Failure Analysis

Drive Gears Forged Material Issues

1. MSC FAILURE MSC FAILURE
ANALYSIS AND ANALYSIS AND
PREVENTIONPREVENTION
VISTEON SIDE GEAR AND PINION VISTEON SIDE GEAR AND PINION
FAILURE ANALYSISFAILURE ANALYSIS
Metallurgical Inspection Metallurgical Inspection
Scott Workman B.S.Scott Workman B.S.
MSC METALLURGICAL eNGINEERMSC METALLURGICAL eNGINEER

2. Subsurface- Origin Macro pittingSubsurface- Origin Macro pitting
Surface , Subsurface Micro Surface , Subsurface Micro
Pitting , Macro pitting Pitting , Macro pitting
and Sub case Fatigue are and Sub case Fatigue are
fatigue failures in which fatigue failures in which
the applied shear stresses the applied shear stresses
have overcome the shear have overcome the shear
strength of the material in strength of the material in
the respective failure the respective failure
zones .zones .

3. Surface Failures Surface Failures
Pits and Spalls are generally considered the Pits and Spalls are generally considered the
most familiar surface failures . most familiar surface failures .
However, they differ in size; spalls are However, they differ in size; spalls are
much larger. Either type can start at the much larger. Either type can start at the
surface , or at the case core interface . surface , or at the case core interface .
Most commonly, pits start along the Most commonly, pits start along the
line of contact where the pressure is line of contact where the pressure is
the heaviest against the tooth of the the heaviest against the tooth of the
mating part .. This line is usually at, or mating part .. This line is usually at, or
very near, the pitch line.very near, the pitch line.
Many times , a fatigue crack will begin Many times , a fatigue crack will begin
at a pitted area and form spalls.at a pitted area and form spalls.

4. Case CrushingCase Crushing
Case Crushing is due to excessive Case Crushing is due to excessive
compressive loads. When the case is to thin compressive loads. When the case is to thin
and the core is to low in hardness, heavy and the core is to low in hardness, heavy
loads break through the case, denting the loads break through the case, denting the
core. When case crushing is considered, case core. When case crushing is considered, case
depth is not necessarily related to tooth depth is not necessarily related to tooth
thickness . Instead , it is more strongly thickness . Instead , it is more strongly
influenced by the radii of curvature at the influenced by the radii of curvature at the
point of contact, the load, and the core point of contact, the load, and the core
hardness. If the load is exceptionally high , hardness. If the load is exceptionally high ,
we can lower it by changing the spiral or we can lower it by changing the spiral or
helix angle to produce a higher ratio or helix angle to produce a higher ratio or
overlap contact. Altering the diametral (or overlap contact. Altering the diametral (or
module) pitch , or reduce the pressure module) pitch , or reduce the pressure
angle may also help.angle may also help.

5. Case Depth- Shear Stress ThreoryCase 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 load
to 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 tooth failure 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.

6. Case Depth-Shear Stress TheoryCase Depth-Shear Stress Theory
St =Depth of Maximum shear stress below the surface
P= 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 or may 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.

7. Back lash FatigueBack lash Fatigue
Back lash fatigue failuresBack lash fatigue failures commonly result commonly result
from shear at the case core interface from shear at the case core interface
along the contact zone. This type of along the contact zone. This type of
failure can be reduced by enlarging or failure can be reduced by enlarging or
rounding the root filets, or by changing rounding the root filets, or by changing
the pressure angles. And , of course , the pressure angles. And , of course ,
we all know that gears last longer we all know that gears last longer
when loads are reduced .when loads are reduced .

8. Subsurface micro crack
Subsurface micro pitting
Inclusion oriented
The Subsurface
micro pitting as
observed to the
right at 400x is
the first affect of
excess torque
the secondary
affect of micro
pitting are the
micro cracks
before major
failure occurs.

9. 400X MICRO PITTING AND SMALL
MICRCRACKS PROPOGATING FROM
PITTING. OBSERVE BLACK AREAS AT
OR NEAR SURFACE.
PINION GEAR

10. MAJOR FAILURE: FOM THE
PROPOGATED MICROCRACK AND
PITTING ON SURFACE. STRESS FROM
EXCESS LOAD OF BACKLASH CAUSE OF
SIDE GEAR FAILURE.
50X 2% NITOL ETCHANT
FORD INTERCEPTOR

11. CASE CRUSHING EXAMPLE . SIDE GEAR VIEWING EDGE
AT 50X . THIS IS THE MAIN CONTACT POINT. THE
ARROW POINTS TO CRUSH AREA OF EDGE AND THE
CASE CORE INTERFACE.THE EDGE OF PRODUCT
SHOULD BE ROUNDED AT MID TOOTH PITCH HEIGHT.

12. MACROPIT IN PROCESS OF MAJOR
FAILURE. OBSERVE INITIAL
PITTING AREA. AREA SHOWS
OUTLINE OF MACRO PIT.

13. MACRO PIT 200X WITH SUBSURFACE MICRO CRACK.

14. CONCLUSION OF STUDYCONCLUSION OF STUDY
OBSERVANCE OF THESE DEFECTS AND THE STUDY OF OBSERVANCE OF THESE DEFECTS AND THE STUDY OF
PRODUCT FAILURE RESULTS WITHOUT DATA FROM CUSTOMER PRODUCT FAILURE RESULTS WITHOUT DATA FROM CUSTOMER
OF LOAD APPLIED, EVALUATION OF SET UP , LUBRICANTS ECT OF LOAD APPLIED, EVALUATION OF SET UP , LUBRICANTS ECT
HAS NARROWED OUR FIELD OF CHOICE AS WHAT TO DO IN HAS NARROWED OUR FIELD OF CHOICE AS WHAT TO DO IN
RESPONSE OF FAILURE. RESPONSE OF FAILURE.
Response to failure: ULTRASONIC TESTING OF BAR PRIOR TO Response to failure: ULTRASONIC TESTING OF BAR PRIOR TO
SECTIONING PROCESS.SECTIONING PROCESS.
Engineering review of part dimensions and heat treatment core hardness and Engineering review of part dimensions and heat treatment core hardness and
case depth.case depth.