Newtonian mechanics in scale of minutia

This article commemorates Newton's contributions to mechanics in small scale. In particular, it deals with Newtonian slow viscous action of fluids in narrow conjunctions leading to hydrodynamics. It is shown that the Newtonian continuum relies on some bulk properties of fluids as opposed to their molecular interactions. When the latter and surface energy effects become dominant, the interaction potentials deviate from the Newtonian continuum. A plethora of largely empirically based force laws are used to describe conjunctional behaviours in nanoscale, usually lightly loaded. Some of these force laws are described, and their applicability to nanoconjunctions of very small devices and some biological systems is noted. In general, a thorough understanding of all the involved kinetics is required. Representative problems in soft nanoscale contacts in normal (humid) atmosphere are highlighted in the article. It is shown that contact load/adhesion depends on several key parameters including surface roughness, surface free energy, atmospheric moisture, and normal approach velocity

Dry and wet nano-scale impact dynamics of rough surfaces with or without a self-assembled monolayer

In nanoscale conjunctions, molecular interactions cannot be ignored, nor surface energy effects, adhesion of surface asperities, or their stiction by any wetting action of an intervening fluidic media. Many experimental micro-electromechanical systems (MEMS) are prone to failure or malfunction due to structural degradation (wear and damage) of their load-bearing and power-transmitting conjunctions. Remedial solutions have been attempted by surface treatments, such as the introduction of self-assembled monolayers (SAMs) that adhere to the surfaces, and which are hydrophobic in nature. The physics of many of the contributing phenomena, such as adhesion, meniscus action, or electrostatics, are reasonably well understood, but their interactions under small-scale impacting condition is less investigated. The problem has a transient nature, owing to inertial dynamics, surface topography, attractive surface energy of asperities, and formation of any condensates. The interplay between these kinetics, using the established models, can be quite complex, particularly with regard to prediction of contact area and pull-off force in rebound of impacting pairs. The behaviour can vary from near classical Hertzian to that dominated by adhesion. The repercussions can be very significant in power transmission or load bearing for these small devices. This paper attempts to contribute to the growing understanding of contact behaviour in smallscale

Physics of ultra-thin surface films on molecularly smooth surfaces

This paper investigates the physics of ultra-thin surface films in nano-scale counterformal conjunctions. A novel approach is proposed in which the transient behaviour of the fluid film is integrated with the contact mechanics of the approaching bodies. The method predicts film thickness and pressure distribution, as well as local elastic deflection and resulting sub-surface stress tensor. It is found that inertial dynamics of bodies, as determined by local squeeze film effect, prevents diminution of film below certain thickness and reduces the solvation effect. This approach has direct applications for micro-scale mechanisms, where prediction of thin surface adhered film thickness is required for tribological purposes, as well as structural in-service integrity of contacts of vanishing dimensions

Multiphysics analysis for the determination of valvetrain characteristics

The ideal function of a valvetrain system is to synchronize the opening and closing of the inlet and exhaust valves with the required thermodynamics of the combustion process. As such, ideally a kinematic-type mechanism is desired. However, the timing requirements in the action of each valve and between any inlet-exhaust pair necessitate the use of contacting pairs of suitable profiles. The very existence of contact renders the problem one of complex non-linear dynamics, which is further exacerbated by the translational imbalance of the reciprocating compliant elements such as the valve itself. The interplay between these various forms of dynamics, inertial, structural, and impact/contact, make the problem quite complex to analyse. As a result, some of the most important problems with valvetrains are only surmised at, rather than fundamentally understood. The multiphysics modelling approach proposed in this paper renders a better understanding, as well as conforming to experimental observations

Impact dynamics of rough and surface protected MEMS gears

The paper provides an analysis of dynamics of micro-gear pairs, typically used in an assortment of microelectromechanical system (MEMS) devices. It includes a mathematical hierarchical model of the impact dynamics of meshing gear teeth. It comprises the nanoscopic effect of asperity tips’ adhesion for relatively rough surfaces on a microscopic level (overall contact domain). The analysis is extended to the depletion of long chain molecule Self-assembled molecules (SAM) in impact behaviour of meshing gear-teeth pairs. The analyses show that for the usual high operating speeds of MEMS gears, due to high impact velocities, the role of asperity tips’ adhesion is quite insignificant. However, the same is not true for lower impact velocities, which would occur under start-up, run-up to normal operating speeds or during deccelerative motions. The paper proposes a novel spectral-based approach to predict the degradation of the protective SAM layer between meshing teeth, while the mechanism is in continual relative motion

