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Diesel engine crankshaft

In this post on the diesel engine crankshaft we will discuss the Crankshaft Of Marine Diesel Engines, I have explained right from the basic design to the crankshaft’s alignment technology. In addition to the crankshaft, I have also described in detail the Crankshaft bearings—this post will be helpful for marine engineers and students.

Crankshaft (Diesel engine crankshaft)

 

Diesel engine crankshaft is the most mysterious, complicated, and crucial component of any internal combustion engine.

During the engine running period, Heavy operational loads are experienced by the crankshaft.

Crankshaft converts the reciprocating motion in to rotary motion developed due to thecombustion of fuel and air in side the combustion chamber. 

This rotary motion is further transmitted for various applications. In the case of Propulsion engines, it is used for rotating the Propeller, and for Generators, it is used for turning the Alternator,

Other components function depends upon the correct revolution of Crankshaft, such as Camshaft for the firing order and fuel timing. The Engine will come to a standstill if a single part of the Crankshaft fails.

Due to the operation’s nature, Diesel engine crankshaft is subjected to several types of forces like bending and twisting action upon the Crankshafts depending upon applications’ nature. These forces are due to various factors not limited to the piston’s weight, combustion loads, axial load from propellers, compression loads of webs on journals, and generators’ heavy torque loads.

Types of the Diesel engine crankshaft

The crankshaft of marine Diesel is as per the following classifications

Sr. No.	Type of Crankshaft
1	Solid Single Piece
2	Semi Built
3	Fully Built
4	Welded

Semi Built Diesel engine crankshaft

In this design pins and crank throw is cast out separately. Same forging is used for throws consisting of 2 webs and the crank pins. Separately machined main journals are shrink fitted in tot he drilled holes of crank webs.

Advantage of this method is the crankpins and webs are single forging and the web can be reduced in thickness reducing the weight without affecting the strength.

But there is a disadvantage that due to the shrink fit developed hoop stresses can lead to cracks. This can be prevented by balancing the unit.

Fully Built Diesel engine crankshaft

In this type of design, all the components are fabircated separately and shrink fitted. Webs, crankpins, and main journals are fabricated separately. Crank pins and journals are machined and shrink-fitted in the heated webs with bored matching holes. Fitting of dowel to locate is not accepted since these dowels are stress riser.

Reasons for Diesel engine crankshaft Failure

1. Over speeding of Engine on Load.
2. Eddy currents/ stray currents being passed from Faulty Alternator 
3. Fatigue failure originated from the lip of a bored oil hole in the crankpin.
4. Heavy vibration created due to torsion may lead to crack in the crankpin and main journals.
5. Starvation of lubrication has been found the most common cause of bearing failures resulting in the significant loss of crankshafts.
6. The cylinder over-pressurized due to the hydraulic lock due to water leakage in the cylinders. 
7. Crack developed on the fillets.
8. Misaligned Crankshaft

Reason of the Diesel engine crankshaft misalignment

1. Damaged main Bearing
2. Disturbed line bore of bedplate in Over slung type of Crankshaft holding arrangement
3. Disturbed line bore of the cylinder block in the underslung type of crankshaft holding arrangements.
4. Incorrect tightness of the foundation bolts
5. Crack in bearing saddle
6. Loose Main bearing bolts
7. Excessive forces from piston assembly are causing bending moment.
8. Crankcase fire/explosion
9. Cracked bearing housing

10. Broken tie bolts

11. Structure weakening due to erosion and corrosion

How to prevent Diesel engine crankshaft failure

1. Take immediate action to eliminate the vibration. Don’t run the engine for a prolonged period with heavy vibration.
2. Check the foundation bolts of the Engine regularly.
3. Keep the Engine Tuned
4. Check the Deflection of the Crankshaft regularly.
5. Keep the Engine and Alternator aligned and should maintain the alignment reading within the prescribed limits.
6. Always look for any metal particle on the lube oil filter disc while cleaning.
7. Get the Lube oil sample checked periodically.
8. Many times Governor Malfunctioning has been the reason for Engine over speeding. Hence carry out the Governor maintenance periodically as per the schedule.
9. Avoid Overloading of Engine

10.Avoid Over speeding of Engine. 

