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Design out maintenance on frequent failure of motor ball bearings-2-3
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Design out maintenance on frequent failure of motor ball bearings-2-3

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  • 1. INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – AND TECHNOLOGY (IJMET) 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online) IJMETVolume 4, Issue 1, January- February (2013), pp. 242-251© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI) ©IAEMEwww.jifactor.com DESIGN-OUT MAINTENANCE ON FREQUENT FAILURE OF MOTOR BALL BEARINGS 1 2 Piyush Gupta* ,Shashank Gupta 1 Mechanical Group,Inter-University Accelerator Centre, New Delhi – 110067, India. 2 Department of Mathematics and Department of Manufacturing Engineering, Birla Institute of Technology and Science, Pilani – 333031, India. ABSTRACT Availability of mechanical equipment is a function of its reliability and maintainability. Reliability of equipment, at any instant of time signifies the probability of its survival. Classically, the reliability is an equipment design attribute. It is, however, experienced that reliability of equipment is also dependent on how well the equipment has been shaped-up in the chain of processes from design to commissioning. A case study on design out maintenance on frequently failing bearings of a pump-motor set, which showed poor reliability, is discussed. A step wise analysis is detailed in this paper. The analysis showed that improper inspection post-manufacturing or lack of emphasis on the manufacturing drawings issued by the design department can lead to low equipment reliability and can create field problems for maintenance personnel. It is suggested that an analytical approach to maintenance culminates into design out maintenance, thereby increasing reliability and availability. The design out maintenance approach applied to the case study increased the mean time to failure of bearings from 37 days to 2066 days. This shows that DOM is capable of significantly reducing operation costs of an organization. Keywords: failure analysis; design-out maintenance; ball bearing; facial run-out 242
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME1. INTRODUCTION Mechanical equipment, before being put to commercial use goes through variousfunctional stages. These are: detailing customer specifications including conditions of use,design, manufacturing, inspection, testing, transportation, erection, commissioning and finaltesting. A high availability [1] and commercial viability of the equipment depends on properexecution of all these functions. A process industry consists of innumerous equipment, whichis critical in the equipment chain. It is therefore, recognized that equipment must have highreliability, and therefore, the mean time to failure (MTTF) of such equipment must be high. Reliability of equipment is classically associated with equipment design attributes. Agood equipment design builds these into the equipment. However, it is experienced that,occasional oversight by designers lead to situations, which cause frequent failure ofequipment with MTTF significantly lower than expected. Such equipment, having passedthrough the different stages as above, is handed over to the operations group. However, it isthe responsibility of the maintenance group to deliver high availability of the equipment,which may carry inherent design defect. Field analysis of failures is, therefore, an option forthe maintenance manager in case of high failure rate of the equipment. Subsequent designcorrections based on the knowledge of maintenance [2] or design-out maintenance (DOM)may, therefore, be resorted to by the maintenance function. It is not strictly maintenance [3],but is a necessity borne out of compulsions from: operations for higher availability andmanagement for cost reduction. Competitive designs ensure that downstream life cycle factors, such as maintenanceare envisaged at the beginning of the design process [4]. This is design for maintenanceapproach. However, another design approach is to design out maintenance [5]. This is costlyand is employed in situations where uptime of equipment is critical to system reliability anddowntime costs are usually high. Choice of design out approach is a trade-off between costsof recurring