Preventing disasters: A roadmap to understanding gear and pinion failures

While consumables, such as liners, are replaced frequently year over year, the mill structural components may last for decades with proper installation, operation and maintenance. However, this proves most difficult when it comes to open gears which are the second most expensive component in a grinding mill and the most vulnerable to failure. Hence, reliability of a grinding mill’s gear has increasingly become the subject of concern in both small or large operations to ensure production and profitability goals are achieved.

Furthermore, as a high cost long lead item, spare gears are very rarely acquired in advance by mining companies, which puts a lot of strain on maintenance supervisors and planners to develop solid preventive maintenance and periodic inspection plans to avoid gear and pinion failure. Hence, a close look at different failure modes allows for a better understanding of the cause and effects as well as how these issues can be corrected when uncovered late.

Failure modes

A failure mode is defined as a cause of failure of an equipment, component or system. Potential failure modes depicted here are based on known industry standard AGMA 1010-F14 for Appearance of Gear Teeth – Terminology of Wear and Failure. While simplified for the purpose of this article, to properly understand how common modes of gear failure are identified and measured, it is important to distinguish Class, Mode and Degree.

Class

A category regrouping different general modes. These include wear, scuffing, plastic deformation, Hertzian fatigue, cracking, fracture and bending fatigue.

Mode or General Mode

Known as general modes, is a family of modes classified within the same Class. As such, adhesion, abrasion and scaling are all categorized as wear.

Degree or Specific Mode

A degree is the level or extent to which the mode is present while a specific mode is an exact classification of a failure mode. For more common Classes such as wear and scuffing, the degree of different modes is classified as either mild, moderate or severe. Other Classes such as plastic deformation, Hertzian fatigue and bending fatigue will be defined with specific modes. Classes such as cracking and fracture do not have their modes classified under a degree nor a specific mode.

Most common failures

Gear failures are often a combination of one or multiple failure modes which can be retraced to one or many root causes related to either improper installation, operation or maintenance. In most cases, failures could be prevented through good practice. For better comprehension, the below summary provides a brief overview of the most common failures recorded on operating mill gears and pinions through empirical observations and experience with the equipment by paying close attention to terminology as described in AGMA 1010.

Wear

Modification to the gear tooth surface resulting in removal of material. Different general modes of wear have different appearances but all result in the disappearance of machining marks on teeth flanks which may serve as an early indication. Different sources of wear can include minor misalignment, loss of lubricant and contamination. While pinion wear can progress significantly faster than wear on the gear, due to the higher number of rotations, signs of wear to the gear will start to appear through transmission from a pinion with moderate wear. Pinion flipping or replacement is recommended to avoid transmitting additional wear to the gear.

Scuffing

Severe adhesion that causes metal transfer from one tooth surface to another due to welding and tearing. Scuffing can occur through lack of gear lubrication, with the worst-case being high heat, causing fusion and material transfer from the gear to the pinion described as a crevasse and weld fillet on one or the other of the mating flanks. Pinion and gear flipping are required in the event of this failure which may exhibit excessive noise and vibrations.

Plastic deformation

A very frequently recorded mode of plastic deformation is indentations. Indentations is damage in the appearance of small tearing caused by foreign material that becomes trapped between the mating flanks during the mesh. Damage to the gear progresses rapidly when size and quantity of indentations on the pinion become significant. This is commonly noticed on the ends of the gear face due to contamination, and can be avoided through proper gear guard sealing.

Hertzian Fatigue – Macropitting and Spalling

Macropitting occurs when craters appear at the surface or closely beneath the surface. Progressive macropitting and point surface macropitting can form significant craters in the tooth flank that lead to spalling, resulting either in material tearing off and/or cracking. Contaminants in lubricants can promote pitting but the main cause is often design and manufacturing related. Regardless, its presence should not be tolerated and pinion flipping, or replacement is required. If localized presence of macropitting is found on the gear, surface grinding for profile relief is required but in the case of macropitting or spalling, a gear flip is recommended.

