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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) © IAEME


ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)                                                       IJMET
Volume 4, Issue 1, January- February (2013), pp. 242-251
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2013): 5.7731 (Calculated by GISI)                  ©IAEME
www.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




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

1. INTRODUCTION

         Mechanical equipment, before being put to commercial use goes through various
functional stages. These are: detailing customer specifications including conditions of use,
design, manufacturing, inspection, testing, transportation, erection, commissioning and final
testing. A high availability [1] and commercial viability of the equipment depends on proper
execution of all these functions. A process industry consists of innumerous equipment, which
is critical in the equipment chain. It is therefore, recognized that equipment must have high
reliability, and therefore, the mean time to failure (MTTF) of such equipment must be high.
         Reliability of equipment is classically associated with equipment design attributes. A
good equipment design builds these into the equipment. However, it is experienced that,
occasional oversight by designers lead to situations, which cause frequent failure of
equipment with MTTF significantly lower than expected. Such equipment, having passed
through the different stages as above, is handed over to the operations group. However, it is
the 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 for
the maintenance manager in case of high failure rate of the equipment. Subsequent design
corrections 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 and
management for cost reduction.
         Competitive designs ensure that downstream life cycle factors, such as maintenance
are envisaged at the beginning of the design process [4]. This is design for maintenance
approach. However, another design approach is to design out maintenance [5]. This is costly
and is employed in situations where uptime of equipment is critical to system reliability and
downtime costs are usually high. Choice of design out approach is a trade-off between costs
of recurring maintenance, downtime and re-design [6]. This approach is necessitated due to
the inadvertent errors that may have occurred in one or more stages, through which the
equipment 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 “cause
of maintenance”. It is an engineering design problem and often forms part of maintenance
department’s responsibility. It is appropriate for items of high maintenance cost, which arises
because of: defective design or operation outside design specifications. It is experienced that
in many cases design out is aimed at items that are not expected to require maintenance. In
this, the choice is between cost of redesign and the maintenance resource cost including the
downtime 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 of
implementing the design out maintenance and the benefits accrued thereof.
         In section 2, the system under study is described. Section 3, gives the details of the
failures and analyses the failure data. In sections 4 and 5, the cost of annual failures is
evaluated and steps for design out maintenance are detailed. Corrective actions and its results
are given in section 6. Section 7 discusses the cause of failure and finally, the last section
concludes.




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

2. DESCRIPTION OF THE SYSTEM

        This case study pertains to a pump-motor set. The purpose of the unit is to pump
chilled water at 7 degree centigrade to a particle accelerator system for removal of heat from
accelerator components. The three phase induction motor, which had repeated bearing failure
acts as a prime mover to the single stage, back pull-out centrifugal water pump. The pump
operating characteristics are: Head = 85 meters of water column (MWC) and discharge of
37.5 m3 per hour. The motor is coupled to the pump by means of a flexible coupling with
rubber spider. The coupling manufacturer permits a radial and angular off-set of alignment on
the coupling flanges as 0.25 mm for both the off-sets. The complete unit is anchored on to the
mild steel channel frame, which in turn is grouted on an inertia block of size 1500 mm(length) x
500 mm (width) x 160 mm (height). The inertia block is resting on vibration isolation pads to
prevent transmission of vibrations to adjacent machinery. The set-up is shown in Fig.1.




                                 Figure 1.pump-motor set-up

The 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 installation
procedures. On commissioning, the unit was found to have severe grinding noise and vibrations
on the motor bearings. However, the unit could not be shut-down for investigation due to
pressures from the operations. The unit tripped on motor overload protection after a continuous
operation span of 51 days. The bearings of the motor were replaced and the unit was re-started in
four hours. However, severe noise and vibrations persisted. Subsequently, similar type of outages


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

occurred after the unit ran for 36, 47, 24 and 28 days. Each outage was due to virtual seizure of
bearings causing the unit to trip on motor overload protection. The analysis of the failure data is
shown in Fig. 2. A 2-parameter Weibull failure distribution plot showed a mean time to failure of
approximately 37 days with a confidence level of 90 %. The reliability function [7] given in
expression (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 that
the bearings had failed within a relatively small time span. The scale parameter (θ) was found
to be 41.16 days. The value of θ indicated that after 41 days, probability of bearing failure
was 63.2 %. The MTTF was found to be 37 days with lower confidence limit of 31 days at a
confidence level of 90%. Therefore, the number of failures per year was (365 / 37) = 9.86.



