The Elimination of Part Break-In by Isotropic Superfinishing

Part break-in cycles and bending fatigue resistance can be improved via isotropic superfinishing.

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Precision metallic components undergo a number of manufacturing processes that can include milling, grinding, and honing as a part of final surface preparation. A primary goal of these processes is to generate an improved final surface finish that conforms to the demands of the part’s long-term performance requirements. However, the surface texture that these machining processes create is less than optimal as evidenced by the common requirement of a break-in or run-in procedure or period.

As a basic example, imagine a set of paired spur gears that have undergone post-carburization grinding to remove quench distortion. A magnified inspection of any one of the ground teeth will reveal a grind line pattern on the tooth’s flank. The pattern consists of parallel rows of asperities corresponding to the direction of the grinding. Traditionally, the gears would be considered complete and ready for use. (Figure 1)

Figure 1: Microscopic view of a machined surface.

However, when placed into operation for the first time, the paired spur gears will commence a break-in period during which the asperity peaks on opposing tooth flanks contact one another. Initially, there will be poor load distribution and high Hertzian contact stress due to concentrated asperity-to-asperity interaction.

If the contact stress loading exceeds the plastic deformation limit of the steel, asperities will be aggressively and uncontrollably removed by abrasion, sheering, breakage, or peening-over. As asperities are broken off, their resultant debris is carried in the lubricant stream. This debris will accelerate and increase surface damage to gear teeth and bearings until it is, hopefully, removed by the lubricant filtration system.

The critical break-in period can be monitored using a thermocouple that will show a continuous, rapid increase of the lubricant temperature for a machined surface. The temperature increase is the result of parasitic friction caused by the asperity-to-asperity contact. The temperature will be at its peak when break-in is mostly complete and will gradually decline to an equilibrium point once regular asperity contact has stopped. (Figure 2)

Figure 2: Machined surface, asperity-to-asperity interaction.

Because break-in occurs in an aggressive and uncontrolled fashion, the resulting post-break-in tooth flank surfaces are locations of significant tribological and metallurgical interest. The characteristics created on the tooth flanks during the break-in phase can result in adverse consequences to longevity and engineered performance of the gears.

In areas where the asperities’ peaks have been torn off, broken off, or snapped off during break-in, the resulting surface divot can morph into a future metal-loss initiation site. For an analogy, consider the divot as a small, springtime pothole in an asphalt roadway. As the wheels of continuously passing traffic pound the pothole, the result is more-and-more asphalt loss and a growing diameter. Now imagine that the wheels of passing traffic are replaced with gear teeth coming into and going out of mesh, and the surface divot represents the pothole. On the gear flank, metal loss and pit propagation represent asphalt loss and pothole growth.

Let’s consider a second consequence of traditional asperity break-in — the inability of the process to improve the surface finish near or in fillet radius areas resulting in residual grind lines. These residual grind lines function similarly to the scribe lines that one uses to cut glass. By applying pressure, the glass snaps on the line. On a gear, these residual grind lines under bending fatigue loading serve as crack initiation locations. However, if all the grind lines have been removed in a controlled fashion, longevity is increased because failure initiation sites are no longer present. (Figure 3)

Figure 3: Lubricant temperature during the break-in period of a honed surface versus an isotropic superfinished surface.

When a part has been prepared to create an isotropic superfinish, the grind line asperities are removed in a controlled and predictable fashion across the tooth flanks and through the root fillet area. As described in the October 2015 Materials Matter column, the word isotropic means uniform or invariant in all directions. An isotropic superfinish has no unidirectional pattern as seen on a ground surface.

Under stress loading from bending fatigue, this type of surface is a significant advantage — as there is no initiation site to foster cracking or tooth failure. If the part has been isotropically superfinished via a process such as REM’s ISF® Process, the surface is not completely smooth; rather, it has a random texture that has been found to be excellent for lubricant retention. In fact, the texture wets better during lubrication because the oil can spread more uniformly as a result of asperity peak removal. With an asperity-free surface, the threat of peak asperity breakthrough of the lubricant layer is virtually eliminated.

Returning to the previously mentioned example of paired isotropic superfinished spur gears, when going into mesh for the first time, it is clear that the opposing tooth flanks have a vastly increased surface contact area. Whereas grind line asperities concentrate Hertzian contact stresses into a few small, asperity-to-asperity areas, the asperity-free surface of the isotropically superfinished tooth flanks more efficiently disperse the Hertzian contact stress over a wider area, thereby greatly reducing contact stress in any one location. (Figure 4)

Figure 4: Pothole with damage propagation.

For a tangible analogy to this operating regime change, consider an individual who is attempting to cross a frozen pond with very thin ice. It is important to uniformly disperse body weight across the thin ice by lying down. Even though the individual’s body weight remains the same, the more efficient distribution of this body weight across the thin ice significantly reduces the downward force in any one location — and greatly enhances the chances of escaping without falling through. A similar situation is occurring during gear mesh when tooth flanks have been isotropically superfinished. The contact stresses at the moment of mesh are more uniformly dispersed across the tooth’s flank as opposed to being concentrated into a few asperity peak locations.

Referring back to the temperature curve charts for the break-in procedure, compare the isotropic temperature curve with the honed break-in phase curve. Note that the isotropic surface has a gentle temperature rise to a steady-state operation temperature that is 10⁰C cooler than the traditionally ground surface. Also note the complete absence of a temperature spike for the isotropically prepared surface. With no asperities present, there is no build-up of excessive parasitic friction and no temperature spike related to the excessive friction generated during asperity removal. Simply stated, isotropically finished parts are asperity-free and break-in free from the first moment of use. (Figure 5)

Figure 5: Microscopic view of an isotropic superfinish.

As discussed in previous Materials Matter columns, chemically accelerated vibratory finishing processes (such as the ISF Process) are ideal and unique methods of improving gear and bearing surfaces to avoid the otherwise inevitable break-in cycle and all of its potentially deleterious effects. Coupling this with the benefits of reduced Hertzian contact stress, improved load distribution, and increased resistance to bending fatigue, isotropic superfinishing can be beneficial to many component applications.

* ISF is a registered trademark of REM Chemicals, Inc.
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received a B.A. from The University of Connecticut and an M.S. in environmental sciences from Long Island University. He has been with REM Surface Engineering since 1989 and currently serves as a sales engineer and as REM’s product manager. Since 1978, Nebiolo has been an active member in the National Association for Surface Finishing (NASF) where he has represented the Connecticut chapter as an NASF national delegate and is the 2010, 2014, and 2015 recipient of the NASF National Award of Merit. From 1996 to 2000, he served as one of SME’s Mass Finishing technical training program instructors. He has published and presented dozens of technical papers and is the author of the SME Mass Finishing Training Book. Nebiolo can be reached at bnebiolo@remchem.com.