An exploration of frictional and vibrational behaviors of textured deep groove ball bearing in the vicinity of requisite minimum load

In case of lightly loaded radial ball bearings, failure mechanisms other than fatigue such as smearing of raceways due to increased frictional torque and vibrations often prevail. Hence, attempts have been made herein for reducing the frictional torque and minimizing the vibrations of a radial deep groove ball bearing employing surface textures at the inner race. Nanosecond pulsed laser was used to create texture (involving micro-dimples having different dimple area density) on the inner race of test bearings. Using an in-house developed test rig, frictional torque and vibrational parameters were measured at different speeds and light loads (i.e. in vicinity of 0.01C, where C is dynamic load capacity of radial ball bearing). Significant reduction in frictional torque and overall vibrations were found in the presence of micro-dimples on inner race at light loads irrespective of operating speeds. Even without satisfying the minimum load needed criteria for the satisfactory operation, substantial reduction in smearing marks was found on the races of textured ball bearings in comparison to conventional cases.


Introduction
Grease lubricated deep groove ball bearings are widely employed in machines and mechanical systems in industries due to involved simplicity in lubrication. However, these bearings possess low inherent damping capacity due to poor retain-ability of grease/lubricant at the concentrated contacts of balls/races. This makes radial ball bearings energy inefficient and prone to vibrations. Figures 1(a) and 1(b) show the photographic views of an assembled radial ball bearing and its components, respectively. However, Figs. 1(c) to 1(f) illustrate the mechanism of grease removal from the track when a ball was moved on the inner race. This explanation demonstrates that how scarcity of grease prevails in practical situations resulting in starved lubricated bearings. Several authors have reported their studies involving the starved lubrication and have found adverse behaviours of it on bearings/ contacts [1][2][3][4][5][6][7][8][9]. In the past several decades, research groups globally have made attempts to improve the effective lubrication in the rolling bearings in spite of lubricant scarcity [10]. Various techniques such as new designs of cage, new materials of cage, surface coatings, and blending of nanoparticles/additives in the lubricants have been employed to improve the performances of rolling bearings and generic concentrated contacts [11][12][13][14][15][16][17][18][19].
In recent past years, surface textures involving certain type of patterning of nano/micro geometries have emerged as a viable technology to improve the tribodynamic behaviors of lubricated generic concentrated contacts as well as contacts found in the machine elements [20][21][22]. Based on the numerical [23][24][25][26][27][28][29][30][31][32] and experimental [33][34][35][36][37][38][39][40][41] investigations on lubricated textured concentrated contacts under unidirectional motion, researchers have reported vital beneficial results in terms of reduction in friction coefficient/frictional torque and decrease in temperature rise. Moreover, substantial reduction in the vibration and noise have also been reported at the lubricated textured contacts for wide range of operating parameters [20,23]. It has been established that texture attributes such as shape, size, and orientation of tiny geometries play significant role in the effective lubrication at the concentrated contacts. Micro-features/dimples of dimensions smaller than the Hertzian contact size have yielded better tribodynamic performances. Researchers have experimentally investigated the role of surface textures on lubricated reciprocating/oscillating contacts. A decrease in the overall vibrations and reduction in friction coefficient have been reported at the lubricated textured concentrated contacts in comparison to conventional reciprocating/ oscillating contacts [42][43][44][45].
Effects of surface textures/dents on the fatigue life of heavily loaded concentrated contacts have been explored using numerical and experimental approaches [36,[46][47][48][49][50][51][52][53]. Broadly it has been concluded that enhancement in fatigue life has occurred due to improved lubrication in the presence of oil filled tiny dimples present in surface textures. Presence of dimples/dents yielded longer pitting durability and prevented the scuffing at higher contact loads [48,49]. Extended durations of experimental studies were conducted to explore the influence of surface textures on the fatigue life of hardened steel surfaces under heavily loaded rolling/sliding motions [36,47,[50][51][52][53]. It has been observed that deep dents (greater than 20 μm depth) caused detrimental effects on fatigue life, whereas the presence of shallow dimples of depth < 1.0 μm have increased the fatigue life even at high loads [53].
It is understood based on the information provided in the literature that the ball bearings must be operated with a requisite minimum load (> 0.02C, where C is dynamic load capacity corresponding to each bearing) in order to achieve the satisfactory operation. If the requisite minimum load is not applied then the tribological and dynamic performances of the rolling bearings deteriorate due to the skidding of balls at the raceways that yield high frictional torque and large magnitude vibrations. Moreover, it is also found based on the literature review that certain surface textures are beneficial in terms of reducing the friction and vibration at lubricated concentrated contacts. Thus, idea came in mind of authors to explore the effect of surface texture on frictional, vibrational, and smearing marks (on raceways) of a radial deep groove ball bearing operating at the loads in the vicinity to the requisite minimum load. Hence, objective of this work was set to experimentally explore the effect of a textured ball bearing (with textured inner race) on frictional, vibrational, and smearing marks (on raceways) in comparison to conventional (without textured inner races) bearing at light loaded conditions. The radial deep groove ball bearing of designation SKF-BB1B 420206 was employed in this experimental study due to their ease (without inception of any defects) in component's disassembly and assembly for/after texturing over the inner race.

