The document analyzes various failure modes in gears, including pitting, spalling, case crushing, backlash fatigue, and subsurface microcracking. It discusses how these failures originate and provides recommendations to prevent them, such as rounding root fillets, changing pressure angles, and adjusting case depth and hardness values based on applied loads and desired life. Metallurgical inspection of failed gears found evidence of microcracking originating from pitting and case crushing at stress concentrations, leading to major failures.

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

5. 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 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 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 10 6 to 10 7 cycle's is the pinion pitch diameter (in) ;o t is the pressure angle ;Yb the base helix angle; and mg is the tooth ratio. Below in metric units.

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.