GETTING CAVITATION DAMAGE UNDER CONTROL

The components and materials used in ocean-going vessels undergo the most rigorous testing in the world. With the right service intervals and by using duly approved service parts and expert service technicians, there is every probability that thrusters, for example, can operate for 30 years or more. At some point, however, problems can arise due to wear and similar phenomena – not only in the electrical but also in the mechanical parts. Cavitation damage is such a problem, and we, ATA’s Gear Doctors have spent countless research hours getting to understand the phenomenon and also finding remedies for our customers.

Although usually associated with propellers and pumps, cavitation damage is sometimes also observed in power transmission components. In gears, cavitation damage is believed to be caused by rapid oil flow through the mesh and the resulting pressure variation, the damage mechanism being the same as in propellers/pumps. The symptoms and problems arising may likewise be similar: vibration, noise and, in the worst-case scenario, loss of performance.

Cavitation damage in gears varies a lot in extent, severity – meaning depth – and location. In one-way drives, cavitation usually appears on the unloaded side, but it may also appear on the loaded side.

Cavitation damage

An example of cavitation damage in a bevel gear of a thruster (and an example of propeller cavitation damage as reference):

Cavitation damage may occur anywhere on the tooth: on the tip, the flank or the root area. ATA’s Gear Doctors classify all cavitation damage into three categories. ’Light’ cavitation damage (STAGE 1) initially appears on the tooth as a kind of a ‘dull’ area, which may look like a locally sandblasted surface area. In ‘more advanced’ cavitation damage (STAGE 2), clear cavities – or craters – have formed, which clearly penetrate below the tooth surface. Cavitation damage that is considered ‘Serious’ (STAGE 3) occurs when the cavitation craters have become so deep that the gear’s structural load bearing capacity is compromised.

STAGE 1


STAGE 2


STAGE 3

When and how can cavitation damage be repaired?

There are various techniques and methods to repair cavitation damage. The success of the repair depends greatly on the extent and location of the damage and requires profound knowledge of the application and gear design. Depending on the case, repair may be possible by remachining the teeth and/or by removing the damaged sections by localized milling or grinding operations. Identifying the loaded flanks is an important first step in the repair, and measures to prevent cavitation damage from recurring should always follow. ATA Gears has decades of experience of repairing gear damage, and today, such repairs are an integral part of the ATA ReViveTM service.

To assess the feasibility of repair, the following rough guidelines can be stated:

STAGE 1 → Can usually be repaired
Repair is usually relatively simple (remachining or local grinding/polishing)
The power transmission capability of repaired gears usually remains the same

STAGE 2 → Can possibly be repaired
The repair may be more challenging, success depending on various factors
The power transmission capability of repaired gears may be reduced

STAGE 3 → Probably cannot be repaired
In some cases repair may be possible but may require significant power reduction in use (i.e. temporary ’emergency repair’)

ATA Gears - Jesse Rontu - BlogJesse Rontu, Chief Gear Engineer

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This is a shortened version of the whitepaper published for SMM 2024. If you are more interested in our recommendations and options regarding the repair/prevention of cavitation damage, feel free to contact ATA’s Gear Doctors to get that special whitepaper on the topic.