# Multicriteria FMECA Based Decision-Making for Aluminium Wire Process Rolling Mill through COPRAS-G

Authors: Nilesh Pancholi; M. G. Bhatt
Journal: Journal of Quality and Reliability Engineering
 (2016)

---

## Abstract

This paper presents a multifactor decision-making approach based on “grey-complex proportional assessment (COPRAS-G) method” in a view to overcome the limitations of Failure Mode Effect and Criticality Analysis (FMECA). In this model, the scores against each failure mode are expressed in grey number instead of crisp values to evaluate the criticalities of the failure modes without uncertainty. The suggested study is carried out to identify the weights of major failure causes for bearings, gears, and shafts of aluminium wire rolling mill plant. The primary findings of the paper are that sudden impact on the rolls seems to be most critical failure cause and loss of power seems to be least critical failure cause. It is suggested to modify the current control practices with proper maintenance strategy based on achieved maintainability criticality index (MCI) for different failure causes. The outcome of study will be helpful in deriving optimized maintenance plan to maximize the performance of process industry.


---

## Body

## 1. Introduction

The reliability and maintenance engineering is important to maintenance practitioners and reliability engineers to keep the system in a state of readiness. Moreover, it helps to identify the condition based faults, compare possible failure patterns, and maximize effectiveness in maintenance plan. There are many techniques available for planning maintenance activities of process industries. Traditional Failure Mode Effect and Criticality Analysis (FMECA) has proved to be prominence tool among maintenance personnel, where failure modes are ranked on risk priority number (RPN), which is the product of chances of failure (C), degree of detectability (D), and degree of severity (S) to prioritize the maintenance activities.Traditional FMECA is a widely accepted methodology for prioritizing failure modes; however, it has some limitations. It does not cover the interdependency of different failure modes and their effects. It considers only limited criteria like C, D, and S and does not cover some important criteria like maintainability (M), spare parts availability (SP), economic safety (ES), economic cost (EC), and so forth which may also influence the failure modes. Moreover, same importance will be given to C, D, and S ignoring their relative importance and even small variation in the value of C or D or S may change the value of RPN significantly due to multiplication rule.It has been observed that past researchers have undergone various modifications for improving FMECA to overcome these drawbacks for different processing units. Sahoo et al. [1] show that failure modes, effects, and critique analysis (FMECA) is an integral part of the technical design of maintenance and it represents a strong tool to evaluate and improve system reliability and therefore reduces costs associated with maintenance that is used in a wide range of industry. Some researchers [2–5] incorporated a new factor called operating conditions in the field of power plant. Anish et al. [5] presented a multifactor decision-making approach for prioritizing failure modes for paper industry as an alternative using TOPSIS. Braglia et al. [6, 7] presented fuzzy TOPSIS and Xu et al. [8] presented fussy assessment based FMEA for engine system. Gargama and Chaturvedi [9] introduced fuzzy RPN applying level sets where the three risk factors are expressed into fuzzy linguistic variables. Adhikary et al. [10] presented multicriteria FMECA for coal-fired thermal power station using COPRAS-G method. Zhang [11] presented integration of both subjective weights and objective weights to avoid failure modes from being underestimated or overestimated based on fuzzy TOPSIS to get the closeness coefficient for each failure mode. Chanamool and Naenna [12] highlight the importance of fuzzy FMEA for prioritization and assessment of failures that likely occur in the working process of an emergency department of hospitals. Liu et al. [13] presented a novel approach for FMEA based on combination weighting and fuzzy VIKOR method where integration of fuzzy analytic hierarchy process (AHP) and entropy method is applied for risk factor weighting in this proposed approach to deal with the uncertainty and vagueness from humans’ subjective perception and experience in risk evaluation process.It has been observed that previous researchers did not consider COPRAS-G based multicriteria decision-making approach to process industries like aluminium wire rolling mill. In this paper COPRAS-G, a multicriteria decision-making tool, is applied to model FMECA in lieu of the traditional multiplication rule of the criticality factors.


