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Division of Materials Technology, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, 126 Pracha Uthit Road, Bang Mod, Thung khru, Bangkok, Thailand
Received: February 25, 2012 / Accepted: March 06, 2012 / Published: April 25, 2012.
Abstract: This paper reports the results of an investigation into the premature failure of a helical gear used in a gearbox of a steel mill in Thailand. The gear failed after about 15,000 h of service which was much shorter than the normal service life of 40,000-50,000 h. It was concluded the helical gear subsequently failed due to fatigue fracture initiated by surface and subsurface damages resulting from excessive contact stress. Excessive contact stress at gear tooth surface resulted from the replacement of a new, more powerful motor. The lesson learned from this case is that one must be careful when replacing key components of machines. The consequences of any replacements must first be thoroughly analysed before the final decisions are made.
Helical gears are widely used in numerous engineering applications including gearboxes. Gearboxes are key components of most heavy duty machines and are extensively used in steel industry. Failure of gears not only results in replacement cost but also in process downtime. This could have a drastic consequences on productivity and, more importantly, on delivery which could possibly result in permanent loss of customers. For example, in this case the ‘downtime’was 12 days and 3,840 metric tonnes of steel output was lost before the failed helical gear could be replaced.
The causes of gear failure are numerous including faulty design, improper applications, and manufacturing errors. Design errors include such things as incorrect gear geometry, incorrect material, poor material quality, and inadequate lubrication system. Application errors include things such as improper mounting and installation, inadequate cooling, improper lubrication, and poor maintenance. Manufacturing errors could be poor machining or faulty heat treatment of parts or poor gear system assembly [1].
Rolling contact fatigue which results in surface pitting and eventual fracture is one of the most common failure modes of mechanical elements that are subjected to rolling contact fatigue loading such as gears, bearings etc. This type of fatigue is often a key factor that governs the service life of such components[2]. The complete contact fatigue process starts with micro-pit formation followed by crack initiation, crack growth, and the breakaway of surface material layer [3]. Damage due to contact fatigue in gear teeth usually occurs in one of three areas; along the pitch line, in the addendum, and in the dedendum [4]. The pitting formed on the surface lead to stress concentrations which, in turn, lead to crack initiation [5]. Pitting under pure rolling can occur even under proper lubrication conditions, since oil, as an incompressible fluid, will merely transmits the contact load [6]. Most gear tooth fatigue failures occur in the tooth root fillet where cyclic stress is much less than the yield strength of the material [7]. The fatigue process leading to pitting is
The failed helical gear was used in a gearbox driving the first rolling stand in a hot re-rolling steel mill in Samutsakorn, Thailand. The steel mill produces steel re-bars 6 mm to 12 mm diameter with a capacity of 20 metric tonnes per hour. The first stand was initially designed for rolling steel billets 100 mm × 100 mm square × 6 meters long. Due to supply shortage of the required billet size, larger 120 mm × 120 mm square steel billets were used as raw materials. In order to increase the power required to roll larger steel billets, the original 300 kW electric motor was replaced by a more powerful 600 kW one. The change in the motor was done without changing the reduction gearbox. The average current and voltage when rolling 120 mm ×120 mm steel billet were 700 amps and 720 volts, respectively. The gearbox failed after approximately 15,000 h which was much lower than the expected working life of 40,000-50,000 h in continuous running conditions [10].
The failed helical gear had 69 teeth, with a face width of 128 mm. The module of the gear was 8 mm, helix angle 13 degrees and the pressure angle (?) 20 degrees. The gearbox ratio and input shaft revolutions were 15.90 and 400 rpm, respectively. The gear was designed to transmit the mechanical power of 300 kW, and the safety factor used in the design was 1.75. Relevant layout of the gearbox is shown in Fig. 1.
The failed gear was first inspected visually then macroscopically using digital camera. Samples of the material in the vicinity of the fracture of the failed gear were taken and metallographic specimens were prepared for optical and electron microscopy examination, and for microhardness measurement.
Dye-penetrant technique (DPT) was employed to enhance visual inspection of the crack and to fully reveal the nature of the crack.
Chemical analysis of the gear material was performed using a spectrolab spectrophotometer(Model: M8, Type: LAVWA 18A). Standard metallographic specimen preparation procedures were employed.
Microhardness across of the gear tooth thickness at the pitch line was measured using a Vickers hardness tester (Mitutoyo model MVKH1) with a 300 gm load.
Microstructures of the specimens were studied, and micrographs were taken using an optical microscope(LECO: IA32-Image analysis system). Fracture surface samples were cleaned ultrasonically in acetone. The samples were examined using a JEOL-JSM 5800 scanning electron microscope.