Details of sample ball bearing and texturing
SKF make radial deep groove ball bearings (BB1B 420206) were employed in this study, whose inner race, outer race, and rolling elements (balls) were made of bearing steel (EN31) while the cage was made of polyamide material. All the sample ball bearings were procured from the same manufacturing batch. This particular bearing was chosen due to the associated ease of disassembly/ reassembly of its components without causing any surface damages. The polyamide cage was designed with a snap-type fit that helped in the disassembly and reassembly of bearing components as photographically illustrated in Figs. 2(a)-2(p). The surface of the inner-race was micro-textured using a nanosecond pulsed fiber laser that delivered up to 50 W of average laser power at 1,064 nm wavelength, with a repetition rate ranging between 25 to 500 kHz. Semi-spherical micro-geometries/dimples were textured on the inner race of the bearings with an average diameter of dimples ranging in 28 ± 4 μm. The patterns of the micro-dimples created on the inner race of test bearings have been schematically illustrated in Fig. 3. The textures having dimple area densities (DAD) of 5%, 15%, and 25% were manufactured for explorations. The texture area density is the ratio of the surface area ablated by laser divided by total area i.e. [(number of dimples x area of each dimple) / total area]. The total   Table 1. The scanning electron microscope (SEM) images of textured and lapped-textured inner races were captured as shown in Figs. 4(a)-4(c) corresponding to 5%, 15%, and 25% DAD. It is worth noting here that the surface roughness (Ra) of conventional (fresh untextured pieces) and textured (pieces after lapping) inner races of test bearings were measured and found in the range of 0.08-0.09 μm.

Test rig and experimental aspects
A test rig was designed and developed for conducting the experiments on test bearings (radial deep groove ball bearings). Frictional torque and vibrational amplitude parameters were measured in-situ using an S-type load cell and two proximity probes, respectively. Figures 5(a) and 5(b) show a CAD model and a photographic view of the test rig, respectively. The DC electric motor and test bearing mounted shafts were connected using a jaw coupling to minimize the transmission of vibrations and heat flow towards the test bearing. The rotational speed was controlled by a DC motor controller. The inner races of the test bearings were press-fitted to the shaft while the outer races were held stationary in a floating housing. The bearing housing rotation was arrested using the S-type load cell for measuring the frictional torque. The vibrational displacements during rotation were measured in vertical and horizontal directions using two proximity sensors. The load cell and proximity sensors were calibrated before each set of experiments. The output from the proximity sensors were in mV. Load cell and proximity sensors were connected to a data acquisition system (DAQ) as can be seen in Fig. 5(c). A laptop connected to the DAQ was used for recording the data during the experiments. The raw data of vibrational amplitude as a function of time is obtained from the sensor. The displacement from voltage output was mapped to enable the measurement of vibrational amplitude. Fast Fourier transform (FFT) was applied to the collected vibrational data to evaluate the vibrational magnitudes as function of frequency.
Commercially available general-purpose grease was used for lubrication of test bearings. The properties of the grease are listed in Table 2. Steady state (i.e. constant temperature at outer race) has been arrived for all sets of operating conditions before recording the results. Depending on the values of input data, steady state temperature reached after lapse of time varying in the range of 30 to 60 min. Based on this observation, it was decided to conduct each experiment for duration of 60 min. The operating parameters are also listed in Table 3. Reaching of steady state temperature has been shown in Figs. 6(a) and 6(b) for the cases of bearings having conventional and textured inner races, respectively, at a set of operating parameter.    It is worth reporting here that before start of each experiment, the components and test bearings were ultrasonically cleaned using propanol followed by drying at room temperature. Well before start of experiment, each test bearing was lubricated using 2 g of grease. Thereafter, the frictional torque and vibrations were recorded for analysis/interpretation after reaching to steady state condition of bearing (i.e. outer race reaching to a constant temperature).