11.Carry out the function test of the following instruments

1. Over speed trip
2. Low lube oil pressure alarm.
3. High lube oil temperature alarm
4. Crankcase pressure alarm

Diesel engine crankshaft Alignment Technology

An arrangement in which the Prime mover and Alternator are positioned in a straight line or parallel to each other ( Axial and radial directions) is called Alignment.

Primary Symptoms of Diesel engine crankshaft Misalignment

1. Increased Vibration
2. High body casting temperature 
3. Frequent loosening/breaking of foundation bolts
4. Shims and dowels are getting loosened up.
5. Excessive leakage at seals
6. High rate of coupling failure
7. Premature & frequent failure of Bearings

Results of Misalignment of Diesel engine crankshaft

1. Failure of  Crankshaft and bearings.
2. Vibrations in Machine 
3. The major breakdown of Components due to stress loading

Reasons for performing a Alignment of Diesel engine crankshaft

1. Whenever a driver or driven equipment is removed for repairs.
2. Excessive vibration in equipment 

Type of Misalignment in Diesel engine crankshaft

• Parallel Misalignment
• Angular Misalignment

Measuring and correcting Diesel engine crankshaft Misalignment

Basic Principles involved in measuring Parallel and angular Misalignment

A vital part of carrying out the Alignment is keeping the reference points. And maintaining the same throughout the process. If the reference points are changed, then there are chances that the Alignment will not be perfect.

There following components in the alignment process.

1. Movable Component: – A components moves during the alignment process is called the Movable component. It is not easy to move the engine, and hence only generator is shifted during the alignment.
2.  Fixed Component: – Prime Mover is considered as the Fixed Component. It remains stationary, and its shaft becomes the reference point.

Shaft Run out

Face Run out

Place the Dial Gauge on a fixed surface and pointer at the face of Wheel. Turn the shaft by one complete revolution. Any change in reading will indicate the untrue beginning. 

Radial Run Out

Position the dial gauge on a fixed surface and the pointer at the periphery of the flywheel to check the centre diameter. Turn the flywheel by one complete revolution to get the reading of off centre diameter.

The parallelism between two components

For measuring the shaft parallelism turn the shaft by one revolution, keeping the dial gauge on a fixed surface and the pointer at the wheel.

Deflection (Diesel engine crankshaft)

The crankshaft journals rest on Main Bearings. These main bearings support the total weight of the Crankshaft. In the prolonged running, there is wear in the bearings. The pattern of this wear is abnormal due to various Engine running conditions. Central crankshaft line will change its position in the event of any abnormal wear down of bearings. This change in status will either be upward or downward. This slight change in the position of Crankshaft will make it run with fatigue in the crank webs. Due to this reason, the measurement of crankshaft web deflection is required. If the deflection is beyond the permissible limit, the fatigue level will be dangerous and cause the crankshaft failure.

Important Instructions ( function of Crankshaft)

1. Deflection readings may vary with the measuring conditions, which need to be the same as possible for the exact comparison.
2. Deflection measurement should be carried out at the Engine cold conditions. The dial gauge should be nearly at the same temperature as the Engine.

Crankshaft Deflection Checking Procedure

1. Rotate the Crankshaft to measuring position 1.
2. Set the dial gauge to zero
3. Rotate the Crankshaft and read the values at positions 2 to 5. (Only the deviation from zero settings of the dial gauge to be recorded.)
4. Evaluate these readings.