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

4. ANNUAL COST OF FAILURES

         The direct cost of each failure was evaluated as INR 3000, which included the cost of
manpower 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 failures
were 9.86 and the cost of these is evaluated as 9.86 x 36,000 = INR 355,000. The
exceptionally high failure rates along with high cost of failures motivated the maintenance
group to systematically analyze the failures and adopt the design out maintenance strategy at
the 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




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

5. DESIGN OUT MAINTENANCE

       Detailed online observations were made with the unit in operation, which
indicated subsequent off-line checks to be made. These are shown in Figure 3. The
temperature measurements were done with digital type thermometer and the vibration
readings were recorded with vibration analyzer model – 5050, make – Baseline.

        The presence of severe grinding noise from the motor bearings along with high
bearing temperature was indicative of excessive pre-loading of the bearings, and or
malfunctioning of the motor cooling fan. This was checked after dis-assembly of the
motor. Further, as per ISO 10816-1:1995standards [8], which are applicable for rigid
rotor systems that yield bearing cap vibrations indicative of shaft motion, the
vibrations recorded belong to Class –D of vibration severity. Therefore, the overall
vibration levels were high. Further, it was noted that the vibration level on out-board
bearing of the motor in the axial direction was high (120µ), and it was more than the
radial component (94µ). Additionally, a high level (68µ) of second harmonic vibration
was present on the motor outboard-bearing. It was, therefore, concluded that the
bearing failure was probably because of the fact that the motor bearings were
subjected 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




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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
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 the
unit assembly. As a next step, in analyzing the cause of bearing failure, the pump- motor set
was shut down. The pump and motor were decoupled. The motor was dismantled and
additional checks were made based on the inferences shown in Figure 3. These are shown
above in Figures 4 and 5.
         The correctness of installation and assembly of the pump and motor was ensured as
above. 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 some
defect 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 was
checked by a dial gage having a least count of 0.01 mm. The maximum value of the run-out
and taper was found to be within a 0.01 mm. The facial run-out of the in-board and out-board
bearing seating was also checked. The facial run-out of out-board bearing seat, as shown in
Figure 6 (face f1) was found to be 0.72mm and that on the in-board bearing seat (face f2) was
0.21 mm. It therefore, appeared that the facial run-out on the bearing seat forced the bearing
to tilt with respect to the shaft axis, instead of being square to it.




                             Figure 6.schematic of the motor rotor


6. CORRECTIVE MAINTENANCE ACTION AND RESULTS OF THE ACTION

        The facial run-out on the bearing seat surfaces f1 and f2 were machined off to an
accuracy 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 operating
parameters were observed and are shown in Table 2. The motor ran without any bearing
failures for 2,066 days after, which they were replaced in accordance with the preventive
maintenance 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




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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

7. DISCUSSION

The 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 on
account of velocity difference between the rolling element, i.e., balls and inner race; the
tangential velocity; and the sliding action between cage and seals. Lubrication, such as
grease is used to minimize the effect of sliding friction. However, an out of square fitting
of the ball bearing on the shaft journal does kill the clearance, which is otherwise required
for the lubrication to fill-in, and to create a lubrication film for metallic separation
between: balls and races; and, cage and seals. This may have caused heat generation due
to metal to metal contact. Repeated collapse of lubrication film and subsequent build up
may 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 per
best practices. However, in the process chain of the equipment; the inspection function
and the manufacturing function both failed to recognize the relevance of the facial run-out
of the bearing seat surface. It appears that this was not emphasized by the designer in the
manufacturing drawings released to the production department. This is further
corroborated 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 operating
speed. A study is presently underway to quantify the defect levels vis-a-vis the vibration
levels and MTTF.

