Experimental and analytical study of gear micropitting initiation and propagation under varying loading conditions
Introduction
Gear design and analysis methods are standardised by many international committees and manufacturing organisations, such as AGMA 2001-D04, ISO/6336 and DIN 3990, to guide the various aspects of gear design and industrial applications. The methods have been developed to investigate and eliminate gear tooth failures such as tooth breakage, surface micropitting, scuffing, sliding wear and spalling (ISO 10825, 1995 and BS 7848, 1996). Gear tooth flanks are subjected to varying loading and relative sliding conditions when the tooth pair engages along the line of action because of changes of the radii of tooth profile curvature at different contact points and varying load sharing factors at the single/double tooth contact regions. These cause variations of some key parameters such as gear tooth contact stresses and sliding velocities between two mating teeth during each engagement cycle, especially when subjected to variable loading and variable rotational speed conditions in operation. Although gear design and failures are well studied, however, prediction of initiation and propagation of gear tooth micropitting to accurately estimate gear service life under complex operational condition is still a challenging problem. It contributes to considerable costs due to early replacement of gears, unplanned shutdowns for carrying out maintenance procedures, such as for wind turbine gearboxes.
During gear engagement, gear teeth experience a complex combination of surface rolling and sliding contact which varies along the tooth flank [1], [2], as shown in Fig. 1. When used as a driving gear, the pinion sliding direction is away from the pitch line, whereas the driven wheel gear slides towards the pitch line [3]. For the pinion gear, this makes the sliding motion apt to pull the material away from the pitch line. At the pitch line of the wheel gear, however, the material is compressed by the sliding motion. The dedendum of both gears has a relatively short contact length and negative sliding as the direction of the sliding velocity is in the opposite direction of the rolling velocity. Furthermore, the contact stress changes continuously throughout the meshing process and high contact stresses can occur at the single tooth contact region where the load is supported by single pair of gear teeth. The variation of contact stresses and sliding directions can cause high temperatures and mixed lubrication conditions at contact surfaces, or even break down the lubrication film along the tooth flank. In addition, the surface roughness of meshing gear teeth will change after certain running cycles under loading. This leads to variations of lubrication condition between asperities of gear contact surfaces contributing to the initiation of micropitting. Gear tooth flank micropitting is characterised by a continuous surface deterioration, owing to various operational and loading conditions. Compared to the size of the contact zone, the micropits are small and shallow, with a size of about 5–10 µm long and 5–20 µm deep [4].
Gear surface failure of micropitting is affected by many factors including gear design, material, surface treatment and finishing, lubricant, and operational conditions such as loading and velocity, and lubrication condition. The following section will briefly review the published research in micropitting and effects of these key factors.
Section snippets
Review of recent micropitting research
Using a back-to-back gear test rig, surface durability of treated and untreated gear surfaces loaded under different torque levels was tested by Krishnamurthy and Rao [5]. The treated gears endured higher contact stresses and had a longer service life than that of untreated gears. Brechot et al. [6] tested many types of gears under different load levels. The main goal of these tests was to detect the occurrence of micropitting when using different industrial lubricant oil samples. Zhang and
Gear test rig
The micropitting experiment is carried out using a back-to-back gear test rig in the Design Unit of the University of Newcastle [11], [21]. The back-to-back gear test rig is designed based on a recirculating power loop principle, which provides a desired amount of fixed torque level through the tested gears, only consuming a small amount of power to drive them. The test rig is capable of testing two sets of identical gear sets with equal gear ratios. The schematic layout of the gear test rig is
Analytical investigation
In this study, the calculation of the tooth contact stresses and specific lubricant film thicknesses is in accordance with ISO/TR 15144-1:2010 [19] and the recently published ISO/TR 15144-1:2014 [20] and ISO 6336-1:2006 [20], by developing a toolkit calculation sheet using MS/EXCEL. The effect of symmetrical lead crowning is ignored in the analysis because the calculation is considered only at the middle of the face width where the crowning has no effect.
By applying the variable torque levels
Surface roughness analysis
The comparison between the values of surface roughness measured after each test cycle run under different torque levels show that the peak values of surface asperities deteriorate, as shown in Fig. 5(a) and (b). The surface roughness of pinion gear (Ra) has a steady trend of decrease with the increase of torque ratio, as shown in Fig. 6 (a). However it increases slightly by about 0.01 µm at torque ratio of 0.7. This may be because of sliding motion opts to pull the material away from the pitch
Conclusions
Experimental and analytical studies have been undertaken to investigate micropitting initiation and propagation under varying torque levels. It has found that the excessive loading, micro-geometry, surface roughness and lubricant film thickness are the main factors affecting micropitting. From the results obtained, the following conclusions can be drawn:
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Micropitting starts to occur at the torque ratio of 0.5 in the dedendum area of pinion. This micropitting is non-progressive, and remains in
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