How to Combat Break-in Wear

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 generates 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 components 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 relative 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. Afterward, 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 underneath and reduced kinetic friction. While this technique may help to address the issue of asperities, it should be used only in moderation, as EP additives can be chemically aggressive and destroy some softer metal compounds, especially alloys containing copper.

Another method involves using a slightly lower viscosity oil during the run-in phase. Along with a lighter load, this can help reduce the severity of the break-in process with generally less wear produced. Although this method works, it also lengthens the break-in period. Combined with the decreased workload, this can cause problems with production goals or process requirements, so it is more often used in situations where spare equipment can be utilized to make up for the loss in capacity.

Others believe that machines should be broken in the same manner in which they are expected to operate. While this is typically the industry standard, it can lead to early machine failure if not monitored. 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.

Oil temperature is also influenced during the break-in period. It will often rise from higher friction and then fall as surfaces smooth and load zones are broadened. This is common with large gearsets.

As with all wear debris monitoring, it is important to look at the rate of change and not just the total volume of wear debris. An initial oil sample from a machine may show high iron from machining debris left during the installation process. After a second sample, which may also be high, the focus should be on the rate of change in the metals. This is why tracking hours of runtime between oil samples as well as after adding makeup fluid becomes very important.

Perhaps the best example of breaking in a piece of equipment comes from the standard vehicle engine. Several break-in fluids are available that help temper wear on bearings, cams and cylinders. These fluids typically have a higher additive load and a different viscosity than the engine oil. A new engine usually is run at a lighter load (lower speed) for a short period before it is considered roadworthy. Most engines are broken in at the factory prior to being installed in a car. However, many mechanics replace engines and must repeat this process to ensure engine health.

A friend of mine experienced this firsthand as he was rebuilding the engine for his drag car. He had just finished putting the final touches on the engine and fired it up. Shortly thereafter, he took the car to the track and lost the engine on his first pass. During the teardown, he discovered most of the bearings were wiped out from what he determined to be a lack of oil. Since the running-in period was so short, the oil didn’t have a chance to build up the protective chemical layer on the surfaces to ensure their longevity.

In the initial stage of a machine’s life, there are a few things to keep in mind. More friction and wear will occur due to the high level of asperities. As the asperities are ground down, the new surface becomes polished, signifying the end of the running-in period. Explore the use of a break-in fluid to help minimize the risk of machine failure and to reduce the amount of wear experienced during this time. By vigilant monitoring of wear debris as well as understanding the forces at play during the break-in period, you can ensure that your machines will have a longer life and experience fewer breakdowns.

References

Salomon, G., DeGee, A.W.J. (1981). “The Running-in of Concentrated Steel Contacts: A System-oriented Approach.” The Running-in Process in Tribology.

Heilmann, P., Rigney, D.S. (1981). “Running-in Processes Affecting Friction and Wear.” The Running-in Process in Tribology.

Blau, P.J. (1981). “Studies of the Friction Transients During Break-in of Sliding Metals.” The Running-in Process in Tribology.

Bloch, H.P. (2009). “Practical Lubrication for Industrial Facilities.” Fairmont Press Inc.