Pitting Reveals Gear Issues

Inspect your gear teeth. If pitting exists, it can tell you a lot about gear performance and condition.

Inspect your
gear teeth. If pitting exists, it can tell you a lot about gear
performance and condition.

Best-practice maintenance varies based on component type.

By Neville Sachs, P.E.

Understanding gear and reducer metallurgies is the first step toward effective inspection and maintenance of these components (see “Hardness, Lubrication are Keys to Gear Inspection, p. 41, Maintenance Technology, Dec. 2015). Once that’s accomplished, the next step is to look at the contact pattern of the teeth and rotate the gear with the following in mind:

• Does the pattern show across the entire tooth?
• Is the pattern consistent all the way around the gear? (It’s usually a sign of poor manufacturing quality control if it doesn’t.)

A good contact pattern is critical to good gear life. A contact pattern that runs out at the end of a tooth means the tooth load is a least twice the design load. If the contact pattern only covers half of a tooth, the stress at the heavily loaded edge is more than four times the design load. On through-hardened teeth, this may not present much of a problem, as the gears will wear in. On case-hardened teeth, however, this reflects a very serious situation—one that can lead to tooth breakage.

Pitted gear teeth?

With through-hardened gears, measure the amount of wear and use that to decide when you need to replace the gears. If the gears don’t involve human safety, run them until the tooth thickness is reduced by 30% and don’t worry about failure.

With case-hardened gears, any pitting is cause for concern. Very fine pitting that looks like sandblasting, i.e., “micropitting,” is indicative of very heavily loaded teeth. If micropitting is consistent across the teeth, gear monitoring should become a moderate priority. (A site’s vibration analyst(s) should keep a close eye on the tooth frequencies and sidebands.) Micropitting that’s heavier on one side indicates a misalignment between the mating gears and calls for further—immediate—investigation before something breaks. Is the housing distorted? Are the components really misaligned?

Larger pitting, i.e., where individual pits are easily seen, indicates that the gears are overloaded or the lubricant isn’t doing its job—and requires very close attention. If there is a series of pits in a line across a case-hardened tooth, the risk of tooth fracture increases. (Note: The presence of one or several large, isolated pits almost always points to a manufacturing error.)

Important last words

Don’t worry about backlash with gears always running in one direction.

If through-hardened gears are pitting, replace the lubricant with the next higher viscosity grade. As long as the temperature doesn’t increase substantially, the higher viscosity will result in a thicker lubricant film that leads to less wear. To be sure you’ve improved the situation, before replacing an existing lubricant, measure its temperature, then take the temperature of the higher viscosity oil under similar operating conditions and compare the two on a viscosity-temperature chart. MT

Neville Sachs has spent many years working in the field of machinery reliability and lubrication for a wide range of industries. The author of two books on failure analysis and a contributor of sections to others, he has also written more than 40 articles on these topics. A Registered Professional Engineer, Sachs holds STLE’s CLS certification, among others. Contact him at sachscracks@att.net.