Frictional torque
The measured values of the frictional torque with conventional (without textured inner race) and textured bearings (with textured inner race having different DAD) at different operating parameters are shown in Fig. 7. The substantial reduction in the frictional torque can be observed with textured bearings as compared to conventional. The textured bearings with 5% DAD have yielded reduction in frictional torque in the range of 5%-35% in comparison to conventional case. However with 15% DAD, the minimum and the maximum reductions in the frictional torque values with respect to the conventional are found around 13% and 47%, respectively. At higher DAD (25%), the minimum and maximum frictional torques reduction have been recorded around 6% and 40%, respectively. In light of these observations, the textured bearing having 15% DAD is found best performing from the reduction in frictional torque perspective. For exploring the reasons behind this performance improvement, after finishing the experiments at a set of operating parameter (Load = 60 N, Speed = 1,400 rpm), the SEM investigation of textured and conventional inner races were performed. The captured SEM micro-images have been shown in Figs. 8(a)-8(d). In these images, it can be seen that inner races of conventional ( Fig. 8(a)) and textured (with 25% DAD, Fig. 8(d)) bearings got severely damaged than the cases of 5% ( Fig. 8(b)) and 15% (Fig. 8(c)) DADs. This has been interpreted as more physical contacts (asperity-toasperity contacts) have taken place between balls and inner races in the cases of conventional and 25% DAD. Large number of asperity-to-asperity contacts yielded deep smearing/scratch marks ( Fig. 8(a)) on conventional race surface resulting in high frictional torque. However in case of 25% DAD (Fig. 8(d)), pitting marks and filling/damage of dimples on inner race are visible. This indicates that it has happened due to increase in contact stress and deterioration in effective lubrication at ball/race interface. Increase in stress at the contact has happened due to removal of relatively more surface area in case of 25% DAD. Moreover, it is understood that best results with DAD of 15% has occurred due to improvement in the lubrication mechanisms as explained in Figs. 9(a)-9(c). In presence of texture on inner race, mainly three modes of lubrication at the interfaces of balls/race were expected as schematically explained in Fig. 9. The pair of dimpled surface (inner race) and running counter surface (balls) might have formed innumerable micro-fluid film bearings ( Fig. 9(a)). The pressure generated at innumerable locations in tiny fluid film bearings might have supported the loaded body over the thin film. In this way, the physical contacts between the balls and textured race might have got avoided (or reduced) resulting in reduction of frictional torque. The dimples also might have acted as innumerable micro-oil reservoirs and debris/contaminants trappers, which must have provided the oil for smearing at the interface in case of severe mixed lubrication or/and in presence of scarcity of oil. These mechanisms have been schematically explained in Figs. 9(b) and 9(c). Cumulatively, the micro-textured surfaces have played vital role in improving the performance behaviors of the ball bearings. The comparison of SEM micrographs in Figs. 10(a) Fig. 9 Mechanisms responsible for lubrication improvement in presence of texture. and 10(b) reveals presence of severe scratches on the balls of conventional bearing as compared to the textured case. This observation also indirectly indicates that film thickness of lubricating oil has improved in presence of texture, which caused lesser asperity to asperity interactions, thus, less scratch marks on the balls of textured bearing. It is worth noting here that the texture has resulted in better retainability of oil at the inner race, which has improved the lubrication and accordingly enhancement in the performance behaviors as compared to conventional case. The retainability of lubricant at the textured inner race has also been observed based on the photographic evidence of inner race after conducting an experiment. Figures 11(a) and 11(b) show the photographic images of conventional and textured inner races, respectively. Figure 11(b) reveals better oil/grease retainability in presence of texture. The oil adherence on inner races  has also been ascertained in terms of the contact angle measured using the Goniometer. Figures 12(a) and 12(b) demonstrate the contact angles measured between oil (base oil of grease) droplet and races before and after the tests. In cases of textured race ( Fig. 12(b)), contact angle has substantially been found less than conventional case. This also supports the better retainability of lubricant at texture race. The transmission   . It can be seen in these figures that relatively less damage of grease's soap fibers has taken place in presence of texture. This indicates increase in grease life with textured bearing. Moreover, the mechanical efficiency of the test bearings at a set of operating parameters was measured. It was found that the efficiency of conventional bearing varied in the range of 96% to 97% at the load of 60 N and speed of 1,400 rpm. However with textured bearing, the efficiency was measured always above 98%.

Vibrations
In order to compress the paper, this section presents only vertical vibration results for the cases of conventional and textured ball bearings measured at different operating parameters. Based on the vibrational analysis in the frequency domain, the amplitudes of the vibrations obtained at combinations of operating parameters (loads and speeds) have been presented in Fig Table 4 for the cases of conventional and textured bearings for ease in comparison from readers' perspective. Table 4 indicates the reduction of vibrations even at the harmonics with textured bearings.
of conventional and textured ball bearings presented in this paper, the following conclusions have been drawn: 1) Significant reduction (15% to 47%) in the frictional torque have been found with textured bearing in comparison to conventional case.
2) Vibration amplitudes have reduced substantially in the range of 25% to 46% in the presence of texture  | https://mc03.manuscriptcentral.com/friction was found at textured inner race. 5) TEM micro-image revealed less damage of the grease soap fibers with textured bearing as compared to conventional case.

Acknowledgements
Authors acknowledge and thank the management of Indian Institute of Technology (IIT) Delhi for allowing to use the various experimental and measuring facilities. Moreover, the first author thanks the management of Osmania University, India, for sponsoring him to pursue his Ph.D. programme at IIT Delhi under quality improvement program (QIP) scheme.
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