What is main bearings ?
Main Bearing also defined as the bearings which directly supports and are in contact with the crankshaft of the engine
The main bearings in a piston engine are the bearings that hold the crankshaft in place and also allow it to rotate within the engine block.
Main bearings are typically plain or journal bearings that are held in place by the engine block and bearing caps. The number of main bearings per engine varies between engines, and is frequently determined by the forces produced by the engine’s operation.
Main bearings (MB ) did the work of Supporting the long running crankshaft throughout the engine’s length. It is necessary to check the condition of bearing at regular interval of time.
Total number of main bearing are :- n+1
What is main bearing clearance ?
Clearance of main barring is the space between the crankshaft and bottom shell of main bearings.
Significance of main bearing
The main bearing clearance measurement determines the amount of wear down the bearing has endured. The bearing clearance will be determined by the engine’s size.
Must read :- Types of lubrication
Why clearance of main bearing is measured ?
The clearance is measured to determine how much bear down if wearing occurs.
The bearing clearance will vary depending on engine size, but for a 900mm bore engine, the clearance is between 0.40 and 0.70 mm, with a maximum allowable of 0.9mm. Similarly, for a 500mm bore engine, the allowable clearance is specified as 0.4mm to 0.55mm.
All modern bearings are typically thin-walled with non-adjustable clearance. If the bearing clearance has reached its maximum or the bearing has become damaged, it cannot be reconditioned and must be replaced.
Main bearing clearance :- 0.3 to 0.4 mm
Note:- Measures of clearance of MB determines the amount if wear down of bearing. It is taken between the journal and web.
Reason for Bearing Clearance
The increase in bearing clearance indicates that the bearing material has worn out, which could be due to one of the following reasons:
1. Journal Defect: A journal is a polished part of the crankshaft that houses the bearings and allows the crankshaft to rotate. If the journal pin is faulty, the bearing clearance will increase more quickly. The cause of journal pin damage:-
- Overheating: Bearing materials are made of materials that are easier to embead and absorb shock. In the event of a defect, the bearing material will wear out, preserving the shaft journal. If the engine is run for an extended period of time after the bearing material has been wiped out, the shaft will become overheated, causing damage to the journal, cracking, and an increase in the hardness of the journal.
- Hairline cracks: When the crankshaft is turned on, the journal pin is stressed. If stresses on the journal are localized and increased, a crack may form in such high stress areas, particularly near fillet radii and oil grooves. An expert technician can remove this by pin grinding. This will change the size of the bearing that will be used for the grinded pin, as well as de-rate the engine.
- Metal Contact: This is a problem similar to abrasive damage.
Squeezing, scoring, cracking, and pitting will occur on the journal pin if there is metal to metal contact due to wiping of the bearing material or dirty lube oil containing small metal particles.
2. Bearing Defect: If the crankshaft journal is fine but the defect is in the bearing itself, it could be due to one of the following reasons:
- High oil temperature: If the lube oil temperature supplied to the bearing is not maintained, the oil thickness will decrease and metal contact will occur.
- Oil viscosity: Correct viscosity oil with a high viscosity index should be used; otherwise, bearing material will not be able to float on the oil film and contact the journal metal.
- Carrying capacity for oil loads: The oil’s load carrying capacity is an extremely important factor. When the ship is operated at a higher load, which is transmitted to the main bearings, a low load carrying capacity oil will fail, resulting in bearing damage.
- Oil pressure: Maintaining a continuous oil supply and proper oil pressure will ensure that the oil film between the bearings and the journal is never depleted, allowing the oil to lubricate the bearings.
- Temperature in the engine compartment: If the engine is started in cold weather, the oil supply to the bearings will be insufficient, resulting in bearing material damage. The bearing closest to the oil supply pump will be the most affected.
- Spark erosion: It can cause overheating of main engine bearings due to insufficient lubrication caused by cavities. More information is available here.
How to Measure main Bearing Clearance of main engine ?
To measure the clearance of a marine engine’s main bearing, various methods are used by different marine engine manufacturers. The following are some of the most common methods used onboard ships to measure main bearing clearance:
- Bridge gauge method
- Telescopic feeler gauge / Swedish feeler gauge
- L.o inlet pipe connection
- Lead wire method.
- Take immobilzaton certificate from company
- Enclosed space permit
- Tool box meeting
- Take the all necessary tools
- Proper PPE to be used
- Stop main engine
- Turning gear engage
- Indicator cock open and shut down starting air supply
- Stop lube oil pump
- Note down draft
- Open crankcase ant ventilate the crankcase
- Note down the firing order and start with first unit. Carry out it in sequence.
