Pitting – Risk factor for gearbox failure

Causes & avoidance strategies

Pitting in professional circles is called "localised corrosion". Pitting mainly comes from a loss of friction on gears and roller bearings and can be recognised by material eruptions and the formation of micro-cracks on the surface of the rolling elements and their raceway. When viewed with the naked eye, the points have a matt structure.

How does pitting occur on metal surfaces?

All technical systems, which consist of so-called passive layer-forming metals can be attacked by pitting or micro-pitting. Usually, these are high-alloy chromium-nickel steels, aluminium and titanium alloys or nickel-based alloys. Passivation is a thin oxide layer on the surface, which protects the material from further oxidation. If this passive layer is destroyed locally by tribological, abrasive, erosive or other mechanical stress, pitting starts to occur. Possible causes of damage to the protective oxide layer are for example, foreign bodies in the particular lubricant, a lack of lubrication or too high humidity.

The chemical properties of the surface environment then reduce the rate of curing the affected points. The result is a pitting seed, from which the dissolution of the metal proceeds into the depth. Over time, other chemical processes are then responsible for ensuring that the re-passivation of the protective oxide layer stops altogether and the corrosion eats unhindered into the depth of the metal. There is stable pit growth leading in the end to irreversible damage to the gears or the roller bearings.

Pitting formation on roller bearings and gears caused by tribological stress

We know from tribology that pitting can be caused by the material strength, defined by the Hertzian contact stress being exceeded. This occurs usually at the gear flanks of gears or in roller bearings between the rolling elements and the outer or inner ring of the bearing. In the surface pressure, the greatest stress on of the component does not occur on the surface but actually at a certain depth of the material. This must be taken into account in the design and manufacture of the particular components.

Other factors for the emergence or spread of pitting and micro-pitting are:

due to the material: the surface hardness and quality, as well as the depth of the hardness penetration
due to the form: fault in the flank form of gears and in the peripheral speed
due to the environment: lack of oil viscosity, unsuitable additives in oil, wrong temperature, high humidity

By the way, the dimples caused by pitting can be seen very easily on railroad tracks with the naked eye. These must be reground regularly to maintain their operational reliability.

How does pitting damage manifest?

In roller bearings, pitting damage causes a change in the performance of the bearing. If the damaged bearings are to be examined more accurately, you need to check not only the bearing itself, but also the components of the adjacent parts, the seals and lubrication. Another cause may also be environmental and operating conditions, for example too high temperatures or too much humidity. If gearboxes, bearings or gear constructions are not checked and maintained regularly, there will be long-term irreparable damage, making a complete replacement necessary and bringing with it very high costs. Similar damage may occur in the case of unusually long running times and permanently high loads.

How can pitting be prevented?

Basically, a clean design, the right material for a particular purpose, and exact dimensions of the individual components are the prerequisite against pitting damage. In mechanical engineering there are recognised and permitted values for the maximum possible surface pressure. Forms and profiles of the components must be calculated accurately to ensure that these values can actually be adhered to. Otherwise, damage can occur very quickly.
Other significant measures to prevent pitting are regular inspection, proper servicing and maintenance of roller bearings, gears and gearboxes to the manufacturer's instructions. Optimising the sealing system, more frequent filtering of the lubricant to remove any foreign bodies or basically using a suitable lubricant all help too.