8. CONCLUSION

       The lack of emphasis by the designer on critical dimensions of equipment may
lead to defective manufacture of its components. In absence of clarity on the detailed
production drawings, the defect in the manufactured product is passed by the inspection
function to the end-users, therefore, creating field problems and, therefore, low reliability
and availability of the equipment. Such instances are dealt by the maintenance function
leading 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, Workshop
Engineer, IUAC, New Delhi for their support in implementation of the works.




                                             250
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

REFERENCES

[1] G.V.A.Vasantha, R. Roy, A. Lelah, andD. Brissaud, A review of product-service systems
design 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-West
Press Pvt. Ltd., 1991).
[4] B. Gagnon, R. Leduc, and L. Savard, From conventional to a sustainable engineering
design 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 Maintainability
Symposium,22-25 January, 2001,Philadelphia, USA.116-122.
[6] A.K. Jain, Influence of modification of design out maintenance & design out information
system for maintenance cost control & a lucrative business (with case study). International
Journal of Engineering Trends and Technology, 4 (1), 2013, 1-9.
[7]Charles E. Ebeling, An Introduction to Reliability and Maintainability Engineering. (New
Delhi, India, Tata McGraw-Hill Education Private Limited, 2000).
[8] ISO 10816-1:1995.          Mechanical vibration -- Evaluation of machine vibration by
measurements on non-rotating parts- Part 1: General guidelines. International Organization
for Standardization, Geneva, Switzerland.
[9] http://freevideolectures. com/Course/3142/Tribology/32….. Accessed on Jan 23, 2013


ABOUT THE AUTHORS

Piyush Gupta, B.Tech. (Mechanical), I.I.T.,Delhi, and M.Tech.(Industrial Tribology
Maintenance Engineering and Machine Dynamics Centre), I.I.T., Delhi, is presently working
as Engineer ‘G’ at Inter University Accelerator Centre, New Delhi, India. He has 33 years of
industrial experience out of which he has 25 years of experience in managing operations and
maintenance of an accelerator based research facility, besides having 8 years of experience
with Bharat Heavy Electricals Ltd., India, in the maintenance and trouble shooting of steam
and gas turbines. He is currently pursuing his doctoral degree from Indian Institute of
Technology, Delhi, India. His interest is in the areas of maintenance and machine dynamics.

Shashank Gupta is an under-graduate, dual degree student of Department of
Mathematics and Department of Manufacturing Engineering at Birla Institute of Technology
and Science, Pilani, Rajasthan, India. He is a scholarship holder from Department of Science
and Technology, Government of India, New Delhi, India.




