Cams typically aren't considered gears, but they do share common attributes like high, cyclic loads, tight tolerances for surface finishes, specific hardening requirements, etc., and they exhibit some similar failure and wear mechanisms:
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Failure Analysis for Gearing
Corrosive wear (Fig. 3) is visible as surface deterioration, caused by the chemical action of active ingredients in the lubricant. These may include acid, moisture, foreign materials, and extreme-pressure additives. During operation, the oil breaks down and allows corrosive elements present in the oil to attack the gear contact surfaces. This action may affect the grain boundaries and cause fine, evenly distributed pitting. Checking the oil for breakdown and changing it at regular intervals can help minimize corrosive wear. Lubricants with high antiscuff, antiwear additive content must be observed even more carefully because they are chemically active. Gear units that are exposed to salt water, liquid chemicals, or other foreign materials should be sealed from their environment
Pitting failures depend on surface contact stress and the number of stress cycles. Initial pitting (Fig. 5), with areas of small pits from 0.015 in. to 0.030 in. in diameter, occurs in localized parts of the gear teeth that are over-stressed. It is sometimes called corrective pitting because it tends to redistribute the load by progressively removing high contact spots, and often stops once the load has been redistributed. Continued operation may polish or burnish the pitted surface and improve its appearance. Pitting can be monitored by periodically putting some bluing on the affected area, then applying some cellophane tape to lift the pattern and put it in a notebook. Comparing the impressions over time will tell whether the pitting has stopped. While accurate manufacturing control of involute profiles is the best method of preventing pitting, a careful break-in at reduced loads and speeds once the unit is installed also will help minimize pitting by improving gear tooth contact.
Destructive pitting (Fig. 6) appears as much larger pits than initial pitting, often in the dedendum section of the gear teeth. These larger craters usually are caused by more severe overload conditions that cannot be relieved by initial pitting. As stress cycles build up, pitting will continue until the tooth profile is destroyed. To correct the cause of destructive pitting, the load on the surface of the gear needs to be reduced below the material’s endurance limit, or the material hardness needs to be increased to raise the endurance limit to where pitting will not occur.
Spalling (Fig. 7) resembles destructive pitting, except that the pits may be larger, quite shallow, and irregularly shaped. The edges of the pits break away rapidly, forming large, irregular voids that may join together. Spalling is caused by excessively high contact stress levels. Remedies include reducing contact stress on the gear surface or hardening the material to increase its surface strength.
Both spalling and destructive pitting are indications that the gears do not have sufficient surface capacity and should probably be redesigned if possible.
Micropitting is a type of contact fatigue that appears as frosting or gray staining under thin film conditions (Fig. 8). The surface acquires an etch-like finish, with a pattern that sometimes follows the slightly higher ridges left by cutter marks or other surface irregularities. It usually shows up first on the dedendum section of the driving gear, although it may begin on the addendum section as well. When viewed under magnification (Fig. 9), the surface is seen as a field of very fine micropits under 0.0001 in. deep. Causes include high surface loads and heat generation, which thins the lubrication film and leads to marginal lubrication. Improving the surface finish is an effective remedy, through either manufacturing techniques such as hard honing and grinding or a careful break-in cycle. These techniques help lower heat generation by improving conformity of tooth contact and equalizing load distribution. Reducing the lubricant temperature and surface loading will also minimize frosting. Sometimes, frosted areas that appear initially will slowly be polished away during subsequent operation if loads and temperatures are not excessive