Crankshaft construction, defects, deflection and rebuilding.
Construction
Crankshafts of marine engines are made up of a number of cranks, which are built-up to form a single shaft. Lubrication of the crank bearing is important, and is usually from the Cross-head, through the connecting rod. This eliminates the bores, which act as stress raisers.
Crankshafts are rotated by forces transmitted through the connecting rods and bottom end bearings. Every crank is made up of two crank webs joined by a common crank-pin, to which the bottom end bearing is fitted.
Each web is connected to a journal to form part of a continuous shaft Main bearings support the shaft, at each journal One end of the shaft is connected to the Flywheel, to transmit the engine power. The other end is called as the 'free-end'.
Crankshafts are rotated by forces transmitted through the connecting rods and bottom end bearings. Every crank is made up of two crank webs joined by a common crank-pin, to which the bottom end bearing is fitted.
Each web is connected to a journal to form part of a continuous shaft Main bearings support the shaft, at each journal One end of the shaft is connected to the Flywheel, to transmit the engine power. The other end is called as the 'free-end'.
Crank shaft Defects and Re-Building
Defects in crankshaft
1. Bearing corrosion, if fuel and lubricating oil combine, resulting in weak acid formation, which could damage the bearing surfaces, resulting in scoring.
2. Vibration due to incorrect power balance, running in Critical band, running vessel in Light condition for long periods leading to impulsive forces from propeller, excessive wear-down of Thrust bearing.
3. Defects during manufacture, such as slag inclusions, incorrect heat treatment and machining defects of oil holes and fillets.
4. Fretting corrosion, which increases with load, amplitude of movement and frequency.
Re-building crankshaft
When a crankshaft is damaged, it may often be un-economical to replace, or even to remove and send ashore for repairs. In that case, a possible alternative is in-situ repairs.' Sometimes it is sufficient to re-grind the journals and fit over-sized bearings.
If repair in-situ is not possible, if the wear is excessive and there is no suitable over-sized bearing available, the crankshaft must be sent for re-conditioning. In that case, the journals are built-up, by arc welding, submerged arc welding or TIG / MIG welding and Chrome plating. The welding machines used are automatic. The crankshaft is first cleaned with an alkaline solution. All scores are removed by grinding. The welding torch is then positioned and the oil holes in the crankshaft are plugged with ceramic plugs, to prevent weld splatter from entering the openings. After completion of welding, the crankshaft must be stress-relieved. This may be done by butane burners. A temperature indicating crayon is used to mark the crankshaft. As the heat is applied, the colour of the mark changes, which indicates when the desired temperature reached. This repair is acceptable for 4 stroke engines, however replacement
still the preferred option.
After repairs, the crankshaft must be checked for kinetic and dynamic balance.
Kinetic unbalance occurs when the shaft is rotated. The un-balanced weight sets up a centrifugal force, which tends to throw the mass out-wards causing bending of the crankshaft. Dynamic un-balance results when there are two weights in separate planes. These weights will set up separate forces and cause the crankshaft to twist on an axis perpendicular to the axis of rotation. In the crankshaft, both kinetic and dynamic forces develop as soon as the engine starts.
3. Defects during manufacture, such as slag inclusions, incorrect heat treatment and machining defects of oil holes and fillets.
4. Fretting corrosion, which increases with load, amplitude of movement and frequency.
Re-building crankshaft
When a crankshaft is damaged, it may often be un-economical to replace, or even to remove and send ashore for repairs. In that case, a possible alternative is in-situ repairs.' Sometimes it is sufficient to re-grind the journals and fit over-sized bearings.
If repair in-situ is not possible, if the wear is excessive and there is no suitable over-sized bearing available, the crankshaft must be sent for re-conditioning. In that case, the journals are built-up, by arc welding, submerged arc welding or TIG / MIG welding and Chrome plating. The welding machines used are automatic. The crankshaft is first cleaned with an alkaline solution. All scores are removed by grinding. The welding torch is then positioned and the oil holes in the crankshaft are plugged with ceramic plugs, to prevent weld splatter from entering the openings. After completion of welding, the crankshaft must be stress-relieved. This may be done by butane burners. A temperature indicating crayon is used to mark the crankshaft. As the heat is applied, the colour of the mark changes, which indicates when the desired temperature reached. This repair is acceptable for 4 stroke engines, however replacement
still the preferred option.
After repairs, the crankshaft must be checked for kinetic and dynamic balance.
Kinetic unbalance occurs when the shaft is rotated. The un-balanced weight sets up a centrifugal force, which tends to throw the mass out-wards causing bending of the crankshaft. Dynamic un-balance results when there are two weights in separate planes. These weights will set up separate forces and cause the crankshaft to twist on an axis perpendicular to the axis of rotation. In the crankshaft, both kinetic and dynamic forces develop as soon as the engine starts.