Applied stress was calculated using the data from actual operating conditions. Electrical power (Po) was calculated using Eq. (1) [11], transmitted torque (T), and tangential load (Wt) were calculated using Eqs. (2) and (3) [12].
4.1 Visual Examination
The appearance of the failed helical gear is shown in Fig. 2. There were two broken teeth as shown in Fig. 2a. The initial and final stage pitting on the contact side are shown in Fig. 2b.
4.2 Dye-Penetration Test
Cracks were observed in the failed gear teeth and crack origins were in the pitting zones of the gear teeth.
The cracks originated at the pitting area, then propagated towards the root circle of the gear as shown in Fig. 3. Similar crack paths were often observed in gear fracture in practice. 4.3 Composition Analysis
The average values of the analysis are shown in Table 1. The composition indicates that the gear was made from low alloy steel to JIS-SCM 415 standard[14], commonly and widely used in making gears [15]. This means that proper material was used for making the gear.
4.4 Hardness Profile
The hardness profile of a gear tooth is shown in
Fig. 4. The maximum hardness at the outer surface of the case and the minimum value at the core were found to be 713.2 HV (60.7 HRC) and 440.5 HV (44.5 HRC), respectively. The hardness values in Fig. 4 indicates that the gear had been case hardened by carburizing which is common practice for gear heat treatment [16]. 4.5 Microstructure Examination
The case microstructure was tempered martensite as shown in Fig. 5a. The core was a mixture of ferrite and pearlite as shown in Fig. 5b. The microstructures indicate that the gear was carburized, quenched, and tempered which is common practice in heat treatment of gears [16]. No abnormality was found in the microstructure.
4.6 Gear Tooth Surface Examination
SEM examination of gear tooth surface revealed that there were pitting and spalling areas on the active side of the gear tooth, as shown in Fig. 6a, b. The presence of extensive sub-surface cracks and spalling at the active surface side of the gear tooth was an indication that the gear tooth was subjected to very high contact stress with a shear component.
(1) The failure of the helical gear was caused by excessive contact stress on the surface of the gear teeth. The calculated contact stress is 1.7 times higher than the allowable contact stress of gear material. This excessive stress was the result of the replacement of original electric motor by a more powerful one.
(2) The fracture starts from pitting area at surface of a gear tooth followed by fatigue crack initiation, crack growth, and final fracture. The pitting occurred as a result of excessive stress.
[1] Elecon Engineering Co., Ltd, Power transmission & Drive Solution, http:/www.elecon.com/gearworld/dat-gwfailure.html, access on 10/01/10.
[2] K.J. Abhay, V. Diwakar, Metallurgical analysis of failed gear, Eng. Fail. Anal. 9 (3) (2002) 359-365.
[3] K.L. Johnson, The strength of surfaces in rolling contact, Mech. Eng. Sci. 203 (3) (1989) 151-163.
[4] A.L. Alban, Systematic Analysis of Gear Failures, 2nd ed., ASM International, Materials Park, Ohio, 1985, pp. 86-94.
[5] P.J.L. Fernandes, Tooth bending fatigue failure in gears, Eng. Fail. Anal. 3 (3) (1996) 219-225.
[6] A.L. Alban, Failure of gears, failure analysis and prevention, american society for metals, Materials Park, Ohio 11 (1996) 587-601.
[7] B. Errichello, J. Muller, How to analyze gear failures, http:/www.machinerylubrication.com/Read/150/gear-fail ures.html, access on 17/04/11.
[8] S. Suresh, R.O. Ritchie, Propagation of short fatigue cracks, International Metals Reviews (1984) 445-476.
[9] D.W. Dudley, Fatigue and Life Prediction of Gear, ASM Handbook, Metal Park, American Society for Metals 19(1996) 872.
[10] Principals of bearing selection and applications, SKF General Catalogue, Germany, 2003, p. 87.
[11] C.L. Erickson, B.M. Jones, Electrical and Electronics Engineering, Marks’ Standard Handbook for Mechanical Engineers, 9th ed., McGraw-Hill, 1987, pp. 15-3.
[12] S.H. Loewenthal, Shafts, Couplings, Keys, ETC, Mechanical Design Handbook, McGraw-Hill, New York.
[13] B.J. Hamrock, S.R. Schmind, O.B. Jacobson, Fundamentals of Machine Elements, 2nd ed., McGraw-Hill, New York, 2005, pp. 640, 678.
[14] Ferrous Materials & Metallurgy I, JIS Handbook, Japan Standards Association, Tokyo, 2002, p. 1304.
[15] Bangkok Special Steel Co., Ltd: Machinery Steel, URL:http://www.bssteel.co.th/product_en. html, access on 17/04/11.
[16] S.K. Lyu, K. Inoue, G. Deng, M. Kato, Effect of surface treatments on the strength of carburized gears, Mech. Sci. Tech. 12 (2) (1998) 206-214.
