What to check on a Intermediate Bearing.
An intermediate bearing, sometimes called a line bearing, supports the length of propeller shaft that runs between the gearbox or main engine coupling and the stern tube. Its role is narrow and specific: carry the shaft's radial weight and keep it running true along its centerline, nothing more. It does not seal against seawater ingress like the stern tube seal, and it does not absorb propeller thrust like the thrust bearing built into the gearbox or a dedicated thrust block. On a short shaft line, a single intermediate bearing or…
What defines an intermediate bearing
An intermediate bearing, sometimes called a line bearing, supports the length of propeller shaft that runs between the gearbox or main engine coupling and the stern tube. Its role is narrow and specific: carry the shaft's radial weight and keep it running true along its centerline, nothing more. It does not seal against seawater ingress like the stern tube seal, and it does not absorb propeller thrust like the thrust bearing built into the gearbox or a dedicated thrust block. On a short shaft line, a single intermediate bearing or even none may be needed; longer shaft lines, common on larger vessels where the engine sits well forward of the stern tube, need one or more intermediate bearings spaced to keep shaft deflection within limits.
Main components
Bearing housing
A split housing bolted to a foundation on the shaft tunnel structure, allowing the bearing to be opened for inspection or shell replacement without disturbing the shaft itself.
Bearing shell / bush
Traditionally white metal (babbitt) on a steel backing, though water-lubricated rubber or synthetic bearings are used in some installations; the shell is the wearing part and is designed to be replaceable.
Lubrication system
Oil-lubricated bearings run in a sealed housing with a self-contained oil bath or ring-oiling arrangement; the lubrication method must match the shaft speed and load, since underlubrication is the most common cause of premature wear.
Temperature monitoring
Most intermediate bearings on vessels above a modest size carry a temperature sensor or thermometer pocket, since rising bearing temperature is the earliest reliable warning of developing wear or lubrication failure.
Alignment shims
Thin shims under the housing set the bearing's vertical and lateral position precisely during shaft alignment, and are a key reference point whenever the shaft line is re-aligned after engine or gearbox work.
Selection and sizing
- Shaft diameter and speed, which set the bearing's bore and the lubrication regime it needs.
- Span between bearings, calculated from shaft deflection limits so the shaft does not sag excessively between support points.
- Load rating versus the shaft's actual weight distribution, confirmed during the vessel's shaft alignment calculation.
- Bearing material - white metal for most oil-lubricated applications, water-lubricated synthetic materials where oil lubrication is impractical or undesirable near the stern tube.
Regulations and class
Class societies require shaft alignment calculations to be submitted and approved as part of the shaft line design, and intermediate bearing positions are fixed by that approved alignment. Periodic survey under class rules includes shaft line inspection at intervals tied to the vessel's survey cycle, commonly opened up for examination at the five-yearly special survey, with bearing clearance and wear measured against the manufacturer's tolerance. Continuous shaft survey arrangements, where fitted, allow extended intervals between openings provided monitoring data such as bearing temperature and oil condition is logged and within limits.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Bearing shell wear beyond tolerance | Inadequate lubrication or contaminated oil over extended running | Increased shaft vibration and risk of shaft-to-housing contact |
| Rising bearing temperature | Lubrication failure, misalignment, or bearing shell breakdown | Accelerated wear, and if unaddressed, bearing seizure |
| Misalignment after dry-docking | Hull deflection change or incomplete re-alignment check after work | Uneven load on bearings, premature wear concentrated at one point |
| Oil contamination | Seal degradation letting water or debris into the housing | Abrasive wear accelerating shell breakdown well ahead of schedule |
What to look for in a supplier
- Bearing shell material and dimensions matched exactly to the vessel's approved shaft alignment calculation.
- Documented load rating and recommended re-lubrication interval for the specific installation.
- Ability to supply matched shims for re-alignment work, not just the bearing housing alone.
- Track record supporting the vessel's class society survey requirements for shaft line components.
Log bearing temperature at every watch rather than relying on an alarm setpoint - a slow upward trend over days is the warning that catches wear before it becomes a seizure, while a fixed alarm only fires once the damage is already underway.