maintenance, downtime and re-design [6]. This approach is necessitated due tothe inadvertent errors that may have occurred in one or more stages, through which theequipment moved, e.g., defective design, improper inspection, faulty installation, etc.Additionally, continuous efforts to improvise profitability also results in DOM.It is, therefore, recognized that DOM is an effective tool, which aims to eliminate the “causeof maintenance”. It is an engineering design problem and often forms part of maintenancedepartment’s responsibility. It is appropriate for items of high maintenance cost, which arisesbecause of: defective design or operation outside design specifications. It is experienced thatin many cases design out is aimed at items that are not expected to require maintenance. Inthis, the choice is between cost of redesign and the maintenance resource cost including thedowntime costs. This paper attempts to discuss a real life case study on the implementation of DOM.The objective of this paper is to demonstrate the effectiveness of the methodology ofimplementing the design out maintenance and the benefits accrued thereof. In section 2, the system under study is described. Section 3, gives the details of thefailures and analyses the failure data. In sections 4 and 5, the cost of annual failures isevaluated and steps for design out maintenance are detailed. Corrective actions and its resultsare given in section 6. Section 7 discusses the cause of failure and finally, the last sectionconcludes. 243
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME2. DESCRIPTION OF THE SYSTEM This case study pertains to a pump-motor set. The purpose of the unit is to pumpchilled water at 7 degree centigrade to a particle accelerator system for removal of heat fromaccelerator components. The three phase induction motor, which had repeated bearing failureacts as a prime mover to the single stage, back pull-out centrifugal water pump. The pumpoperating characteristics are: Head = 85 meters of water column (MWC) and discharge of37.5 m3 per hour. The motor is coupled to the pump by means of a flexible coupling withrubber spider. The coupling manufacturer permits a radial and angular off-set of alignment onthe coupling flanges as 0.25 mm for both the off-sets. The complete unit is anchored on to themild steel channel frame, which in turn is grouted on an inertia block of size 1500 mm(length) x500 mm (width) x 160 mm (height). The inertia block is resting on vibration isolation pads toprevent transmission of vibrations to adjacent machinery. The set-up is shown in Fig.1. Figure 1.pump-motor set-upThe specifications of the motor are given in Table 1 below: Table – 1 Specifications of the prime mover S. N. Details Specifications S. N. Details Specifications 1 Type Induction 8 Duty S1 2 Make Reputed 9 Ambient 50 deg. C 3 Capacity 22 KW 10 Phase/Frequency 3 Ph. / 50 Hz 4 Rating Continuous 11 Insulation Class B, IP-22 5 Frame Size 160L 12 Drive-end bearing SKF 6310 6 RPM 2920 13 Non-drive end SKF 6210 bearing 7 Amperage 42 Amps.3. DESCRIPTION OF FAILURES AND ANALYSIS OF FAILURE DATA The pump motor set was commissioned after observing the correct installationprocedures. On commissioning, the unit was found to have severe grinding noise and vibrationson the motor bearings. However, the unit could not be shut-down for investigation due topressures from the operations. The unit tripped on motor overload protection after a continuousoperation span of 51 days. The bearings of the motor were replaced and the unit was re-started infour hours. However, severe noise and vibrations persisted. Subsequently, similar type of outages 244
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEoccurred after the unit ran for 36, 47, 24 and 28 days. Each outage was due to virtual seizure ofbearings causing the unit to trip on motor overload protection. The analysis of the failure data isshown in Fig. 2. A 2-parameter Weibull failure distribution plot showed a mean time to failure ofapproximately 37 days with a confidence level of 90 %. The reliability function [7] given inexpression (1) gave significantly low reliability of only 36.8% after 41 days of operation. R(t) = e - (t / θ)β ... … .. (1)Where, θ = characteristic life or scale parameter, β = shape parameter. Figure 2. bearing failure, 2 parameter Weibull probability plot Reliasoft Weibull ++7 (www.Reliasoft.com) The value of shape factor (β) was found to be 4.06. The high value of β indicated thatthe bearings had failed within a relatively small time span. The scale parameter (θ) was foundto be 41.16 days. The value of θ indicated that after 41 days, probability of bearing failurewas 63.2 %. The MTTF was found to be 37 days with lower confidence limit of 31 days at aconfidence level of 90%. Therefore, the number of failures per year was (365 / 37) = 9.86. 245