## 2. COPRAS-G Methodology

The concept of grey number was basically derived from grey theory, which deals with the decisions of uncertainty experienced in real-world environment [14–19]. The grey number is having upper and/or lower limits whose exact value is unknown but the interval within which the value falls is known [15–17]. Hwang and Yoon, 1981 [20], highlight importance of multicriteria decision-making (MCDM) where multiple and conflicting criteria are under consideration in different areas like personal, public, academic, or business contents.The COPRAS-G method for criticality evaluation of failure modes is expressed through the following steps [15–17]:(1)
Select the set of various criteria and failure modes and arrange them along the columns and the rows, respectively, in the decision matrix.
(2)
Construct the decision-making matrixX which shows the criteria ranking in grey number intervals:(1)
X
=
x
i
j
;
y
i
j
=
x
11
;
y
11
⋯
x
1
n
;
y
1
n
⋮
⋱
⋮
x
m
1
;
y
m
1
⋯
x
m
n
;
y
m
n
,
wherex
i
j is the lower value and y
i
j is the upper value of the interval. i
=
1,2
,
…
,
m which represents the failure modes along the row and j
=
1,2
,
…
,
n which represents the criteria along the column in decision matrix.
(3)
Normalize the decision matrixX, as follows:(2)
x
1
i
j
=
x
i
j
1
/
2
∑
j
=
1
n
x
i
j
+
∑
j
=
1
n
y
i
j
,
y
1
i
j
=
y
i
j
1
/
2
∑
j
=
1
n
x
i
j
+
∑
j
=
1
n
y
i
j
.
Normalized decision matrixX
1 is as follows:(3)
X
1
=
x
1
11
;
y
1
11
⋯
x
1
1
n
;
y
1
1
n
⋮
⋱
⋮
x
1
m
1
;
y
1
m
1
⋯
x
1
m
n
;
y
1
m
n
.
(4)
Calculate weight of each criterion based on Shannon’s entropy concept where initially we have to calculate entropye
j and from it weight w
j for jth criteria as follows:(4)
e
x
j
=
-
1
ln
⁡
m
∑
i
=
1
m
x
i
j
ln
⁡
x
i
j
,
e
y
j
=
-
1
ln
⁡
m
∑
i
=
1
m
y
i
j
ln
⁡
y
i
j
,
w
x
j
=
1
-
e
x
j
∑
j
=
1
n
1
-
e
x
j
,
w
y
j
=
1
-
e
y
j
∑
j
=
1
n
1
-
e
y
j
.
(5)
Determine weighted normalized matrix as per the following equations:(5)
x
2
i
j
=
x
1
i
j
·
w
i
j
,
y
2
i
j
=
y
1
i
j
·
w
i
j
.
Weighted normalized decision matrixX
2 is as follows:(6)
X
2
=
x
2
11
;
y
2
11
⋯
x
2
1
n
;
y
2
1
n
⋮
⋱
⋮
x
2
m
1
;
y
2
m
1
⋯
x
2
m
n
;
y
2
m
n
.
(6)
Calculate the weighted mean normalized sumsP
i for beneficial criteria whose larger values are preferable and R
i for nonbeneficial criteria whose smaller values are preferable as follows:(7)
P
i
=
-
1
2
∑
j
=
1
k
x
2
i
j
+
y
2
i
j
,
R
i
=
-
1
2
∑
j
=
k
+
1
k
x
2
i
j
+
y
2
i
j
,
wherei
=
1,2
,
…
,
m, “k” is the number of beneficial criteria, and (m
-
k) is the number of nonbeneficial criteria. All the beneficial criteria are placed in the decision-making matrix first and then the nonbeneficial criteria are placed.
(7)
Calculate the relative significance/weight MCI of each alternative as follows:(8)
M
C
I
=
P
i
+
R
m
i
n
∑
i
=
1
m
R
i
R
i
∑
i
=
1
m
R
m
i
n
/
R
i
=
P
i
+
∑
i
=
1
m
R
i
R
i
∑
i
=
1
m
1
/
R
i
,
whereR
m
i
n is the minimum value of all weighted mean normalized sums “R
i” of nonbeneficial criteria.
The criticality ranks (priorities) of alternatives are ranked according to the value ofM
C
I in increasing order; that is, larger value of M
C
I is having higher priority than other alternatives. M
C
I
m
a
x is the maximum value of relative significance/weight among all alternatives.
(8)
Calculate the degree of unity in percentage (%) contributionC
i for ith failure cause and assign rank based on value of MCI:(9)
C
i
=
M
C
I
i
M
C
I
m
a
x
∗
100
,
whereM
C
I
m
a
x is the maximum value of relative significance/weight among all alternatives.


