Root cause analysis of field failure concern for improvement

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

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

232

ROOT CAUSE ANALYSIS OF FIELD FAILURE CONCERN FOR

IMPROVEMENT IN DURABILITY OF VEHICLE SYSTEM

A. D. Lagad1, Dr. K. H. Inamdar

2

1Department of Mechanical Engineering,

Walchand College of Engineering, Sangli, Maharashtra, India,

2Department of Mechanical Engineering,

Walchand College of Engineering, Sangli, Maharashtra, India

ABSTRACT

Durability and reliability of an automobile are equally important from industry as well

as customer point of view. Automotive industries are still riddled with significant warranty

costs that incur due to premature failure of their products in the customer hands. The key to

reduce the design and development expenses and warranty expenses is to subject the product

for reliability and/or durability tests for failure modes investigation. Design of the vehicle

components must be adapted as accurately as possible to the operating conditions. In order to

achieve these goals, durability tests are performed through a combination of physical testing,

on road at a proving ground test track, and using a servo-hydraulic road test simulator in

laboratory. In this study, root cause analysis is carried out for bonnet cable failure in vehicle

model of an automotive industry. Various possible causes are identified using fishbone

diagram, why-why analysis is carried to find root cause and solution is proposed. Modified

design of hood latch reduced operating efforts for hood latch and avoided bonnet cable

failure.

Keywords: Durability, Reliability, Root cause analysis, Fishbone diagram.

1. INTRODUCTION

In automotive industry, vehicle safety and product quality are prime factors on which

automotive manufacturers give significant attention. During vehicle manufacturing, defects

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arise during manufacturing, assembly and other processes. For continuous quality

improvement and customer satisfaction, plant quality functions are planned, integrated and

executed to detect these defects and make action plans, preventive measures to resolve them.

Currently, automotive industries are dedicating a lot of attention to improve product

quality, durability and reliability already in a virtual simulation environment. A major issue

that designers and manufacturers in this field have to face is to improve vehicle lifetime [1,2].

Durability and reliability of a vehicle are equally important from a customer point of view.

Automotive industries are still riddled with significant warranty costs that incur due to

premature failure of their products in the customer hands [3]. Vehicle manufacturers discover

issues through their own vehicle tests, inspection procedures, or information gathering [2]. In

order to achieve improved durability and reduced warranty costs, durability tests are

performed through a combination of physical testing, on road at a proving ground test track,

and using a servo-hydraulic road test simulator in laboratory [4].

Modern vehicles exhibit a variety of performance, safety and comfort features in

which they fundamentally differ from previous models. Durability and reliability in vehicle

components are prime considerations for successful functioning of vehicles in all aspects

mentioned above [5, 6].

In this study, root cause analysis is carried out for bonnet cable failure in vehicle

model of an automotive industry and solution is implemented accordingly. Various finished

vehicle audits performed in automotive industry considered are ANOVA-C (Advanced New

Overall Vehicle Audit), GD (Global Demerit) test and Durability ANOVA-C. The purpose of

Durability ANOVA-C is to check and capture data on internal/known customer vehicles

periodically to get early feedback on field failure and make action planning [7,8].

2. CONCERN RESOLUTION- BONNET CABLE FAILURE

Concern identified and considered for resolution is ‘High Operational Effort for Hood

Latch Release Lever’ which results in bonnet cable failure. This concern is identified during

durability ANOVA-C and GD audit. Effort readings taken on internal customers’ vehicle are

shown in table 1. Specifications are 40-80 N. Table 2 shows hood latch release lever effort

readings for 60 Ready for Inspection (RFI) vehicles. Higher operational efforts are observed

for some vehicles.