Crankshaft deflection.
Reasons of crankshaft Misalignment
Misalignment of the crankshaft occurs due to wear of main bearings or distortion of the engine bedplate. This can also occur due to running aground, from damage to the ship's hull. It can be detected by measuring deflections of crankshaft webs, for each unit of the engine.
It may he advisable to rotate the turning gear slightly in reverse direction after stopping for readings, this will ensure free positioning of the cranks, particularly for those adjacent to the turning gear.
All readings are recorded and these should he compared with previous values, preferably with the ship in a similar load condition and at similar temperatures.
Total deflection vertically and horizontally is calculated for each crank. The vertical total will be proportional to misalignment between the bearings due to wear-down. The horizontal total indicates side wear in the bearings. By plotting all vertical deflections for the whole engine, it is possible to obtain information as to which main bearings are 'high' and which are 'low'.
This may be assisted by bridge gauge readings from the bearings, but these do, not take possible distortion of the bedplate into account. It must be checked that the journal is in contact with the bottom surface of the bearing.
Limiting values for maximum deflection are set by engine builders. These depend upon the stiffness of crankshaft, the engine's stroke/bore ratio, and so on. They indicate the limits to which misalignment may be permitted-before renewal of bearings and realignment are necessary.
Excessive misalignment will cause bending of the crankshaft and webs with fluctuating and altemating stresses, causing fatigue and the possibility of shaft failure. It will set up vibration and cause damage to main bearings.
The readings may be tabulated as follows:
Checking of crankshaft Misalignment
Frequency of inspections
a. Crankshaft deflections should be recorded at the time of installation.
b. After the first 1000 hrs. of running.
c. Annual ( during over-hauls). During this period, ship should be in the loaded condition and the engine warm.
d. After running aground, or when Holding down bolts are found damaged
a. Crankshaft deflections should be recorded at the time of installation.
b. After the first 1000 hrs. of running.
c. Annual ( during over-hauls). During this period, ship should be in the loaded condition and the engine warm.
d. After running aground, or when Holding down bolts are found damaged
Precautions:
a. Wear proper clothing, safety shoes and helmet
b. If the engine has been running, wait at least 20 minutes after the engine has been finally stopped before opening the crankcase doors. Keep the lubricating oil circulating pump in operation in the meantime. These precautions are necessary in order to avoid risks of a. crankcase explosion
c. Shut off starting air supply and fuel to the engine
d. Engage turning gear, obtain clearance from deck department for turning engine on turning gear during the time of deflection measurement
e. A person should remain stand by outside the crankcase when taking measurements
f. No smoking and no naked lights
a. Wear proper clothing, safety shoes and helmet
b. If the engine has been running, wait at least 20 minutes after the engine has been finally stopped before opening the crankcase doors. Keep the lubricating oil circulating pump in operation in the meantime. These precautions are necessary in order to avoid risks of a. crankcase explosion
c. Shut off starting air supply and fuel to the engine
d. Engage turning gear, obtain clearance from deck department for turning engine on turning gear during the time of deflection measurement
e. A person should remain stand by outside the crankcase when taking measurements
f. No smoking and no naked lights
Procedure:
a. Gauge positioning :- the micrometer gauge should be positioned between adjacent crank webs at the extreme edge of the journals opposite side-away from the crankpin as shown; normally a small circle is punched on the crankpin for convenience.
b. Place the gauge in position with the crank just past the bottom dead centre in the direction of turning so the connecting rod is clear of the gauge. c. Adjust the micrometer reading to show '0" in this position. The gauge should not be removed or touched till a full set of readinqs has been taken. If needed, a mirror may be used to read
d. Turn the engine in the direction of arrow and record successive readings port (P), top (T), starboard (S) and bottom (B2).
a. Gauge positioning :- the micrometer gauge should be positioned between adjacent crank webs at the extreme edge of the journals opposite side-away from the crankpin as shown; normally a small circle is punched on the crankpin for convenience.
b. Place the gauge in position with the crank just past the bottom dead centre in the direction of turning so the connecting rod is clear of the gauge. c. Adjust the micrometer reading to show '0" in this position. The gauge should not be removed or touched till a full set of readinqs has been taken. If needed, a mirror may be used to read
d. Turn the engine in the direction of arrow and record successive readings port (P), top (T), starboard (S) and bottom (B2).
Remove gauge from position B2 before the connecting rod touches it.
