MSC FAILURE MSC FAILURE ANALYSIS AND ANALYSIS AND PREVENTIONPREVENTIONVISTEON SIDE GEAR AND VISTEON SIDE GEAR AND

PINION FAILURE ANALYSISPINION FAILURE ANALYSIS

Metallurgical Inspection Metallurgical Inspection Scott Workman B.S.Scott Workman B.S.

MSC METALLURGICAL eNGINEERMSC METALLURGICAL eNGINEER

Subsurface- Origin Subsurface- Origin Macro pittingMacro pitting

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

Surface Failures Surface Failures Pits and Spalls are generally Pits and Spalls are generally

considered the most familiar considered the most familiar surface failures . However, surface failures . However, they differ in size; spalls are they differ in size; spalls are much larger. Either type can much larger. Either type can start at the surface , or at the start at the surface , or at the case core interface . Most case core interface . Most commonly, pits start along the commonly, pits start along the line of contact where the line of contact where the pressure is the heaviest pressure is the heaviest against the tooth of the mating against the tooth of the mating part .. This line is usually at, or part .. This line is usually at, or very near, the pitch line.very near, the pitch line.

Many times , a fatigue crack Many times , a fatigue crack will begin at a pitted area and will begin at a pitted area and form spalls.form spalls.

Case CrushingCase Crushing Case Crushing is due to Case Crushing is due to

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

Case Depth- Shear Stress Case Depth- Shear Stress ThreoryThreory

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.

Case Depth-Shear Stress Case Depth-Shear Stress TheoryTheory

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.

Back lash FatigueBack lash Fatigue

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

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.

400X MICRO PITTING AND SMALL MICRCRACKS 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 OF SIDE GEAR FAILURE.

50X 2% NITOL ETCHANT FORD INTERCEPTOR

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.

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

MACRO PIT 200X WITH SUBSURFACE MICRO CRACK.

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 PRODUCT FAILURE RESULTS WITHOUT DATA FROM CUSTOMER OF LOAD APPLIED, EVALUATION OF SET CUSTOMER OF LOAD APPLIED, EVALUATION OF SET UP , LUBRICANTS ECT HAS NARROWED OUR FIELD OF UP , LUBRICANTS ECT HAS NARROWED OUR FIELD OF CHOICE AS WHAT TO DO IN RESPONSE OF FAILURE. CHOICE AS WHAT TO DO IN RESPONSE OF FAILURE.

Response to failure: ULTRASONIC TESTING OF BAR Response to failure: ULTRASONIC TESTING OF BAR PRIOR TO SECTIONING PROCESS.PRIOR TO SECTIONING PROCESS.

Engineering review of part dimensions and heat treatment Engineering review of part dimensions and heat treatment core hardness and case depth.core hardness and case depth.