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME4. ANNUAL COST OF FAILURES The direct cost of each failure was evaluated as INR 3000, which included the cost ofmanpower and spares. The downtime cost for each failure was evaluated as INR 33,000.Therefore, each failure costs the organization INR 36,000. The number of annual failureswere 9.86 and the cost of these is evaluated as 9.86 x 36,000 = INR 355,000. Theexceptionally high failure rates along with high cost of failures motivated the maintenancegroup to systematically analyze the failures and adopt the design out maintenance strategy atthe first available maintenance window. Check noise levels Severe grinding noise Bearings pre-loaded Check: bearing fits; motor cooling Measure motor bearing fan Driven end bearing = 72 deg. C temperatures Driving end bearing = 87 deg. C Overall bearing vibrations (with filter-out): In-board (IB) bearing – 60 µ in vertical direction Record motor IB bearing – 6.6 mm/s in vertical direction IB bearing – 90 µ in axial direction bearing vibrations Outboard (OB) bearing – 94 µ in vertical direction OB bearing – 9.2 mm/s in vertical direction OB bearing – 120 µ in axial direction Filtered vertical bearing vibrations: Presence of significant 1x RPM – 52 µ (IB), 80 µ (OB) component of second 2 x RPM – 15 µ (IB), 68 µ (OB) harmonic vibration 3 x RPM – 5 µ (IB), 12 µ (OB) indicates presence of misalignment forces / looseness in assembly Check:misalient; foundation and motor Check: misalignment; foundation and end cover bolts for tightness; coupling motor end cover bolts for tightness; fits; looseness of motor - rotor stamping; coupling fits; looseness motor and- pump flexible-coupling gap; of motor rotor stamping; flexible-coupling gap; motor slope. and pump slope. Figure 3. on-line observations on the pump – motor set and inferences 246
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME5. DESIGN OUT MAINTENANCE Detailed online observations were made with the unit in operation, whichindicated subsequent off-line checks to be made. These are shown in Figure 3. Thetemperature measurements were done with digital type thermometer and the vibrationreadings were recorded with vibration analyzer model – 5050, make – Baseline. The presence of severe grinding noise from the motor bearings along with highbearing temperature was indicative of excessive pre-loading of the bearings, and ormalfunctioning of the motor cooling fan. This was checked after dis-assembly of themotor. Further, as per ISO 10816-1:1995standards [8], which are applicable for rigidrotor systems that yield bearing cap vibrations indicative of shaft motion, thevibrations recorded belong to Class –D of vibration severity. Therefore, the overallvibration levels were high. Further, it was noted that the vibration level on out-boardbearing of the motor in the axial direction was high (120µ), and it was more than theradial component (94µ). Additionally, a high level (68µ) of second harmonic vibrationwas present on the motor outboard-bearing. It was, therefore, concluded that thebearing failure was probably because of the fact that the motor bearings weresubjected to high axial forces. TheThe shaft journals were shaft journals were measured to have an measured to have an Abnormality Abnormality interference fit fit 0.015 mmmm interference of of 0.015 on and the inner race of the bearings. and on the inner race of the pre-loading The bearingbearings. were housings pre-loading non-existent non-existent measured to have 0.01 mm The bearing housings were interference-fit with outer race measured to have 0.01 mm of interference-fit with outer the bearings. Grinding noise Check: race of the bearings. and high Bearing fits; bearing temp. motor cooling fan Record motor still bearing vibrations The motor coolingfan was unexplained? The motor cooling fan was visually checked for visually checked for breakage No No breakage and looseness on and looseness on the motor abnormality abnormality the motor shaft shaft wasnoticed. was noticed. Analyse Analyse further … further … Figure 4.off-line checks and further inferences 247