## 3. Case Study

### 3.1. Introduction

The proposed model is applied to the aluminium wire rolling mill processing plant situated in Gujarat, India. The detailed layout of process is given in Figure1. The aluminium wire is produced through Properzi Process where solid aluminium bar of 40 mm is fed into stands to gradually reduce diameter to 6 mm rod through fifteen stands in series. At each stand diameter of rod decreases by about 15–20%. It is concluded that bearings, gears, and primary and secondary shafts are identified as most critical components based on historical comprehensive failure and repair data.Figure 1
Layout of aluminium wire rolling mill process flow.To decide the score for each individual failure mode for every process input of critical components, the following methods are used:(i)
Historical failure data which gives comprehensive behavioral study of failure pattern of critical components.
(ii)
Questionnaires to floor operators, managers, and maintenance personnel.The score for chances of failure, detectability, maintainability, spare parts, economic safety, and economic cost is ranked as per Tables 1, 2, 3, 4, 5, and 6, respectively.Table 1
Scores for chance of failure (C).
Occurrence
MTBF
Score
Almost never
More than three years
1
Very rare
Once every 2-3 years
2
Rare
Once every 1-2 years
3
Very low
Once every 11-12 months
4
Low
Once every 9-10 months
5
Medium
Once every 7-8 months
6
Moderate high
Once every 5-6 months
7
High
Once every 3-4 months
8
Very high
Once every 1-2 months
9
Extremely high
Less than 1 month
10Table 2
Scores for detection of failure (D).
Chances of detection
Likelihood of nondetection (%)
Score
Immediate
<10
1
Best
10 to 20
2
Better
21 to 30
3
Good
31 to 40
4
Easy
41 to 50
5
Occasional
51 to 60
6
Late
61 to 70
7
Difficult
71 to 80
8
Very difficult
81 to 90
9
Impossible
91 to 100
10Table 3
Scores for maintainability (M).
Maintainability scope
Criteria for measure
Score
Extremely high
<10
1
Very high
10 to 20
2
High
21 to 30
3
Moderate high
31 to 40
4
Medium
41 to 50
5
Low
51 to 60
6
Very low
61 to 70
7
Rare
71 to 80
8
Very rare
81 to 90
9
Almost nil
91 to 100
10Table 4
Scores for spare parts (SP).
Criteria for availability and requirement
Score
Easily available & desirable
1
Easily available & essential
2
Easily available & very essential
3
Hard to procure but desirable
4
Hard to procure but essential
5
Hard to procure but very essential
6
Scarce and desirable
7
Scarce and essential
8
Scarce and very essential
9
Impossible and urgent
10Table 5
Scores for economic safety (ES).
Criteria for economic safety
Score
Extremely low
1
Very low
2
Low
3
Fair
4
Average
5
Medium
6
Moderately high
7
High
8
Very high
9
Extremely high
10Table 6
Scores for economic cost (EC).
Criteria for economic cost
Score
Extremely low
1
Very low
2
Low
3
Fair
4
Average
5
Medium
6
Moderately high
7
High
8
Very high
9
Extremely high
10


### 3.2. Importance of Use of COPRAS-G

During brainstorming session, maintenance personnel score a criticality factor into different criticality levels so it is challenging to do criticality analysis of failure modes accurately. Hence this practical difficulty can be solved by expressing the scores of a criticality factor in an interval (grey number) instead of certain and exact value (white number). In this problem, COPRAS-G method, a multifactor decision-making tool, is used by expressing criticality factors with grey numbers in lieu of the traditional multiplication rule. The main idea of COPRAS-G method is to express the criteria values in intervals, which comes from real situation of decision-making.


