in mixed friction.     

A Three-Dimensional Navier-Stokes-Based Numerical Model for Squeeze Film Dampers. Part 2—Effects of Gaseous Cavitation on the Behavior of the Squeeze Film Damper

The damping coefficients for a squeeze film damper (SFD) were determined and discussed in Part 1 using the full Navier-Stokes equations (NSE) coupled with a homogeneous cavitation model. In this continuation, Part 2, article these coefficients are introduced into the governing equations of motion to determine the trajectory of the rotor and its stability. The nonlinear response of the damper predicted by the NSE model is compared to results obtained from the application of the Reynolds equation. The influences of gas mass concentration as well as that of the amount of imbalance on transmissibility and eccentricity of the damper are also investigated.     

Hydrodynamic Effects on the Boiling Interface in a Misaligned,Two-Phase,Mechanical Seal—A Qualitative Study

A qualitative study of the hydrodynamic effect on the radius at which boiling occurs in two-phase, misaligned, mechanical seals with coning is presented. The more important parameters affecting the problem are identified and analyzed. It is found that hydrodynamic effects depend mainly on a modified Sommerfeld number. It is shown that in most practical cases the hydrodynamic effects can be neglected men at large face misalignment. It is further shown that the boiling interface in many cases is practically axisymmetric and at almost the same radius as for perfectly aligned faces. A factor that reduces the tendency to cavitate in misaligned, two-phase seals is also pointed out.     

Theoretical Analysis of Piston-Ring Lubrication Part II—Starved Lubrication and Its Application to a Complete Ring Pack

Most studies of piston-ring lubrication assume fully flooded lubrication in the cylinder-bore and piston-ring conjunction. However, the lubricant supply is not always adequate for fully flooded lubrication. This study incorporates the effects of lubricant starvation into the one-dimensional piston-ring analysis developed earlier and applies it to a complete ring pack. A system of three nonlinear equations is derived to solve the starved lubrication problem. A postulate for lubricant transport in a complete ring pack is also proposed. A computer code is developed to apply the starved lubrication model to a complete ring pack.

The findings reveal that lubricant starvation has an important effect on the minimum film thickness of a ring pack, especially at mid-stroke for compression rings. The role of oil-control rings and the evaluation of ring performance as a complete ring pack are also discussed.     

Coloured adaptive structured Petri-net Part II: deduction of the structured graph from the pregraph and application — modelling of a flexible workshop

In Part II of this paper, the authors describe the transformation of the pregraph into a structured coloured Petri-net. This step illustrates the different elementary processes as sequential control systems of the workshop and communications and binding types between these processes.     

The Durability of Gear and Disc Specimens—Part II: Post Failure Examination and Gear-Disc Correlation

In this paper the authors present a post failure examination of the gear and disc tests reported in Ref (I) on a standard carburized steel (BS SI56), a temper resistant carburized steel (AISI M50NiL), and a carburized and nitrided temper-resistant steel (AISI M50NiL Duplex).

An analysis of the relationship between the gear and disc tests is also presented. The fatigue lives of the gear tests correlated well with the chamfered disc tests. However, this was thought to be fortuitous, due to the different stressing of the specimens. The fatigue lives of the crowned disc tests gave longer lives than the gears for equivalent stresses. As the slide-roll ratio was increased towards that of the highest point of single tooth contact (?0.26) the correlation improved. At a slide-roll ratio of ?0.28 on the disc specimens, where the correlation of pitting life might have been expected to be the best, scuffing of the disc occurred, possibly due to the higher temperatures. Several factors thought to be responsible for the difference in lives between gear and disc specimens have been discussed. The roughness and slide-roll ratio have been deemed the most important.     

Formulation,rheology and thermal ageing of polymer greases—Part I: Influence of the thickener content

The aim of this work is to show the correlation between polymer greases’ rheology and its formulation. The tested polypropylene (PP) thickened greases were evaluated regarding their thickener content and its effect on the rheological properties. An artificial ageing procedure was performed by heating fresh grease samples in an oven to study the thermal degradation. The ageing evaluation was performed through rheological measurements, FT-IR spectra, oil loss, bleed-oil viscosity changes and bleed rate. The rheology measurements were performed on a rotational rheometer, emphasizing on the storage and loss moduli values at the Linear Visco-Elastic (LVE) region. The flow curve of each grease was also measured. A modified Herschel–Buckley model was applied and the data was correlated to the thickener content.     

Scuffing Behavior of Piston-Pin/Bore Bearing in Mixed Lubrication—Part II: Scuffing Mechanism and Failure Criterion

Surfaces and subsurfaces of test specimens that failed in scuffing were examined. A scuffing criterion is developed stating that scuffing failure would occur when the maximum surface tangential traction is larger than the modified shear strength. A scuffing uncertainty factor is introduced to reflect the influence of factors that affect wear and scuffing and the inaccuracy in modeling.     

Rheological Behavior of Greases: Part II—Effect of Thermal Aging,Correlation with Physico-Chemical Changes

The thermal degradation of greases was studied from the viewpoint of both rheological and physico-chemical changes. Rheological investigations showed that lithium greases are weakened by thermal aging while urea greases are strengthened. Structural, chemical, and composition modifications were the most likely changes in the grease samples.

The occurrence of structural changes was confirmed by Transmission Electron Microscopy (TEM). experiments. Lithium networks were degraded by aging, but urea networks were not degraded and even seemed to be reinforced. Chromatography (SEC), FT-IR spectrometry, and filtration techniques were used to obtain information on the composition and on the chemical aspects. The main phenomenon observed in these tests concerns the lithium greases, which show a relatively important loss of thickener.

Finally, this work showed that parameters obtained in continuous and dynamic rheological tests seem to be qualified to represent the modifications occurring in greases.