Received: February 25, 2012 / Accepted: March 06, 2012 / Published: April 25, 2012.
Abstract: This paper reports the results of an investigation into the premature failure of a helical gear used in a gearbox of a steel mill in Thailand. The gear failed after about 15,000 h of service which was much shorter than the normal service life of 40,000-50,000 h. It was concluded the helical gear subsequently failed due to fatigue fracture initiated by surface and subsurface damages resulting from excessive contact stress. Excessive contact stress at gear tooth surface resulted from the replacement of a new, more powerful motor. The lesson learned from this case is that one must be careful when replacing key components of machines. The consequences of any replacements must first be thoroughly analysed before the final decisions are made.
Helical gears are widely used in numerous engineering applications including gearboxes. Gearboxes are key components of most heavy duty machines and are extensively used in steel industry. Failure of gears not only results in replacement cost but also in process downtime. This could have a drastic consequences on productivity and, more importantly, on delivery which could possibly result in permanent loss of customers. For example, in this case the ‘downtime’was 12 days and 3,840 metric tonnes of steel output was lost before the failed helical gear could be replaced.
The causes of gear failure are numerous including faulty design, improper applications, and manufacturing errors. Design errors include such things as incorrect gear geometry, incorrect material, poor material quality, and inadequate lubrication system. Application errors include things such as improper mounting and installation, inadequate cooling, improper lubrication, and poor maintenance. Manufacturing errors could be poor machining or faulty heat treatment of parts or poor gear system assembly [1].
Rolling contact fatigue which results in surface pitting and eventual fracture is one of the most common failure modes of mechanical elements that are subjected to rolling contact fatigue loading such as gears, bearings etc. This type of fatigue is often a key factor that governs the service life of such components[2]. The complete contact fatigue process starts with micro-pit formation followed by crack initiation, crack growth, and the breakaway of surface material layer [3]. Damage due to contact fatigue in gear teeth usually occurs in one of three areas; along the pitch line, in the addendum, and in the dedendum [4]. The pitting formed on the surface lead to stress concentrations which, in turn, lead to crack initiation [5]. Pitting under pure rolling can occur even under proper lubrication conditions, since oil, as an incompressible fluid, will merely transmits the contact load [6]. Most gear tooth fatigue failures occur in the tooth root fillet where cyclic stress is much less than the yield strength of the material [7]. The fatigue process leading to pitting is
The failed helical gear was used in a gearbox driving the first rolling stand in a hot re-rolling steel mill in Samutsakorn, Thailand. The steel mill produces steel re-bars 6 mm to 12 mm diameter with a capacity of 20 metric tonnes per hour. The first stand was initially designed for rolling steel billets 100 mm × 100 mm square × 6 meters long. Due to supply shortage of the required billet size, larger 120 mm × 120 mm square steel billets were used as raw materials. In order to increase the power required to roll larger steel billets, the original 300 kW electric motor was replaced by a more powerful 600 kW one. The change in the motor was done without changing the reduction gearbox. The average current and voltage when rolling 120 mm ×120 mm steel billet were 700 amps and 720 volts, respectively. The gearbox failed after approximately 15,000 h which was much lower than the expected working life of 40,000-50,000 h in continuous running conditions [10].
The failed helical gear had 69 teeth, with a face width of 128 mm. The module of the gear was 8 mm, helix angle 13 degrees and the pressure angle (?) 20 degrees. The gearbox ratio and input shaft revolutions were 15.90 and 400 rpm, respectively. The gear was designed to transmit the mechanical power of 300 kW, and the safety factor used in the design was 1.75. Relevant layout of the gearbox is shown in Fig. 1.
The failed gear was first inspected visually then macroscopically using digital camera. Samples of the material in the vicinity of the fracture of the failed gear were taken and metallographic specimens were prepared for optical and electron microscopy examination, and for microhardness measurement.
Dye-penetrant technique (DPT) was employed to enhance visual inspection of the crack and to fully reveal the nature of the crack.
Chemical analysis of the gear material was performed using a spectrolab spectrophotometer(Model: M8, Type: LAVWA 18A). Standard metallographic specimen preparation procedures were employed.
Microhardness across of the gear tooth thickness at the pitch line was measured using a Vickers hardness tester (Mitutoyo model MVKH1) with a 300 gm load.
Microstructures of the specimens were studied, and micrographs were taken using an optical microscope(LECO: IA32-Image analysis system). Fracture surface samples were cleaned ultrasonically in acetone. The samples were examined using a JEOL-JSM 5800 scanning electron microscope.
Applied stress was calculated using the data from actual operating conditions. Electrical power (Po) was calculated using Eq. (1) [11], transmitted torque (T), and tangential load (Wt) were calculated using Eqs. (2) and (3) [12].
