Mechanical engineering consultancy
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A small company giving 110% service
A consultancy providing specialist technical engineering assistance with understanding and resolving rotating machinery problems. Serving Operators/ Owners in the marine and energy sectors.
Services include; failure/ damage investigation surveys on all types of main and auxiliary mechanical machinery, ship propulsion shaft alignment measurements/ calculations, strain gauge measurements as well as engineering assessment/ design. Brabon Engineering Services is independent of any machinery manufacturer or shipyard.
Concerned about contracting with a small company?
With the depth of 30+ years of experience, academic knowledge as well as advanced tools, Brabon Engineering Services can assist you in understanding and resolving difficult situations. Having undertaken many challenging projects, I have the perseverance and grit to identify and assist in rectifying the root cause of your machinery problem.
As a small engineering consultancy we are flexible to your requirements. There are also no unnecessary overheads in our fees which are focused on working time, not waiting or travel. Brabon Engineering Services can deliver better value for money than a larger company.
Technical engineering services
- Root cause analysis of mechanical damage/ failure to machinery using a highly disciplined logic process. Examples include; diesel engines, shaftline bearings, suspected excitation of shaft dynamics or problems with lube oil/ cooling systems. An effective investigation is the means to prevent further incidents and avoid compounding total costs associated with the failure.
- Detailed ship shaft alignment modelling/ design calculations of bearing load distribution. Involves advanced modelling/ analysis tools and techniques.
- Ship shaft alignment measurements and assessments using advanced tools and techniques. Satisfactory alignment is the foundation for maximum bearing life (reliability). Tasks include:
- Sterntube bore alignment offsets using the Taylor-Hobson micro-alignment telescope and targets. Experience has shown the Taylor-Hobson optical kit is superior as a complete system to laser alignment. Post-processing image analysis/ scaling assures an accuracy of better than 0.01mm.
- Shaftline bearing loads by shaft jacking using strain gauge load cell and displacement transducer with computer data logging. Automated analysis of jacking data (results available immediately).
- Main engine, main bearing loads by crankshaft jacking. Engine crank web deflection measurements.
- Bearing loads by shaft bending strains (strain gauges). Advanced analysis techniques are applied to derive bearing loads beyond the accessible spans.
- Measurement of alignment/ offsets of elastic couplings and cardan shafts. Gap and sag of uncoupled intermediate shaft sections (preliminary alignment).
- Assessment/ interpretation of measured bearing loads (versus design).
- Calculations of bearing offset adjustments and predicted load changes for optimised local alignment and shaftline bearing load distribution.
- Review of (sterntube) bearing operating and condition monitoring data to provide a risk assessment of bearing damage.
- Strain gauge field measurements as well as any calculations of stress/ applied force. Components/ structure may be situated outdoors. Best practice is applied in gauge configurations/ installations along with best quality digitizers/ measurement devices. We can also supply complete measurement systems from gauges through to indication device or monitoring system interface.
- Engineering calculations, mathematical modelling and design. Examples include; classical mechanics, pipework systems, finite element modelling, data analysis (using Scilab/ Python) as well as CAD drawings.
- Preparation of formal written reports of the highest standard.
Have you suffered an inexplicable machinery problem?
Brabon Engineering Services will conduct a meticulous investigation and help prevent further incidents.
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“He who learns but does not think, is lost! He who thinks but does not learn is in great danger.” Confucius
Ship machinery discussion topics
(Practical tips from experience over the years.)
5 May 2021, Ship propulsion shaft alignment
sterntube bearing loads in the stopped and running conditions
Propeller shaft alignment bearing loads are measured with the shaft stopped. However, the objective is satisfactory loads when running. Differences when running underway include:
- Propulsive hydrodynamic forces acting on the propeller, i.e. mainly axial thrust plus smaller transverse forces and moments. The centre of steady (mean) thrust is typically above the propeller centre due to the natural asymmetry of the hull and inflow wake. (Except perhaps on a submerged submarine.)
- Shaft displacement, vertical and horizontal, in each bearing changes (upwards) as each journal rides on a lubricant film. The propeller shaft aft end typically rises in the sterntube aft, aft end bearing clearance with increasing shaft speed (due to the thrust eccentricity). Bearing reaction loads vary accordingly.
