degradation of grease service life. Lee and Amarnath (2014) conducted experimental investigations to evaluate the surface fatigue wear failure of spur gear.
Experimental investigations to assess surface fatigue failure in rolling contact bearing M. Balaji Naik, M. Amarnath, Shashikanth Pandey Tribology and Machine Dynamics Laboratory , Department of Mechanical Engineering, Indian Institute of Iinformation Technology Design and Manufacturing Jabalpur, Jabalpur – 482005, India. Abstract: Grease is the most important lubricant which plays significant role in reduction of friction and wear in machine elements viz. gears, cams, ball / roller bearings and journal bearings. Cylindrical roller bearings have high speed and radial load carrying capacity. The life of the bearing is affected by various operating conditions such as load, speed and minimum film thickness. In the present work, experiments were carried out to assess wear propagation in roller bearing using specific film thickness and vibration parameter analysis methods. Results obtained from the experimental investigations highlight the suitability of above mentioned methods in assessment of incipient faults on contacting surfaces of roller bearing. Key words: Bearing, Film thickness, Vibration signal, Grease, Gears, Cams
1. Introduction Cylindrical roller bearings are widely used in high speed, heavily loaded rotating machines such as aircraft engines, gas turbines, rolling mills etc. These bearings are usually lubricated with silicon based greases. Variations in operating conditions result transition in lubrication regimes thereby causing wear propagation on rolling contact surfaces of bearings (Cann and Spikes, 1992; Farcas and Gafitanu (1999)).
Karacy and Akturk (2009) carried out experimental
investigations to detect faults in roller bearing. Statistical parameters of vibration signals were
considered to detect the severity of faults developed over a period of 1400 hours. Statistical features of vibration signals viz. root mean square (RMS), crest factor and kurtosis value showed overall increasing trend with respect to time. Further, spectrum analysis of vibration signals was also considered to locate the fault. Author highlighted the suitability of spectrum analysis along with statistical parameters analysis to detect fault propagation on various components of roller bearing. Farcas and Gafitanu (1999) carried out experiments to investigate service life of the grease. Authors have correlated the reduction in specific film thickness of grease with degradation of grease service life. Lee and Amarnath (2014) conducted experimental investigations to evaluate the surface fatigue wear failure of spur gear. Statistical parameters of vibrations signals along with specific film thickness values were considered to detect fault propagation over a period of 1000 hours, the results were correlated to stribeck curve. Choudhury and Tandon (2000) carried out experimental investigations to detect and diagnose the defects in roller bearings. Bearing defects were simulated using spark erosion method. Acoustic emissions (AE) techniques were used to detect faults on rolling contacts surfaces. Authors have highlighted significance of acoustic emission parameters such as ring down counts and peak amplitudes in bearing fault detection.
2. Experimental setup and procedure The main objective of the experiment was to assess surface fatigue wear on rolling contact surfaces of grease lubricated roller bearing. The experimental setup is shown in Fig 1. It consists of a 5 HP three phase induction motor which drives a shaft through belt and pulley arrangement. The bearing shaft was operated at a constant speed of 800 rpm using variable frequency drive. The shaft is mounted on two bearings i.e. support and test bearings, the detailed specifications of the bearing and grease are given in Table 1. A radial load of 1kN is applied to the test bearing,
which is off center towards the test bearing as shown in Fig 1. Lithium based mineral oil grease NLGI 3 is used to lubricate the bearing. Table1. Specification of test bearing used in Experiment: Parameter
Specifications
Bearing No
NJ 307E
Category
Cylindrical roller
Inner race diameter (mm)
35
Outer race diameter (mm)
72
Width (mm)
15
Roller diameter (mm)
12
Bearing material
AISI 52100 steel
Grease
Li soap / mineral oil grease
Base oil
Mineral oil (naphthalene)
Kinematic viscosity (mm2/s) (400c / 1000c)
120-130 / 12
Atmospheric density (g/cm3)
0.890
Table 2. Fault frequencies of rolling element bearings Ball frequency
(fb)
(Hz)
Outer race frequency (fo)
(Hz)
Inner race frequency (fi)
(Hz)
Cage frequency
(Hz)
(fc)
PC with DAQ Card installed
Data Acquisition system
B&K amplifier
Frequency controller Thermo couple Power supply unit
Support Bearing
Main power supply Load bearing
Housing
Test Bearing
Three phase induction motor
Load Fig 1.schematic diagram of cylindrical roller bearing test rig
3.
