Gray spotting

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Gear with micro-pitting

Gray spots are signs of wear and tear in areas of highly stressed metallic components close to the surface. It occurs primarily on gear wheels in transmissions , but the sliding partner camshaft  - tappets of modern valve trains are also affected.

The affected area on the tooth flank appears dull gray to the naked eye. Under high magnification it can be seen that a large number of tiny breakouts and pores are the cause of the gray appearance. Mixed and sliding friction occurs at the contact surfaces under high loads . The areas close to the surface are plastically deformed and microcracks form. Under certain circumstances, micro-pitting can lead to deeper cracks and then to large-scale breakouts on the gearwheel (called pits or pitting ). The micro-pitting also causes a change in the tooth flank shape and can change the gear dynamics and the noise behavior.

Microspotting occurs when the contact between two components is subject to high sliding speeds and low lubricant film thicknesses (see: Lubrication ).

The most important causes for the development of gray spots are:

  • high load
  • low speeds or high lubricant temperatures (they result in lower lubricant film thicknesses)
  • unfavorable tooth geometry (which leads to high sliding speeds or high local loads at individual points on the tooth flank)
  • Great roughness of the tooth flank (the greater the roughness, the greater the lubricant film thickness must be in order to avoid metallic contact)
  • unfavorable lubricants ( additives in the oil can have a very strong influence on the tendency to form micro-stains ( corrosion ), and low viscosities also result in lower lubricant film thicknesses)

Micro-pitting as gear damage is rarely found in motor vehicle transmissions. It occurs mainly in highly stressed industrial gearboxes, e.g. B. in rolling mills, mill drives or in wind turbines .

Since the geometry, the load and the speed are mostly determined by external requirements, the choice of a suitable lubricant is crucial. High-performance lubricants are tested in a standardized test for their suitability for avoiding gray stains.

The roughness on the tooth flank can be measured with optical ( focus variation or AFM ) or tactile measuring methods ( profilometer ). Due to the complexity of the task, area measurement ( EN ISO 25178-2) is preferable to profile measurement.

The term "micropitting" or "gray staining" is used in English for the term gray staining. The ISO / TR 15144-1: 2010 standard deals with the determination of gray spots.

literature

  • Berthold Schlecht: Machine elements 2. Pearson Education, Munich 2010, ISBN 978-3-8273-7146-1 .
  • Karl Sommer, Rudolf Heinz, Jörg Schöfer: Wear of metallic materials. Assessing forms of appearance reliably, Springer Vieweg Verlag, Wiesbaden 2012, ISBN 978-3-83482-463-9 .
  • Fritz Klocke, Christian Brecher: Gear and transmission technology. Design - production - investigation - simulation, Carl Hanser Verlag, Munich 2017, ISBN 978-3-446-43068-6 .

Individual evidence

  1. ↑ H. Winter, G. Schönnenbeck: Gray spots on case-hardened gears. Fatigue of the material edge layer with possible consequential damage. Online (accessed August 2, 2018)
  2. ↑ E. Bauer: Typical flank fatigue damage on case-hardened spur gears. Online (accessed August 2, 2018)
  3. ↑ Research Association for Drive Technology: Brief test procedure to investigate the influence of lubricants on pitting in gears. Online (accessed August 2, 2018)
  4. ↑ Facts, worries and considerations about the cause of damage storage, micro-pitting and high-performance oil (accessed on August 2, 2018)
  5. ↑ Khashayar Nazifi: Influence of the geometry and the operating conditions on the gray spotting of large gears. Dissertation at the Ruhr University Bochum, Bochum 2010, online (accessed on August 2, 2018)
  6. ↑ ISO TR 15144: Method A and B of the micro-pitting calculation in KISSsoft (accessed on August 2, 2018)

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