Author; Brett Weintz. Updated March 2021.
What causes a sterntube bearing high temperature?
A high temperature typically indicates compromise of the hydrodynamic lubricant film over a portion of the bearing. This may be a transient increase in shaft load say during heavy manoeuvring, or low maximum load margin due to poor alignment between bearing and shaft journal, or lubricant condition degradation, e.g. water contamination due to seal leakage. Note however, that bearing problems often involve multiple contributory factors.
Sterntube bearings are inaccessible and typically have minimal monitoring. There may only be subtle indications of damage. However, there are significant cost and operational impacts if inspection/ repair is considered necessary. The evidence needs careful review in any decision to book a dry-dock.
Ship sterntube bearings have a crucial function to support the propeller shaft and react the hydrodynamic propulsive transverse forces/ mass exerted on the propeller. This page includes descriptions of symptoms associated with propeller shaft sterntube/ A-bracket bearing problems as well as suggested mitigation actions.
You can find the following Brabon Engineering Services site pages and case studies regarding ship propulsion shaft bearings:
- Ship sterntube aft bearing operations and problems discussion (this page).
- Hydrodynamic bearing design and damage discussion page.
- Ship vibration (shaft dynamics) discussion page.
- Shaft alignment measurements case study.
- Strain gauge shaft alignment measurement case study.
- Sterntube aft bearing machining offset accuracy case study.
- Fair curve alignment design example.
- Sterntube bearing problem case study 1.
- Sterntube bearing problem case study 2.
- Shaft alignment measurements Brabon Engineering Services undertake.
- Shaft alignment measurement details discussion page. Includes factors affecting derived results/ uncertainty.
- Shaft alignment principles discussion page.
Sterntube bearing condition monitoring
(How to detect sterntube bearing damage?)
Given the vital role of the stern tube aft bearing in ship propulsion, the following actions are suggested as prudent in order to monitor for problems/ damage:
- Continuous monitoring of bearing temperature. To enable comparisons between different sea areas and seasons, the bearing temperature can be normalised for variations in ambient conditions by taking the difference between the indicated bearing temperature and current sea water temperature, e.g. at the inlet to the central cooler supply.
- Periodic review/ trending of the oil analysis results (where applicable), e.g. concentration of lining material wear particles as well as water content. Note that water content without an associated increase in sodium/ magnesium concentration may be due to condensation in header/ drain tanks. If an environmentally acceptable lubricant (EAL) is used in the stern tube, then total acid number (TAN), i.e. pH, should also be trended. As a basic check the sterntube lubricant strainer basket (off-line unit) may be inspected on each change-over of duty and stand-by pumps.
- Routine monitoring of lubricant level in stern tube header tank(s) as well as lubricant circulation, for either natural or pumped systems. In the absence of sensors, there can be a temperature difference between the supply and return pipes which would indicate a circulation exists. Where operational practice is to circulate the stern tube lube oil through a (main gearing lube oil system) purifier, say temporarily when shut-down in port, then the sterntube header tank level should be checked a short time after the lube systems are changed-over to the normal (sailing) configuration.
- Net/ rope cutters should be inspected for condition and the clearance/ alignment checked at each dry-docking activity. A fine rope/ line may pass the cutters and cause damage to the aft stern seal possibly resulting in bearing damage (due to sea water ingress). The aft stern seal(s) should be examined and overhauled at periodic intervals depending on the vessel operating hours. Not overhauling the stern seal(s) as opportunities present is suggested to be a false economy.
- If an air type aft stern seal is fitted, then the drain tank should be checked for content and emptied as necessary, e.g. water and/ or oil. If the aft stern seal has a header tank, then the oil should be sampled and analyzed or examined (as a minimum), e.g. discolouration due to debris and/ or water.
Sterntube bearing operational precautions
- Care is needed when operating in a light ballast/ ballast condition (with the shaft speed and observing bearing temperature) as shallow immersion of the propeller can produce (increased) inflow wake variations (around the propeller sweep) possibly resulting in adverse propulsive hydrodynamic forces/ moments on the propeller and associated loads on the stern tube aft bearing. This can be a problem during heavy weather particularly as the need to maintain the way and heading of the vessel may preclude the option of reducing shaft speed.
