A marine engine comprises several types of bearings that support the rotating and reciprocating crankshaft, camshaft, and crosshead of the engine, enabling the marine engine to perform the desired operation of power generation. It is very important to maintain good quality and quantity of lubricating oil for all these bearings to avoid the breakdown of the engine.

In this article, we will explore the most acceptable and approved ways to taking measurements or dimensions in main bearing clearance of marine diesel engines

What is the Main Bearing?

Bearings, which directly support and are in contact with the crankshaft of the engine are known as main bearings.

The main bearing of a marine engine supports the long-running crankshaft throughout the engine length. This makes it imperative to check the condition of the bearing at regular intervals of time.

Why Clearance is Measured?

The clearance measurement of the main bearing determines the amount of wear down the bearing has been subjected to.

The clearance of the bearings will depend on the size of the engine 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 provided as between 0.4mm and 0.55mm

All the modern bearings are usually of the thin wall type with nonadjustable clearance. If the bearing clearance has reached its maximum limit or the bearing got damaged, it cannot be reconditioned and needs to be changed.

Reason for Bearing Clearance

The increase in bearing clearance indicates the worn-out bearing material, which may be due to the following reasons:

1. 1 | Journal Defect: A journal is a polished part of the crankshaft accommodating the bearings, helping the crankshaft setup to rotate. If the journal pin is defective, the bearing clearance will increase at a faster rate.

Reason for journal pin damage:

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• Overheating: Bearing materials are made from material that has better embeddability shock-absorbing properties because of any defects, the bearing material will wear out saving the shaft journal. If the engine is run long even after the bearing material is wiped out, the shaft will get overheated leading to damage to the journal developing cracks, and an increase in the hardness of the journal.
• Hairline Cracks: The journal pin is under stress when the crankshaft is in operation. If the stresses on the journal are localized and increased, a crack may form in such high-stress areas, especially near fillet radii and oil grooves. To get rid of this, the expert technician can do pin grinding. This will change the size of the bearing to be used for the ground pin and also de-rate the engine.
• Metal Contact: Similar problem to the abrasive damage. If there is metal-to-metal contact due to wiping of the bearing material, or dirty lube oil containing small metal particles- squeezing, scoring, cracking, and pitting will occur on the journal pin.

1. 2 | Bearing Defect: If the journal of the crankshaft is fine but the defect is in the bearing itself, it may be due to the following reasons:

• • High oil temperature: Not maintaining the lube oil temperature supplied to the bearing will decrease the oil thickness and lead to metal contact.
• • Oil viscosity: Correct viscosity oil should be used with a high viscosity index else bearing material will not be able to float on the oil film and contact the journal metal.
• • Oil Load carrying capacity: The load-carrying capacity of the oil is extremely important, When the ship is operated with a higher load, which in turn is transmitted to the main bearings, a low load-carrying capacity oil will fail in the situation, resulting in bearing damage.
• • Oil pressure: Maintaining the continuous oil supply and correct oil pressure will ensure oil film between bearings and journal is never run out to maintain the oil lubrication.
• • Engine ambient temperature: If the engine is started at a cold ambient temperature, the oil supply during the start to the bearings will not be sufficient leading to damage to the bearing material. The bearing which is placed farthest from the oil supply pump will be affected largely.
• • Spark erosion: It may lead to overheating of main engine bearings caused by improper lubrication resulting from cavities.

Methods to Measure Bearing Clearance

There are various types of methods adopted by different marine engine manufacturers to measure the clearance of the main bearing of marine engines. Following are some of the most prominent methods used onboard ships to measure the clearance of the main bearing:

1. Bridge with Depth Gauge

This method is used in SULZER 2-stroke marine engines where the bearing‘s shell is removed along with the keep (the bearing shell is lined with the keep). After that a bridge is fitted over the top of the journal pin, from port to starboard, making a bridge over the crankshaft with two ends supported on the cross girder.

A simple vernier-type depth gauge is then inserted in the hole provided on the bridge and the scale of the depth gauge is rested on the crankshaft pin. The total depth on the scale is measured and compared with the previous reading and the reading in the manual for calculating the wear down of the bearing.

In old model SULZER engines, a collar is provided in the bearing shell along with a small hole. Thus, without removing the keep, the bridge is fitted adjacent to the keep, and the depth gauge is used from the hole provided in the shell to measure the shell wear down.

2. Bridge with Feeler Gauge

In some engines, after removing the shell and the keep, the bridge is installed as explained in the above point. Also, in place of a depth gauge, a feeler gauge is used to measure the clearance between the journal pin top and the bridge bottom. The bridge used here is different in terms of height and the gap between the pin and the bridge is very small as compared to that of the bridge used in the above-mentioned method.

3. Telescopic or Swedish Feeler Gauge

In engines like MAN B&W, this is the most common method used to measure the bearing clearance of the top shell. In this method, there is no need to remove any connection or keep it for measuring the clearance.

The telescopic gauge is inserted between the gap of the crank web and the bearing keep. When the tip reaches the shell top, the feeler is inserted between the shell and the pin to check the clearance.

4. Dial type Depth Gauge

This method is used in new MAN B&W engines (SMC-C) which does not require the top keep to be removed. The lube oil pipe connection screw hole is in the bearing keep which can be accessed from the hole on the bearing shell.

The dial gauge is inserted in this screw hole and the reading is taken as the clearance for the upper shell.

5. Lead wire – The Traditional Method

This is a traditional method and is to be used when no other alternative or tools are present. In this method, lead wire is inserted at different positions between the bearing and pin. The bearing housing is tightened. Ensure not to over-squeeze the wire more than 1/3rd of the original diameter.

General Method for taking clearance using lead wire:

• • Turn the crankshaft to set the crank at the TDC position.
• • Remove the locking and slacken the nut to lower the bottom half with bolts.
• • Three lengths of lead wires are to be inserted or laid circumferential in the bottom half at three different places.
• • Close the bottom half into position and tighten the nut to its rated tightening torque.
• • Open and lower down the bottom half again.
• • Remove the lead wires and take the measurements.
• • It must be within the limit, if out of the limit, the bearing shell must be replaced with new ones, or the clearance by adjusting the shims.

Contact us now to discuss how to help you optimize the health of your marine diesel engines.

Contact Ugpeo Technique at +234-80778-12370 or visit www.ugpeotecnique.com and embark on your journey to seamless, safe, and successful maritime ventures!