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Scorpio Lug Nut Torque loss (Field Issue)

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Concern

Torque loss an overview

Cause and effect analysis

Failure analysis Design

Lug profile design

Benchmarking

Failure analysis Method

Tightening pattern

Torque specification

Re-torque frequency

Failure analysis Process

Variation in conical seat

Mounting face concavity high

Paint in mating surface

Failure analysis Material

Dissimilar materials Low hardness

Potential causes

Recommendations

Contents

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Concern

Torque loss observed in the vehicles as reported by mail by M&M on March 21st 2010

Repeated issue followed up by M&M SQA audit by May 28, 2010

Customer version:Customer version: Lug nut torque loss happens in alloy wheels mounted to the Scorpio W 408 vehicle;

wheel gets detached from the vehicle while in operation.

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DESIGNDESIGN

TORQUE LOSSTORQUE LOSS

METHODMETHOD

MATERIALMATERIALPROCESSPROCESS

Lug profile design

Benchmarking

Tightening pattern

Torque specification

Variation in conical seat

MF concavity - high

Paint in mating surface

Mating surface Low Hardness

Cause and effect analysis

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Design

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Lug profile design

Lug profile to be designed in accordance to global lug specification criteria requirements. ref. SCL non

compliance to the criteria requirement will have an impact on the torque.

S #S # PARAMETERPARAMETER M&M SPECIFICATIONM&M SPECIFICATION SCL CRITERIASCL CRITERIA COMPLIANCECOMPLIANCE

1 Lug hole diameter 18.50 18.50

2 Tolerance Lug hole diameter - 0.12 0.503 Lug hole counter diameter 34.50 40.90

4 Tolerance - Lug hole counter diameter + 0.50 0.50

5 Lug hole conical seat angle 60.00 60.00

6 Tolerance Lug hole conical seat angle 0.50 0.50

7 Conical seat section thickness 14.50 14.40

8 Tolerance Conical seat section thickness 0.20 0.80

9 True position tolerance 0.15 0.30

Based on the comparative report it is confirmed that design of lug hole profile does not constitute the

failure, hence the cause is ELIMINATEDELIMINATED

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Design

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Benchmarking

Benchmarking the lug profile based on the vehicle load application for similar category vehicles to

understand the differences between load, bolt pattern, profile parameters and lug seat type.

Based on the comparative report it is confirmed that there is no difference in the profile parameters

w.r.t to the similar category vehicles, hence the cause is ELIMINATEDELIMINATED

S#S# WHEEL CODEWHEEL CODE WHEEL SIZEWHEEL SIZE WHEEL LOADWHEEL LOAD SEAT TYPESEAT TYPE LUG DIALUG DIA LUG LENGTHLUG LENGTH FASTNERFASTNER

1 CX 822 17 x 7J 726 Kg Conical 15.80 17.50 M14 x 1.75

2 CX 823 17 x 7J 726 Kg Conical 15.80 17.50 M14 x 1.75

3 TS 559 16 x 6.5J 1100 Kg Spherical 18.50 14.40 M14 x 1.50

4 TS 558 16 x 5.5J 1000 Kg Spherical 18.50 14.40 M14 x 1.50

5 TS 556 17 x 6.5J 860 Kg Spherical 18.50 14.40 M14 x 1.50

6 TS 555 16 x 6.5J 860 Kg Spherical 18.50 14.40 M14 x 1.50

7 TK 669 15 x 6JJ 660 Kg Flat type 19.00 14.50 M12 x 1.50

8 TK 235 15 x 6JJ 560 Kg Flat type 19.00 14.50 M12 x 1.50

9 FO 376 15 x 6J 450 Kg Conical 21.00 16.00 M 12 x 1.5010 W 408 16 x 6.5J 1150 Kg Conical 18.50 14.50 M14 x 1.50

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Method

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Tightening pattern

Lug nut torque pattern during wheel assembly is one of the potential cause for the torque loss, a non-sync

pattern will constitute failure.

Lug nut torque pattern as per the specified star pattern for the 5 hole Scorpio alloy wheel. Hence the

cause is ELIMINATEDELIMINATED

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Method

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Torque specification

Lug nut torque applied during the assembly is also one of the potential failure mode, it is known that either

over torque or under torque will have an impact for failure.

Based on the comparative report is it is identified that the torque applied during wheel assembly is

comparatively low than the standard specified. Hence it is one of the potential ROOT CAUSEROOT CAUSE

S #S # WHEEL CODEWHEEL CODE WHEEL SIZEWHEEL SIZE FASTNERFASTNER TORQUE SPECTORQUE SPEC STANDARDSTANDARD COMPLIANCECOMPLIANCE

1 CX 822 17 x 7J M14 x 1.75 115 Nm 115 ~ 122 Nm

2 CX 823 17 x 7J M 14 x 1.75 115 Nm 115 ~ 122 Nm

3 TS 559 16 x 6.5J M14 x 1.50 117.5 ~ 147 Nm 115 ~ 122 Nm

4 TS 558 16 x 5.5J M14 x 1.50 117.5 ~ 147 Nm 115 ~ 122 Nm

5 TS 556 17 x 6.5J M14 x 1.50 117.5 Nm 115 ~ 122 Nm

6 TS 555 16 x 6.5J M14 x 1.50 117.5 Nm 115 ~ 122 Nm

7 TK 235 15 x 6JJ M12 x 1.50 88 ~ 118 Nm 95 ~ 108.5 Nm

8 TK 669 15 x 6JJ M12 x 1.50 95 ~ 125 Nm 95 ~ 108.5 Nm

9 FO 376 15 x 6J M12 x 1.50 108 Nm 95 ~ 108.5 Nm10 W 408 16 x 6.5J M14 x 1.50 83 ~ 104 Nm 115 ~ 122 Nm

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Process

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Variations in conical surface

Variations in the conical seat surface is one of the potential cause for the torque loss, any profile variation in

the lug seat will not mate properly with the lug nut and will have an impact on the failure.

Lug nut conical seat parameter is arrived using a single tool and inspection frequency defined in the

process is at par to identify all the non conformities, hence this cause is ELIMINATEDELIMINATED

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Process

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Mounting face concavity high

If the mounting face concavity is beyond the specified limit, the torque loss my happen due to the vibration

of wheel during operation.

Mounting face convexity parameter is arrived in a single operation and inspection frequency defined in

the process is at par to identify all the non conformities, hence this cause is ELIMINATEDELIMINATED

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Process

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Paint in lughole

Paint in lug seat surface is one of the cause for the torque loss, paint between the surface of conical seat

and lug nut will have an impact for failure.

Paint is observed on the lug seat area , this is one of an important factor needs to be considered for

torque loss. Hence it is one of the potential ROOT CAUSEROOT CAUSE

Paint observed in the conical seat

area

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Material

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Mating surface

Mating surface of the conical seat is one of the important factor needs to be considered for the torque loss,

the chemical bonding between the mating surfaces is an important factor.

The lug surface is chrome, will have not

have a good chemical bonding with the

aluminium surface

The lug nut seat surface is chrome plated surface, suspect the chemical bonding with the mating surface

wheel seat will not be good. Hence it is one of the potential ROOT CAUSEROOT CAUSE

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Material

With the process controls in place to identify the non-conformities in terms of low hardness, this cause

is ELIMINATEDELIMINATED

POTENTIAL FAILURE MODE:POTENTIAL FAILURE MODE: Low hardness

Low hardness of the lug profile area is one of the reason to suspect the torque loss. Low hardness will lead

to material deformation during torquing and will have an impact for failure.

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Thank you