- Filling the aft peak tank in order to increase the aft draught and improve the propeller immersion in the ballast condition on a vessel not fitted with a stern tube forward bearing and having a flexible hull, say in excess of 100,000 DWT, could result in an adverse variation of the sterntube aft bearing load distribution. If there is no service experience of operation in this condition, then the risks should be carefully considered. Underway tests could be conducted with the aft peak tank only partially filled. Shaftline bearing load measurements with and without the aft peak tank filled may be available from original construction.
Sterntube bearing damage indications
(How to ensure indications of bearing damage are not unnoticed?)
Temperature of bearings
A rapid (exponential) rise in bearing temperature above the alert level would be a clear indication of bearing distress. For oil bath lubricated, whitemetal lined, thick wall, plain type bearings a typical alert temperature is 65°C. (This includes an assumed temperature gradient between the running surface and the sensor head as well as a margin on the maximum/ melting temperature of the whitemetal lining of 120°C to 150°C.) Note that due to lower thermal conductivity, non-metallic bearings and metallic bearings having support liners set in (epoxy) chock may operate at a slightly higher temperature and have a slower rate of cooling. In cases of severe damage the lube oil may become contaminated and emulsified within a short time as well as there being a progressive increase in vibration around the vessel aft end (due to the increased bearing clearance). Stern seal operation should be checked.
Pressure oil fed, thin wall, plain type bearings generally have a higher alert temperature. This is due to the greater cooling afforded by a forced supply of fresh lube oil, the reduced distance between running surface and sensor head as well as the possible application of alloys with higher temperature tolerance.
If bearing damage is suspected, although a high temperature event may not be apparent, then bearing temperature records should be examined for indications of a change in bearing condition. To consider a long period of time, then a trend of the difference between bearing temperature and the sea water (at the same time) can be generated. Following damage bearing temperature may be lower during steady motoring (due to possible increased clearance), although the bearing temperature may be more sensitive to manoeuvring or heavy ship motions. The temperature difference between the stern tube aft and stern tube forward bearings can be compared for before and after the event (for the same shaft speed and aft draught). For twin screw vessels, the difference in bearing temperature between port and starboard shafts can be compared for before and after the event.
Lube oil condition
Should the stern tube lube oil analysis trend trigger an alert for bearing lining metal(s), then further investigation is warranted. If sodium and magnesium concentration have increased, then sea water contamination due to seal damage might be suspected. If an environmentally acceptable lubricant (EAL) is in use, then additional laboratory tests can indicate the process(es) of bearing damage. Contact Brabon Engineering Services for further information.
The International Association of Classification Societies (IACS) provide the following recommended alert levels for sterntube lube oil contaminants (from analysis):
- Water 1% (10,000 ppm)
- Sodium 80 ppm (sea water)
- Magnesium 30 ppm (sea water)
Note, water contamination without high sodium and magnesium levels may be condensation in a header tank.
- Lead 10 ppm (bearing lining)
- Tin 10 ppm (bearing lining)
- Copper 50 ppm (stern seal box, cooler)
- Iron 30 ppm (propeller shaft, sterntube internal surface)
- Chromium 10 ppm (stern seal liners)
- Nickel 10 ppm (stern seal liners)
- IACS also advise that the oil is also analyzed for oxidation characteristics, e.g. TAN (acid number).
Note that the sampling method is crucial to condition monitoring. The shaft should be running and at normal operating temperature. Enough oil should be run off the sample cock to thoroughly clear the ‘dead leg’ in order to obtain a representative sample. Note that variations of say ±2 ppm (±2 mg/kg) in the oil analysis contaminant results can indicate different different trends, so a few grains of dirt in a 150 ml sample can be significant. There are numerous sources of further guidance on lube oil sampling best practice, such as ReliabilityWeb and Ametek Spectro Scientific.
