64 S E P T E M B E R 2 0 0 6 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

ast January, at the 15th Inter-national Tribology Colloqui-um in Esslingen, Germany, a

number of very interesting paperswere presented on the methods ofmanufacturing gears and the impacton their lubrication.

Engineers at one of the world’sleading manufacturers of gears, ZFFriedrichshafen, explained that differ-ent methods are used to produce gearsfor vehicle transmissions, dependingon the specifications and volumesinvolved. While mainly shaved gearswere made previously, today’s low-noise requirements mean that gearsare increasingly finish-machined,using shot peening or honing afterhardening. Different production meth-ods give differing gear quality, surfacecondition and internal stresses.

ZF conducted basic tests to com-pare the influence of surface condi-tion and lubricant viscosity and addi-tive type on the service life of rollersand tooth flanks, using appropriatetransmission tests, notably manufac-turing-oriented pitting tests. Theyfound that gear tooth flank servicelife is influenced decisively by thequality of the marginal zone (surfacestructure and material), the lubricant(viscosity and additives) and theoperating conditions (temperatureand speed). In the mixed-frictionrange, tribological boundary-layerformation plays an important role.

The engineers also found that theDIN factors pertaining to roughnessinfluence, ZR, and lubricant influ-ence, ZL, are not sufficient for appli-cations in vehicle transmissions.With new, lower-viscosity transmis-sion oils, gear tooth flanks need to be

made with increasingly low levels ofroughness and irregularity. Relativelubricating-film thickness can be auseful parameter, especially if lubri-cating-film thickness is calculatedwith the help of measured pressureviscosities.

Researchers at NMIin Germany and AftonChemical Corp. in theUnited States andJapan are looking atthe effects of oil viscos-ity and surface rough-ness on tribologicallayers in gear contacts.They reported that low-er oil viscosity and high gear toothsurface roughness reduce tribologicallayer thickness, while high oil viscosi-ty and lower gear tooth surface rough-ness increase it. Previous work, usingsecondary neutral mass spectrometryand a nano-indenter to analyze tribo-logical contact layers, had found thatthe alkyl structure of ZDDP and thetype of cation have a profound effecton the thickness and nano-hardnessof the tribological layer.

The latest study extended thatwork by varying oil viscosity and sur-face roughness, while keeping theadditive chemistry constant, to deter-mine their impact on the tribologicallayer. The new study also used Scan-ning Electron Microscopy togetherwith focused ion beam imaging of therectangular, well-shape cross-sectionto look at the surface layer.

The researchers observed thatthinner tribological layer thickness isthe result of greater micropitting,which removes surface material.Greater micropitting implies that the

surface experiences shorter tribo-logical load history because morefresh material is exposed to the tribo-logical load of the surface. In addi-tion, greater micropitting also brings

the maximum nano-hard-ness closer to the surface.

Researchers at the Tech-nical University of Munich inGermany used an FZG twindisk test rig to better under-stand the effect of surfaceroughness and texture onthe formation of a lubricat-ing oil mean film thicknessand pressure distribution

over the Hertzian contact zone. Theyinvestigated the influence of rough-ness and surface texture as a functionof load and sum velocity. The resultsshowed that surfaces with circumfer-ential grinding have a decreasing filmthickness with increasing surfaceroughness, while surfaces with trans-verse grinding show almost no influ-ence of roughness on film thickness.

With increasing roughness, thebandwidth of measured pressure max-ima is increased, and peaks andtroughs of roughness result in peaksand troughs of pressure. An influenceof film thickness on the pressure band-width was not observed for thick filmconditions, and researchers also founda good correlation of pressure meas-urements with initial calculations.

Together, the studies indicate that,for the lubrication of gears, thephysics and the chemistry of surfacesand lubricants appear to be intimate-ly connected. <<

David Whitby is chief executive of Pathmas-

While mainly shavedgears were made previ-ously, today’s low-noiserequirements meanthat gears are increas-ingly finish-machined,using shot peening orhoning after hardening.

Worldwide

LBy R. David Whitby

Advances in manufacturingand lubricating gears