Micropitting and related phenomena in case carburised gears
Adrian Oila
Micropitting is a form of surface contact fatigue encountered in bearings and gears, under lubricating conditions, which lead to their premature failure. All gears are susceptible to micropitting, including spur, helical and bevel. Micropitting can occur with all heat treatments applied to gears with both, synthetic and mineral lubricants. It can occur after a relatively short period of operation and, after a certain number of cycles, gears need to be replaced due to the increased noise and vibrations caused by the deviations of the tooth profile. Continuing operation of affected gears can lead to a catastrophic type of failure (i.e., tooth breakage). These considerations explain the increasing current interest in micropitting.
It has been reported that micropitting in bearings is associated with specific microstructural transformation in steel, i.e. martensite decay. However, to the author's knowledge, this transformation has not been reported in gears. In this work, extensive metallurgical investigations have been carried out and they revealed that similar transformation occurs in gears.
The aim of this project was to determine the mechanism of micropitting by taking into account the influence of several controlling factors such as material, surface finish, lubricant, load, temperature, speed and, slide-to-roll ratio [2]. Their influence is assessed with a fractional factorial experimental design. Several non-destructive techniques have been used in order to monitor the specimen condition during and after running, such as X-ray diffraction, optical profilometry, light microscopy. The mechanical properties of the products of martensite decay, known as dark etching regions, white etching bands and butterflies are highly relevant to the fatigue behaviour of the steel. Nanoindentation and AFM techniques have been used to determine these properties [1].
A micropitting mechanism correlated with the mechanism of martensite decay in gears is suggested based on these analyses [3-4].
[1] Nanoindentation testing of gear
steels, A.Oila and S.J. Bull, Zeitschrift fur Metallkunde, 94 (2003) 793-797.
[2] Assessment of the factors
influencing micropitting in rolling/sliding contacts, A. Oila and S.J. Bull, Wear, 258
(2005) 1510-1524.
[3] Phase transformations associated with
micropitting in rolling/sliding contacts, A. Oila and S.J. Bull, J. Mater.
Sci., 40 (2005) 4767-4774.
[4] Martensite decay in micropitted gears, A.
Oila, S.J. Bull, B.A. Shaw, C.J. Aylott,
J. Eng. Tribology (Proc. IMechE Part J) 219 (2005) 77-83.