### 3.3. Failure Mode Effect and Criticality Analysis with Assignment of Score in Grey Number Range

The potential failure modes, their causes, and failure effect of bearings, gears, and primary and secondary shafts are generated through the root cause analysis method. The scores for chances of failure (C), degree of detectability (D), degree of maintainability (M), spare parts (SP), economic safety (ES), and economic cost (EC) for various failure causes are ranked on scale of 1–10 as per concept of grey number range in [x
i
j
;
y
i
j] based on Tables 1–6, where x
i
j is the lower value and y
i
j is the upper value of the interval as reflected in Table 7. The scales of 1 to 10 signify from least to most consideration of impact of criteria and are assigned on basis of questionnaires and brainstorming session to floor operators, shop floor managers, and maintenance personnel for various individual failure causes (C1 to C14).Table 7
COPRAS-G based FMECA.
Particulars
⁢Decision matrix
Weighted mean normalized sum⁢
Relative significance/weight⁢
% contribution⁢
Criticality rank⁢
Key process input
Potential failure mode
Potential causes
Potential failure effects
Current controls
⁢C
⁢D
⁢M
⁢SP
⁢ES
⁢EC
What is the process input?
In what ways can the process input fail?
What causes the key input to go wrong?
What is the impact on the key output variables once it fails (customer or internal requirements)?
What are the existingcontrols and procedures that prevent either the cause or the failure mode?
⁢Chance ⁢of ⁢failure
⁢Detection ⁢probability of ⁢failure
⁢Maintainability ⁢criteria
⁢Spare ⁢parts ⁢criteria
⁢Economic ⁢safety ⁢criteria
⁢Economic ⁢cost ⁢criteria
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
P
i
MCI
C
i
Rank
Bearing failure⁢
Bearing high temperature
Improper lubrication & defective sealing
Bearing gets jammed/bearing housing jammed
Lubricating the parts when occurred
8
9
7
8
1
2
2
3
3
4
3
4
0.1297
0.1297
60
9
Bearing corrosion
Higher speed than specified
Increase in vibration & noise
Proper coolant
8
9
5
6
1
2
2
3
3
4
4
5
0.1244
0.1244
58
10
Bearing fatigue
Design defects & bearing dimension not as per specification
Life reduction
Bearing replacement
9
10
7
8
6
8
3
5
9
10
9
10
0.2156
0.2156
100
1
Roller balls wear-out
Foreign matters/particles
Sudden rise in thrust
Regular cleaning of parts
7
9
6
7
4
5
3
5
7
8
5
6
0.1662
0.1662
77
4
Bearing misalignment & improper mounting
Sudden impact on the rolls
Shaft damage & impact damage on other parts
Routine check-up
8
10
5
6
6
7
5
7
9
10
9
10
0.2079
0.2079
96
2
Electrical damage
Loss of power
Operation interrupted
Electrical wiring check-up
7
9
1
2
1
2
3
4
5
6
2
3
0.1062
0.1062
49
12
Rolling mill gearing failure⁢
Gear teeth wear-out
Inadequate lubrication-dirt, viscosity issues
Rough operation & considerable noise
Routine check-up of lubrication
7
8
2
3
5
6
3
4
7
8
4
5
0.1401
0.1401
65
8
Gear teeth surface fatigue (pitting)
Improper meshing, case depth & high residual stresses
Gear life reduction
Preventive maintenance
8
9
4
5
5
6
3
4
4
5
5
6
0.1444
0.1444
67
7
Gear teeth scoring
Overheating at gear mesh
Interference & backlash phenomenon
Lubricating when needed
4
5
3
4
2
3
2
3
2
3
3
4
0.0863
0.0863
40
14
Gear teeth fracture
Excessive overload & cyclic stresses
Sudden stoppage of process plant
Breakdown maintenance
9
10
2
4
6
7
3
4
7
8
7
8
0.1700
0.1700
79
3
Gear teeth surface cold/plastic flow
High contact stresses due to rolling & sliding action of mesh
Slippage & power lose
Gear replacement when needed
3
4
6
7
3
4
3
4
2
3
3
4
0.1022
0.1022
47
13
Rolling mill shaft (primary & secondary) failure
Shaft fretting
Vibratory dynamic load from bearing
Leads to sudden failure
Breakdown maintenance
5
6
4
5
3
5
3
4
3
4
3
4
0.1096
0.1096
51
11
Shaft misalignment
Uneven bearing load
Vibration & fatigue
Preventive maintenance
8
9
4
5
5
6
4
5
4
5
5
6
0.1477
0.1477
68
6
Shaft fracture (fatigue)
Reverse & repeated cyclic loading
Sudden stoppage of process
Preventive maintenance
9
10
2
3
6
7
3
4
5
6
6
7
0.1526
0.1526
71
5
100
117
58
73
54
70
42
59
70
84
68
82


