Gear tooth wear is commonly seen in service due to the high loads and repeated contact. The convoluted profile of gear teeth ensures a smooth transition of contact across the gear, reducing the wear. In this post, we will discuss the different types of gear wear and how they arise.

The Different Types of Gear Wear

Running in wear

This type of gear wear occurs as soon as the gears start meshing during operation. Running wear allows the gear teeth to settle during meshing and distributing the load by removing irregularities. The surface texture of the tooth flank becomes smooth because of the wear. The visible appearance of this wear is a mirror-like smooth surface.

Moderate Gear wear

Again this is one the expected types of gear wear. Moderate wear occurs once the gear has undergone a considerable time in service, gears in the second half of design life. Moderate wear should consist of consistent wear resulting in all gear teeth losing material from both the addendum and dedendum.

Abrasive wear

Abrasive wear is the removal of material due to the presence of hard particles e.g. metallic debris. The hard particles will be suspended in the lubricant and picked up by the gear tooth when travelling through the lubricant. Visually, this wear mechanism is represented by radial scratches which appear on the mating gear. In extreme cases, it can lead to a sharpening of the gear tooth.

Looking deeper into abrasive wear we can find scratching/scoring. Scratching/scoring gives fine grooves which are in the direction of sliding and have an irregular appearance. The ploughing action from hard asperities can sometimes be the cause of scratching/scoring.

Scuffing

Scuffing results from the lubricant layer over the tooth failing during operations occurring from a high load. The break down of the lubricant results in excessive temperature leading to localised welding with a transfer of material. Scuffing appears as rough edges which generally appear at the tooth tops, the end of contact where sliding is at its peak.

Indentation

Indentation is plastic deformation to the tooth surfaces covering small areas. Similar to abrasive wear, indentation results from a hard particle in the lubricant being rolled over by contacting gears. The high contact stress results in the hard particle plastically deforming the tooth and indenting the gear. Indentations are straight forward to identify with their unique appearance. The exact appearance will depend on the properties and size of the particle trapped. Indentation is commonly a secondary damage mechanism due to other component failures, e.g. bearings resulting in debris entering the lubricant.

Rippling 

Rippling are ridges that appear in the perpendicular motion of sliding, commonly found on the tooth flanks. The ridges have an appearance to mud that has been disturbed by wind, although on a much smaller scale. The ridges are not straight but wavy in nature. The cause of rippling is high loading on the tooth.

Pitting

Pitting is a surface fatigue phenomenon which occurs during the presence of rolling or a mix between both rolling and sliding contacts. Small parts of the material break out and leave an appearance of scattered holes. High contact stresses that surpass the fatigue strength of the material and repeated loading, cause pitting. Pitting can have a detrimental knock-on effect if it spreads across the entire contact surface, leading to seizing surfaces.

Summary

Here we have discussed many different types of gear wear, however, there are some I have not covered in this post. Inspecting gears is a job I have undertaken during my graduate scheme, so I wanted to pass on a small fraction of gear tooth wear.

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