Table 1: Hood latch release lever efforts for internal customers

Customer Lower Limit

(N)

Upper Limit

(N)

Phase 1

(N)

Phase 2

(N)

Phase 3

(N)

1 40 80 55 110 122

2 40 80 55 110 85

3 40 80 69 72 75

4 40 80 76 98 100

5 40 80 99 108 105



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Table 2: Hood latch release lever effort readings for 60 vehicles

Sr. No Effort

(N) Sr. No

Effort

(N) Sr. No

Effort

(N) Sr. No

Effort

(N)

1 73 16 64.9 31 71.3 46 70

2 67.8 17 69.7 32 69.3 47 74.3

3 56.8 18 54.6 33 58.4 48 71.9

4 59.3 19 64.4 34 66.7 49 76.5

5 78.1 20 86.1 35 77.8 50 62.8

6 76.1 21 61.4 36 74.3 51 64.1

7 78 22 78 37 55.4 52 64.4

8 66.9 23 64 38 67.3 53 71.3

9 63.3 24 80.6 39 67.8 54 69.1

10 70.8 25 71.2 40 66.1 55 64.6

11 69.4 26 65.2 41 77.5 56 66.1

12 77.3 27 59.3 42 70.6 57 72.7

13 74.2 28 63.2 43 62 58 71.4

14 66.1 29 73.1 44 72.2 59 100

15 77.4 30 76.1 45 77.5 60 69.2

Bonnet cable failure is third in the list of significant field failure concerns as shown

in pareto diagram in fig. 1. Analysis is already carried out for first two concerns.

2.1 Process Flow for Bonnet Cable and Lock assembly

Bonnet cable and bonnet lock are fitted and assembled on assembly line in TCF. Steps

involved in this process are mentioned below,

1.Bonnet cable fitment is done on body with grommet and plugs (Trim Line).

2.Bonnet cable bracket is fitted on cockpit (Trim Line).

3.Hook of bonnet cable is assembled with bottom side hood latch (Underbody Line)

4.Bottom side hood latch is fitted on front grill upper body panel with 3 bolts by giving full

torque (Underbody line)

5.Loose mounting of top side hood latch on bonnet bottom side with 2 bolts by pre torque.

(Final line)

6.Alignment of bonnet as well as both hood latch parts. Tightening of top side hood latch by

giving full torque to bolts (Final line)

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

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May

Fig. 1 Pareto diagram for significant field failure concerns

Alignment process is done in following steps,

1.Alignment of hood by retightening hinges (both locks are in loose condition).

2.Gap & Flushness corrected by giving jerk to the hood.

3.Bonnet is locked first time.

4.Opening of bonnet by giving slight stroke to hood with mallet first

due to stuck. Top side lock assembly is again aligned with respect to bottom side lock

assembly.

5.Full tightening of the both hood lock assemblies (body side & bonnet side)

6.Again bonnet is locked & opened 2 to 3 times.

Fig. 2 shows hood latch assembly on vehicle

(b)

Fig. 2 (a) Hood latch assembly on vehicle


6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

235

Pareto diagram for significant field failure concerns

Alignment process is done in following steps,

ent of hood by retightening hinges (both locks are in loose condition).

Gap & Flushness corrected by giving jerk to the hood.

Opening of bonnet by giving slight stroke to hood with mallet first time if it is not opened

stuck. Top side lock assembly is again aligned with respect to bottom side lock

Full tightening of the both hood lock assemblies (body side & bonnet side)

Again bonnet is locked & opened 2 to 3 times.

shows hood latch assembly on vehicle as well as top and bottom hood latch.

(a)

(b) (C)

Hood latch assembly on vehicle (b) Top side hood latch (c) Bottom si

hood latch


June (2013) © IAEME

it is not opened

stuck. Top side lock assembly is again aligned with respect to bottom side lock

s well as top and bottom hood latch.

Bottom side



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3. ROOT CAUSE ANALYSIS

After brainstorming, various possible causes contributing to bonnet cable failure are

enlisted. Cause and effect diagram (Fishbone diagram) by using relevant possible causes

identified in brainstorming is shown in fig. 3. Probable causes for bonnet cable failure are

highlighted in figure.

Fig. 3 Cause and effect diagram for bonnet cable failure

3.1 Verification of probable causes

1. Damage to cable while fitment:

i. Fitment process is observed on assembly line.

ii. No damage is observed to cable while fitment

iii. Remark: Invalid

2. Improper cable assembly:

i. Standard operation procedure is followed during cable assembly.

ii. No abnormalities found during cable fitment on body as well as cable assembly with

cockpit and hood latch.