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME All anchor bolts of foundation Check: and motor end covers found Foundation and adequately tightened motor end cover bolts for tightness; The hub of the coupling halves had sliding fit on both the Coupling fits; motor and pump shafts. Looseness of motor - rotor The motor rotor stamping was Installa- Further stamping; found adequately anchored tion analysis onto the motor shaft with no defects required Flexible- not …… coupling gap; visible signs of axial movement found Motor and pump slope; The coupling halves had adequate axial gap of 2.2 mm Misalignment; (recommended value was between 2-3 mm) The slopes of pump and motor shafts were measured by precision level and were within 0.10 mm per meter 0 -0.04 +0.02 +0.03 -0.01 0 +0.05 +0.03 (radial alignment) (axial alignment) (All readings in mm) Radial alignment checked with dial gage (DG) of 0.01 mm least count. The DG anchored on motor shaft with its pointer on pump shaft. Inside micrometer of least count 0.01 mm used for measuring axial alignment. Double shaft rotation method was used to nullify the effect of facial run- out on coupling faces and the effect of axial shift of the motor /pump shafts within bearing axial clearances. Figure 5. Additional off-line checks and further inferences 248
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME Therefore, it became necessary to carry out further investigations by dismantling theunit assembly. As a next step, in analyzing the cause of bearing failure, the pump- motor setwas shut down. The pump and motor were decoupled. The motor was dismantled andadditional checks were made based on the inferences shown in Figure 3. These are shownabove in Figures 4 and 5. The correctness of installation and assembly of the pump and motor was ensured asabove. It did not conclude on any specific cause of repeated bearing failures. As a next step,it was decided to check for defects in the motor rotor, keeping in mind that there was somedefect that caused the high axial vibrations. The motor- rotor shown in Figure 6 was loaded on a lathe between centers. The run-out on the journal diameters and taper of the bearing journals along the journal length waschecked by a dial gage having a least count of 0.01 mm. The maximum value of the run-outand taper was found to be within a 0.01 mm. The facial run-out of the in-board and out-boardbearing seating was also checked. The facial run-out of out-board bearing seat, as shown inFigure 6 (face f1) was found to be 0.72mm and that on the in-board bearing seat (face f2) was0.21 mm. It therefore, appeared that the facial run-out on the bearing seat forced the bearingto tilt with respect to the shaft axis, instead of being square to it. Figure 6.schematic of the motor rotor6. CORRECTIVE MAINTENANCE ACTION AND RESULTS OF THE ACTION The facial run-out on the bearing seat surfaces f1 and f2 were machined off to anaccuracy of 0.01 mm to make the faces square with the shaft axis. The motor was re-assembled and coupled with the pump and run. The operatingparameters were observed and are shown in Table 2. The motor ran without any bearingfailures for 2,066 days after, which they were replaced in accordance with the preventivemaintenance schedule. Table 2. Operation parameters of the pump-motor set after design-out maintenance action Operation parameter In-board bearing Out-board bearing Noise Smooth Smooth Temperature 36 deg. C 38 deg. C Overall Vibration amplitude 32 µ 38µ 249
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME7. DISCUSSIONThe total friction (F) for rotating shaft mounted on anti-friction bearings [9]is given by: F ൌ F୪୭ୟୢ ൅ Fୱ୮ୣୣୢ ൅ Fୱୣୟ୪ … … . ሺ2ሻ F୪୭ୟୢ ൌ Load dependent friction, Fୱ୮ୣୣୢ ൌ Lubrication and speed dependent friction, Fୱୣୟ୪ ൌ Seal dependent friction. It is recognized that ball bearings do have sliding friction due to: sliding onaccount of velocity difference between the rolling element, i.e., balls and inner race; thetangential velocity; and the sliding action between cage and seals. Lubrication, such asgrease is used to minimize the effect of sliding friction. However, an out of square fittingof the ball bearing on the shaft journal does kill the