## 3.1. Introduction

The proposed model is applied to the aluminium wire rolling mill processing plant situated in Gujarat, India. The detailed layout of process is given in Figure1. The aluminium wire is produced through Properzi Process where solid aluminium bar of 40 mm is fed into stands to gradually reduce diameter to 6 mm rod through fifteen stands in series. At each stand diameter of rod decreases by about 15–20%. It is concluded that bearings, gears, and primary and secondary shafts are identified as most critical components based on historical comprehensive failure and repair data.Figure 1
Layout of aluminium wire rolling mill process flow.To decide the score for each individual failure mode for every process input of critical components, the following methods are used:(i)
Historical failure data which gives comprehensive behavioral study of failure pattern of critical components.
(ii)
Questionnaires to floor operators, managers, and maintenance personnel.The score for chances of failure, detectability, maintainability, spare parts, economic safety, and economic cost is ranked as per Tables 1, 2, 3, 4, 5, and 6, respectively.Table 1
Scores for chance of failure (C).
Occurrence
MTBF
Score
Almost never
More than three years
1
Very rare
Once every 2-3 years
2
Rare
Once every 1-2 years
3
Very low
Once every 11-12 months
4
Low
Once every 9-10 months
5
Medium
Once every 7-8 months
6
Moderate high
Once every 5-6 months
7
High
Once every 3-4 months
8
Very high
Once every 1-2 months
9
Extremely high
Less than 1 month
10Table 2
Scores for detection of failure (D).
Chances of detection
Likelihood of nondetection (%)
Score
Immediate
<10
1
Best
10 to 20
2
Better
21 to 30
3
Good
31 to 40
4
Easy
41 to 50
5
Occasional
51 to 60
6
Late
61 to 70
7
Difficult
71 to 80
8
Very difficult
81 to 90
9
Impossible
91 to 100
10Table 3
Scores for maintainability (M).
Maintainability scope
Criteria for measure
Score
Extremely high
<10
1
Very high
10 to 20
2
High
21 to 30
3
Moderate high
31 to 40
4
Medium
41 to 50
5
Low
51 to 60
6
Very low
61 to 70
7
Rare
71 to 80
8
Very rare
81 to 90
9
Almost nil
91 to 100
10Table 4
Scores for spare parts (SP).
Criteria for availability and requirement
Score
Easily available & desirable
1
Easily available & essential
2
Easily available & very essential
3
Hard to procure but desirable
4
Hard to procure but essential
5
Hard to procure but very essential
6
Scarce and desirable
7
Scarce and essential
8
Scarce and very essential
9
Impossible and urgent
10Table 5
Scores for economic safety (ES).
Criteria for economic safety
Score
Extremely low
1
Very low
2
Low
3
Fair
4
Average
5
Medium
6
Moderately high
7
High
8
Very high
9
Extremely high
10Table 6
Scores for economic cost (EC).
Criteria for economic cost
Score
Extremely low
1
Very low
2
Low
3
Fair
4
Average
5
Medium
6
Moderately high
7
High
8
Very high
9
Extremely high
10


## 3.2. Importance of Use of COPRAS-G

During brainstorming session, maintenance personnel score a criticality factor into different criticality levels so it is challenging to do criticality analysis of failure modes accurately. Hence this practical difficulty can be solved by expressing the scores of a criticality factor in an interval (grey number) instead of certain and exact value (white number). In this problem, COPRAS-G method, a multifactor decision-making tool, is used by expressing criticality factors with grey numbers in lieu of the traditional multiplication rule. The main idea of COPRAS-G method is to express the criteria values in intervals, which comes from real situation of decision-making.