3. Improper cable routine:

i. Bonnet cable routine is observed from release lever end to cable hook end during

fitment, also on finished vehicles

ii. Cable routine was found ok in vehicles. All clips were found on position.



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iii. Cable routine found ok in vehicles for which higher efforts reported to operate hood

latch release lever

iv. Remark: Invalid

4. Improper alignment of bonnet as well as latch-striker:

i. Alignment is done as per standard operation procedure.

ii. Gaps are measured for bonnet on finished vehicles and alignment found ok.

iii. However, during first locking-unlocking operation in alignment process, cable failure

incidences were observed on final line

iv. As alignment is done in 2-3 attempts by operator judgment- chances that striker gets

stuck in latch in first cycle.

v. Remark: Valid

5. Nut runner not applying required torque/ bolt looseness:

i. Nut runner calibration done as per scheduled frequency

ii. Bolts used for tightening of both latch and striker were checked on finished vehicles for

looseness- Found ok

iii. Torque is measured on finished vehicles, also on vehicles for which higher efforts

reported- torque found as per specification.

iv. Remark: Invalid

6. Deviation of cable length from required length:

i. Bonnet cable length is measured for cables in storage area at side of assembly line

ii. No variation in cable length, length found as per specifications


7. Cracks on cable flap or bracket:

i. Bonnet cables were examined visually for any cracks or damages to cable flap or

bracket

ii. No cracks or damage observed


8. Less thickness for cable flap or bracket:

i. Previous actions: Bracket wall thickness increased by 1.2 mm, Reinforcement added to

flap

ii. Cable flap and bracket thickness measured- Found ok as per modified dimensions


9. Bottom hood latch spring has more stiffness:

i. Parameters such as free length, number of turns, coil diameter contributing spring

stiffness were checked for bottom hood latch spring.

ii. No variation in stiffness- Found ok


10. Cable routine disturbed:

i. Cable routine observed in RFI area, also on customer vehicles during dealer visit.

ii. Cable clips were ensured for position (not fallen)



iii. No abnormalities found

iv. Remark: Invalid

11. Bonnet as well as latch-striker alignment getting disturbed over usage of ve

i. Bonnet alignment observed, gap measurements done and bonnet efforts were taken on

customer vehicles during dealer visit

ii. Gap variation is observed on right and left side (misalignment) also higher efforts were

noted

iii. In some vehicles (at dealer and

jerk from side (because striker get stuck in latch )

iv. Remark: Valid

3.2 Valid causes Bonnet cable failure occurs

i. Bonnet as well as latch-striker alignment get

ii. During first locking-unlocking operation in alignment process, chances that striker get

stuck in latch.

Stack up analysis of top hood latch and bottom hood latch shows that only 2 mm

clearance given between the top and

not getting maintained due to contribution of hinges

On a vehicle which had run 7000 kilometers, front grill is removed; bonnet is

slammed few times and observed that bonnet is not

lever. Refer fig. 5 for explanation.

Fig. 4 Stack up analysis of top hood latch and

bottom hood latch



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striker alignment getting disturbed over usage of vehicle:

Bonnet alignment observed, gap measurements done and bonnet efforts were taken on

customer vehicles during dealer visit

Gap variation is observed on right and left side (misalignment) also higher efforts were

In some vehicles (at dealer and production shop) bonnet got opened after giving slight

jerk from side (because striker get stuck in latch )

occurs because striker gets stuck into bottom latch.

striker alignment getting disturbed over usage of vehicle

unlocking operation in alignment process, chances that striker get


clearance given between the top and bottom hood latch for alignment as shown in fig

not getting maintained due to contribution of hinges and in process.


slammed few times and observed that bonnet is not opening by operating hood latch release

for explanation.