A change in the propulsion shaft bearing loads might provide circumstantial evidence of sterntube bearing damage. However, careful measurements are required and the ship conditions of draught aft and prime mover temperature need to be similar to the reference case. Note that it would be usual for the sterntube aft bearing load bias to move towards forward with an associated decrease in the sterntube forward bearing load. However, in the event of a minor bearing wipe it is also possible for the overall alignment condition to improve.
A change in the propeller shaft aft poker gauge measurement results would be a clear indication of aft bearing damage. However, it is generally prudent to double-check poker gauge measurements. A poker gauge arrangement with measurement points, both above and below the shaft, provide greater assurance of an accurate result. The poker gauge measurement reliably indicate damage/ wear on a non-metallic bearing. However, measurement variations and uncertainty in poker gauge results typically exceed/ mask the propeller shaft deviation with a damaged oil lubricated white metal bearing.
Brabon Engineering Services can assist in cases of suspected stern tube bearing problems with alignment measurements including:
- Detailed ship shaft alignment modelling/ design calculations of bearing load distribution.
- Bore alignment offsets using the Taylor-Hobson micro-alignment telescope and targets.
- Bearing loads by shaft jacking using strain gauge load cell and displacement transducer.
- Main engine, main bearing loads by crankshaft jacking. Main engine crank web deflections.
- Measurement of alignment/ offsets of elastic couplings and cardan shafts as well as gap and sag of uncoupled shaft sections.
- Bearing loads by shaftline bending strains (strain gauges).
- Assessment/ interpretation of measured bearing loads (versus design).
- Calculations of bearing offset adjustments and predicted load changes for an improved local alignment and/ or optimised shaftline bearing load distribution.
Sterntube aft bearing problems – case study 1
Vessel and propulsion shaft arrangement
The case concerned a product tanker about 40,000 tonne deadweight having a slow-speed, two-stroke diesel direct drive propulsion system. The engine had a maximum continuous rating of about 8,500 kW at 125 rpm. The fixed pitch propeller rotated clockwise during ahead motoring as viewed from aft.
The propeller shaft was supported by a stern tube aft and stern tube forward bearing as well as having a nominal diameter of 460mm. The stern tube bearings were both tin based whitemetal lined, plain cylindrical sleeve type having oil washways at 3 and 9 o’clock. The sterntube aft bearing (only) was fitted with a temperature sensor. The intermediate shaft was supported by a whitemetal lined, plain, half shell bearing with a self-contained oil lubrication system.
The vessel was fitted with an air type aft stern seal. The stern tube lube oil system included a 0.5 m^3/ hour circulation pump normally delivering oil from a tank to the forward cavity of the aft stern seal, flowed under the forward seal lip into the stern tube. The stern tube lube oil tank and aft stern seal centre/ drain cavity were pressurised using air. An automatic control system maintained a constant flow through the stern seal drain cavity (to sea via the aft seal lips) and a constant pressure differential in the sterntube (lube oil tank) over the stern seal drain cavity.
Bearing damage indications
The stern tube aft bearing had been re-metal repaired some years previously after having suffered thermal wipe and fatigue damage. The Owners had noticed an increasing trend in copper concentration up to about 30 ppm and water contamination up to about 0.1% (1,000 ppm) in the stern tube lube oil analysis over a period of about 18 months. A small amount of metallic particles were also found in the stern tube lube oil system strainers. Tin, lead and iron concentration had remained steady at less than 10 ppm. There were no reported high temperature incidents for the sterntube aft bearing.
Shaftline bearing load measurements were conducted with the vessel in a laden condition and indicated an elevated load bias on the stern tube aft bearing (heavy) to the aft end, with a light load on the stern tube aft bearing – forward end. A heavy load bias to the aft end of the stern tube aft bearing can make the bearing susceptible to damage in operation. Propeller shaft poker gauge measurements were inconsistent/ inconclusive.
Dry-dock bearing inspection
The Owners were uncertain of the stern tube aft bearing condition, being suspicious of the previous re-metal repair, and decided to dry-dock the vessel in order to undertake bearing inspections. The propeller shaft was withdraw and the sterntube aft and forward bearings were found to be in a satisfactory condition. The aft bearing was polished in the lower half in the area of the aft end as well as the forward end.
