Pitting - risk factor for gearbox failures

Causes & avoidance strategies

Causes & avoidance strategies

Pitting can be translated as "pitting", but is also referred to as "pitting" or "pitting corrosion" by experts. Pitting is mainly caused by friction losses on gears and rolling bearings and can be recognised by material breakouts and the formation of microcracks on the surface of rolling elements and their raceways. When viewed with the naked eye, the areas have a matt structure.

How does pitting occur on metal surfaces?

All technical systems consisting of so-called passive layer-forming metals can be affected by pittings or micropittings. These are usually high-alloy chromium-nickel steels, aluminium and titanium alloys or nickel-based alloys. Passivation is a thin oxide layer on the surface that protects the material from further oxidation. If this passive layer is locally destroyed by tribological, abrasive, erosive or other mechanical stress, pitting occurs. Possible causes of damage to the protective oxide layer include foreign bodies in the respective lubricant, inadequate lubrication or excessive humidity.

The chemical properties of the surface environment then reduce the speed at which the affected areas heal. A pitting nucleus is formed, from which the dissolution of the metal progresses in depth. Over time, further chemical processes are then responsible for the repassivation of the protective oxide layer coming to a complete standstill and corrosion eating its way unhindered into the depths of the metal. This leads to stable hole growth, which ultimately results in irreversible damage to the gears or rolling bearing.

Pitting formation on rolling bearings and gears due to tribological stress

It is known from tribology that pitting can be caused by a localised exceeding of the material strength, defined by the Hertzian pressure. This usually occurs on the tooth flanks of gears or in rolling bearings between the rolling element and the outer or inner ring of the bearing. In the case of surface pressure, the greatest stress of the component does not occur on the surface, but at a certain depth of the material. This must be taken into account when designing and manufacturing the respective components.

Other factors for the formation and spread of pittings and micropittings are

• material-related: the surface hardness and quality, as well as the depth of hardening
• shape-related: Errors in the flank shape of gears and in the peripheral speed
• Environmental factors: insufficient oil viscosity, unsuitable additives in the oil, incorrect temperature, high humidity

Pitting caused by pitting can be seen very easily with the naked eye on railway tracks. These must be reground regularly to maintain their operational safety.

How does pitting damage manifest itself?

In rolling bearings, pitting damage changes the operating behaviour of the bearing arrangement. If you want to examine the damaged bearings more closely, you need to check not only the bearing itself, but also the components of the surrounding parts, the sealing and the lubrication. Environmental and operating conditions can also play a role, such as excessively high temperatures or excessive humidity. If gearboxes, roller bearings or gear constructions are not checked and maintained regularly, irreparable damage will occur in the long term, necessitating a complete replacement and resulting in very high costs. A similar damage pattern can occur with unusually long running times and permanently high loads.

How can pitting be prevented?

Basically, it can be said that a clean design, the right material for a specific application and precise dimensions of the individual components are the first prerequisites for preventing pitting damage. In mechanical engineering, there are recognised and permissible values for the maximum possible surface pressure. The shapes and profiles of the components must be calculated precisely to ensure that these values can actually be adhered to. Otherwise, damage can occur very quickly.
Other essential measures to prevent pitting are regular inspection, proper maintenance and servicing of rolling bearings, gears and gearboxes in accordance with the manufacturer's specifications. It can also help to optimise the sealing system, filter the lubricant more frequently to remove any foreign bodies or generally use a more suitable lubricant.