Cracking

Cracking is a separation event caused by breaking which is  observed at the surface of the tooth flank. Cracking may have many sources whether design, manufacturing, installation or operation and it is not always simple to narrow down its root cause. Indeed, cracks may occur from excessive tooth surface damage such as macropitting, where cracks propagate from, giving it a “dirty” look. Cracks may also propagate from beneath the surface such as subcase cracking in surface hardened gears or through bending fatigue stress when under the influence of repeated stresses, giving it a “cleaner” look. Hence, this type of look is an easy indication on whether the failure mechanisms have originated from the surface or below the surface. Cracks found through visual or non-destructive testing such as wet MPI or Eddy Current must be attended to and corrective actions taken. A pinion where cracking is observed, regardless of order of magnitude, poses a threat to the gearset and must be replaced to eliminate all risk. However, cracks found on gears must be treated on a case by case basis to determine the proper corrective action. When the crack is believed to be sub-surface, excavation is not recommended as cracks may be larger below the surface. On the other hand, surface cracks tend to become smaller in length below the surface after excavation. However, in all cases, grinding the surface of the area where the crack is located is preferred to avoid stresses applied to the area during pinion to gear mesh. A pinion with cracks should be replaced.

Fracture

Much like cracking, a fracture can come from different sources but is often the result of overload, such as a pinion misalignment for instance, where it may fail from plastic deformation or fracturing. Fractures may be brittle or ductile.  A brittle fracture is preceded with little plastic deformation, while it would be more significant in the case of a ductile fracture. In the case of fatigue failure, fatigue cracks grow over time which ultimately leads to fractured teeth on gears and pinions. Although a brittle fracture may not provide any observable warning, ductile fractures can be detected through observation of plastic deformations over time. Likewise, fatigue cracks are also an early indication of fracture. Fractured teeth, whether on the pinion or the gear, can severely damage the gearset and grinding mill.

Corrective actions

Pinion replacement

When significant damage is observed on the pinion teeth, replacement eliminates all risks of damaging the gear. If no spare is available and the operating pinion has a virgin flank, flipping could also be done as an alternative. However, the clutch hub and inching drive hub would need to be inverted from one end to the other. Afterwards, the pinion must be aligned to the gear through measuring the gear root clearance, not the backlash. This is due to the backlash being unreliable due to profile wear on the used gear and/or the used pinion. Proper attention to the pinion to gear offset must also be considered. This is process is referred to as a static alignment.

Lapping Compound

When unusual noise and/or vibrations are detected at the pinion to gear mesh, probable causes range between a simple pinion misalignment to mild to moderate profile wear to the pinion and gear profiles. Therefore, proper mechanical verification to ensure proper pinion alignment should be carried out along with a visual inspection of the active flanks. When a static alignment proves ineffective to remove vibrations, applying a lapping compound during operation will correct the profile mismatch. This is process is referred to as a dynamic alignment.

Tooth grinding

When cracks are found to the gear teeth through any non-destructive testing methods, profile relief through grinding with a buffing disk allows  removal of contact in the affected area and therefore, remove all stresses applied during meshing. Excavating cracks is also not recommended unless used to smooth sharp edges due to the cracking. Furthermore, it’s recommended to remove a pinion with cracks rather than attempting tooth grinding.

Gear flip and replacement

A gear flip is recommended when a significant number of cracks are detected on its teeth’s active flanks. Flipping the gear to use the flank opposite to the cracks will result in these sustaining compressive bending and therefore, avoid further propagation. This operation needs to be carried out before cracks evolve into a fracture, when flipping is no longer possible. When multiple failure mechanisms having resulted in cracking are detected, the gear and pinions need to be replaced and aligned. It is also recommended to perform a complete mechanical verification and mill survey to identify and eliminate unnecessary issues at the time of performing the gear replacement.

Metso outotec expert solutions bringing safety and sustainability

Although many failure modes exist, causes can be narrowed down to contamination, improper lubrication and misalignment, both of which could be avoided through the implementation of good operational and maintenance practices. Metso Outotec is a provider of expert solutions to ensure reliability, integrity and longevity of a grinding mill’s most critical component. Indeed, through close monitoring with the use of advanced instrumentation, periodic inspections and preventative maintenance services, our service offering holds no equal to bring safety and sustainability to your operation.