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

  • 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) © IAEME ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) IJMET Volume 4, Issue 1, January- February (2013), pp. 242-251 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2013): 5.7731 (Calculated by GISI) ©IAEME www.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) © IAEME 1. INTRODUCTION Mechanical equipment, before being put to commercial use goes through various functional stages. These are: detailing customer specifications including conditions of use, design, manufacturing, inspection, testing, transportation, erection, commissioning and final testing. A high availability [1] and commercial viability of the equipment depends on proper execution of all these functions. A process industry consists of innumerous equipment, which is critical in the equipment chain. It is therefore, recognized that equipment must have high reliability, and therefore, the mean time to failure (MTTF) of such equipment must be high. Reliability of equipment is classically associated with equipment design attributes. A good equipment design builds these into the equipment. However, it is experienced that, occasional oversight by designers lead to situations, which cause frequent failure of equipment with MTTF significantly lower than expected. Such equipment, having passed through the different stages as above, is handed over to the operations group. However, it is the 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 for the maintenance manager in case of high failure rate of the equipment. Subsequent design corrections 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 and management for cost reduction. Competitive designs ensure that downstream life cycle factors, such as maintenance are envisaged at the beginning of the design process [4]. This is design for maintenance approach. However, another design approach is to design out maintenance [5]. This is costly and is employed in situations where uptime of equipment is critical to system reliability and downtime costs are usually high. Choice of design out approach is a trade-off between costs of recurring maintenance, downtime and re-design [6]. This approach is necessitated due to the inadvertent errors that may have occurred in one or more stages, through which the equipment 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 “cause of maintenance”. It is an engineering design problem and often forms part of maintenance department’s responsibility. It is appropriate for items of high maintenance cost, which arises because of: defective design or operation outside design specifications. It is experienced that in many cases design out is aimed at items that are not expected to require maintenance. In this, the choice is between cost of redesign and the maintenance resource cost including the downtime 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 of implementing the design out maintenance and the benefits accrued thereof. In section 2, the system under study is described. Section 3, gives the details of the failures and analyses the failure data. In sections 4 and 5, the cost of annual failures is evaluated and steps for design out maintenance are detailed. Corrective actions and its results are given in section 6. Section 7 discusses the cause of failure and finally, the last section concludes. 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) © IAEME 2. DESCRIPTION OF THE SYSTEM This case study pertains to a pump-motor set. The purpose of the unit is to pump chilled water at 7 degree centigrade to a particle accelerator system for removal of heat from accelerator components. The three phase induction motor, which had repeated bearing failure acts as a prime mover to the single stage, back pull-out centrifugal water pump. The pump operating characteristics are: Head = 85 meters of water column (MWC) and discharge of 37.5 m3 per hour. The motor is coupled to the pump by means of a flexible coupling with rubber spider. The coupling manufacturer permits a radial and angular off-set of alignment on the coupling flanges as 0.25 mm for both the off-sets. The complete unit is anchored on to the mild steel channel frame, which in turn is grouted on an inertia block of size 1500 mm(length) x 500 mm (width) x 160 mm (height). The inertia block is resting on vibration isolation pads to prevent transmission of vibrations to adjacent machinery. The set-up is shown in Fig.1. Figure 1.pump-motor set-up The 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 installation procedures. On commissioning, the unit was found to have severe grinding noise and vibrations on the motor bearings. However, the unit could not be shut-down for investigation due to pressures from the operations. The unit tripped on motor overload protection after a continuous operation span of 51 days. The bearings of the motor were replaced and the unit was re-started in four 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) © IAEME occurred after the unit ran for 36, 47, 24 and 28 days. Each outage was due to virtual seizure of bearings causing the unit to trip on motor overload protection. The analysis of the failure data is shown in Fig. 2. A 2-parameter Weibull failure distribution plot showed a mean time to failure of approximately 37 days with a confidence level of 90 %. The reliability function [7] given in expression (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 that the bearings had failed within a relatively small time span. The scale parameter (θ) was found to be 41.16 days. The value of θ indicated that after 41 days, probability of bearing failure was 63.2 %. The MTTF was found to be 37 days with lower confidence limit of 31 days at a confidence 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) © IAEME 4. ANNUAL COST OF FAILURES The direct cost of each failure was evaluated as INR 3000, which included the cost of manpower 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 failures were 9.86 and the cost of these is evaluated as 9.86 x 36,000 = INR 355,000. The exceptionally high failure rates along with high cost of failures motivated the maintenance group to systematically analyze the failures and adopt the design out maintenance strategy at the 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) © IAEME 5. DESIGN OUT MAINTENANCE Detailed online observations were made with the unit in operation, which indicated subsequent off-line checks to be made. These are shown in Figure 3. The temperature measurements were done with digital type thermometer and the vibration