• A reading at position 5 should be nearly the same as reading at position 1. In case of significant difference, then repeat the steps from 2 to5.
• Vertical and horizontal deflections are as follows.
• Vertical deflection; dv =T-(x+y)/2Horizontal deflection ; 
• dh= P-S

Instructions

• If the vertical and horizontal deflections have exceeded the Equipment manufacturer’s limits, realign the Engine and driven machinery. 
• Before realigning, check the bearings metal for any abnormal wear.
• Recheck the deflection by repeating steps 1 to5.
• Record the measurements 

Benefits ofDiesel engine crankshaft Alignment

1. Increased reliability and operating life of the machine.
2. Reduction in the following areas.

• Spare Parts Consumption
• Maintenance labor cost
• Production loss
• Need for standby arrangements
• Safety
• Cost of Power Consumption

Crankshaft Bearings of Diesel engine crankshaft

The rotational power of the Propeller or Alternator to turn the Crankshaft is considered by the engine. Connecting rod bearings and main bearings support the crankshaft via connecting rod. Main bearings transfers the load with zero friction with the metal components

Special bearing material plays a vital role in keeping the rotating main journal floating in the Main bearings in the film of lubricating oil.

The Bearing in a marine diesel engine is subj

Following forces act on the bearings in a marine Diesel Engine.

1. Gas pressure developed inside the cylinder.
2. Due to the development of inertia forces by Reciprocating and rotating parts. 
3. Reciprocating and rotating parts generate centrifugal forces.
4. Engine vibration creating friction between crankshaft journal and Barings.

The main Bearing design is such that it overcomes the various forces along with the support of the crankshaft revolving at high speed. Hence the material for the Bearing is chosen to support the journal and have the embedding qualities.

Main Bearing Metallurgy

1. Should be corrosion resistance
2. High resistance to friction to reduce losses 
3. Should have enough strength to carry the dynamic load
4. Nonreactive with lubricating oil
5. Should behave the best running inability
6. Should be lubrication oil film supportive of maintaining the hydrodynamic lubrication.
7. Should have the best embed ability quality to embed any small particles carried along with lubricating oil
8. The tensile and compressive strength of material should be of excellent level
9. Thermal resistance withstands the temperature in some abnormal conditions if the Bearing is running hot.

Types of Bearing

1. Lead Bronze Bearing
2. Bi-metal Bearing
3. Tri-Metal Bearing

Types of bearing failure (function of Crankshaft)

1. Fatigue failure
2. Wiping
3. Erosion
4. Spark Erosion

Use the new bearings during maintenance as per the OEM recommendations. However, an inspection of bearing and assessment of the condition may ask for the premature change of bearings. Analyzing the reading of used bearing will reveal the cause of deterioration.

Generally, all bearing will show signs of some wear. It will reveal scoring, wiping, dirt, or embedding of the foreign particle during the close inspection. Some other problem which needs to take care will be indicated by the damage or uneven wear in one or more crankshafts.

CAUSES OF BEARING FAILURE

The aim of this topic is for the reader to enable the diagnosis of engine bearing failure. Maximum common causes of Bearing Failure are listed below.

• DirtLubrication system dirt
• Dirt at the bearing back
• Inadequate Lube Oil

• Lubrication system problems
• Oil seal leakage
• Wrong Assembly

• Bearing assembled in reverse position
• Connecting Rod bearing caps assembled in reverse position
• Components machined improperly

• Housing fillet radius grounded improperly
• Journals machined out of shape-hour-glass shaped, barrel-shaped, or tapered shape.
• Misalignment of housing and shaft ( distorted line bore of housing)
• Crush not provided sufficiently.
• Overloading
• Corrosion
• Cavitations
• Dirt

Dirt in the Lubrication System of Diesel engine crankshaft

Ingress of the Dirt particles in the lubrication system is one of the most frequent and common causes of severe damage to the bearing. Dirty Engine is the main problem. If the girt of dirt is more significant than the embedding surface, then the dirt particles will cause the scratch and, in severe cases, deep lining on the journal and crank pins.

Guidance: – Ensure the complete careful cleaning of the engine after the repairs 

Dirt at bearing back

The presence of foreign particles in the dirt at the bearing back of Diesel engine crankshaft will raise the bearing from that point, making contact with the crankshaft journal. Can be witnessed localized wear at the area opposite to particle and the inner surface of the bearing.