- Stiffness of shaft support in each bearing decrease due to the lubricant film (softer than the bearing and supporting structure). Support of the propeller shaft aft journal in the sterntube aft bearing changes with shaft loading, displacement/ curvature and the lubricant film. When stopped the journal should be supported at two points, the aft end and forward end of the sterntube aft bearing.
Thrust force and eccentricity vary with shaft and ship speed. Thrust eccentricity may be from 5% to 20% of propeller tip radius. At service speed, the vertical loads on the sterntube aft bearing tend to decrease at the aft end while increasing at the forward end. The sterntube forward bearing load also tends to increase. For optimum sterntube bearing alignment we arrange a modest load bias to the aft end of the bearing in the stopped condition.
29 April 2021, Strain gauge measurements
standing on the shoulders of giants
Electrical resistance strain gauges were invented in 1938 by Prof Arthur Rudge of MIT & separately by E.E. Simmons of Caltech in 1937.
Strain gauges are a means of finding actual strains in a component under trial/ service conditions, obviating assumptions of boundary conditions inherent in modelling analysis.
Gauges can be affixed on a component in any orientation (vertical or inverted surfaces are slightly more difficult). The gauge factor (sensitivity) is used to find strain from the change in resistance. Ideally, the component should be subject to a known load to calibrate the output. Alternatively, the bridge circuit & indicated output can be checked by shunting the gauges with a precision resistor.
A Wheatstone bridge is used to measure gauge resistance variations. The circuit was invented in 1833 by Samuel Christie & ‘popularized’ by Charles Wheatstone around 1843. When there are less than four active gauges, the bridge is completed with temperature compensation gauges or bridge completion resistors.
Measured strains can be converted to principal strains using Mohr’s circle. Principal stresses can be derived from strain by the generalized Hooke’s law & the material modulus.
21 April 2021, Machinery failure analysis
Ship sterntube bearings, inexplicable damage/ failures
Identifying the causes of a machine failure can help avoiding further incidents and compounding costs.
Seemingly intractable machinery problems often involve multiple contributory factors. In considering failure hypotheses greater weight should be given to commonly encountered scenarios, i.e. unusual phenomena are less frequently encountered and thus a less likely cause.
Plain bearings have a limited number of damage modes as listed below, however, these can be the result of a multitude of scenarios.
- Loss of hydrodynamic lubricant film due to starvation.
- Loss of hydrodynamic lubricant film due to overload. The shaft journal ‘wandering’ into the washway could be included in this group.
- Loss of hydrodynamic lubricant film due to edge loading, leading to localized damage.
- Fatigue damage to bearing surface due to cyclic loading.
- Wear (material loss) of bearing surface and/ or shaft journal, e.g. as a result of particles contamination of the lubricant.
- Breakdown/ degradation of the lubricant and its properties, e.g. by oxidation or water contamination.
- A metallurgical imperfection in the bearing lining.
- Damage to the bearing surface due to corrosion or electrical discharge.
Evidence and intermediate conclusions should be critically reviewed noting that the appearance changes with progression of damage. This step is crucial to any successful root cause analysis. The jigsaw puzzle pieces should fit together without the aid of a mallet, i.e. without more than one presumed element/ extrapolation.
14 April 2021, Ship shaft alignment
Sterntube forward bearing load in spec, so is propeller shaft alignment satisfactory?
This is not true. The sterntube forward bearing load, derived by jacking, does not correlate to one unique shaft alignment condition, i.e. a satisfactory alignment (load bias) of the propeller shaft in the sterntube aft bearing.
The sterntube forward bearing load is a result of the propeller shaft aft loading combined with the intermediate (forward) shafts and bearings. The interaction is more significant with short stiff shaftline systems, e.g. bulk carriers. It can be shown that a family of alignment conditions for the sterntube bearing and propulsion engine produce the same sterntube forward bearing load.
Strain gauge bending measurements offer one of the few means of establishing if there is satisfactory alignment in the sterntube bearing. An accurate jacking measurement can also be interpreted to indicate the loaded condition in the sterntube aft bearing, but, not the actual load bias.
This is important in the case of vessels where the stern tube is not fitted with a forward bearing. In such installations the load distribution on the stern tube aft bearing is mainly controlled by the intermediate bearing offset (chock height), rather than the line bored geometry of the stern tube.
