You should always take into consideration the requirements of SOLAS 74, as amended, and the manufacturer's manual
Crankshaf
Crankshaf
These are most useful when taken with the engine “hot” and the vessel in a similar loading condition to the previous or reference readings.
Readings taken in drydock are of no significant use since the ship is supported by blocks and not taking it’s natural shape as in water.
Excessive deflections may be caused by :
Bearing wear (adjust or renew bearings)
Misalignment (re-align)
Loose or fretted chocks (re-chock engine).
Since there are many diverse designs of engines, gears etc, the Surveyors should consults the manufacturer’s instructions for recommended limits of wear, In some cases manufacturers recommend renewal of bearings and bolts based on running hours.
How to take Crankshaft Deflections?
The figure shown below gives an idea of which measurement is taken to find the deflections of the crankshaft.
As you can see from the picture a dial gauge is inserted between the crank-webs to find out the distance between them. If the deflection is measured after the specified interval, it is necessary that it is taken at the same point otherwise the reading will not give a real reflection about the degree of deflection. Normally a center punch is used to make markings so that each time the deflection is taken at the same point.
Apart from using the same point on the crank-web for measuring deflection there are other factors which need to be kept in mind and these include load on the ship, trim, hog, sag etc. Ideally the deflection needs to be taken at four points of the crank namely top, bottom and the two sides. In actual practice however the bottom reading is not taken due to chances of fouling by the connecting rod and instead reading is taken on both sides of the bottom position, thereby in total 5 readings are taken from each crank-web at the positions shown by the following figure.
In other cases bearing renewal is dependent on measured crown thickness or % age wear of overlay plating or visibility of bearing backing.
Additionally such factors as spelling or fretting of the back of shells are reason for renewal. In some engines it may be recommended to renew main bearings in sets, in others it may be possible to renew bearings by halves.
Additionally such factors as spelling or fretting of the back of shells are reason for renewal. In some engines it may be recommended to renew main bearings in sets, in others it may be possible to renew bearings by halves.
In large engines adjustment of clearances may be possible.
When manufacturer’s recommendations are not available the following Guidance may be followed:
Clearances Of Diesel Engines (Fitting and maximum clearances)
The table gives the fitting diameter clearance Jo and the maximum clearance
allowed Jm. Any clearance above Jm is reason enough for renewal even if the
Babbitt-metal is satisfactory.
Jm = 1.75 x Jo for top end of connecting rods with bearings on 2 stroke engines
Jm = 2 x Jo for bearings with joints and rotating completely
(bearings, bottom ends of connecting rods)
Jm = 3 x Jo for bushes
Bottom and top ends (connecting rods) of four (4) stroke engines.
Jm = 1.5 to 1.75 x Jo for top ends of connecting rods with bearings (depending on the engine speed high or low speed)
Jm = 2 x Jo for bottom ends of connecting rods
Jm = 2.5 x Jo for top ends of connecting rods with bushes
Note: Bearing shells with relatively thick Babbitt-metal will always show wear-down after the engine is run-in.
For bearing shells with lead bronze which is more resistant than Babbitt-metal, the flitting clearances must be adjusted accordingly.
A few high spots or hard spots will be sufficient to cause over-heating.
Thin shell bearings (medium speed engines) Reasons for rejection.
Layer configuration of tri-metal lead bronze bearings (as used in medium-speed diesel engines):
Finding
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Action
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1. Uniform running pattern, minor scratches and corrosion in the plated overlay (proper L.O. treatment)
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Satisfactory.
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2. Minor corrosion of the plated overlay. small area of nickel dam exposed one side.
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Satisfactory further use.
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3. Heavier corrosion of the plated overlay. Nickel dam exposed both sides. The shell has reached the wear limit, a good adaptation is no longer assured.
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Satisfactory for limited use.
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4. A large area of plating area corroded, but no bronze showing, running pattern good. But bearing should be renewed because it can no longer embed dirt and adapt itself.
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Renew.
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5. Heavy corrosion and abrasion of the plated overlay. L.O. contaminated by solid foreign particles causing wear and scratches Into the bronze. Check journal for scratches and smooth with oil stone ff necessary. This bearing condition is considerable risk for engine.
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Renew.
Inspect other bearings
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6. Plated overlay almost completely removed by corrosion and wear. Partial scoring (Dirt in L.O.). Bearing may fall and cause heavy damages. Check journal for scratches and smooth with oil stone.
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Renew.
Inspect other bearings.
Inspect L.O. filters & separators.
Clean L.O. circuit.
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Connecting rod bottom ends
For some engines there are special procedures to tension the bottom end bolts which must be carefully followed. - Consult maker’s instructions. For some engines bottom end bolts have a limit life - consult maker’s instructions.
For engines with serrated couplings the lit of the bearing caps should be checked and the serrations crack detected.
In all cases refer to maker’s instructions.
Crankshaft deflections
These are most useful when taken with the engine “hot” and the vessel in a similar loading condition to the previous or reference readings.
Readings taken in drydock are of no significant use since the ship is supported by blocks and not taking it’s natural shape as in water.
Excessive deflections may be caused by :
Bearing wear (adjust or renew bearings)
Misalignment (re-align)
Loose or fretted chocks (re-chock engine).
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Low oil supply or oil starvation doesn’t just sound bad; it does bad things inside the engine.
Cylinder Liner
Reasons for Cylinder Liner Wear The wear in the cylinder liner is mainly because of following reasons:- 1) Due to friction. 2) Due to corrosion. 3) Abrasion 4) Scuffing or Adhesion |
