Mark T. Devlin

Fundamentals of anti-shudder durability: Part I - Clutch plate study

In automatic transmission technology development the degradation of paper friction plates has often been considered a major failure mechanism by which transmissions lose their anti-shudder characteristics. One of the most common degradation processes for paper friction plates is known as glazing. In this study, we focus on the relationship between friction plate glazing and anti-shudder durability in the Japanese Automobile Standards Organization (JASO) low velocity friction apparatus (LVFA) rig test following the procedure M349-98. We also investigate the impact of used friction plates and used oil on torque capacity durability as measured by an SAE No. 2 machine following the JASO procedure M348-95. We find that friction plate glazing has no correlation with anti-shudder durability. A completely glazed plate can have long anti-shudder durability but a barely glazed plate can have short anti-shudder durability. The basic reason for the lack of correlation is that friction plate glazing does not affect boundary friction coefficients. In almost all the cases studied, changes in fluid properties have a greater influence on boundary friction than does surface glazing.

Friction Reduction of Lubricant Base Oil by Micelles and Crosslinked Micelles of Block Copolymers

Diblock copolymers with one block soluble and the other block insoluble were dispersed in an industrial base oil (BO) to yield spherical micelles (SMs). SMs were also prepared in more manageable solvents that had similar solubility properties as the BO towards the copolymers but had lower viscosities and lower boiling points and absorbed less in the near UV region. The photocrosslinking of the cores of the latter micelles yielded crosslinked micelles or nanospheres. We have tested the lubrication properties of the micelle and nanosphere samples in BO under conditions simulating those found in automobile engines. Solutions of micelles and nanospheres with 2-cinnamoyloxyethyl acrylate units in their cores exhibited a unique friction reduction pattern and had friction coefficients that were significantly lower in the boundary lubrication regime (BLR) than in the mixed lubrication regime. Such particles reduced the friction of the BO by > 70% in the BLR and performed substantially better than the widely used industrial anti-friction agent glyceryl monooleate. The factors affecting this unique friction reduction behavior were investigated and a possible reason for it was proposed.

Enhancement of Engine Oil Wear and Friction Control Performance through Titanium Additive Chemistry

Traditionally, wear protection and friction modification by engine oil is provided by zinc dithiophosphate (ZDDP) or other phosphorus compounds. These additives provide effective wear protection and friction control on engine parts through formation of a glassy polyphosphate antiwear film. However, the deposition of phosphorus species on automotive catalytic converters from lubricants has been known for some time to have a detrimental effect of poisoning the catalysts. To mitigate the situation, the industry has been making every effort to find ZDDP-replacement additives that are friendly to catalysts. Toward this goal we have investigated a titanium additive chemistry as a ZDDP replacement. Fully formulated engine oils incorporating this additive component have been found to be effective in reducing wear and controlling friction in a high-frequency reciprocating rig (HFRR), 4-ball bench wear, Sequence IIIG, and Sequence IVA engine tests. Surface analysis of the tested parts by Auger electron spectroscopy, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS) have shown that Ti species have been incorporated into the wear tracks and can only be found on the wear tracks. We used synchrotron based near edge X-ray absorption fine structure (NEXAFS) to investigate the chemical bonding mechanism of the Ti additive with the metal surface that affects the wear improvement mechanism. We postulate that Ti provides antiwear enhancement through inclusion in the metal/metal oxide structure of the ferrous surface by forming FeTiO3.

ATF Friction Properties and Shift Quality

Multiple plate disk clutches are used extensively for shifting gears in automatic transmissions. In a shift from one gear to another one or more clutches is engaging or disengaging. In these active clutches the automatic transmission fluid (ATF) and friction material experience large changes in pressure P, temperature T, and sliding speed v. The coefficient of friction, μ, of the ATF and friction material depends on v, P and T, and also changes during clutch engagement. Changes in μ can lead to vibration and poor shift quality if the ATF and clutch friction material are improperly selected. An in-depth theoretical understanding of the cause of vibration in shifting clutches is crucial in the development of a suitable ATF to work with a particular friction material. To understand the relationship between ATF friction properties and shifting clutch vibration we present a theoretical model that identifies several possible causes: (1) self-excitation instability, (2) a reduction in friction holding torque during engagement, or (3) resonance caused by periodic pressure oscillations. The ATF and friction material properties that affect these sources of vibration include the friction level, μ, and the friction slopes with respect to sliding speed ∂μ/∂v, pressure ∂μ/∂P, and temperature ∂μ/∂T. These properties must be carefully balanced to ensure that a clutch will deliver good shift quality and high torque capacity with effective vibration suppression.

An Investigation of Hydraulic Motor Efficiency and Tribological Surface Properties

Hydraulic motor efficiency does not depend upon viscosity alone. Under low-speed, high-torque conditions, hydraulic motors operate in the boundary regime and, therefore, surface interactions of lubricant additives can affect friction and efficiency. This article presents an investigation of boundary film formation, friction, and surface topography in benchtop tribometers and hydraulic motors. Fluids investigated included those with varied antiwear packages (zinc dialkyldithiophosphate [ZDDP], ashless) and friction modifiers (with and without) and base oil (Group I, Group III). The mechanical efficiencies of geroler, axial piston, bent-axis, and radial piston motors were measured under low-speed, high-torque conditions. The addition of a friction modifier to an ashless hydraulic fluid increased the efficiency of the motors at low speed. Energy-dispersive X-ray spectroscopy (EDX) analysis of motor surfaces after testing revealed the presence of tribochemical films from the hydraulic fluid additives. In benchtop tribometer testing, the friction modifier reduced friction significantly but also increased wear. This could be related to surface competition of the friction modifier and antiwear chemistries, as evidenced by the reduced concentration of phosphorus on the surface. These findings are significant because they provide insights toward the development of fluids that can enhance motor efficiency but also demonstrates the need for a well-balanced additive package so that improved motor efficiency can be achieved without affecting other important properties of the fluid.