8 April 2021, Ship propeller shaft alignment
Slope and offsets for replacement sterntube aft bearing
An oil lubricated sterntube aft bearing is 2x shaft diameter in length. This is for the static mass load and propulsive hydrodynamic forces acting on the propeller. A long bearing also aids in damping the vibratory element of propulsive forces due to the propeller blades acting in the unsteady wake in-flow.
Thermal wipe damage at the extreme aft end, 6 o’clock position is a common failure mode. Whitemetal bearings are not tolerant of excessive edge load. With a long bearing, the difference in reaction loads at the fwd and aft ends is sensitive to bearing slope (and relative to the shaft). Shaft bending due to mass load and propulsive forces of the propeller also affect the reaction loads at each end of the bearing.
For optimum sterntube bearing alignment we arrange a modest load bias to the aft end of the bearing in the stopped condition during shipyard dry docking repair, i.e. the forward end is not unloaded. This provides satisfactory operation at slow, and full away, shaft speeds. At high shaft speed/ power the thrust eccentricity is typically above the propeller centre resulting in a load bias to the forward end of the bearing.
1 April 2021, Machinery failure analysis and troubleshooting
Damage mechanism categories
Seemingly intractable mechanical machinery damage problems often involve multiple contributory factors.
Moreover, mechanical failures can typically involve multiple, commonly encountered, factors/ phenomena. Unusual anomalies, by their nature, are less frequently encountered. For example, bearing failure due to oil starvation is more common than say shaft lateral vibration resonance (whirling). The timing/ succession of each event can be pivotal in the (overall) failure. The challenge is to correctly discover the root cause(s) from all the available evidence.
Considering various damage mechanisms:
- Fatigue failure. This can occur irrespective of a conservative margin between yield strength and mean loading. This margin is generally indicated by the relative size of the final overload area. A fatigue fracture surface is characteristically different to that of an overload failure at microscopic and macro scale, e.g. beach, arrest and ratchet marks. The shape/ orientation of fracture face indicates the type of stress having caused propagation, e.g. a spiral fracture face would be associated with torsional fatigue. Although the dominant stresses may change as the crack propagates through the component section.
- Material deficiency or change. This may involve metallurgical imperfections such as forging lap, non-metallic inclusions, casting voids or corrosion.
- Assembly problems. This can include fasteners not tightened, looseness of fit/ seizure.
- Elastic/ plastic deformation. This is generally readily identified although the cause of overload may not be clear. It is also possible for heavy vibration to generate excessive stresses, although this occurs infrequently. In this scenario it is important to distinguish between high levels of (source) excitation forces, and resonance (response) conditions.
- Leakage/ compromised fluid containment. Seal leakage may be due to normal wear and tear or lack of maintenance. In some cases, shaft vibration can manifest with seal leakage.
- Wear and deterioration of running or mating surfaces, including fretting and corrosion.
24 March 2021, Ship propeller shaft
Non-metallic sterntube bearing failure trends
When considering non-metallic/ composite and whitemetal sterntube bearings, there are reasons for and against each option. For example, whitemetal bearings have a slightly lower failure rate, whereas non-metallic bearings tolerate bedding-in and a damaged bearing may possibly continue operation until a dock is available.
- Non-metallic bearing linings have a low/ poor heat transfer coefficient. The lube oil, or water, then has a greater role in heat dissipation. Any large vessel fitted with a non-metallic sterntube bearing can suffer a high temperature event as a result of heavy manoeuvring, e.g. emergency stop. (Note, the shaft should not be stopped in the event of damage being suspected.)
- Non-metallic bearing linings can suffer gradual cumulative damage due to loading and heat. This can lead to cracking, delamination and material loss. Poor alignment can be a factor in damage to non-metallic bearings.
- Non-metallic, water lubricated bearings can suffer damage by heavy abrasive wear due to contamination of the lube water (closed circuit system), or operation in silt laden estuaries (open systems). Vessels having open A-brackets and an operating profile involving extended idle time may be susceptible to abrasive wear due to hard marine growth on shaft journals forming during shut down periods.
It is also interesting to note that whitemetal lined bearings tend to run at slightly higher temperature where the sterntube is fixed with epoxy chock in the stern frame. This is typically due to a reduced heat transfer to the ambient water via the epoxy layer.
