Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/108300
Gear Lubrication IN THE TRENCHES JOSH PICKLE NORIA CORPORATION PREVENTING Micropitting and SURFACE Fatigue Many gears can be affected by a phenomenon known as micropitting. This condition is seen when microscopic cracks form on gears and through time and stress result in microscopic pits. These pits grow larger and eventually break away. This can even be a primary failure mode for gears. Micropitting generally occurs under elastohydrodynamic lubrication (EHL). When the oil film thickness under EHL becomes too thin at the gear pitchline, surface asperities will begin to come into Selecting the right viscosity is key in reducing micropitting and surface fatigue. contact. When these asperities contact each other on opposing h h ii h h i surfaces and under high load, they cause elastic or plastic deformation, which leads to micropitting. Surface fatigue is very similar. Under elastohydrodynamic lubrication, surface fatigue often results from denting on a surface due to hard or soft particles. The dents in the surface create what are known as berms. Over time and with repeated high loading, pits develop where the surface breaks apart. With continued high loading, the pits become larger. The Effects Surface fatigue and micropitting are influenced by the particular lubricant being used, including its base oil, additives, viscosity selection and particle contamination. While micropitting or surface fatigue can occur with synthetic or mineral oil lubricants, synthetics can provide better protection at higher temperatures than mineral oils with the same viscosity grade and additive 48 | January - February 2013 | www.machinerylubrication.com package. This is due to the fact that synthetics can have a higher viscosity index. In other words, the viscosity of synthetics may change less with an increase in temperature. Although extreme pressure (EP) additives are often necessary, in certain cases they can be very chemically aggressive to surfaces and cause micropitting. These types of additives also become more active with higher temperatures. Some researchers claim oils that do not have EP additives will exhibit a maximum resistance to micropitting. An oil's ability to protect against micropitting can be determined using the FZG FVA 54 test. High-viscosity oils also have a greater resistance to micropitting because of their thicker EHL films. However, going to a higher viscosity is not always the best option because it can cause higher operating temperatures, energy loss and/or an increased rate of oil oxidation. Particles that are much larger than the EHL film thickness can become entrained between surfaces due to a rolling action. Once these particles are in the contact area, they are subjected to massive amounts of contact pressure. Particles with lower compressive strength under this contact pressure can break into smaller pieces, with some embedding in the surfaces and others passing through the contact zone. Harder particles that are larger than the EHL film thickness can pass through the contact zone by denting the softer surface. As mentioned previously, these dents create berms (shoulders) and, over time with more contact pressure, can dislodge from the surface. Controlling Micropitting and Surface Fatigue Selecting the right viscosity is key in reducing micropitting and surface fatigue. Higher loads will require higher viscosity, while lower loads allow for lower viscosities.