Fatigue failure (pitting) - Gear defects

Fatigue failure (pitting) - Gear defects - Other defects

Figure 1: Fatigue failure (pitting) in the contact surface of a gear tooth. Beach marks are visible in some of the larger pits.

Defect name: Fatigue failure (pitting)
Record No.: 2094
Type of defect (Internal/Surface): Surface
Defect classification: Gear defects, other defects
Steel name: Steel
Steel composition in weight %: No data.
Note: When an important gear failure occurs, someone becomes responsible for analyzing the failure, determining its cause, and recommending a solution. A company can select its own engineer, an outside consultant, or both. If a consultant is called in, this should be done as early in the process as possible. Though similar procedures apply to any failure analysis, the specific approach can vary depending on when and where the inspection is made, the nature of the failure, and time constraints.

Contact fatigue. In another failure mode, called contact or Hertzian fatigue, repeated stresses cause surface cracks and detachment of metal fragments from the tooth contact surface, Figure 1. The most common types of surface fatigue are macropitting (visible to the naked eye) and micropitting.
Macropitting occurs when fatigue cracks start either at or below the surface. As the cracks grow, they cause a piece of surface material to break out, forming a pit with sharp edges. Based on the type of damage, macropitting is categorized as nonprogressive, progressive, spall, or flake. The nonprogressive type consists of pits less than 1 mm diam in localized areas. These pits distribute load more evenly by removing high points on the surface, after which pitting stops.
Progressive macropitting consists of pits larger than 1 mm diam that cover a significant portion of the tooth surface. In one type, called spalling, the pits coalesce and form irregular craters over a large area.
In flake macropitting, thin flakes of material break out and form triangular pits that are relatively shallow, but large in area.
Micropitting has a frosted, matte, or gray stained appearance. Under magnification, the surface is shown to be covered by very fine pits (less than 20 mm deep).
Metallurgical sections through these pits show fatigue cracks that may extend deeper than the pits.
Reference: Not shown in this demonstration version.