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MSC FAILURE

ANALYSIS AND

PREVENTION VISTEON SIDE GEAR AND PINION

FAILURE ANALYSIS

Metallurgical Inspection

Scott Workman B.S.

MSC METALLURGICAL eNGINEER



Subsurface- Origin Macro pitting

Surface , Subsurface Micro

Pitting , Macro pitting

and Sub case Fatigue are

fatigue failures in whichthe applied shear stresses

have overcome the shear

strength of the material

in the respective failurezones .



Surface Failures

Pits and Spalls are generally considered the

most familiar surface failures .

However, they differ in size; spalls are

much larger. Either type can start at the

surface , or at the case core interface .

Most commonly, pits start along theline of contact where the pressure is

the heaviest against the tooth of the

mating part .. This line is usually at, or

very near, the pitch line.

Many times , a fatigue crack will begin

at a pitted area and form spalls.



Case Crushing

Case Crushing is due to excessive

compressive loads. When the case is to thin

and the core is to low in hardness, heavy

loads break through the case, denting the

core. When case crushing is considered, case

depth is not necessarily related to tooth

thickness . Instead , it is more stronglyinfluenced by the radii of curvature at the

point of contact, the load, and the core

hardness. If the load is exceptionally high ,

we can lower it by changing the spiral or

helix angle to produce a higher ratio or

overlap contact. Altering the diametral (or

module) pitch , or reduce the pressure

angle may also help.



Case Depth- Shear Stress Threory

Determination of case depth – shear stress theory

Of the several parameters used in optimizing a

gear design, case depth, surface hardness, and core hardness

of a tooth are of significant importance. These three

parameters are normally selected on the basis of applied loadto a gear and it’s required life under the service conditions.

Research carried out shows that a proper combination of

case depth, surface hardness, and core hardness provides the

maximum gear life. Analytically, these parameters are

determined as outlined sudsequently. In transmitting torque,

a gear tooth is subjected to at least two types of major

stresses: contact and bending. These stresses cause toothfailure due to metal fatigue. Gear tooth failure due to contact

stress, commonly known as pitting occurs when small pits

initiated by fatigue cracks are formed on or below the tooth

surface. These pits emanate at the highest point of single

tooth contact .(hpstc) for pinion and at the lowest point of

single tooth contact (LPSTC) for the mating gear. HPSTC

and LPSTC are the main contact stress points.



Case Depth-Shear Stress Theory

St =Depth of Maximum shear stress below the surfaceP= Pitch point Sc =Total Case Depth

Recommended Case Depth at Pitch Line.

For critical gears such as ours, it is advisable to

hold case depth toward the maximum and the

minimum limit raised. The Case depth so

specified is the total case. (see DP below) The

effective case (hec ) for surface durability is

taken at approximately 75% of the total case ormay be estimated by the following equation:

Where Sc is the maximum contact stress ,psi, in the region

of 106 to 107 cycle's is the pinion pitch diameter (in) ;ot is

the pressure angle ;Yb the base helix angle; and mg is the

tooth ratio. Below in metric units.



Back lash Fatigue

Back lash fatigue failures commonly result

from shear at the case core interface

along the contact zone. This type of

failure can be reduced by enlarging or

rounding the root filets, or by changing

the pressure angles. And , of course ,

we all know that gears last longer

when loads are reduced .



Subsurface micro crackSubsurface micro pitting

Inclusion oriented

The Subsurface

micro pitting as

observed to theright at 400x is

the first affect of

excess torque

the secondary

affect of micro

pitting are themicro cracks

before major

failure occurs.



400X MICRO PITTING AND SMALLMICRCRACKS PROPOGATING FROM

PITTING. OBSERVE BLACK AREAS AT

OR NEAR SURFACE.

PINION GEAR



MAJOR FAILURE: FOM THE

PROPOGATED MICROCRACK AND

PITTING ON SURFACE. STRESS FROM

EXCESS LOAD OF BACKLASH CAUSE OFSIDE GEAR FAILURE.

50X 2% NITOL ETCHANT

FORD INTERCEPTOR



CASE CRUSHING EXAMPLE . SIDE GEAR VIEWING EDGE

AT 50X . THIS IS THE MAIN CONTACT POINT. THE ARROWPOINTS TO CRUSH AREA OF EDGE AND THE CASE CORE

INTERFACE. THE EDGE OF PRODUCT SHOULD BE

ROUNDED AT MID TOOTH PITCH HEIGHT.



MACROPIT IN PROCESS OF MAJOR

FAILURE. OBSERVE INITIALPITTING AREA. AREA SHOWS

OUTLINE OF MACRO PIT.



MACRO PIT 200X WITH SUBSURFACE MICRO CRACK.



CONCLUSION OF STUDY

OBSERVANCE OF THESE DEFECTS AND THE STUDY OF

PRODUCT FAILURE RESULTS WITHOUT DATA FROM CUSTOMER

OF LOAD APPLIED, EVALUATION OF SET UP , LUBRICANTS ECT

HAS NARROWED OUR FIELD OF CHOICE AS WHAT TO DO IN

RESPONSE OF FAILURE.

Response to failure: ULTRASONIC TESTING OF BAR PRIOR TOSECTIONING PROCESS.

Engineering review of part dimensions and heat treatment core hardness and

case depth.

failureanalysis-090416013608-phpapp02

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