Do you suspect a problem with a vessel’s sterntube or shaftline bearing?
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Sterntube aft bearing problems – case study 2
Vessel and propulsion shaft arrangement
The case concerned an ultra-large ore carrier with a slow-speed, two-stroke diesel direct drive propulsion system. The engine had a maximum continuous rating of about 25 MW at 75 rpm. The fixed pitch propeller rotated clockwise during ahead motoring as viewed from aft.
The propeller shaft was supported by a stern tube (aft) bearing only and had a nominal diameter of 800mm. The sterntube bearing was a whitemetal lined, plain cylindrical sleeve type having oil washways at 3 and 9 o’clock. The stern tube bearing was fitted with a (dual head) temperature sensor. With no stern tube forward bearing fitted, the next bearing forward was a whitemetal lined tilting pad type intermediate bearing supporting the intermediate shaft. The intermediate bearing had a self-contained oil lubrication system but was cooled with water from the main engine low temperature cooling water circuit.
The vessel was fitted with an air type aft stern seal. The stern tube lube oil system included a 0.5 m^3/ hour circulation pump delivering oil from a tank to the mid area of the stern tube as well as a circulation flow through the aft stern seal forward cavity. The stern tube lube oil tank and aft stern seal centre/ drain cavity were pressurised using air. An automatic control system maintained a constant flow through the stern seal drain cavity (to sea via the aft seal lips) and a constant pressure differential in the sterntube (lube oil tank) over the stern seal drain cavity.
Bearing damage incident
The vessel was underway steady motoring in the fully laden condition. The sea state was noted as moderate and the sea temperature was 22°C. The shaft speed was 66 rpm having been reduced from 71 rpm 15 minutes earlier. The vessel was executing a course change through 90° with a turn to starboard reportedly using less than 10° helm. Near the end of the turn port helm was applied to steady the vessel on the new course when a high temperature incident occurred on the stern tube bearing with an indicated peak bearing temperature of 105°C. The main engine order was reduced to 29 rpm and then stopped with the turning gear engaged. The stern tube bearing temperature decreased to less than 46°C (the alert temperature) over a period of about two hours.
After a further five hours on the turning gear the main engine was re-started. Stern tube bearing temperatures were reported as satisfactory during the rest of the voyage without any abnormally high temperature increase during manoeuvring.
Metallic particles (of unknown material) had been found in the stern tube lube oil system strainers. However, it was not known when the strainers had been previously cleaned. Lube oil samples drawn when the vessel arrived at the discharge port were sent for analysis which found tin concentration of 9 ppm while levels of iron and lead were well below 10 ppm. Water content was also low at 370 ppm. The particle count was found to be elevated at ISO 23/23/19.
Propulsion shaftline bearing load measurements were conducted at the discharge port in the laden condition with the engine warm. Previous measurements also in the laden condition with the engine were available for comparison. Shaftline bending strain measurements indicated the load distribution on the stern tube (aft) bearing had changed with a decrease in the load bias aft, i.e. load had transferred from the aft to forward end of the bearing. The load on the intermediate bearing had also decreased (although this load is noted as being sensitive to the main engine condition).
Sterntube bearing damage
The Owners placed the vessel in a shipyard to undertake any necessary repairs. The propeller shaft was withdraw and the stern tube (aft) bearing was found to have suffered a thermal (over-temperature) wipe at the aft end. The lower surface was damaged from the starboard to the port washway and extended forward about 300mm from the extreme aft edge. Lining metal was extruded/ deposited in the starboard and port washways as well as partly around the upper half. In addition, there were polished areas at the 6 o’clock position immediately forward of the thermal wipe and at the extreme forward end of the bearing.
Suggested actions when damage is suspected
In the event of (suspected) bearing damage the following is suggested:
- Reduce shaft speed. The shaft should be maintained at the highest speed that still allows a decreasing bearing temperature (towards the typical operating condition). Normalisation of the sterntube bearing temperature may take several hours. The shaft should not be stopped. Clearly however, navigational safety must take precedence over bearing condition.
