The influence of transient thermo-elastohydrodynamic conjunctions on automotive transmission rattle
2011-02-16T17:20:51Z (GMT) by
Automotive transmission rattle is the noise generated due to impacts between manual transmissions meshing gear teeth in the presence of backlash. It is considered to be a Noise, Vibration and Harshness (NVH) phenomenon and is originated due to combustion irregularities (engine order vibrations), especially in diesel vehicles. This thesis focuses in the case of creep rattle for the MMT6 Ford Getrag transmission (six speeds plus reverse) with a DW10b, 4-cylinder, 4-stroke, 2.0 litres diesel engine. This particular rattle condition is fundamentally similar to any other where an engaged gear is pertained (drive, over-run or float), with the 1st or 2nd gear engaged at a very low engine speed. The numerical models include an initial single degree of freedom (DoF) simulation. It comprises either of the engaged gear pair under Hertzian contact conditions or of a loose gear pair under hydrodynamic regime of lubrication. Once the validity of this model is established and correlated with the results obtained from a single gear pair test rig, simulations of increasing complexity can be envisaged. A 7 DoF numerical model is, therefore, developed. The Hertzian contact model still prevails for the engaged gear pair, whereas an analytical hydrodynamic solution is implemented for the remaining 6 loose gear wheels and Petrov s law is applied to the needle bearings retaining the gear wheels. With the aim of accommodating a fully lubricated model of all the tribological conjunctions, an analytical elastohydrodynamic (EHL) Grubin type algorithm is employed. Also, the energy equation is analytically solved for hydrodynamic and elastohydrodynamic conjunctions, based on the assumptions dictated by the Peclet number. Therefore, under hydrodynamic conditions, the energy equation is governed by viscous heating and convective cooling, whereas in the EHL conjunctions the governing terms are viscous and compressive heating, together with conductive cooling. The retaining needle bearings follow the same heat generation mechanism as journal bearings. The effective viscosity, as obtained from the Houpert s equation accounting for pressure and thermal effects, is fundamental for the study of the friction in the contact. The hydrodynamic contacts are only governed by viscous friction, whereas EHL conjunctions exhibit asperity iv interactions as well as viscous effects. The results obtained from this new 7 DoF model are then compared to the experimental measurements taken from the vehicle tests and various purpose-built drivetrain rigs. A metric named Impulsion Ratio is hereby introduced, aiming to shed some light into the predictions obtained by the various models presented. This metric is the ratio of driving over resistive forces acting on each individual gear wheel. Its use is tested to predict single or double-sided rattle scenarios and, therefore, ascertaining higher and lower rattle levels. The 13 DoF model from which these conclusions were obtained includes shafts planar translation and rocking moments. The rolling element bearings supporting the shafts are, therefore, modelled to capture the inherent frequencies arising from their motion. The final model introduces the effects of transient thermo-elastohydrodynamics. This 7 DoF dynamic model accounts for a numerical solution of Reynolds equation with Elrod s cavitation algorithm for simultaneous teeth in mesh. The results obtained validate the previously used Grubin assumption by comparing the predicted central film thickness along the full mesh of one tooth. Also, the effect of starved input conditions and thermal and isothermal solutions are studied.