clearance, which is otherwise requiredfor the lubrication to fill-in, and to create a lubrication film for metallic separationbetween: balls and races; and, cage and seals. This may have caused heat generation dueto metal to metal contact. Repeated collapse of lubrication film and subsequent build upmay be responsible for the predominant second harmonic vibration in the axial direction. The analysis of the bearing failures revealed that the installation was done as perbest practices. However, in the process chain of the equipment; the inspection functionand the manufacturing function both failed to recognize the relevance of the facial run-outof the bearing seat surface. It appears that this was not emphasized by the designer in themanufacturing drawings released to the production department. This is furthercorroborated by the fact that, many motors operating in the plant at IUAC had this defect,though to a lesser degree and the failures were not immediate because of lower operatingspeed. A study is presently underway to quantify the defect levels vis-a-vis the vibrationlevels and MTTF.8. CONCLUSION The lack of emphasis by the designer on critical dimensions of equipment maylead to defective manufacture of its components. In absence of clarity on the detailedproduction drawings, the defect in the manufactured product is passed by the inspectionfunction to the end-users, therefore, creating field problems and, therefore, low reliabilityand availability of the equipment. Such instances are dealt by the maintenance functionleading to design out maintenance, which is costly and maybe inconvenient to implement.ACKNOWLEDGEMENTS The authors would like to thank the Director, IUAC and Sh. S.K. Saini, WorkshopEngineer, IUAC, New Delhi for their support in implementation of the works. 250
  • 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEMEREFERENCES[1] G.V.A.Vasantha, R. Roy, A. Lelah, andD. Brissaud, A review of product-service systemsdesign methodologies. Journal of Engineering Design, 23(9), 2012, 635-659.[2] Allen Kent Allen,Encyclopedia of Computer Science and Technology. CRC Press, 1990.[3] A. Kelly, Maintenance Planning and Control (New Delhi, India, Affiliated East-WestPress Pvt. Ltd., 1991).[4] B. Gagnon, R. Leduc, and L. Savard, From conventional to a sustainable engineeringdesign process: different shades of sustainability. Journal of Engineering Design, 23(1), 2012,49-74.[5] T. Markeset and U. Kumar, R&M and risk-analysis tools in product design, to reduce life-cycle cost and improve attractiveness.Proceedings of Annual Reliability and MaintainabilitySymposium,22-25 January, 2001,Philadelphia, USA.116-122.[6] A.K. Jain, Influence of modification of design out maintenance & design out informationsystem for maintenance cost control & a lucrative business (with case study). InternationalJournal of Engineering Trends and Technology, 4 (1), 2013, 1-9.[7]Charles E. Ebeling, An Introduction to Reliability and Maintainability Engineering. (NewDelhi, India, Tata McGraw-Hill Education Private Limited, 2000).[8] ISO 10816-1:1995. Mechanical vibration -- Evaluation of machine vibration bymeasurements on non-rotating parts- Part 1: General guidelines. International Organizationfor Standardization, Geneva, Switzerland.[9] http://freevideolectures. com/Course/3142/Tribology/32….. Accessed on Jan 23, 2013ABOUT THE AUTHORSPiyush Gupta, B.Tech. (Mechanical), I.I.T.,Delhi, and M.Tech.(Industrial TribologyMaintenance Engineering and Machine Dynamics Centre), I.I.T., Delhi, is presently workingas Engineer ‘G’ at Inter University Accelerator Centre, New Delhi, India. He has 33 years ofindustrial experience out of which he has 25 years of experience in managing operations andmaintenance of an accelerator based research facility, besides having 8 years of experiencewith Bharat Heavy Electricals Ltd., India, in the maintenance and trouble shooting of steamand gas turbines. He is currently pursuing his doctoral degree from Indian Institute ofTechnology, Delhi, India. His interest is in the areas of maintenance and machine dynamics.Shashank Gupta is an under-graduate, dual degree student of Department ofMathematics and Department of Manufacturing Engineering at Birla Institute of Technologyand Science, Pilani, Rajasthan, India. He is a scholarship holder from Department of Scienceand Technology, Government of India, New Delhi, India. 251