## 3.3. Failure Mode Effect and Criticality Analysis with Assignment of Score in Grey Number Range

The potential failure modes, their causes, and failure effect of bearings, gears, and primary and secondary shafts are generated through the root cause analysis method. The scores for chances of failure (C), degree of detectability (D), degree of maintainability (M), spare parts (SP), economic safety (ES), and economic cost (EC) for various failure causes are ranked on scale of 1–10 as per concept of grey number range in [x
i
j
;
y
i
j] based on Tables 1–6, where x
i
j is the lower value and y
i
j is the upper value of the interval as reflected in Table 7. The scales of 1 to 10 signify from least to most consideration of impact of criteria and are assigned on basis of questionnaires and brainstorming session to floor operators, shop floor managers, and maintenance personnel for various individual failure causes (C1 to C14).Table 7
COPRAS-G based FMECA.
Particulars
⁢Decision matrix
Weighted mean normalized sum⁢
Relative significance/weight⁢
% contribution⁢
Criticality rank⁢
Key process input
Potential failure mode
Potential causes
Potential failure effects
Current controls
⁢C
⁢D
⁢M
⁢SP
⁢ES
⁢EC
What is the process input?
In what ways can the process input fail?
What causes the key input to go wrong?
What is the impact on the key output variables once it fails (customer or internal requirements)?
What are the existingcontrols and procedures that prevent either the cause or the failure mode?
⁢Chance ⁢of ⁢failure
⁢Detection ⁢probability of ⁢failure
⁢Maintainability ⁢criteria
⁢Spare ⁢parts ⁢criteria
⁢Economic ⁢safety ⁢criteria
⁢Economic ⁢cost ⁢criteria
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
x
i
j
y
i
j
P
i
MCI
C
i
Rank
Bearing failure⁢
Bearing high temperature
Improper lubrication & defective sealing
Bearing gets jammed/bearing housing jammed
Lubricating the parts when occurred
8
9
7
8
1
2
2
3
3
4
3
4
0.1297
0.1297
60
9
Bearing corrosion
Higher speed than specified
Increase in vibration & noise
Proper coolant
8
9
5
6
1
2
2
3
3
4
4
5
0.1244
0.1244
58
10
Bearing fatigue
Design defects & bearing dimension not as per specification
Life reduction
Bearing replacement
9
10
7
8
6
8
3
5
9
10
9
10
0.2156
0.2156
100
1
Roller balls wear-out
Foreign matters/particles
Sudden rise in thrust
Regular cleaning of parts
7
9
6
7
4
5
3
5
7
8
5
6
0.1662
0.1662
77
4
Bearing misalignment & improper mounting
Sudden impact on the rolls
Shaft damage & impact damage on other parts
Routine check-up
8
10
5
6
6
7
5
7
9
10
9
10
0.2079
0.2079
96
2
Electrical damage
Loss of power
Operation interrupted
Electrical wiring check-up
7
9
1
2
1
2
3
4
5
6
2
3
0.1062
0.1062
49
12
Rolling mill gearing failure⁢
Gear teeth wear-out
Inadequate lubrication-dirt, viscosity issues
Rough operation & considerable noise
Routine check-up of lubrication
7
8
2
3
5
6
3
4
7
8
4
5
0.1401
0.1401
65
8
Gear teeth surface fatigue (pitting)
Improper meshing, case depth & high residual stresses
Gear life reduction
Preventive maintenance
8
9
4
5
5
6
3
4
4
5
5
6
0.1444
0.1444
67
7
Gear teeth scoring
Overheating at gear mesh
Interference & backlash phenomenon
Lubricating when needed
4
5
3
4
2
3
2
3
2
3
3
4
0.0863
0.0863
40
14
Gear teeth fracture
Excessive overload & cyclic stresses
Sudden stoppage of process plant
Breakdown maintenance
9
10
2
4
6
7
3
4
7
8
7
8
0.1700
0.1700
79
3
Gear teeth surface cold/plastic flow
High contact stresses due to rolling & sliding action of mesh
Slippage & power lose
Gear replacement when needed
3
4
6
7
3
4
3
4
2
3
3
4
0.1022
0.1022
47
13
Rolling mill shaft (primary & secondary) failure
Shaft fretting
Vibratory dynamic load from bearing
Leads to sudden failure
Breakdown maintenance
5
6
4
5
3
5
3
4
3
4
3
4
0.1096
0.1096
51
11
Shaft misalignment
Uneven bearing load
Vibration & fatigue
Preventive maintenance
8
9
4
5
5
6
4
5
4
5
5
6
0.1477
0.1477
68
6
Shaft fracture (fatigue)
Reverse & repeated cyclic loading
Sudden stoppage of process
Preventive maintenance
9
10
2
3
6
7
3
4
5
6
6
7
0.1526
0.1526
71
5
100
117
58
73
54
70
42
59
70
84
68
82


