The stern tube is at the heart a traditional shaft driven propulsion system. It provides the conduit from inside the vessel and the engine to outside and the driven propeller.
Sterntube bearings are located in the Sterntube and serve to support the rotating tail shaft onto which the vessels propellors are fitted.
ATZ have the parts, service and experience to survey, supply and provide technical guidance on both white metal and composite derivatives.

Sterntube Bearing

Sterntube bearings are of the journal type. That is bearings in the shape of tubes fitted to the inside of the Sterntube itself. They are lubricated by the bath of lubricant that they are submerged in and operate hydrodynamically when the shaft is rotating.

Two main types of Sterntube bearings exist, which are:

  • Oil lubricated white metal bearings – fitted to most ships and filled with oil to provide lubrication and cooling
  • Water lubricated – fitted to newer ships and those operating within sensitive marine areas as well as military vessels requiring resistance to underwater explosions. These usually have only a single seal on the inboard side. Lubrication and cooling of composite bearings in the tube are provided by seawater.

Although essentially tubes of material they have various features that allow them to perform. These include:

  • External diameter slots that allow:
    • the passing of pipework to serve the operation of the aft seals (oil, draining and air systems are supported in this way) in the case of oil lubricated
    • the passing of pipework for bearing temperature sensors in both oil and water lubricated arrangements
    • the passing of aft bearing wear down gauges in water lubricated arrangements
  • Internal diameter cut outs (wash ways) that allow better through flow of lubricating and cooling oil or water.

Both white metal and composite bearings are an interference fit in their respective stern tubes which means they must be press-fitted with significant hydraulic force when fitted. In the case of composite bearings the interference is much greater, and it is often best practise to shrink the composite bearings by cryogenic freezing before fitting.

The importance of reliable operation of the bearings is paramount. Through failure comes the possibility of:

  • Loss of use of a shaft line
  • Damage to the Sterntube tail shaft
  • Leakage of oil and associated environmental impact if the aft seal is affected.

Lubricant Quality

It is important that the lubricant quality is maintained:

For an oil lubricated bearing arrangement that means clean filtered oil that is regularly sampled and changed at the correct service interval. It also means good maintenance and monitoring of the aft seal performance to avoid any seawater ingress into the bearings lubricating oil.

For a water lubricated bearing arrangement that means, clean filtered seawater pumped to the Sterntube at a flow rate at least that recommended by the maker.

If monitoring and maintenance protocols are carried out, then the bearings should last the life of the vessel. Of course, exceptional circumstances can occur leading to accelerated wear or failure of the bearings. In those situations, replacement is required. 

In most cases bearing replacement require withdrawal of the tail shaft to gain access to the Sterntube and allow bearing removal and refitting. However, some composite bearing designs are not tubes but two half shells forming a tube – these can be exchanged with the tail shaft in position.

Whatever your bearing needs, ATZ have extensive experience with both types and can supply new bearings or provide advice on the operation, maintenance and troubleshooting of your equipment.


FAQ’s

I have seawater ingress into my Sterntube. How do I know if my white metal bearings have suffered damage?

If the seawater has caused loss of hydrodynamic film, then you should see temperature spikes from your bearing sensors. Also, check your Sterntube oil sample in a laboratory, if there are traces of tin (from the white metal) it is likely they are damaged.

Is wear down of the bearing acceptable?

In a white metal bearing generally no. The white metal material is relatively thin and the oil lubrication should prevent anything other than minimal wear.
Composite bearings have much more wear allowance. The entire wall thickness of the material can support the shaft and wear is expected. Seawater is not nearly as good a lubricant as oil.

What should I consider when changing a bearing?

Ensure good condition of the Sterntube bore in which the bearing will be fitted. Measure well to calculate the outside diameter of the new bearing – to be machined on site. Note that the stern tube bore may need machining or repairing.
Check and double check the orientation of the bearing before fitting. There are external and internal features which must line up.
Check the condition of the shaft, any machining here will require machining a smaller than anticipated bearing inside diameter.
Cleanliness, lubrication and removal of sharp edges is key when hydraulically pushing up a white metal bearing. Any pickup on the bearing outside diameter could ruin your new bearing (and damage the stern tube too).

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