Several failure modes may be present and you need to identify which is the primary mode, and which are secondary modes that may have contributed to failure.lists six general classes of gear failure modes, of which the first four are the most common. An understanding of these four common modes will enable you to identify the cause of failure.

1. Bending fatigue. This common type of failure is a slow, progressive failure caused by repeated loading. It occurs in three stages:

• Crack initiation. Plastic deformation occurs in areas of stress concentration or discontinuities, such as notches or inclusions, leading to microscopic cracks.
• Crack propagation. A smooth crack grows perpendicular to the maximum tensile stress.
• Fracture. When the crack grows large enough, it causes sudden fracture.

As a fatigue crack propagates, it leaves a series of “beach marks” — visible to the naked eye — that correspond to positions where the crack stopped. The origin of the crack is usually surrounded by several concentric curved beach marks.

Most gear tooth fatigue failures occur in the tooth root fillet, where cyclic stress is less than the yield strength of the material and the number of cycles is more than 10,000. This condition is called high-cycle fatigue. A large part of the fatigue life is spent initiating cracks, whereas a shorter time is required for the cracks to propagate.

Stress concentrations in the fillet often cause multiple crack origins, each producing separate cracks. In such cases, cracks propagate on different planes and may join to form a step, called a ratchet mark.

2. 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. 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.

3. Wear. Gear tooth surface wear involves removal or displacement of material due to mechanical, chemical, or electrical action. The three major types of wear are adhesion, abrasion, and polishing.

Adhesion is the transfer of material from the surface of one tooth to that of another due to welding and tearing.

Adhesion is categorized as mild or moderate, whereas severe adhesion is termed scuffing (described later).

Typically, mild adhesion occurs during gearset run-in and subsides after it wears local imperfections from the surface. To the unaided eye, the surface appears undamaged and machining marks are still visible. Moderate adhesion removes some or all of the machining marks from the contact surface. Under certain conditions, it can lead to excessive wear.

Abrasion is caused by contaminants in the lubricant such as sand, scale, rust, machining chips, grinding dust, weld splatter, and wear debris. It appears as smooth, parallel scratches or gouges.

Abrasion ranges from mild to severe. Mild abrasion consists of fine scratches that don’t remove a significant amount of material from the tooth contact surface, whereas moderate abrasion removes most of the machining marks.

Severe abrasion, which removes all machining marks, can cause wear steps at the ends of the contact surface and in the dedendum. Tooth thickness may be reduced significantly, and in some cases, the tooth tip is reduced to a sharp edge.

Finally, polishing is fine-scale abrasion that imparts a mirror-like finish to gear teeth, Figure 7. Magnification shows the surface to be covered by fine scratches in the direction of sliding. Polishing is promoted by chemically active lubricants that are contaminated with a fine abrasive.

Polishing ranges from mild to severe. Its mild form, which is confined to high points on the surface, typically occurs during run-in and ceases before machining marks are removed. Moderate polishing removes most of the machining marks.

Severe polishing removes all machining marks from the tooth contact surface. The surface may be wavy or it may have wear steps at the ends of the contact area and in the dedendum.

4. Scuffing. Severe adhesion or scuffing transfers metal from the surface of one tooth to that of another. Typically, it occurs in the addendum or dedendum in bands along the direction of sliding, though load concentrations can cause localized scuffing. Surfaces have a rough or matte texture that, under magnification, appear to be torn and plastically deformed.

Scuffing ranges from mild to severe. Mild scuffing occurs on small areas of a tooth and is confined to surface peaks. Generally, it is nonprogressive.

Moderate scuffing occurs in patches that cover significant portions of the teeth. If operating conditions do not change, it can be progressive.

Severe scuffing occurs on significant portions of a gear tooth (for example, the entire addendum or dedendum). In some cases, surface material is plastically deformed and displaced over the tooth tip or into the tooth root. Unless corrected, it is usually progressive.