Bearings literally keep the world turning with any two components operating in contact with each other needing them. Vessels are no exception with the majority of ships running a traditional shaft based propulsion system with an oil lubricated stern tube containing white metal bearings.
As a plain rather than a rolling element, or ball bearing – good alignment, adequate cooling and lubrication is essential to maintain a lifetime of performance.
Being essential to ship operation and safety – white metal bearings are a particular area of interest for Classification Society Surveys.

White Metal Bearings

White metal bearings as the name suggests are light in colour owing to the running surface of the bearing being made from lead historically and now tin based alloys. The backing element of the bearing is a harder shell made from steel which has the softer tin-based lining material deposited on it using a rotational moulding process with molten metal known as babbiting. The stiffer backing material provides structural support for the bearing while the softer lining gives optimum friction and wear properties.

Designed, installed and maintained correctly – white metal bearings can last the lifetime of a vessel – perhaps as long as 25 years. While the wear surfaces of the bearings are relatively thin compared to their thickness, when operating correctly the shaft is supported on a full fluid film which prevents any contact between the surfaces other than when they are at rest.

Bearings will be fitted within the vessel stern tube as well as along the shaft line within the machinery space in standalone pedestals, termed line-shaft bearings.

Performance

Achieving good performance and life from the bearings is dependent on several factors which include:

  • Correct design and modelling of the shaft line – bearings are built with slope and offset as required to maximise shaft support and load on the bearing itself.
  • Accurate installation and alignment of the bearings – installations differing from the required configuration as a result of bending or hull distortion when afloat can lead to overload.
  • Adequate system cooling and filtration – cleanliness of the lubricating oil is essential to maintain its load bearing and anti-friction properties. Debris and heat will contribute to accelerated wear. Some systems rely on thermal convection alone for circulation, others include pressurised oil feeds.
  • Seal maintenance – any seawater entering the bearings will cause corrosion and increased surface contact. In extreme cases this can overheat the bearings causing the shaft to seize resulting in damage to the bearings and shaft. This will usually result in emergency dry dock, bearing and possibly shaft replacement

Supporting Systems

A vessel fitted with white metal bearings will include several supporting systems which would be included within an on-board survey to assess system condition.

Typically, this would comprise:

  • Oil pumping, filtration and cooling system
  • System seals
  • Thermocouple probes providing live operating temperatures
  • Bearing wear down measurements

Common Problems

Most common problems are seen in white metal bearings as a result either of system misalignment or inadequate lubrication. Some instances of reduced performance and failure have also been linked to the switch to alternative, emulsifying and EAL oils where different oil properties have led to system overloading. 

An all too common fix seen for a leaking stern tube is to make use of a thicker or emulsifying oil to manage incoming seawater or outgoing oil. In cases where reduced thermal conductivity of these products has not been taken into consideration – accelerated wear or bearing failure can result.

Bearing Survey

Within a bearing survey, AtZ would gather all available operating information from the vessel, before planning a more detailed inspection. This would include:

  • Onboard monitoring data and drawing review
  • Pumping and cooling system inspection
  • Checking bearing clearance (where possible afloat / in dock)
  • Checking lubrication log and estimating any leakage rates
  • Draw conclusions on system condition and remaining life
  • Support required parts, service or upgrade proposal

A full and detailed survey of the white metal bearings and liners could usually only be completed in dry dock with partial or full removal of the tail shaft and pedestal covers.

We would always recommend a full bearing inspection with replacement as required should the shaft be withdrawn and replacement of all seals.


FAQ’s

There is debris in the lubricating system oil, but we have fitted additional filtration to remove this. Is this a problem?

Yes.
Debris within an oil system is sign either of a component failed and breaking down, or high wear. The top of each pedestal can be removed to carry out a basic visual inspection of the bearings. It is worth analysing the particles to establish the material being deposited, but preparations needs to be made for either a line shaft bearing change or tail shaft withdrawal.
Line shaft bearing shells can be re-metalled and replaced in service, but most stern tube bearings will require dry dock and shaft removal to change.

We want to change to an EAL oil – is this a straight swap?

No – each part of the system needs to be considered for compatibility first. A system flush may be required, and some seals and other equipment might need to be changed for a compatible material.
AtZ can review your system and advise any required steps for upgrade.

Our vessel is newly constructed and while all appears in order, we have some increased system bearing temperatures in heavy weather. What could be the cause?

Bearing temperatures are related directly to load, and in cases the temperature rises rapidly, this is usually indicative of overloading. We have seen many cases in which equipment has been installed with poor alignment accounting for loaded parameters of the vessel which may only become apparent in heavily loaded conditions. It is also possible that a leaking aft seal causing water ingress could affect lubricant performance resulting in temperature spikes.
An urgent review of the system together with an alignment check would be the best course of action to identify root cause – and could be offered as a complete solution from AtZ.

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