Ramkumar P

Premature bearing failures commonly occur in applications that operate under extreme boundary conditions. The lubrication engineers and tribologists are confronting a key challenge in these bearing failures associated with microstructure decay, such as White Etching Cracks (WECs) and White Etching Areas (WEAs). Lubricant degradation is one of the critical factors for the subsurface decay in the bearing steel. This work evaluates the performance of mineral oil (heavy paraffin oil) and synthetic poly alkyl glycol (PAG) against WEAs formation under pure sliding with cyclic compressive loading. The performance of lubricants was evaluated using Infrared Fourier Transform (FTIR) and Electron Spin Resonance (ESR) spectroscopy. The outcomes reveal that the free radical formation rate is higher for paraffin than PAG. This study found that WEAs formation in the bearing steel is delayed in the PAG tested samples compared to paraffin.

Wear of components is a critical factor influencing the service life of a product. Thus wear prediction and simulation has become an important part of engineering. One of the most common forms of wear in mechanical components is sliding wear. Factors affecting dry sliding wear include normal load, relative speed, geometry (both macroscopic and microscopic), temperature, material properties and environmental conditions. Sliding wear experiments are done using tribometers. However, such experiments are expensive and time consuming. In the past few decades, a Finite Element Method (FEM) based wear simulation approach has gained popularity. The objective of the present work is the numerical wear prediction of 2D steel-on-steel pin-on-disc dry sliding contact using Finite Element Method (FEM). Initially, the 2D elastic contact problem is solved using non linear finite element method to obtain the pressure at the contact nodes. The contact pressure obtained is validated with the Hertz solution. The Archard's wear law is used along with the Euler's integration scheme to obtain the material worn out at each wear incremental step. Wear is taken into account by updating the pin geometry after each step. A user-defined FORTRAN subroutine UMESHMOTION is used to apply the wear at the contact nodes. Arbitrary Lagrangian-Eulerian (ALE) technique, an adaptive meshing technique in ABAQUS, is used. This technique prevents element distortion and maintains a high quality mesh throughout the analysis. In order to reduce the computational time, the extrapolation technique is used. Pin-on-disc (PoD) experiments are performed using SAE 304 stainless steel as pin material and SAE 52100 hardened steel as disc material to validate the wear simulation results. The coefficient of friction and the wear coefficient used in the simulations are determined from the experiments. Mesh convergence study is done and the contact pressure obtained from FEM is compared with the Hertz analytical solution. The wear depth obtained from simulations is compared with the experimental results.

Wear is a complex phenomenon, which depends on various parameters such as load, velocity, material properties, surface, environmental conditions, etc. Hence, wear prediction is a challenging part of engineering. This paper focuses on numerically predicting the wear of 304 stainless steel pin sliding against AISI 52100 bearing steel disc, using pin-on-disc tribometer setup. The experiments are performed for loads of 10 N, 30 N, and 50 N and a sliding speed of 0.4 m/s. The wear coefficient and coefficient of friction obtained from the experiments are given as input to a 2D elastic finite element method model using a commercially available finite element method-based software ABAQUS. The differential form of the Archard’s wear law is used to obtain the wear depth at the contact nodes. The UMESHMOTION+ Arbitrary Lagrangian–Eulerian technique is used to update the contact geometry after each wear increment. The major drawback of wear simulation is the large computational time requirement. To address this, three extrapolation techniques are used namely, the constant extrapolation, the linear extrapolation, and the constant pressure extrapolation technique. A new criterion for using extrapolation during sliding wear simulation was proposed. The extrapolation techniques take into consideration the evolution of the contact pressure and contact geometry during sliding wear. The effectiveness of these techniques based on the computational time and accuracy are analysed. Based on the accuracy, the linear extrapolation technique was found to be most effective, while the constant pressure extrapolation technique was most useful in reducing the computational time. The numerical results obtained are validated with the experimental results.

The tribological performance of piston ring-cylinder liner conjunction (PRCLC) notably depends on the surface finish of cylinder liner. This study investigates the mixed lubrication condition near dead centres considering non-Gaussian roughness and cavitation. The non-Gaussian flow factors are incorporated in the average Reynold's equation to include the effect of non-Gaussian roughness. Whereas, the Weibull probability distribution function is employed to model the asymmetry in asperity heights. It is found that surfaces with more negative skewness exhibit lower engine total frictional force in the vicinity of the dead centres. No substantial effect of the skewness and surface roughness is observed in the mid-stroke of the piston. It is shown that PRCLC operates in mixed-EHL regime near the dead centres. The lambda ratio is found to increase with an increase in skewness of the rough surface.

