AGMA 12FTM15

A failure mode called “flank breakage” is increasingly observedin different applications of cylindrical and bevel gears. Thesebreakages typically start from the active flank approximately inthe middle of the active tooth height and propagate to the toothroot of the unloaded flank side. Crack initiation can be localizedbelow the surface in the region between case and core of surfacehardened gears. This failure mode can neither be explained by theknown mechanism of tooth root breakage nor by the mechanism ofpitting. Even bevel gears in truck and bus applications are at therisk to suffer from subsurface fatigue, if the optimum utilizationof the material should be achieved. In this case a balance betweenthe flank breakage and pitting risk has to be found. The purpose ofthis paper is to describe a new material physically basedcalculation method to evaluate the risk of flank breakage versusthe risk of pitting. The verification of this new method byexperimental tests is exemplarily shown. In cooperation with “ZG ­Zahnräer und Getriebe GmbH” (ZG) “MAN truck and bus AG” (MTB)developed a new method for the calculation of the risks of flankfailure by flank breakage and pitting. The calculation method hasbeen adjusted and approved by experimental tests on powertrain testrigs of MAN. The ten different test gear variants had an outerdiameter of de2 = 390 mm to 465 mm, a ratio i = 4,5 to 5,7 and anormal module of mmn = 6 mm to 8 mm. Also variants with the samemain geometry but different Ease­Off designs were examined. Allgear sets were tested under a defined load spectrum. Based on theresearch work at the FZG (Gear Research Center at the TechnicalUniversity of Munich in Germany) of Oster, Hertter and Wirth acalculation method for bevel gears was established. The principleof the calculation model is the local comparison of the occurringstresses and the available strength values over the whole toothvolume. Therefore it is possible to evaluate the risk of initialcracks beyond the surface of the flank. Close to the surface cracksmay grow and cause pitting ­ especially in the flank area withnegative specific sliding. Cracks in the transient area betweencase and core lead to a high flank breakage risk. First the localstresses and forces on the flank are determined by a loaded toothcontact analysis followed by the calculation of the maximumexposure (regarding yielding) and dynamic exposure (regardingfatigue) of the material inside the tooth. Thereby the stresscomponents from the Hertzian contact, bending, thermal effects(flash temperature) and friction are considered. Furthermore thepositive effect of residual compressive stresses and accordinglythe disadvantageous effect of the residual tensile stresses can beimplicated. Finite elements method investigations have been carriedout in order to achieve a sufficient approximation of the residualstress distribution in the transverse tooth section. The strengthvalues are locally considered, depending on the material depth andthe position on the flank. The recalculation of the test gearsshowed a good correlation between the occurred type of damage andthe determined material exposure inside the tooth. The variantsfailed with flank breakage could be reliably distinguished from thevariants failed by pitting by the new material­physical method.With this knowledge it is now possible to optimize the maingeometry parameters of the gear set (e.g. number of teeth, spiralangle, pressure angle) as well as the micro geometry (Ease­Off)that influences the load distribution on the flank. Altogether thisnew method leads to an insured increase of the permissible materialutilization and hence to smaller gear sizes while keeping the loadcapacity on a constant level.

Edition:

12

Published:

10/01/2012

Number of Pages:

21

File Size:

1 file , 930 KB