Showing papers by "Homer Rahnejat published in 2002"

Vibration monitoring of high speed spindles using spectral analysis techniques

Abstract: A significant source of noise and vibration in precision high speed machine tool spindles is bearing induced vibration, which is caused by inherent geometrical characteristics, as well as out-of-balance assembly and interactions between rolling mating members with surface irregularities. The multitude of causes often makes a diagnostic approach quite arduous, particularly due to insufficient information obtained from frequency domain analysis alone. Furthermore, the averaging nature of Fourier analysis and its poor frequency resolution make the task in complex vibration spectra more difficult. The paper presents a new approach in use of auto-correlation analysis with good frequency resolution, combined with windowing capability to isolate significant causes for subsequent remedial actions. The method is applied to a routing spindle with special reference to primary induced bearing frequencies.

Transient elastohydrodynamic lubrication of finite line conjunction of cam to follower concentrated contact

Abstract: The paper presents a combined solution for multi-body dynamics of a modified cycloidal cam to flat follower with transient lubricated finite line contact conjunction. The solution to the dynamics of the valve train system is undertaken in the Euler frame of reference, which includes valve surge due to the non-linear behaviour of the spring. The elastohydrodynamic transient contact conjunction is analysed in each integration step time by simultaneous solution of the Reynolds' equation in space-time co-ordinates with the instantaneous elastic film shape equation, taking into account the generalized contact elasticity for finite line contact geometry. This approach, not hitherto reported in the literature, yields high generated pressures and thin films at contact extremities that govern wear and fatigue spalling performance of such contacts. Results show that the usual approaches in both quasi-static analyses and transient line contact solutions yield results that are not in line with actual prevailing conditions.

Aligned and misaligned contacts of rollers to races in elastohydrodynamic finite line conjunctions

Abstract: The paper provides a solution for finite line concentrated contact of a roller-to-race under aligned and misaligned conditions. The lubricated contact conjunction is subject to an elastohydrodynamic regime of lubrication under isothermal conditions. Of particular interest are the edge stress discontinuities, represented by large secondary pressure spikes at the side constriction and to the rear exit in the contact domain. These pressure 'pips' are considerably larger in magnitude than those occurring in the central exit of the contact. The presence of pressure peaks inhibits the flow of lubricant in their vicinity, causing islands of minimum lubricant film thickness at the sides of the contact, referred to as the end closure films. The paper shows that the film shape and pressure distribution at the extremities of a finite line contact are not revealed by the traditional line contact solutions usually undertaken. The flow pattern becomes more complex with roller misalignment and the edge effects described are exacerbated. The paper provides the first ever solution of misaligned roller-to-race contact for moderate to high loaded elastohydrodynamic conjunctions. The numerical predictions conform well with both experimental and numerical findings of others for the cases where similar work has been reported.

Vibration Modeling of Rotating Spindles Supported by Lubricated Bearings

Abstract: This paper outlines a five degrees of freedom model of a rotating spindle supported by a pair of lubricated angular contact ball bearings. The ball to raceway contacts are simulated by non-linear contact springs, representing the elastic deformation of the mating rolling members and nonlinear spring/dampers, corresponding to the contact elastohydrodynamic oil film thickness. A regression formula is used to model the latter and includes the damping contributed by the squeeze film effect caused by the mutual convergence of bearing rings. Some results of simulation studies with the model are also presented, in both the time and frequency domains. They include the overall system response, when subjected to varying spindle mass or the number of balls in the support bearings. Furthermore, comparisons are made between the simulated response of the dry and lubricated models. The overall contribution to damping of the elatstohydrodynamic oil films between the rolling elements and their raceways is shown to be slight.

Physics of lubricated impact of a sphere on a plate in a narrow continuum to gaps of molecular dimensions

Abstract: This paper investigates the phenomenon of lubricated impact dynamics of ellipsoidal bodies upon semi-infinite elastic solids, giving rise to Hertzian contact conditions. The analysis conforms to the numerical predictions and experimental findings of others, when the physics of motion of the lubricant can be described through Newtonian continuum mechanics, with the dominant viscous action embodied in the transient solution of Reynolds' equation. The equivalence of squeeze film action under impacting conditions with that of a converging gap in pure entraining motion is shown. This concept is extended to study the accelerative nature of the lubricant film surface, and its concordance with Reynolds' assumption through use of a relativistic frame of reference and hyperbolic geometry. When the investigation is extended to the case of ultra-thin film conjunctions of the order of a few to several molecular diameters of the intervening fluid layer, the physics of fluid film motion through impact involves more complex kinetic interactions. These include the effect of structural force of solvation, as well as that of a meniscus force, formed in such narrow conjunctions. The former, through active dispersion, tends to promote a structureless environment, whilst the latter through wetting action encourages the formation of a coherent film. This paper shows the interplay between these competing kinetics.

Elasto-multi-body dynamics of a multicylinder internal combustion engine

Abstract: This paper presents a multi-body model of a four-cylinder, four-stroke diesel engine, incorporating component flexibility. The model also includes engine firing order and experimentally measured combustion time history. The paper presents numerical predictions for conical motion of the flywheel as a result of combined torsion-deflection modes of the flexible crankshaft system. The half-engine order responses induce complex three-dimensional whirling motion of the flywheel, which is responsible for repetitive shock loading of the drivetrain system through impact with the clutch system. This leads to an assortment of noise and vibration concerns, one of which is in-cycle vibration of the clutch system. Numerical prediction of this vibration response agrees well with experimental findings

Optimization of the high-frequency torsional vibration of vehicle driveline systems using genetic algorithms

Abstract: Vehicle drivelines with manual transmissions are exposed to different dynamic engine torques under driving conditions. Engine torque can dramatically vary with throttle demand from coast to drive condition and, conversely, with throttle release from drive to coast. Abrupt application or release of throttle in slow moving trafe c or rapid engagement of the clutch can be followed by an audible response, referred to in industry as the clonk noise. This paper presents a complete dynamic model of a vehicle driveline for the optimization of high-frequency torsional vibration by the distributed-lumped (hybrid) modelling technique (DLMT). The model used is e rst validated against experimental tests. Parameter sensitivity studies have been carried out using the model to identify the important components affecting clonk. Three key parameters have been chosen from the parameter study. To optimize these key factors, genetic algorithms (GAs) have been used in this multi-parameter optimization problem. The GAs show signie cant reduction in the driveline noise, vibration and harshness (NVH).

High frequency torsional vibration of vehicular driveline systems in clonk

Abstract: The lightly damped driveline system has many lash zones, represented by low-stiffness components and thin shell elements with large modal densities. The high-frequency noise and structural dynamic modes, excited under impact conditions through low lash rate zones are of a short transitory nature, but a major concern to vehicle drivers and other road users alike. Abrupt application or release of throttle in slow moving traffic or rapid engagement of the clutch can be followed by an audible response, referred to in the industry as the clonk noise. This paper presents a complete model of a vehicle driveline for investigation of high-frequency torsional vibration using a distributed-lumped (hybrid) modelling technique (DLMT). It is shown that DLMT predictions agree well with the experimental results.

Transient elastohydrodynamic lubrication of piston skirt to cylinder liner under combined reciprocating and slapping motions

Abstract: Transient elastohydrodynamic lubrication of piston skirt to cylinder liner under combined reciprocating and slapping motions