## 4. Results and Discussion

Table8 shows the relative significance/weight of each alternative MCI and the degree of unity in percentage (%) contribution (C
i) for ith failure cause which is derived as per COPRAS-G methodology discussed in Section 2.Table 8
Criticality ranking based on MCI.
Failure cause versus criteria
Weighted mean normalized sum
Relative weight
% contribution
Criticality rank
Failure cause
Notation
P
i
MCI
C
i
Rank
Bearing high temperature
C1
0.1297
0.1297
60
9
Bearing corrosion
C2
0.1244
0.1244
58
10
Bearing fatigue
C3
0.2156
0.2156
100
1
Roller balls wear-out
C4
0.1662
0.1662
77
4
Bearing misalignment & improper mounting
C5
0.2079
0.2079
96
2
Electrical damage
C6
0.1062
0.1062
49
12
Gear teeth wear-out
C7
0.1401
0.1401
65
8
Gear teeth surface fatigue (pitting)
C8
0.1444
0.1444
67
7
Gear teeth scoring
C9
0.0863
0.0863
40
14
Gear teeth fracture
C10
0.1700
0.1700
79
3
Gear teeth surface cold/plastic flow
C11
0.1022
0.1022
47
13
Shaft fretting
C12
0.1096
0.1096
51
11
Shaft misalignment
C13
0.1477
0.1477
68
6
Shaft fracture (fatigue)
C14
0.1526
0.1526
71
5It has been observed from Table8 that design defects and bearing dimension not as per specification (C3) seems to be most critical failure cause and overheating at gear mesh (C9) seems to be least critical failure cause. It is suggested to modify the current control practices as listed in Table 1 that failure causes (C3, C5, C10, C4, and C14) with large value of MCI should be kept under predictive maintenance, failure causes (C13, C8, C7, C1, and C2) with moderate value of MCI should be kept under preventive maintenance, and failure causes (C12, C6, C11, and C9) with low MCI should be kept under corrective maintenance.Moreover, it has been observed that almost 70% down time is due to bearing failure and replacement practice is 100%, so it is recommended to select standardized bearing with appropriate specifications and mount them properly during every replacement to avoid bearing misalignment (C5) and minimizing reverse and repeated cyclic loading; thus shaft fatigue (C14) and gear tooth fracture (C10) can be avoided. Appropriate condition monitoring is suggested to continuously record the condition of bearing damage and shaft damage to prevent sudden breakdown and starting thrust on these components. Also, the condition of lubricants should be checked and replaced whenever necessary rather than routine clean-up. Hence, sudden impact on the rolls (C5), design defects with bearing dimension/specification (C3), foreign matters/particles (C4), excessive overload and cyclic stresses (C10), and reverse and repeated cyclic loading (C14) can be covered under recommendations. Failure causes with moderate and low MCI are controlled under preventive and corrective maintenance practices. Comparison matrix for deciding maintenance strategy is shown is Table9.Table 9
Comparison matrix for deciding maintenance strategy.
Sr. number
Failure cause
Suggested maintenance strategy
Impact of MCI & (C
i)
1
C3, C5, C10, C4, C14
Predictive (condition based) maintenance
High MCI & (C
i)
2
C13, C8, C7, C1, C2
Preventive maintenance
Moderate MCI & (C
i)
3
C12, C6, C11, C9
Corrective maintenance
Low MCI & (C
i)


## 5. Conclusion and Scope

This paper highlights multicriteria decision-making approach based on COPRAS-G to overcome the limitations of FMECA. The case study presented in this paper shows how to deal with the problems encountered in aluminium wire rolling mill processing plant with mix of maintenance practices. It is concluded that the study will be helpful in deriving optimized maintenance plan to improve plant efficiency as a whole. The similar work can be extended for process industries of same or of different kinds in a view to decide suitable maintenance strategies in coordination with failure analysis.



---

*Source: 8421916-2016-07-14.xml*