The reading B2 should be '0'. For small difference between 81 and 82 record bottom reading B as (B1 + B2)/2. In case of large difference repeat measurement
The reading B2 should be '0'. For small difference between 81 and 82 record bottom reading B as (B1 + B2)/2. In case of large difference repeat measurement
If misalignment exists the crank webs will open and close slightly as the engine is rotated; this is measured by means of a clock or dial gauge fitted between adjacent webs at a point in line with the outside of the journals furthest from the crank pin. A spring extension rod will hold this in position. The first measurement is taken with the engine just beyond bottom dead centre position with the gauge close to the side of the connecting rod. It is usual to set the gauge to zero. The engine is now rotated by the turning gear and stopped at each quarter turn, where gauge readings are taken as plus or minus values.
The final reading is taken near bottom centre, with the connecting rod on the opposite side of the gauge to the first reading. The first and last readings are averaged to use as an approximation for bottom centre position. This procedure is repeated for each unit in turn. Feeler gauges should be used to ascertain that the crankshaft has not lifted.It may he advisable to rotate the turning gear slightly in reverse direction after stopping for readings, this will ensure free positioning of the cranks, particularly for those adjacent to the turning gear.
All readings are recorded and these should he compared with previous values, preferably with the ship in a similar load condition and at similar temperatures.
Total deflection vertically and horizontally is calculated for each crank. The vertical total will be proportional to misalignment between the bearings due to wear-down. The horizontal total indicates side wear in the bearings. By plotting all vertical deflections for the whole engine, it is possible to obtain information as to which main bearings are 'high' and which are 'low'.
This may be assisted by bridge gauge readings from the bearings, but these do, not take possible distortion of the bedplate into account. It must be checked that the journal is in contact with the bottom surface of the bearing.
Limiting values for maximum deflection are set by engine builders. These depend upon the stiffness of crankshaft, the engine's stroke/bore ratio, and so on. They indicate the limits to which misalignment may be permitted-before renewal of bearings and realignment are necessary.
Excessive misalignment will cause bending of the crankshaft and webs with fluctuating and altemating stresses, causing fatigue and the possibility of shaft failure. It will set up vibration and cause damage to main bearings.
Recording of results
A positive reading of the micrometer gauge means opening up of the crank webs and negative means closing in. One division of the micrometer reading is usually 0.01 mm. Thus a reading of 5 means 0.05 mm and - 13 means ~O.13mm The difference (T - B) is deflection; the difference (P - S) is for referenceThe readings may be tabulated as follows:
| 1 | 2 | 3 | 4 | 5 | 6 | |
| Bottom (B1+B2) | ||||||
| Port(P) | ||||||
| Top(T) | ||||||
| Starboard(S) |
The following data should be recorded at the time of taking deflection:
a. Name of port and dateb. Draughts forward, aft and. amidships
c. Engine room and sea water temperatures
Study and interpretation of results:
a. When comparing the deflection readings' with previous ones, this must be done under similar draught conditions as the readings change according to the way the ship is loaded or ballastedb. The engine makers give the normal and permissible limits of deflection. These values increase as the stroke increases.
Causes of high deflection and checks to be made
a. Wear in the lower half of the main bearing or its saddleb. Structural deformation in way of the engine room due to grounding (permanent) or improper loading or ballasting (temporary)
c. Slack foundation bolts, shifted bedplate
Checks to be made in case of high deflection
A visual inspection may be made of the bedplate, foundation bolts and chocks and the, tank top surrounding areas for any slackness, damage or structural deformation that could cause distortion of the bedplate.A bridge gauge reading may be taken to check if the journal centerline is as original.
After removing the main bearing top cover a bridge gauge is placed on top machined surface of the bedplate and the clearance 'A' between the bridge gauge and the top of the journal is measured by a feeler gauge. Any increase in the reading 'A' compared to previous represents the wear down i.e. distance by which the journal has moved lower.
Some times due to improperfitting the main bearing lower half may work inside the bearing saddle resulting fretting of the saddle increasing the depth C of the saddle.
Further check may be made by removing the main bearing lower half and measuring the crown thickness.
Measuring the wear down is also possible by means of a depth gauge without removing the main bearing cover. In such cases the lubricating oil supply pipe to the main baring is removed and a micrometer depth gauge inserted through the oil hole to reach the journal.
By taking the depth gauge reading and comparing with the previous, the wear down can be ascertained.
If the wear is in the bearing, it is a simple matter to replace the bearing. However if the wear is in the bearing saddle then it is a major work dismantling the engine to remove the crankshaft, line boring the bedplate and fitting thicker bearing shell.
Bedplate can be checked for straightness by stretching a piano wire about 0.5 mm diameter with weights about 40 kg passing over pulleys on stands and measuring the wire heights above the bedplate upper surface at intervals along the length as shown in sketch.
The true heights are obtained after making corrections for the piano wire sag from the tables. By checking the heights with those measured at the time Of installation of the engine, it can be established if the bedplate has distorted. Remedial measures may need the engine to be disconnected from shaft line, repairs carried out and then complete realignment and re-chocking of the engine.,
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