readings were recorded with vibration analyzer model – 5050, make – Baseline. The presence of severe grinding noise from the motor bearings along with high bearing temperature was indicative of excessive pre-loading of the bearings, and or malfunctioning of the motor cooling fan. This was checked after dis-assembly of the motor. Further, as per ISO 10816-1:1995standards [8], which are applicable for rigid rotor systems that yield bearing cap vibrations indicative of shaft motion, the vibrations recorded belong to Class –D of vibration severity. Therefore, the overall vibration levels were high. Further, it was noted that the vibration level on out-board bearing of the motor in the axial direction was high (120µ), and it was more than the radial component (94µ). Additionally, a high level (68µ) of second harmonic vibration was present on the motor outboard-bearing. It was, therefore, concluded that the bearing failure was probably because of the fact that the motor bearings were subjected 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 the unit assembly. As a next step, in analyzing the cause of bearing failure, the pump- motor set was shut down. The pump and motor were decoupled. The motor was dismantled and additional checks were made based on the inferences shown in Figure 3. These are shown above in Figures 4 and 5. The correctness of installation and assembly of the pump and motor was ensured as above. 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 some defect 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 was checked by a dial gage having a least count of 0.01 mm. The maximum value of the run-out and taper was found to be within a 0.01 mm. The facial run-out of the in-board and out-board bearing seating was also checked. The facial run-out of out-board bearing seat, as shown in Figure 6 (face f1) was found to be 0.72mm and that on the in-board bearing seat (face f2) was 0.21 mm. It therefore, appeared that the facial run-out on the bearing seat forced the bearing to tilt with respect to the shaft axis, instead of being square to it. Figure 6.schematic of the motor rotor 6. CORRECTIVE MAINTENANCE ACTION AND RESULTS OF THE ACTION The facial run-out on the bearing seat surfaces f1 and f2 were machined off to an accuracy 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 operating parameters were observed and are shown in Table 2. The motor ran without any bearing failures for 2,066 days after, which they were replaced in accordance with the preventive maintenance 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) © IAEME 7. DISCUSSION The 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 on account of velocity difference between the rolling element, i.e., balls and inner race; the tangential velocity; and the sliding action between cage and seals. Lubrication, such as grease is used to minimize the effect of sliding friction. However, an out of square fitting of the ball bearing on the shaft journal does kill the clearance, which is otherwise required for the lubrication to fill-in, and to create a lubrication film for metallic separation between: balls and races; and, cage and seals. This may have caused heat generation due to metal to metal contact. Repeated collapse of lubrication film and subsequent build up may 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 per best practices. However, in the process chain of the equipment; the inspection function and the manufacturing function both failed to recognize the relevance of the facial run-out of the bearing seat surface. It appears that this was not emphasized by the designer in the manufacturing drawings released to the production department. This is further corroborated 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 operating speed. A study is presently underway to quantify the defect levels vis-a-vis the vibration levels and MTTF. 8. CONCLUSION The lack of emphasis by the designer on critical dimensions of equipment may lead to defective manufacture of its components. In absence of clarity on the detailed production drawings, the defect in the manufactured product is passed by the inspection function to the end-users, therefore, creating field problems and, therefore, low reliability and availability of the equipment. Such instances are dealt by the maintenance function leading 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, Workshop Engineer, 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) © IAEME REFERENCES [1] G.V.A.Vasantha, R. Roy, A. Lelah, andD. Brissaud, A review of product-service systems design 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-West Press Pvt. Ltd., 1991). [4] B. Gagnon, R. Leduc, and L. Savard, From conventional to a sustainable engineering design 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 Maintainability Symposium,22-25 January, 2001,Philadelphia, USA.116-122. [6] A.K. Jain, Influence of modification of design out maintenance & design out information system for maintenance cost control & a lucrative business (with case study). International Journal of Engineering Trends and Technology, 4 (1), 2013, 1-9. [7]Charles E. Ebeling, An Introduction to Reliability and Maintainability Engineering. (New Delhi, India, Tata McGraw-Hill Education Private Limited, 2000). [8] ISO 10816-1:1995. Mechanical vibration -- Evaluation of machine vibration by measurements on non-rotating parts- Part 1: General guidelines. International Organization for Standardization, Geneva, Switzerland. [9] http://freevideolectures. com/Course/3142/Tribology/32….. Accessed on Jan 23, 2013 ABOUT THE AUTHORS Piyush Gupta, B.Tech. (Mechanical), I.I.T.,Delhi, and M.Tech.(Industrial Tribology Maintenance Engineering and Machine Dynamics Centre), I.I.T., Delhi, is presently working as Engineer ‘G’ at Inter University Accelerator Centre, New Delhi, India. He has 33 years of industrial experience out of which he has 25 years of experience in managing operations and maintenance of an accelerator based research facility, besides having 8 years of experience with Bharat Heavy Electricals Ltd., India, in the maintenance and trouble shooting of steam and gas turbines. He is currently pursuing his doctoral degree from Indian Institute of Technology, Delhi, India. His interest is in the areas of maintenance and machine dynamics. Shashank Gupta is an under-graduate, dual degree student of Department of Mathematics and Department of Manufacturing Engineering at Birla Institute of Technology and Science, Pilani, Rajasthan, India. He is a scholarship holder from Department of Science and Technology, Government of India, New Delhi, India. 251