Stack up analysis of top hood latch and

bottom hood latch

(a)

(b)

Fig. 5 (a) Bonnet is not opened

striker got stuck into latch (b)

marks on bottom latch



hicle:

Bonnet alignment observed, gap measurements done and bonnet efforts were taken on

Gap variation is observed on right and left side (misalignment) also higher efforts were

) bonnet got opened after giving slight

ting disturbed over usage of vehicle.

unlocking operation in alignment process, chances that striker get


bottom hood latch for alignment as shown in fig. 4. It is


opening by operating hood latch release

(a) Bonnet is not opened because

striker got stuck into latch (b) Stuck up

latch



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During dealer visit, customer vehicles were observed for bonnet alignment. Hood

latch release lever efforts were taken and bonnet gaps were measured. Table 3 shows effort

and gap measurement readings. Higher operational efforts were seen in customer vehicles.

Table 3: Effort and gap measurement readings on customer vehicles

Sr.

No.

Hood latch

lever effort

(N)

Gaps (mm)

RH Side LH Side

1 2 3 4 5 6 1 2 3 4 5 6

1 83.9 5 4.9 4.6 5.5 5.2 5.2 4.1 4 4.8 3 3.1 4.1

2 77.1 5.1 2.5 2.1 4.2 4.6 6 6.3 6.3 6.5 4.1 4.7 4.8

3 95.6 5 2.1 1 2.9 3.7 4.4 5.6 5.9 5 3.8 4 5.8

4 114.5 5.8 5 4.7 7.5 4.8 4.9 5.2 3.8 6 2.3 2.8 6

5 103 5 4.8 4.1 5 4.5 4.7 6.1 3.6 4.5 2.8 3.5 4.6

6 80.3 3.8 2.9 1.3 4.1 3.9 6 5.9 5.4 5.2 2.6 3.4 5

7 78.1 6.3 4.9 3.8 5.7 5.7 6.1 6.1 4.8 6.2 2.5 3.9 4

8 82.7 6 4.8 4.2 6.2 5.8 6 6 4.7 5.1 2.8 3 4.6

9 87.5 5.2 4.4 3.8 5.3 5.1 5.1 6 5.1 6.1 3.2 3.2 4.1

10 91.4 5.5 3.2 2.2 4.6 2.8 5.3 5.6 4.9 5.9 4 4.1 5.1

3.3 Why-Why analysis Cause for bonnet cable failure is explained below,

i. Why: Higher operational efforts for hood latch release lever

ii. Why: Striker get stuck into bottom latch

iii. Why: Striker and bottom latch not remain inline

iv. Why: Latch-striker alignment getting disturbed over usage of vehicle or during first cycle

of alignment

v. Why: This factor is not considered during design

4. PROPOSED SOLUTION

As there is problem that striker of top hood latch assembly getting stuck into bottom

latch due to misalignment, there is a need of keeping striker and bottom latch in one line i.e.

to avoid misalignment (oblique travel of striker in latch).

After observation and study of structural design of hood latch parts of vehicle,

proposal is given to implement a groove in the striker of top hood latch and guide on bottom

hood to avoid misalignment and stuck up issue because of which bonnet cable failure occurs.

For this, structural benchmarking was done with hood latch of another vehicle as

shown in fig. 6 and 7.



(a) Fig. 6 (a) Provision of groove in striker with proper cap to control the height after alignment

(b) No provision of groove and simple cap

(a) (Fig. 7 (a) Provision of guide cup on bottom hood latch

According to proposal, design changes

vehicle. Following are the modifications in the design of hood latch bottom part, (Fig

i. Base plate center hole diameter is cha

ii. Flange is added in support plate

iii. Cable slot in cable holding bracket is changed

iv. Locking bracket (guide cup) is added

For hood latch top part, outer diameter of washer is reduced from 32 mm to 25 mm as

shown in fig. 9.