 Guidance;-Clean the bearing housing properly before the assembly.

Inadequate Lubrication

Complete or partial failure of oil supply to bearing journals of Diesel engine crankshaft will lead to a severe bearing seizure. In this case, the amount of oil supplied to the bearing cannot hydrodynamic lubrication film and results in boundary lubrication. Operation under such circumstances will lead to catastrophic failure of principal components.

 Guidance;-Clean the bearing housing properly before the assembly of Diesel engine crankshaft

Inadequate Lubrication

Guidance:- carry out the complete check of the Lube oil system to ascertain the cause of the problem. The results of such investigation will lead to clogged lube oil passage, wrongly installed bearing, or the lube oil pump failure.

Oil Seal failure;- The Crankshaft seal’s failure will result in oil escape from the lubricating point. Due to the broken oil film, the bearing will seize

. Guidance:- Check and replace the oil seals at such places.

Bearing assembled in reverse position

If the bearing shell halves are assembled in the wrong position, the oil hole will be covered by the shell half having the oil hole. This will lead to a significant seizure.

Guidance;- Double check the position of bearing shells before the positioning in location.

Distorted bore hole in housing: – Due to vibrations, the shape of the bore changes. This will have an impression on the bearing shell to take the same shape. This may be oval or some other shape not keeping the perfect roundness, and Will lead in the Bearing seizer.

Guidance: – Check the accurate machining of bore and shaft.

Wrong machining of components

This phenomenon occurs if the grinding stone has worn excessively and adopted a different shape. This shape will affect the part’s grounded portion, resulting in the Oval, Barrel shape, or tapered shape of Pin and journals. The overloaded area of the region will suffer from oil starvation and lead to seizures.

Guidance: – Check the shape of the grinding tone before the grinding operation. If needed, carry out the dressing of stone.

Misaligned housing and shaft– function of crankshaft

Shaft and housing will get misaligned due to the following factors.

• Wrong machining
• Bent crankshaft
• Distorted cylinder block
• Distorted bedplate.

All the above abnormalities will result in Lubricating oil failure at the localized spot and lead to bearing seizure.

Guidance:- Guidance- Ensure the machining tolerances of the components are well within the Manufacturer’s prescribed limit.

Improper crush

 The improper crush will not let thebe ring seat correctly on location and move with the movements. This will create the patches and ultimate failure of bearing in case of a prolonged run.

Guidance:-Follow the Manufacturer’s recommendations for the correct size of the housing bore and torque.

 Overloading

Excessive load due to operating conditions will lead the bearing failure due to metal fatigue.

Guidance;- The engine should be well-tuned and. Assembly clearances should be maintained properly as per prescribed tolerances.

Corrosion

The poor condition of oil can damage the bearing surfaces. This effect is the result of lead getting diluted in the alloy.

Guidance

Guide lines on the grade of oil are to be followed as per the OEM recommendations. The oil changing schedule should be followed strictly as per the oil change schedules of Diesel engine crankshaft.

Cavitations

Due to some arduous running conditions, oil pressure drops and produces vapors that cause cavitations failure. 

Guidance;-Check the Oil pressure and flow rates.

Lubricating oil properties

Main Engine Crankcase

• TBN should below
• Good Detergency
• Good stability to thermal and oxidation.
• Water separable property

Cylinder Lubrication Properties

• TBN retention capability with high TBN
• High stability to oxidation and thermal
• Anti-wear property should be perfect
• Good detergency
• Burning without residue formation.