From multi-body to many-body dynamics

This article provides a brief historical review of multi-body dynamics analysis, initiated by the Newtonian axioms through constrained (removed degrees of freedom) Lagrangian dynamics or restrained (resisted degrees of freedom) Newton–Euler formulation. It provides a generic formulation method, based on system dynamics in a reduced configuration space, which encompasses both the aforementioned methods and is applicable to any cluster of material points. A detailed example is provided to show the integration of other physical phenomena such as flexibility and acoustic wave propagation into multi-body dynamics analysis. It is shown that in the scale of minutiae, when the action potentials deviate from Newtonian laws, the forces are often described by empirical or stochastic functions of separation and the medium of interactions. These make for complex analyses and distinguish a host of many body problems from Newtonian laws of motion. A simple example is provided to demonstrate this. It is suggested that unification of many-body analysis with that of multi-body dynamics is incumbent on the fundamental understanding of interaction potentials at close separations

Multi-physics analysis of valve train systems: from system level to microscale interactions

The paper highlights a holistic, integrated, and multi-disciplinary approach to design analysis of valve train systems, referred to as multi-physics. The analysis comprises various forms of physical phenomena and their interactions, including large displacement inertial dynamics, small amplitude oscillations due to system compliances, tribology, contact mechanics, and durability at the cam-tappet contact. Therefore, it also represents a multi-scale investigation, where the phenomena can be investigated at system level and referred back to underlying causes at subsystem or component level, in other words, implications of an event at microcosm can be ascertained on the overall system performance. This approach is often referred to in industry as down-cascading and up-cascading. The particular case reported here to outline the merits of this approach concerns a four-stroke single-cylinder engine. This promotes a system approach to engineering analysis for integrated noise, vibration, and harshness, durability and frictional assessment (efficiency). Experimental validation is provided with a motored test rig, using laser doppler vibrometry

Thermoelastohydrodynamics of grease-lubricated concentrated point contacts

Isothermal and thermoelastohydrodynamic lubrication (TEHL) analyses of grease lubricated bearings are presented. A grease plug flow is formed in the conjunction that, with no shear at the boundaries with the solid surfaces, adheres to them in the region of high pressures under isothermal conditions. The elastohydrodynamic lubrication grease pressure distribution conforms fairly closely to that of its base oil alone, with the exception of inlet trail and pressure spike regions. The dependency of film thickness on speed (rolling viscosity) and load parameters for the base oil agrees with previously reported findings of the research community. For grease there are subtle differences with the base oil film thickness load and speed dependencies. However, it is clear that extrapolated oil film thickness formulae for oils can be used reasonably for the prediction of grease films, at least as a first approximation. The results presented agree well with optical interferometric measurements reported in the literature for grease-lubricated contacts at low temperatures and low surface velocities. TEHL analysis shows breakdown of the plug flow and significant reduction in film thickness, which can lead to changes in the regime of lubrication to mixed or boundary conditions

Elastodynamic transient analysis of a four-cylinder valvetrain system with camshaft flexibility

This paper presents an analysis of a line of valvetrains in a four-cylinder, four-stroke in-line diesel engine. The method highlighted in this paper predicts the vibration signature together with the prevailing contact conditions and frictional characteristics exhibited in the valvetrain system. This integrated dynamic and tribological investigation provides a practical approach that can be used during the design or the evaluation phase of automotive valvetrain systems

Transient mixed thermo-elastohydrodynamic lubrication in multi-speed transmissions

Noise, vibration and harshness (NVH) refinement as well as thermo‐mechanical efficiency are the key design attributes of modern compact multi‐speed transmissions. Therefore, unlike simple gear pair models, a full transmission model is required for a simultaneous study. The prominent NVH concern is transmission rattle, dominated by the intermittent unintended meshing of several lightly loaded unselected loose gear pairs arising from the system compactness. These gear pairs are subject to hydrodynamic impacts. The thermo‐mechanical efficiency is dominated by the engaged gears, with simultaneous meshing of teeth pairs subject to thermo‐elastohydrodynamic regime of lubrication, with often quite thin films, promoting asperity interactions. Therefore, a full transmission model is presented, comprising system dynamics, lubricated contacts, asperity interactions and thermal balance. Generic multi‐physics models of this type are a prerequisite for in‐depth analysis of transmission efficiency and operational refinement. Hitherto, such an approach has not been reported in literature