(a)

Fig. 8 Change in bottom hood latch



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(b)

groove in striker with proper cap to control the height after alignment

(b) No provision of groove and simple cap

(a) (b)

Provision of guide cup on bottom hood latch (b) No provision of guide cup on

bottom hood latch

According to proposal, design changes were made in striker and bottom hood latch of

. Following are the modifications in the design of hood latch bottom part, (Fig

Base plate center hole diameter is changed

Flange is added in support plate

Cable slot in cable holding bracket is changed

Locking bracket (guide cup) is added


(a) (b)

Change in bottom hood latch (a) Existing (b) Proposed



groove in striker with proper cap to control the height after alignment

ovision of guide cup on

made in striker and bottom hood latch of

. Following are the modifications in the design of hood latch bottom part, (Fig. 8)




241

(a) (b)

Fig. 9 Change in top hood latch (a) Existing (b) Proposed

5. IMPLEMENTATION AND RESULTS

Sample hood latches were received from supplier with said design modifications in

given proposal.

5.1 Concept trial

Concept trial was conducted on three vehicles with new hood latch samples. Table 4

shows comparison of hood latch release lever operational efforts for three vehicles with

existing and modified hood latch parts.

Table 4: Hood latch release lever operational efforts during concept trial

Vehicle

No.

Specification

(N)

Readings with existing

hood latch (N)

Readings with modified

hood latch (N)

1 2 3 1 2 3

1 40-80 78.4 76.1 76.3 53.7 53.2 53.2

2 40-80 79.1 78.4 78 56 56.2 55.5

3 40-80 76.8 76.5 75.9 50.2 49.8 49.1

i. During concept trial, fitment was observed for any difficulties in fitment process as well

as alignment.

ii. No issue regarding fitment or alignment was noticed.

iii. After operating hood latch release lever and primary lock getting released, secondary lock

(to be released by operating lever of top side hood latch) was found ok in all three

vehicles.

iv. Bonnet opening-closing cycle was performed for 500 times on three vehicles with

modified hood latch parts to check whether there is bonnet cable failure. Bonnet cable

failure incidence was not observed and cable found ok.

5.2 Fitment trial

After receiving modified hood latch samples, fitment trial was conducted and

completed for five numbers of vehicles.



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Table 5: Hood latch release lever operational efforts during fitment trial

Vehicle

No.

Specification

(N)

Readings with modified hood

latch (N)

1 2 3

1 40-80 56.5 56.2 55.9

2 40-80 59.3 58.4 58.6

3 40-80 50.2 50.2 50.1

4 40-80 53.8 53.5 53.1

5 40-80 60.4 60.9 60.7

No fitment related issue was observed during trial. Bonnet opening-closing cycle was

repeated for five vehicles and no abnormality was observed (No higher efforts, secondary

locking ok). Hood latch release lever operating efforts were measured on five vehicles and

found within specifications. Readings are shown in table 5.

5.3 Validation and part trial

Validation process is conducted for modified hood latch samples. In validation,

bonnet cable withstands and passed target of 5000 cycles of bonnet opening and closing for

all samples. Part trial is conducted on 200 vehicles and readings were taken. All vehicles

have shown effort readings within specification. No bonnet cable failure incidence was

occurred. Range of effort readings was found between 45 to 62 N (Specification is 40-80 N).

This shows significant operational effort reduction for hood latch release lever.

6. CONCLUSION

1. Root cause analysis is carried out for the concern “Higher operational efforts for hood latch

release lever which results in bonnet cable failure.”

i. Bonnet cable failure occurs due to disturbance in alignment of striker-latch which results

in stuck up condition of striker in latch.

ii. To avoid stuck up condition, design modifications are suggested in hood latch with

provision of locking bracket (guide cup).

2. Trials with modified hood latch samples showed reduction in hood latch release lever

operational effort than that in case of customer vehicles as well as new vehicles with existing

design.

3. In validation, bonnet cable withstand for 5000 cycles of bonnet opening and closing.

During trials as well as implementation for 200 vehicles, no bonnet cable failure incidence

occurred with modified hood latch.

7. REFERENCES

[1] R.d'Ippolito, M. Hack, S. Donders, L. Hermans, N. Tzannetakis, D. Vandepitte,

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from social media, Decision Support Systems, 2012, pp. 1-11.

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[7] ANOVA-C Guidelines in automotive industry.

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0976-6499.

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