Gearboxes are subjected to a lot of strain, especially when used to run heavy machinery, and therefore need to be maintained regularly so as to avoid frequent breakdowns. When a gearbox failure occurs, it is essential to accurately detect the failure mode so that remedial actions can be taken to avoid re-occurrence of the same failure. Here is a look at some of the most notorious causes of gearbox failures and effective ways to prevent them.
Lack of proper lubrication is a primary reason behind numerous gear problems. Low lubricant levels can lead to abrasive wear when metal components come into contact. The high temperatures resulting from structural friction can cause tooth surface damage. If a gear is left to function without proper lubrication, damage will continue until the profiles of the gear's teeth deteriorate to the point where replacement is required.
What is more, abrasive wear is sometimes caused by the presence of contaminants in the lubricant, as oil that is contaminated won't lubricate the various gear components properly. Therefore, always make sure there is enough oil in the gears and that the oil is changed when according to machine manufacturer specifications.
This type of gear failure mode takes place when the stress load at the root fillets of the gear teeth is more than the load capacity of the gear material. This can result from excessive loading, thermal instability, or both. How the fractures present on the surface of gear teeth look like will depend on the magnitude of the failure (a high or a low cycle fatigue).
So-called beach marks, for example, may appear on idler gears where both sides of a tooth are subjected to a reverse stress cycle. Bending fatigue failure modes can be prevented by reducing load, increasing the strength of gear material or improving the geometry of the gear root fillets.
Gear damage resulting from misalignment is noticeable as a crack occurring on a diagonal line and initiating at one end of a gear tooth. Misalignment is a common reason behind damage caused to helical and bevel gear teeth. In most cases, misalignment happens due to loose bearings, which lead to shaft deflection and subsequently a gear tooth fracture. The effects of misalignment tend to be harsher on large machinery because the necessary accuracy of bearing alignment does not change significantly with size, making the difficulty of achieving the accuracy greater with large units.
Faulty bearings are a major cause of misalignment. As part of your preventative maintenance plan, inspect gear bearings to ensure they are in good shape and they are running properly. Otherwise, extensive gear damage can occur.