Modeling Viscous Isothermal Piston Skirts EHL in Very Low Initial Engine Start Up Speeds

In the normal low-speed engine operation, elastohydrodynamic lubrication (EHL) of piston skirts and lubricant rheology reduce friction and prevent wear. In a few initial start up cycles, a very low engine speed and absence of EHL cause adhesive wear. This study models hydrodynamic and EHL of piston skirts in the initial very low cold engine start up speed by using a high viscosity lubricant. The 2-D Reynolds equation is solved and inverse solution technique is used to calculate the pressures and film thickness profiles in the hydrodynamic and EHL regimes, respectively. The work is extended to investigate the effects of three very low initial engine start up speeds on the transverse eccentricities of piston skirts, film thickness profiles and pressure fields in the hydrodynamic and EHL regimes. Despite using a viscous lubricant, thin EHL film profiles are generated at low start up speeds. This study suggests very low speed optimization in the cold initial engine start up conditions to prevent piston wear under isothermal conditions.

A Fast Method for Solving Contact Plasticity in a Half Space

This paper presents a new method of contact plasticity analysis based on Galerkin vectors to solve the eigenstresses due to eigenstrain. The influence coefficients relating eigenstrains to eigenstresses thus can be divided into four terms the one due to the eigenstrains in the full space, others due to the mirrored eigenstrains in the mirror half space. Each term can be solved fast and efficient by using the three-dimensional discrete convolution and fast Fourier transform (DC-FFT) or the three-dimensional discrete correlation and fast Fourier transform (DCR-FFT). The new method is used to analyze the contact plastic residual stresses in half space.

On the Static-Kinetic Friction Transition in Dry Contact of Rough Surfaces

This paper shows that the Mindlin problem involving two spheres in contact under the action of oscillating tangential force can lead to the account of static-kinetic friction transition. In Mindlin’s problem two spheres experience partial slip as a result of application of oscillating tangential load. When the problem is extended to multi-sphere contact, i.e. two rough surfaces, the application of tangential oscillating load results in partial slip for some asperity contacts while others experience full slip. Increase in the amplitude of the oscillating tangential force results in more contacts experiencing full slip, thereby decreasing the number of contacts in parial slip. Constitutive relation proposed by Mindlin at small scale, governing asperity interaction, is used to obtain the large scale slip function through a statistical summation of asperity scale events. The slip function establishes the fraction of asperity contact in full slip. The complement of the slip parameter is a fraction of asperities in partial slip. Through slip function it is shown that it is possible to define a slip condition for the entire surface. The derivation of the slip function allows the account of transition between static friction and kinetic friction.

Optimization of Magnetorheological Brake

Quick response and rheological properties as a function of magnetic field are well known features of MR fluids which inspire their usage as brake materials. Controllable torque and minimum weight of brake system are the deciding functions based on which the viability of the MR brake against the conventional hydraulic brake system can be judged. The aim of this study is to optimize a multi-disk magneto-rheological brake system considering torque and weight as objective functions and geometric dimensions of conventional hydraulic brake as constraints. The electric current accounting magnetic saturation, MR gap, number of disk, thickness of disk, and outer diameter of disk have been considered as design variables. To model the behavior of MR Fluid, Bingham and Herschel Bulkley models have been compared. To implement these models in estimating the braking torque a modification in shear rate dependent component has been proposed. The overall design of MR brake has been optimized using a hybrid (Genetic algorithm plus gradient based) optimization scheme of MATLAB software.

Wear Mechanisms of PTFE in Humidified Hydrogen Gas

PTFE is used as sealing material of machine elements in hydrogen utilizing machine systems, such as fuel cell vehicles and related infrastructures. It is necessary to know the tribological property of sealing materials in hydrogen gas to realize safety and reliability of machine elements operated in hydrogen environment. In this study, humidity in gases was focused on and its effects on the friction and wear of rubbing pair of PTFE pin and AISI 316L disk was investigated in pin-on-disk wear apparatus. The result indicated that the humidity in hydrogen gas had little effect on the friction coefficient between PTFE and AISI 316L. However, the specific wear rate of unfilled PTFE was clearly affected by the humidity. The amount of PTFE transfer film formed on the stainless surface gradually decreased with decreasing the humidity in hydrogen gas. The similar results could be obtained in inert argon gas as well. Water molecules remained in gaseous environments would be included in the formation process of PTFE transfer film affect on formation of PTFE transfer film. The humidity in hydrogen gas should be regulated to ensure the tribological behavior of the PTFE/stainless sliding pair being used in the hydrogen environment.

Study of Elastohydrodynamic Film Shape Under Different Directions of Velocity Vectors

Some machines elements such as hypoid gears and Wildhaber-Novikov gears works under conditions where the surface velocities are variously oriented, thus the entrainment and sliding velocities act at different directions. The influence of sliding velocity vector direction with respect to entrainment velocity vector has not been sufficiently determined yet. So, the aim of current experimental study is to investigate effect of slide-to-roll ratio and direction of sliding velocity vector with respect to entrainment velocity vector on elastohydrodynamic (EHD) film thickness using thin-film colorimetric interferometry in smooth glass-steel point contact.

Friction and Wear Characteristics of H25, H188, H214 Against Hastelloy X

Increasing, demand for more efficient power generation forces turbomachinery to operate at higher temperatures and compression ratios. High speeds combined with high temperatures make turbomachinery sealing applications even more challenging. In order to confirm sufficient service life, seal material pairs should be tested similar to engine operating conditions. The high temperature friction and wear characteristics of cobalt superalloys, Haynes 25, 188 and 214 sheets, rubbed against Hastelloy X pins are presented in this work. Tests are conducted at 25, 200 and 400°C with a validated custom design linear reciprocating tribometer. Sliding speed and distance are 1Hz and 1.2 Km respectively. Friction coefficients are calculated with friction force data acquired from load cell and the dead weight where wear coefficient is calculated through mass loss after the tests.

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.

Enhancement of Tribological Performance of 6061 Aluminum Alloy by Second Phase Hardening via Friction Stir Processing

The properties of metallic alloys can be substantial modified by the addition of a second phase particles. This is especially noticeable when hard particles are incorporated in a relatively soft matrix, often resulting in improved mechanical and tribological performance. This paper presents the results of our study on mechanical and tribological performance enhancement of 6061 Aluminum alloys by incorporation of B4C particle via Friction stir processing (FSP). Unidirectional ball on flat friction and wear tests were conducted with a base material, friction stir processed 6061-Al and 6061-Al doped with B4C particles via FSP against 52100 bearing steel balls under dry sliding conditions. The incorporation of particles not only reduced friction by 30% but also reduced wear by 2 orders of magnitude compared to unprocessed base and FSP material without particles incorporation. FSP alone without particles addition did not have a significant effect on the tribological behavior of the tested aluminum alloy.

Adhesive Contacts for Graded-Elastic Coatings Using Fourier Series Decomposition

We propose a semi-analytic solution technique to determine the subsurface stresses and local deflections resulting in an adhesive contact of graded elastic layers. Identical pressure distributions, typical for a Maugis parameter λ = 1, were applied to a range of graded elastic coatings. The principal stresses and surface deflection in both regions (graded elastic layer and substrate) are computed in terms of Fourier series. This control case has the advantage that the response of different coatings can be easily monitored and compared.