Gearboxes are complicated machines, but they are durable and critical to your operations. However, this equipment is not immune to problems. Micropitting and surface fatigue are two of those problems that you may be familiar with if you own a gearbox. It is important for any gearbox owner or operator to understand these issues and how to prevent them from happening.
Micropitting is a condition that is expressed by microscopic cracks on the surfaces of your gears. Over time, these become larger until they flake away. Micropitting is one of the primary causes of gearbox failure.
This issue occurs under a breakdown of the elastohydrodynamic lubrication (EHL). It may seem counterintuitive that lubrication would result in micro stress fractures and pits. However, the oil film under the EHL must stay at a certain thickness to prevent components from rubbing against one another. If the film is too thin at the gear pitch line, the surfaces will come into contact with one another and result in stress fractures. The problem is especially pronounced in equipment under high loads, which results in plastic or elastic deformation, causing micropitting.
Surface fatigue is similar to micropitting. Under fading EHL, surface fatigue results when surfaces dent due to particle contact. The dents eventually cause flat paths. Over time these will develop pits and surface breakage, as well as cause bearing failure.
Micropitting and surface fatigue are influenced by lubricant, additives, particle contamination, and viscosity. These issues are more common in standard mineral oils, compared to synthetics. Synthetic lubricants are engineered to provide more protection under higher temperatures and stress. These products can provide added protection because their viscosity index is higher. Essentially, the viscosity of synthetics is influenced less by temperature variation.
High-viscosity oils are also better at preventing surface fatigue and micropitting because they have thicker EHL films, allowing larger particulate to pass. However, keep in mind that higher viscosity is also associated with higher operating temperature, an increased rate of oxidation, and more energy loss. The key is to find the right balance between lubrication factors when searching for gearbox oils.
Anywhere in your gearbox that rolling contact occurs is a potential problem area for surface fatigue and micropitting. Rolling-element bearings, the area around the pitchline, rollers, and cams are all examples of high-risk contact areas where these problems can occur.
Micropitting and surface fatigue happen because particles that are larger than the EHL film get stuck between the surfaces from rolling movements. Once these particles are trapped in a contact area, they are subjected to extreme amounts of pressure. The particles with less compressive strength break apart and are embedded in the surface area or rolled out. Harder particles retain their shape and dent the surface of the contact area, resulting in micropits.
The right viscosity is critical to reducing micropitting and surface fatigue. The higher the load, the more viscosity the lubricant will need. You should also take into account the speed of machine movement. Lower speeds result in reduced thickness, while higher speeds cause increased thickness. Finally, the operating temperature is another concern. Higher operating temperatures reduce viscosity, which decreases EHL film thickness. You should balance each of these factors (viscosity, load, speed, and temperature) and consider how each one affects the other.
If you need affordable, high-quality industrial gearbox inspection, repair, or maintenance, contact the experts at Amarillo Gear Service in Amarillo, Texas today. We are a division of Amarillo Gear Company, which has been in continuous operation since 1917. You can call us at (806) 622-1273 or Contact Us by email to learn more about our Amarillo Gear™ and Marley™ gearbox repair services. We will be glad to tell you more about the Regions We Service and the quality workmanship we can bring to the repair or Renewal of your drive.