DESIGN-OUT MAINTENANCE ON FREQUENT FAILURE OF MOTOR BALL BEARINGS

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 1, January - February (2013) © IAEME

242

DESIGN-OUT MAINTENANCE ON FREQUENT FAILURE OF

MOTOR BALL BEARINGS

Piyush Gupta*1 ,Shashank Gupta

2

1Mechanical 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

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING

AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) 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) www.jifactor.com

IJMET

© I A E M E



243

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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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.

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

bearing

SKF 6210

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

Figure 1.pump-motor set-up



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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.

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.

Figure 2. bearing failure, 2 parameter Weibull probability plot

Reliasoft Weibull ++7 (www.Reliasoft.com)



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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.

Figure 3. on-line observations on the pump – motor set and inferences

Check noise levels

Measure motor bearing

temperatures

Presence of significant

component of second

harmonic vibration

indicates presence of

misalignment forces /

looseness in assembly

Overall bearing vibrations (with filter-out):

In-board (IB) bearing – 60 µ in vertical direction

IB bearing – 6.6 mm/s in vertical direction

IB bearing – 90 µ in axial direction

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:

1x RPM – 52 µ (IB), 80 µ (OB)

2 x RPM – 15 µ (IB), 68 µ (OB)

3 x RPM – 5 µ (IB), 12 µ (OB)

Driven end bearing = 72 deg. C

Driving end bearing = 87 deg. C

Severe grinding noise Bearings pre-loaded Check: bearing fits;

motor cooling

fan

Check:misalient; foundation and motor

end cover bolts for tightness; coupling

fits; looseness of motor - rotor stamping;

flexible-coupling gap; motor and pump

slope.

Record motor

bearing vibrations

Check: misalignment; foundation and

motor end cover bolts for tightness;

coupling fits; looseness of motor - rotor

stamping; flexible-coupling gap; motor

and pump slope.



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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.

Figure 4.off-line checks and further inferences

Check:

Bearing fits;

motor cooling fan

Record motor

bearing vibrations

The shaft journals were

measured to have an

interference fit of 0.015 mm

on the inner race of the

bearings.

The bearing housings were

measured to have 0.01 mm

interference-fit with outer

race of the bearings.

Grinding noise

and high

bearing temp.

still

unexplained? The motor cooling fan was

visually checked for

breakage and looseness on

the motor shaft

Abnormality

and

pre-loading

non-existent

No

abnormality

wasnoticed.

Analyse

further …

The shaft journals were

measured to have an

interference fit of 0.015 mm on

the inner race of the bearings.

The bearing housings were

measured to have 0.01 mm

interference-fit with outer race

of the bearings.

Abnormality

and

pre-loading

non-existent

The motor cooling fan was

visually checked for breakage

and looseness on the motor

shaft

No

abnormality

was noticed.

Analyse

further …



248

Figure 5. Additional off-line checks and further inferences

Check: Foundation and

motor end cover

bolts for

tightness;

Coupling fits;

Looseness of

motor - rotor

stamping;

Flexible-

coupling gap;

Motor and pump

slope;

Misalignment;

All anchor bolts of foundation

and motor end covers found

adequately tightened

The slopes of pump and motor

shafts were measured by

precision level and were within

0.10 mm per meter

The coupling halves had

adequate axial gap of 2.2 mm

(recommended value was

between 2-3 mm)

The motor rotor stamping was

found adequately anchored

onto the motor shaft with no

visible signs of axial movement

The hub of the coupling halves

had sliding fit on both the

motor and pump shafts.

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.

Installa-

tion

defects

not

found

Further

analysis

required

……



249

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.

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

Figure 6.schematic of the motor rotor



250

7. DISCUSSION

The total friction (F) for rotating shaft mounted on anti-friction bearings [9]is given by:

F � F�� F�� F�� …… . �2�

F�� Load dependent friction,

F�� Lubrication and speed dependent friction,

F�� 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.



251

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[2] Allen Kent Allen,Encyclopedia of Computer Science and Technology. CRC Press, 1990.

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design process: different shades of sustainability. Journal of Engineering Design, 23(1), 2012,

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cycle cost and improve attractiveness.Proceedings of Annual Reliability and Maintainability

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Journal of Engineering Trends and Technology, 4 (1), 2013, 1-9.

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[8] ISO 10816-1:1995. Mechanical vibration -- Evaluation of machine vibration by

measurements on non-rotating parts- Part 1: General guidelines. International Organization

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[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.

Documents

DESIGN-OUT MAINTENANCE ON FREQUENT FAILURE OF MOTOR BALL BEARINGS