4.1 Visual Examination
The appearance of the failed helical gear is shown in Fig. 2. There were two broken teeth as shown in Fig. 2a. The initial and final stage pitting on the contact side are shown in Fig. 2b.
4.2 Dye-Penetration Test
Cracks were observed in the failed gear teeth and crack origins were in the pitting zones of the gear teeth.
The cracks originated at the pitting area, then propagated towards the root circle of the gear as shown in Fig. 3. Similar crack paths were often observed in gear fracture in practice. 4.3 Composition Analysis
The average values of the analysis are shown in Table 1. The composition indicates that the gear was made from low alloy steel to JIS-SCM 415 standard[14], commonly and widely used in making gears [15]. This means that proper material was used for making the gear.
4.4 Hardness Profile
The hardness profile of a gear tooth is shown in
Fig. 4. The maximum hardness at the outer surface of the case and the minimum value at the core were found to be 713.2 HV (60.7 HRC) and 440.5 HV (44.5 HRC), respectively. The hardness values in Fig. 4 indicates that the gear had been case hardened by carburizing which is common practice for gear heat treatment [16]. 4.5 Microstructure Examination
The case microstructure was tempered martensite as shown in Fig. 5a. The core was a mixture of ferrite and pearlite as shown in Fig. 5b. The microstructures indicate that the gear was carburized, quenched, and tempered which is common practice in heat treatment of gears [16]. No abnormality was found in the microstructure.
4.6 Gear Tooth Surface Examination
SEM examination of gear tooth surface revealed that there were pitting and spalling areas on the active side of the gear tooth, as shown in Fig. 6a, b. The presence of extensive sub-surface cracks and spalling at the active surface side of the gear tooth was an indication that the gear tooth was subjected to very high contact stress with a shear component.
(1) The failure of the helical gear was caused by excessive contact stress on the surface of the gear teeth. The calculated contact stress is 1.7 times higher than the allowable contact stress of gear material. This excessive stress was the result of the replacement of original electric motor by a more powerful one.
(2) The fracture starts from pitting area at surface of a gear tooth followed by fatigue crack initiation, crack growth, and final fracture. The pitting occurred as a result of excessive stress.
[1] Elecon Engineering Co., Ltd, Power transmission & Drive Solution, http:/www.elecon.com/gearworld/dat-gwfailure.html, access on 10/01/10.
[2] K.J. Abhay, V. Diwakar, Metallurgical analysis of failed gear, Eng. Fail. Anal. 9 (3) (2002) 359-365.
[3] K.L. Johnson, The strength of surfaces in rolling contact, Mech. Eng. Sci. 203 (3) (1989) 151-163.
[4] A.L. Alban, Systematic Analysis of Gear Failures, 2nd ed., ASM International, Materials Park, Ohio, 1985, pp. 86-94.
[5] P.J.L. Fernandes, Tooth bending fatigue failure in gears, Eng. Fail. Anal. 3 (3) (1996) 219-225.
[6] A.L. Alban, Failure of gears, failure analysis and prevention, american society for metals, Materials Park, Ohio 11 (1996) 587-601.
[7] B. Errichello, J. Muller, How to analyze gear failures, http:/www.machinerylubrication.com/Read/150/gear-fail ures.html, access on 17/04/11.
[8] S. Suresh, R.O. Ritchie, Propagation of short fatigue cracks, International Metals Reviews (1984) 445-476.
[9] D.W. Dudley, Fatigue and Life Prediction of Gear, ASM Handbook, Metal Park, American Society for Metals 19(1996) 872.
[10] Principals of bearing selection and applications, SKF General Catalogue, Germany, 2003, p. 87.
[11] C.L. Erickson, B.M. Jones, Electrical and Electronics Engineering, Marks’ Standard Handbook for Mechanical Engineers, 9th ed., McGraw-Hill, 1987, pp. 15-3.
[12] S.H. Loewenthal, Shafts, Couplings, Keys, ETC, Mechanical Design Handbook, McGraw-Hill, New York.
[13] B.J. Hamrock, S.R. Schmind, O.B. Jacobson, Fundamentals of Machine Elements, 2nd ed., McGraw-Hill, New York, 2005, pp. 640, 678.
[14] Ferrous Materials & Metallurgy I, JIS Handbook, Japan Standards Association, Tokyo, 2002, p. 1304.
[15] Bangkok Special Steel Co., Ltd: Machinery Steel, URL:http://www.bssteel.co.th/product_en. html, access on 17/04/11.
[16] S.K. Lyu, K. Inoue, G. Deng, M. Kato, Effect of surface treatments on the strength of carburized gears, Mech. Sci. Tech. 12 (2) (1998) 206-214.