Lubricant Chemistry and Rheology Effects on Hydraulic Motor Starting Efficiency

The starting efficiency of hydraulic motors is critically important because it is often a limiting criterion in hydraulic system design. One means of improving starting efficiency is to optimize the chemistry of the lubricating fluid. To understand the potential effect of new hydraulic fluid formulations, it is necessary to characterize their physical properties and performance from multiple perspectives. In this work we report a comprehensive characterization of five lubricating fluids with the same viscosity but different additive and base stock compositions. Analysis includes measurement of frictional behavior, thermophysical and rheological properties, and mechanical efficiency in three different hydraulic motors. Trends were analyzed and correlations between the various fluid properties were identified. The results illustrate the importance of operation-specific fluid formulation and comprehensive lubricant characterization.

Characterization of Deposits Formed on Sequence IIIG Pistons

In the latest passenger car motor oil specifications the Sequence IIIG engine test is used to determine the ability of lubricants to control piston deposits. We have analyzed the chemical composition of Sequence IIIG deposits in order to determine the source of the piston deposits and determine if the mechanism for deposit formation in the Sequence IIIG engine test is similar to previously published mechanisms for formation of high temperature engine deposits. These previous mechanisms show that combustion by-products react with lubricant in the piston ring zone. The mixture of combustion by-products and lubricant are oxidized to form deposit precursors which are further oxidized to form deposits. Since the Sequence IIIG engine test uses lead-free fuel it is important to reexamine the nature of piston deposits formed in gasoline engines and in particular in the Sequence IIIG engine test. Using thermogravimetric, infrared and SEM/EDS analyses we discovered that Sequence IIIG deposits contain a significant amount of carbonaceous material. This carbonaceous material appears to be a deposit formed by the Sequence IIIG fuel. In addition, the Sequence IIIG deposits are quite different from Sequence IIIE deposits since they do not appear to be nitrated or contain lead sulfate.

Effects of Gear Oil Properties on Pitting Life in Rolling Four-Ball Test Configuration

There is a connection between the efficiency of oils and their wear and/or surface damage protective properties, an area not so well described in the literature. One such damage mode is macroscale contact fatigue on gear tooth flank surfaces, also called pitting. The present study is aimed at investigating the correlation between gear oils’ physical properties, important in terms of gear transmission losses, and pitting life. Eight gear oils were formulated giving different combinations of base oil, viscosity, and concentration of friction modifiers. All eight oils also contained an additive package designed to meet GL-5 specifications. This study consists of three parts. In the first, the oils’ physical properties were measured using a set of bench tests. In the second, the pitting lives of the oils were evaluated using rolling four-ball tests. The third part deals with the correlation between the measured physical properties of the oils and their pitting lives. This is achieved through multiple linear regression, with a view to finding the salient properties that have a significant influence on pitting life. The results show that gear oils’ physical properties do have a large influence on the pitting lives. Oil properties that lower interfacial tangential stresses are beneficial in enhancing pitting life.

Temporary and Permanent Viscosity Loss Correlated to Hydraulic System Performance

ABSTRACT Straight- and multigrade fluids were evaluated in a hydraulic dynamometer that incorporated a pressure-compensated axial piston pump and a fixed displacement axial piston motor. Pump, motor, pressure compensator, and directional control valve internal flow losses were determined under various conditions of pressure, speed, and temperature. Fluid samples were collected before and at various times during the dynamometer experiments, and viscosity measurements were performed to probe for correlations between viscosity, operating time, and system leakage flow losses. The low shear rate viscosities of the multigrade fluids decreased linearly throughout the duration of testing due to polymer degradation. However, system flow losses did not exhibit a statistically significant increase as the multigrade fluids sheared down. The fluids were also characterized by their permanent viscosity loss produced in sonic shear and tapered bearing tests and by their temporary shear thinning measured in an ultra-high-shear viscometer at several temperatures. The effects of these viscous properties were analyzed using an empirical model to identify which measures of viscosity were most correlated with flow loss. The results suggested that the relative contributions of temporary and permanent viscosity loss change as the fluid is used. Further, analysis of torque loss and input power revealed that input power and losses are more useful indicators of the effect of fluids on hydraulic system performance than pump efficiency.

Tribological Film Formation in Hydraulic Motors

This paper presents an investigation of the tribological films formed in hydraulic motors. Hydraulic motors convert the fluid power energy produced by positive displacement pumps into rotary motion. Earlier research found that the efficiency of this energy transformation can be enhanced by reducing boundary friction. In order to study the nature of the boundary films formed in an orbital motor, a prototype ashless hydraulic fluid was evaluated in a low-speed high-torque dynamometer. The resulting tribofilm was probed via Energy Dispersive X-Ray Spectroscopy. The results reveal that increasing the hydraulic system temperature raised the relative phosphorus level of the tribofilm.Copyright