- Turn the crankcase throw to one exhaust side.
Bridge gauge method
Bridge with depth gauge
Sulzer 2 strike marine diese engines uses this method for measuring main bearing clearance.
Step to measure
- First remove the shell of bearing along with the keep ( the shell of bearing is lined with the keep).
- After removing the bearing shell, a bridge is fitted over the top of journal pin as shown in the figure below.
- A bridge is fitted from Port to starboard, which makes a bridge over the crankshaft of the engine with two enda supported on cross girder.
- Take the simple vernier type depth gauge.
- Insert the depth gauge in the hole which is provided on the bridge and the scale of the depth gauge which is rested on the pin of crankshaft.
- Measured the total depth of the scale of depth gauge.
- Compare measured length with the previous measured reading or the reading given in the manual.
- Difference between measured and previous calculated gives the wear down of main bearing.
Note :- In older SULZER engines, a collar and a small hole are provided in the bearing shell. Thus, without removing the keep, the bridge is installed adjacent to the keep, and the depth gauge is used to measure shell wear down from the hole provided in the shell.
Bridge with feeler gauge
In some engines, the bridge is installed after the shell and keep have been removed, as described above. In addition, a feeler gauge is used instead of a depth gauge to measure the clearance between the journal pin top and the bridge bottom. The bridge used here is different in height, and the gap between the pin and the bridge is much smaller than the bridge used in the previous method.
A bridge gauge reading may be taken as shown 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.
3. Telescopic or Swedish feeler gauge
This is the most common method of measuring the bearing clearance of the top shell in engines like MAN B&W. There is no need to disconnect or keep any connections or keepsakes when using this method to measure clearance.
The telescopic gauge is placed between the crank web gap and the bearing keep. When the tip reaches the top of the shell, the feeler is inserted between the shell and the pin to ensure proper clearance.
4. Dial type Depth gauge method
This method of measuring the clearance of M.B is used in new MAN B&W engines (SMC-C) and does not necessitate the removal of the top keep. The lube oil pipe connection screw hole is located in the bearing keep, which can be accessed through the bearing shell hole.
The dial gauge is inserted into this screw hole, and the reading is used to determine upper shell clearance.
5. Lead wire method
This is a traditional method that should be used when no other options or tools are available. In this method, lead wire is inserted at various points between the bearing and the pin. The bearing housing has been snugged up. Make sure you haven’t oversqueezed the wire by more than a third of its original diameter. General Procedure for Obtaining Clearance Using Lead Wire:
- Firstly, turn the crankshaft of the engine to TDC postion for the crank.
- Remove locking arrangements and Hydraulically, slacken the nut for lowering the bottom half shell with bolts.
- Three lead wire lengths to be inserted or laid circumferentially in the bottom half at three different locations.
- After inserting lead wire , fit the bottom half shell into position and tighten the nut at rated torque.
- Again open the nut and lowered the bottom half shell.
- Take out the lead wire and measure it’s length.
- Compare it with previous one.
Note:- Clearance of bearing should be within limit. If it is more i.e out if limit , need to replace the bearing shell.
How to measure Bottom End Bearing Clearance
Bottom end bearing is the bearing where bottom of connecting rod connected with crank pin of the crankshaft.
Bottom end bearing also known as crank pin bearings
Method of measuring bottom end bearing clearance
- Lead wire method
- Telescopic gauge method
- Feeler gauge method
Lead wire method
- First, turn the crankshaft of the engine and set the crank at the TDC position.
- After that remove locking arrangements and mark the postion of nut.
- Start slackens the nut and lower the botom half shell of connecting rod.
- Take three lead wire of three length and placed it circumferentially at three positions on the bottom half shell
- After that fit the bottom half in position and tighten it with the same rated torque
- Again slacken the nut and lower the bottom half shell
- Remove lead wires and take its measurements.
- Compare it with previous measured clearance.
It should be in acceptable limit otherwise need to replace bottom shell of connecting rod.
Telescopic gauge method
It is very simple method.
- First, turn the crankshaft and let the postion of crank at BDC postion
- Insert the telescopic gauge in between the lower half and crank pin
- Take the measurements readings.
Feeler gauge method
- Turn the crankshaft of the engine and set the crank at BDC position
- Start inserting the feeler gauge between the lower and and crank pin.
- Take the measurements.