Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/258778
50 | January - February 2014 | www.machinerylubrication.com When a machine is first put into service, there is a period of time known as the "break-in" period. During this time, the machine creates wear debris as components begin their initial motion. While there are differing opinions on break-in times and methods, a few constants remain. Certain variables come into play such as surface coatings, time in storage and lubricant selection. By balancing all of these variables, you can achieve fewer failures during the early stages of machine life. The primary reason machines lose their usefulness is due to the degradation of working surfaces. This can occur for a variety of reasons with a multitude of effects to the rest of the machine. However, when a machine is first put into service, it typically gener- ates more wear than it does after months of use. The rate of wear starts high initially and then gradually declines over time. Break-in wear can vary based on the surface finish of the compo- nents in motion. The surface profile of bearing and gear surfaces can be very smooth or rough depending on the grade purchased as well as the load characteristics that they must support. The small surface irregularities known as asperities exist on all working surfaces. The depth and amount of these asperities will make a difference in the amount of initial running-in wear that will ensue. The smoother the surface (fewer/shorter asperities), the less break-in wear will occur. In boundary conditions (no lubricating film to separate the surfaces), the asperities come in contact with each other more frequently, leading to increased wear and possible metal transfer from one surface to another. This metal transfer is common during the process known as adhesive wear, which occurs when two surfaces are in contact and continue to move past each other. With more asperities, adhesion is also initially high during the break-in phase. All of this leads to greater friction during the infant stage of machine life. As the asperities are ground down against each other, the friction associated with the surfaces moving against each other decreases slightly. This friction is negligible but can account for increased power requirements during startup. As friction decreases and the asperities begin to smooth out against each other during the running-in phase, a small amount of machine polishing occurs. Consider a machine shaft as it spins rela- tive to a journal bearing. As the bearing's asperities are ground against the shaft, the bearing's surface profile becomes smoother. The resulting polishing effect contributes to the higher levels of wear debris that are seen in a machine when it is first put into service. In extreme cases where the break-in process is carried out incorrectly (excessive loads, speeds and/or lubricant starvation), the polishing effect can be destructive and escalate to galling and severe two-body abrasion. This can lead to the loss of surface profile and thus machine failure during the component's infancy. Additionally, surface undulations can concentrate loads to small contact areas. These high points wear down during the break-in period, allowing the load to be dispersed over a larger area. After- ward, the break-in wear zone (high points) will often heal over. To temper or control this surface polishing, many users utilize different lubricants to reduce the amount of wear developed during the first few weeks of machine life. Use of moderate loads and speeds also helps. One strategy includes the use of oils with extreme-pressure (EP) additives, which can reduce the amount of asperities and smooth the surface profile. As these EP additives build up on machine surfaces, they produce a chemical film. This film contains a small amount of the surface material, which becomes sacrificial. As this layer is rubbed away, it results in a smoother profile under- neath and reduced kinetic friction. While this technique may help to address the issue of asperities, it should be used only in moderation, as EP addi- tives can be chemically aggressive and destroy some softer metal compounds, espe- cially alloys containing copper. w e a r d e b r i s A n a l y s i s wes CAsh NorIA CorPorAtIoN how to ComBAt BreAk-IN weAr BACk PAGe BAsICs 58% of lubrication professionals say break-in wear has caused problems in the machines at their plant, according to a recent poll at machinerylubrication.com All machines will generate wear during the break-in phase; it is how you manage this phase that will determine how long the machine will operate afterward.