Results and discussion
3.1. Specific lubricant film thickness and vibration signals The analysis of specific film thickness between rolling / sliding contact is very complex. It involves two rough surfaces in relative motion separated by a lubricant film. The lubricant film between two rolling contacts is subjected to variation in surface velocities and increase in temperature undergoes a change its physical properties. The lubricant film thickness between lubricated contacts is very thin usually of the same order of magnitude as the surface roughness of contacting surfaces. The criteria for possible contact damage can be explained under three conditions such as boundary lubrication, elasto-hydrodynamic lubrication and hydrodynamic lubrication regimes. The grease film thickness between inner race and roller was calculated using Dowson’s equations given by [7-8]: hmin 1.714U r
G 0.568W 0.128
0.694
(1) (2)
Where Ur. G, W is speed, geometric and load parameters respectively. Ra and Rb are surface roughness of inner race and roller. Fig 2 (a) - (d) show lubricant film thickness estimated between inner race and roller over a period of 900 hours. Hydrodynamic lubrication regime
>3
was observed during 0-300 hours showing in 2 (a) and (b). Further, increase in load cycles on contacting surfaces result in wear propagation which led to higher surface roughness values and decrease in specific film thickness trend as shown in Fig 2 (c) and (d). A transition in lubrication regime was observed during 600-900 hours which is represents elastohydrodynamic lubrication regime. A reduction in grease lubricant film thickness between the inner race and rollers result in metal to metal contact thereby causing wear propagation on contacting surfaces of roller bearing. Vibration signals acquired under such operating conditions showed variation in their signal
characteristics. Fig 3 (a) - (d) show vibration spectra acquired during 0, 300, 600, and 900 operating hours. Fig 3 (a) shows frequency spectra obtained under healthy condition of roller bearing, bearing characteristics frequencies are calculated using fault characteristic frequency equations. Fault propagation was appeared on inner race as a result of reduction in specific film thickness which was crucial between inner race and rollers. Hence inner race fault characteristic frequency is predominant in frequency spectra obtained after 300, 600, and 900 operating hours, as depicted in 3 (b) – (d), A gradual increasing in amplitudes of inner race fault characteristic frequency and its higher harmonics can be observed in the vibration spectra. (a)6
(b)
Hydrodynamic lubrication
Hydro dynamic lubricantion 3 5
Specific film thickness ''
Specific film thickness()
5 4 3 2 1 0 0
(c)
6
2 4 Operating time (Hours)
6
4 3 2 1 0
8
0
2 4 6 Operating Time (Hours)
(d) 6
6
Elesto Hydro dynamic lubrication 1.43
Elasto-Hydro dynamic lubrication 1.4 5
5
Specific film thickness ''
Specific film thickness ''
8
4 3 2 1 0 0
2
4
6
Operating Time (Hours)
8
4 3 2 1 0 0
2
4
6
Operating Time (Hours)
Fig. 2 Specific film thickness vs. operating time (a) 0 hours (b) 0 - 300 hours (c) 300 - 600 hours (d) 600 - 900 hours
8
(a)
(b) 1.5
2
Amplitude (m / s )
Amplitude (m / s2)
1.14
2fi 0.76
0.38
0.00
2fi
1.0
0.5
fo
fi
0.0
0
100
200
300
400
0
500
100
200
300
400
500
Frequency (Hz)
Frequency (Hz)
(c)
(d)
1.5
2.0
2fi 1.5
Amplitude (m / s )
1.0
2
2
Amplitude (m / s )
2fi
0.5
fi
1.0
fi 0.5
0.0
0.0 0
100
200
300
Frequency (Hz)
400
500
0
100
200
300
400
500
Frequency (Hz)
Fig. 3 Vibration spectra (a) healthy, (b) 300 hours, (c) 600 hours and (d) 900 hours 3.2.
Wear mechanism
Increasing fatigue load cycles and reduction in lubricant film thickness led wear propagation on inner race. After 300 hours of operation, a fatigue spall was observed on inner race surface as shown in Fig 4 (b). Further, prolonged period of operation and increasing fatigue load cycles
result in more fatigue spalls on contacting surfaces as show in Fig 4 (c) and (d).
Fig 4 Wear images at (a) healthy bearing (b) 300 hours (c) 600 hours (d) 900 hours 4.
Summary and conclusion
Experimental investigations were carried out to asses surface fatigue wear on rolling contact surfaces of roller bearing. Operating conditions such as temperature, viscosity, film thickness, specific film thickness in conjunction with vibration signal parameters were considered to detect and diagnose fault propagation in roller bearing. The following conclusions were drawn based on the experimental results. 1. Specific film thickness between inner race and roller was estimated using Dowson’s equation which correlated with damage severity on rolling contact surfaces.
2. Hydro dynamic lubrication regime was obtained during 0 – 300 hours. Further, increase in fatigue load cycles resulted in wear propagation on contacting surfaces thereby causing increase in surface roughness values which led to decrease in lubricant film thickness. 3.
The reduction in specific film thickness between rolling contact surfaces resulted in increase in wear propagation and vibration signal levels.
4. Frequency spectrum analysis revealed the diagnostic information of wear propagation on inner race, a gradual increase in bearing characteristic frequency amplitudes highlighted the severity of faults developed on inner race. Reference: 1. Cann, P.M., Spikes, H.A (1992): The behavior of greases in elasto-hydrodynamic contacts. J. Phys.D Appl. Phys. 25, A 124-A 132. 2. Farcas.F, Gafitanu.M.D. (1999) Some influence parameters on grease lubricated rolling contacts service life. Wear. 225-229: 1004-1010. 3. Tuncay karacay, and Nizami Akturk (2009) Experimental diagnostics of ball bearings using statistical and spectral methods. Tribology international 42. 836-843. 4. Sang-Kwaon Lee, Amarnath.M. (2014) Experimental investigations to establish correlation between stribeck curve, specific film thickness and statistical parameters of vibration and sound signals in a spur gear system. Journal and vibration control. 1 - 15. 5. Chaudary. A, Tandon. N. (2000) Application of acoustic emission technique for the detection of defects in rolling element bearings. Tribology International. 33: 39 - 45. 6. Dowson, D and G.R. Higginson. (1977) Elasto-hydrodynamic lubrication. New York: Pergamon Press Ltd. 7. Bartz, J.W. (1993) Lubrication of gearing. London: Mechanical Engineering Publication Ltd.
8. Amarnath .M, Sujatha. C, Swarnamani. S. (2009) Experimental studies on the effects of reduction in gear tooth stiffness and lubricant film thickness in a spur geared system. Tribology International; 42: 340-352. 9. Tandon. N, Chaudary. A. (1999) A review of vibration and acoustic measurement methods for the detection of defects in rolling element bearings. Tribology International. 32: 469 - 480.