- Change-over the duty/ stand-by lube circulation pumps and inspect the suction strainer. Reflective flakes, see Figure 1, or the smell of burnt phenolic would be respective indications of damage to whitemetal or non-metallic bearings.
- The sterntube lube oil should be sampled for analysis.
- Check the stern seal drain tank for oil or water, if an air type seal is fitted.
- Check the stern seal oil and air flow rates for variation from usual values, if an air type seal is fitted.
- Machinery monitoring system trends for shaft speed, sterntube aft and forward bearing temperature, sea water temperature and any stern seal data should be saved/ printed or even photographed. Some monitoring systems work on a moving window and automatically delete data after a preset period.
- Continue monitoring bearing and sterntube lube oil temperature, as would be intuitive. An extremely long time period for the bearing temperature to decrease, say 12 hours, and/ or further high-temperature incidents may indicate significant damage.
In the event of a minor thermal wipe on the sterntube aft bearing, then the bearing may continue to give satisfactory service. However an extensive wipe, or fatigue damage, would typically progress. Continued problems with sterntube bearing temperature would typically signify the need for repair action.
Planning dry-docking shaft bearing repairs
In the event that sterntube bearing damage occurs, or is suspected, it is generally prudent to eliminate a poor shaft alignment condition as a possible factor. Brabon Engineering Services can undertake the necessary bearing load measurements and assessments with the vessel afloat during a suitable hiatus in ship operations. Brabon Engineering Services can also undertake a thorough investigation to discover the root cause of the bearing damage.
Experience suggests that the majority of stern tube aft bearing damage cases involve adverse local alignment (slope) of the stern tube aft bearing plus other contributory factors, e.g. heavy manoeuvring at ballast condition. Problems of adverse local alignment are common. Cases of bearing damage involving a single cause are the exception.
If replacement of a sterntube bearing in a shipyard is contemplated, Brabon Engineering Services can undertake the necessary shaft alignment measurements and assessments. It is recommended to undertake bearing load measurements with the vessel afloat prior to dry-docking as this informs any bearing offset adjustments needed to optimize the shaft alignment condition.
Brabon Engineering Services then conduct bore alignment measurements of the sterntube/ A-bracket bearing and housing after the propeller shaft is withdrawn in dry-dock. A visual inspection of the damaged bearing is usually also conducted. This can indicate the damage mechanism and guide rectification action and advisable adjustments. A bearing that has suffered only minor damage might not require replacement.
Brabon Engineering Services can provide recommended design offsets (and slope) of the replacement bearing to optimize the bearing load distribution.
Duration of measurements:
- For jacking measurements, about one hour should be allowed per bearing (including re-positioning equipment). Each bearing load result is available immediately following the measurement. A portable jacking stand may need to be manufactured in order to position the jack under the shaft adjacent to the bearing.
- For strain gauge shaft bending measurements, then about one day should be allowed for fitting strain gauges and associated cables (on one shaft). About one hour should be allowed per shaft alignment bending strain measurement (one shaft). Calculation to derive the bearing loads takes less than 20 minutes. Thus, the results are available a short while after the measurement.
- Sterntube bore alignment measurements generally require about seven hours. However, bore alignment is generally conducted during the night in order to mitigate any possible variations due to the sun acting on the hull. Thus, this task can be scheduled so as to not interrupt day-shift progress.
Further case studies
The following case studies are available on request:
- Ship Main Propulsion Stern tube Bearing Problems – A Risk Assessment. (An outline of the principal damage risk scenarios for ship propeller shaft stern tube aft and stern tube forward bearing as well as describe the shaft alignment process.)
- Propeller Shaft Fatigue Failure, August 2020.
- Stern Seal and A-bracket Bearing Problems, February 2020.
- Sterntube Aft Bearing Whitemetal Fatigue Damage, May 2019.
The above notes are applicable to typical generic events affecting the propeller shaft sterntube bearing, however, please note that every actual event is unique and requires individual consideration. Any action taken upon the information on this website is strictly at your own risk. Brabon Engineering Services are not responsible in any way whatsoever for your use of the information.