Auxiliary Engine (What is the function of crankshaft)

1. Low formation of ash and carbon
2. High stability to oxidation and thermal
3. Good water separable property
4. High anti-wear property
5. TBN retention capability with high TBN
6. High disperse-ability and detergency
7. Should be zinc free for the protection of sliver bearing
8. With high viscosity index

Inspection and Change of Main Bearing 

Inspection and Change of Main Bearing

1. Remove Crankcase covers
2. Turn the Crankshaft to the position of work carrying out convenience. 
3. Dismount the Bracing screws ( Side Screws )
4. Mount the hydraulic tool and loosen the main bearings stud nuts.
5. Remove the hydraulic tool and loosen the nuts a little bit.
6. Fit the eyebolts on both sides diagonally in the bearing cap threaded holes.
7. Insert the wire rope on both the eye bolts to hold the bearing cap in place while removing the bearing nuts.
8. Loosen the main bearing cap from the engine frame by taping it with a lead hammer.
9. Unscrew the bearing nuts by slightly lifting the main bearing cap.
10. Lower down bearing cap on the guide tube. Bearing cap should rest properly on the guide tube, which is fitted on the studs of the bearing.
11. Take out the bearing shell by removing the locking piece on the bearing cap.
12. Remove the bearing shell with the help of upper bearing shell removing tool.
13. Inspect the Main bearing shell /Note;– (The bearings are marked according to the size, and the new bearing also should match the same size)
14. Push the upper bearing shell in the upper part of housing as far as possible
15. Insert the bearing removal tool into the oil hole and rotate the Crankshaft slowly to turn the bearing shell in the housing. Ensure that the shell enters the correct position and remove the tool for the upper main bearing.
16. Use the molycote paste to lubricate the end of the bearing shell. 
17. Lift the bearing cap to its position. Take out the guide tubes, and fit the bearing studs with proper lubrication. (Ensure the correct position of bearing shell and bearing cap)
18. Remove the wire straps.
19. Tighten the nut to the specified limit by fitting the hydraulic tool.
20. Coat the Backside of bracing screws with siltstone.
21. Fit the nut and carry out the tightening at proper torque..

Methods for Checking the Bearing Clearance 

There are two methods for checking the bearing clearances.

1. Lead wire method
2. Feeler gauge method

Lead wire method

The lead wire method is the most popular method of checking the bearing clearances. In this method following procedure is adopted.

1. Bring the crankpin to TDC
2. Remove the locking arrangements from the bolts. Mark the position of the nut.
3. Install the hydraulic tool and slacken the nuts
4. Slide down the bottom half
5. Lay the three-fold or plastic stick.
6. Place the bottom half of the bearing in place and tighten to full hydraulic pressure or the torque spanner.
7. Loosen the nuts
8. Slide down the bottom half
9. Take out the compressed Lead or plastic piece.
10. Measure the thickness
11. Compare the measurement with the engine manufacturer’s data.
12. Replace the bearing shells in case the clearance is measured to be excessive than the prescribed limits.

Feeler gauge Method to check the clearance of Diesel engine crankshaft

Though this method is quicker than the Lead wire method, it has to be checked with utmost care since there are chances of bearing surfing scratch marks while inserting the feeler gauge.

Carry out the check as per the following procedure.

1. Bring the crankshaft to BDC position.
2. Take the measurement readings by feeler gauge insertion between lower half and crankpin.

This method will not provide the exact readings. Since the bearing and shaft measurement area is round in surface and the feeler gauge is straight.

How to check performance after replacement of Diesel engine crankshaft?

After the maintenance work on Crankshaft following checks are done to assess the performance of Crankshaft

Static Checks (crankshaft)

• Turn the Crankshaft to check the hardness in turning.
• Check the deflection of Crankshaft on all webs. Readings of the cold and hot deflection should be at par with the Equipment manufacturer.
•  Alignment readings should match or near the readings at the time of the new installation of this machine.
• Check the end float of the Crankshaft. End float should be almost near to the original readings of Crankshaft.

Dynamic checks

Carry out and record the following checks during the trial run of machinery

• Lube oil pressure
• Lube oil Temperature
• Main bearing Temperatures
• Any abnormal noise from the free end and alternator end
• Symptoms of any hot spot inside the crankcase ( After stopping the engine)

MAN B&W Engines Bearing Sizes

I hope the post on  Diesel engine crankshaft has been useful to the reader. I will welcome the Reader’s comment for the improvement of this post-Diesel engine crankshaft

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