A novel modified dynamic load Pin-on-disc tribometer design along with the real time hydrogenating lubrication methodology is experimented to replicate “white etching area” (WEA) formation on bearing steel in laboratory level testing. Controlled formation of white etching area within a short duration has been achieved with the help of this new approach. Investigation of subsurface microstructural changes has opened a new insight in analysing the sequential formation of white etching area associated with premature rolling element bearings failure. The dynamic loading under hydrogenation establishes the sequential microstructural changes such as clustering, agglutination and deformation followed by dissolution of cementites to form WEA in the bearing steel.

The test duration of the each experiment is fixed based on the microstructure investigation and until the WEA formation is observed in the sample. The summery of test results of three lubricants is given in Table 1. Metallographic inspection of the samples tested in three different oil is illustrated in Figure 1. Several irregular shaped WEA formations observed in mineral oil sample is shown in Figure 1 (a). Long linear cementite formation in PAO sample shown in Figure 1(b) indicate the precursor of long WEA formation in samples. Similarly, one butterfly shaped WEA formation observed in PAG sample is shown in Figure 1 (c).

Despite of more than two decades of research in white etching areas (WEAs), this phenomenon still persists in wind turbine gearbox bearings leading to a reduction of L10 life by 90 percent. There are various drivers for WEAs formation among which hydrogen is considered one of the root causes which accelerates the microstructural degradation. Diffusible atomic hydrogen reduces the threshold for dislocation motion leading to localized strain accumulation. The tendency to form WEAs in bearing steel depends on the stability of carbide precipitates during plastic deformation and resistance to diffusible hydrogen. In this research work, the role of size and volume fraction of carbides on their relative stability against decomposition and hydrogen embrittlement is studied. The performance of bearing ball samples with different carbide sizes and distribution is tested in the presence of two different lubricants in the dynamic load pin-on-disc (PoD) test rig. The results from the study show that reducing the size of carbide precipitates improved their stability against plastic deformation leading to the stagnation of WEAs formation.

This study investigates the effect of loading frequency and hydrogenation to instigate the White Etching Areas (WEAs) formation under severe sliding condition of bearing steel. Experiments were carried out at 1.5 Hz and 4.5 Hz loading frequencies by varying the contact pressure from 1 to 2 GPa using dynamic load Pin-on-Disc (PoD) tribometer. Steel-on-steel tribo-pair was lubricated by two different lubricants, PAO and real-time hydrogenating oil, under boundary lubrication. Synergic interaction between hydrogen enhanced and mechanical stress induced drivers on WEAs formation was established. Subsequently, quantitative insights on the catalysing behaviour of the individual and concurrent actions of drivers were elucidated. Sequential evolution of butterfly around the non-metallic inclusion from the primitive stage was revealed in pure sliding through metallographic investigation.

A novel multi-objective optimization of planetary gearbox is presented using a discrete version of Non-Dominated Sorting Genetic Algorithm (NSGA-II). Minimization of weight and total power loss of planetary gearbox are two objective functions. Number of teeth in the sun, planet and ring gears, module, face width, input shaft and planet pin diameter are design variables. Regular mechanical, bearing selection and scuffing are various design constraints considered. Investigations were done for three different gear profiles on an industrial planetary gearbox. The results were compared with an industrial gearbox provided in the AGMA standard, which showed not only significant reduction of weight and power loss, but were also found safer in scuffing. Pareto fronts of different ISO grade oils were compared for all gear profiles, and ISO VG 460 was found to be the best oil. Further analysis without a scuffing constraint showed moderate to high levels of scuffing risk for lower-grade ISO oils and lower level risk for higher-grade ISO oils. Ultimately, this study helps to reduce weight, enhance the efficiency and prevent early failures in planetary gearbox.

For predicting the sliding wear in Pin-on-Disc (PoD) tribometer contact, a numerical wear simulation technique is presented in this paper. It is based on the Finite element method (FEM) which incorporates the Archard's wear law and the UMESHMOTION subroutine for calculating the wear depth for a 3-D PoD tribometer contact. FEM is utilized in solving the 3-D contact problem. The geometry is updated using the UMESHMOTION subroutine coupled with the Augmented Lagrangian-Eulerian (ALE) remeshing technique of ABAQUS. But, a significant disadvantage of FEM wear prediction is the enormous computational time required for performing 3-D analysis. Hence, to minimize the computational time, an approximation technique is introduced which accounts for the contact pressure evolution at the contact region. It decreased the simulation time and also preserved the accuracy of 3-D wear prediction. Finally, the results obtained from the simulations are compared with the experiments for brass-on-bearing steel PoD contact. An accuracy of 98.81% was obtained for the 10N and 83.10% for the 30N load.