18 March 2021, Ship propeller shaft
White metal sterntube bearing failure trends
Failures of oil lubricated sterntube bearings are rare events with low occurrence rates across the global fleet. With many bearing failure incidents the causes are unknown due to incomplete investigation.
- Tankers and bulk carriers tend to be more susceptible to bearing problems, i.e. having highest failure rate. This is due to typical design having shorter shaftlines and spans between bearings, resulting in bearing loads sensitive to hull flexure and having less margin versus alignment.
- Any large vessel can experience bearing damage due to adverse/ large sterntube bearing loads during heavy manoeuvring, e.g. zig-zag or emergency stop.
- Any class of vessel may succumb to sterntube bearing damage due to stern seal failure/ deterioration and sea water contamination of the lube oil. Deferral of a scheduled seal overhaul should be carefully considered.
- Sterntube bearing problems occurring within the construction warranty period clearly suggests sub-optimum alignment. For example, bore alignment measurements in the construction shipyard using the piano-wire method without correction for sag in the wire.
In addition, cases where sterntube bearing failure has been attributed to the use of EAL lube oil are more likely, in reality, to be the result of poor alignment possibly compounded with other contributory factors. Shipyards have been known to blame the EAL oil as a convenient contrivance to evade a warranty claim.
Non-metallic sterntube bearings, tend to have a higher overall failure rate than whitemetal lined bearings. However, trends are difficult to identify with little variation between individual vessels and vessel types.
10 March 2021, Ship shaft alignment measurements of bearing loads by jacking or strain gauges?
The methods used by the alignment engineer are only one aspect of undertaking an alignment assessment during a dry docking repair. It is important that shaft alignment measurements of bearing loads are conducted with the ship afloat and at an operational draught aft. If there are no measurements with the vessel afloat, then any alignment adjustments are less certain.
Strain gauges can be affixed to the shaft in the accessible spans. Shaft bending inboard near the sterntube is related to load distribution on the sterntube aft bearing (vertical and horizontal). Very few shipyards are capable of this measurement technique. Bending strain measurements offer a means of establishing if there is satisfactory alignment in the sterntube aft bearing.
Bearing loads can also be derived by shaft jacking measurements. However, the jack needs to be located immediately adjacent to the bearing, i.e. accessible. A jacking measurement where the load and lift are manually recorded from jack pressure and a dial indicator, as favoured by many shipyards, may only give a handful of data points with the indicated load being affected by friction in the jack. Loads measured using a strain gauge load cell and data logger provide more accurate results. Application of the correct jack correction factor also helps with the load results.
4 March 2021, Ship sterntube bearing bore alignment measurements using optical or laser equipment?
The tools used by the alignment engineer are only one aspect of the tasks within a typical dry docking attendance to assist with a ship propulsion shaft alignment problem. Other equally important tasks include assessment of all measurement results/ observations as well as deriving recommended offsets/ adjustment for (any) replacement sterntube bearing. It is important that the calculations of bearing (offset) adjustments follows a rigorous procedure with identification of all uncertainties.
Brabon Engineering Services opt to use the Taylor-Hobson micro-alignment telescope and target jigs. Experience shows this is superior as a complete measurement system to laser alignment. Bearing/ housing offsets relative to the datum/ reference line of sight are measured with the vertical/ horizontal direction micrometers of the alignment telescope. Post-processing image analysis/ scaling also assures an accuracy of better than 0.01mm.
25 February 2021, Ship propulsion shaft alignment procedure
If sterntube bearing damage is suspected, then a co-ordinated process is needed to ensure a satisfactory repair and return the vessel to service/ charter. For a sterntube bearing operating with little margin, then reinstating the original design bearing offsets/ slope would likely incur significant risk of further damage.
Important steps in the process:
- Accurate measurement of shaftline bearing loads in the afloat condition prior to vessel docking. Poor jacking measurements can fail to identify adverse bearing loads. Strain gauge shaft bending measurements are also generally recommended as this provides additional information of bearing loads.
- Optical/ laser measurement of (existing, damaged) sterntube bearing bore slope as well as bearing housing slope (after bearing extraction). An accurate, rather than rapid, measurement should be the primary objective for this step. Use of appropriate reference datums for both measurements is needed to provided data that can be compared to design.
- Assessment of measurement results along with the observed condition on the sterntube bearing in order to prepare any recommended alignment adjustments. While this step always involves judgement, it should be based on a proven methodology.
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