Enhancing the Performance of Gears with Isotropic Superfinishing

Grinding and machining processes are commonly the final forming step for gears. However, many studies have shown that the surface roughness and surface texture created by grinding processes is detrimental to optimal gear performance. Given ever increasing performance demands, gear designers must look to advanced surface treatments such as isotropic superfinishing in order to increase gear performance.

Isotropic Superfinishing Compared to Grinding and Mirror-Polishing

Isotropic superfinishing is a process that is capable of producing a low surface roughness (typically less than 4 µin/0.1 µm), non-directional micro-texture. It is this low surface roughness micro-texture that separates isotropic superfinishing from grinding or machining processes as well as from mirror-polishing (non-textured) processes. The periodic texture of ground surfaces has been found to be susceptible to various gear failure modes. The nature of the ground surface itself has been theorized to be the cause of this increased failure risk due to both peak asperity interaction and increased potential for hydraulic cracking leading to micropitting1.  Conversely, mirror-polished surfaces with extremely low surface roughness values have been found to have greater risk of scuffing than isotropic superfinished surfaces2. See Figure 1 for a comparison of these three surface types.

Figure 1: A ground surface showing directional machining lines and distressed metal zone (left), a “mirror-polished” surface with little to no surface texture (middle), and an isotropically superfinished surface showing a non-directional surface texture (right).

Validated Benefits of Isotropic Superfinishing: Contact Fatigue Resistance, Scuffing Resistance, and Increased Load Carrying Capacity

Isotropic superfinishing has been found to have numerous performance benefits in gearing applications. Some of the more interesting performance benefits for gear design consideration and problem resolution include increases to:

  • Contact fatigue resistance
  • Scuffing resistance
  • Load carrying capacity

Isotropic superfinished surfaces have been shown to have a 3x increase in pitting life as compared ground surfaces3. Micropitting FZG testing demonstrated superior results for isotropically superfinished specimens as compared to ground specimens. Isotropically superfinished FZG gears completed all designated loading stages as well as an endurance stage without any visual evidence of micropitting and <1 µm of profile form deviation4.

Scuffing testing by the University of Cardiff produced impressive results while also establishing the importance of the micro-texture as compared to mirror-polished surfaces2. Isotropically superfinished scuffing specimens survived ~80% higher operating loads (the maximum rig loading) as compared to ground specimens during normal testing and completed an additional 30 minute endurance cycle at maximum load without scuffing. Additional scuffing and scoring evaluations have been completed by various aerospace and rotorcraft OEM’s on isotropically superfinished specimens, showing similar performance enhancements5, 6.

Increases to contact fatigue resistance and scuffing resistance are potential upgrades to existing or new gear designs, allowing for greater operating safety margins and longer component life. Increases to load carrying capacity also offer increases to operating safety margin relative to shock loads and the like, but they also offer the ability to modify overall system design. Multiple companies have pursued increases to load carrying capacity (sometimes referred to as power density) in order to upgrade gear systems relative to input power increases, material and weight reductions, or a combination of both. The advantages of increased load carrying capacity can be very useful in weight sensitive applications including the aerospace and wind turbine industries as well as ground vehicles. See Figure 2 for an example of an isotropically superfinished gear.

Figure 2: Isotropically superfinished gear by the ISF® Process.

Isotropic Superfinishing as a Gear Repair Tool

In addition to the benefits of isotropic superfinishing on new gears, the technology is capable of repairing damaged gears to a “better than new” condition7. Isotropic superfinishing has been shown to be effective in repairing gears from wind turbines, helicopter8, 9, and heavy equipment transmissions (see Figure 3 for repair examples). Isotropic superfinishing is capable of repairing damage types such as:

  • Micropitting
  • Scuffing
  • Fretting
  • Corrosion
  • Abrasive Damage
  • Particle Impact/Indentation (FOD) Damage

Figure 3: Damaged and repaired Boeing CH-46 Sun Gear (top left & right), damaged and repaired 1.5 MW Wind Turbine Sun Pinion (bottom left & right).

Isotropic Superfinishing and Complex Applications

Isotropic superfinishing has another benefit in that it is able to simultaneously improve multiple features and process complex components/applications.  Unlike grinding and machining operations, isotropic superfinishing processes are capable of finishing multiple gear features in the same processing cycle. Examples include double helical gear designs and planet gears with integral bearing races.

Size is generally not an issue as the technology has demonstrated the capability of processing gears smaller than the tip of one’s figure and possessing a diametral pitch of 96. Additionally, double helical bull gears weighing > 10,000 pounds (~4,500 kilograms) and annuluses with diameters of >7 feet (roughly 2 meters) have been successfully processed. Nitrided components can be processed for the purpose of removing the detrimental white layer and improving the surface finish. Lastly, exotic and high-hardness steels are capable of being processed without issue. Specific processes have been developed for many of the most advanced gear steels including Pryowear® 53 (AMS 6308), Pyrowear® 675 (5930), Ferrium® C61 (AMS 6517), and Ferrium® C64 (AMS 6509), as well as numerous other gear steels, stainless steels and other alloys.

Conclusion

Fundamentally, there are many potential advantages, benefits, and applications for isotropic superfinishing in the gear industry. Numerous technical studies and case studies exist to justify the consideration of this type of technology for new and existing gear designs. Performance improvements and operating cost reductions are common results (and reasons) for applying the technology. In the ongoing effort to advance gear technology, isotropic superfinishing should be a growing tool for all gear designers and engineers.

References:

  1. Errichello, R., “Morphology of Micropitting,” AGMA 11FTM17, 2011.
  2. Snidle, R. W., Alanou, M. P., Winkelmann, L., Michaud, M., “Effect of Superfinishing on Scuffing Resistance,” ASME 2003 Design Engineering Technical Conference, Chicago, IL, September 2003.
  3. Michaud, M., Sroka, G., Winkelmann, L., “Chemically Accelerated Vibratory Finishing for the Elimination of Wear and Pitting of Alloy Gear Steels, “ AGMA 01FTM7, 2001
  4. Winkelmann, L., El-Saeed, O., Bell, M., “The Effect of Superfinishing on Gear Micropitting, Part II”, AGMA 08FTM10, 2008.
  5. Niskanen, P. W., Hansen, B., Winkelmann, L., “Evaluation of the Scuffing Resistance of Isotropic Superfinished Precision Gears,” AGMA 05FTM13, 2005.
  6. Ehinger, R. T. and Kilmain, C.J., “Evaluation of Isotropic Superfinishing on a Bell Helicopter Model 427 Main Rotor Gearbox,” American Helicopter Society 63rd Annual Forum, Virginia Beach, VA, May 2007.
  7. Michaud, M., Sroka, G., Benson, R., “A Novel Approach to the Refurbishment of Wind Turbine Gears,” AGMA 10FTM03, 2010.
  8. Rao, S., McPherson, D., Sroka, G., “Repair of Helicopter Gears,” AGMA 05FTM15, 2005.
  9. Rao, S., McPherson, D., Sroka, G., “Repair of Helicopter Gears-Phase II”, 10th Joint DoD/NASA/FAA Conference on Aging Aircraft, Palm Springs, CA, April 2007.

ISF is a registered trademark of REM Chemicals, Inc.

For more information visit www.remchem.com

Author of the article

Justin Michaud , is the CEO of REM Surface Engineering with expertise in Lean Manufacturing, operational and financial management, and strategic planning; Justin is a graduate of the University of Notre Dame’s Mendoza College of Business where he was also a member of the university’s men’s soccer team. He can be reached at jmichaudremchemcom.