How to Check the Bearing Clearances of Marine Engines
Bearings are common components of marine vessels and their engines, implemented for the means of supporting rotational and reciprocating movements of crankshafts, camshafts, and the cross head of the engine. Bearings play a crucial role in facilitating power generation operations for various marine engine types, and their well-being and lubrication will regularly dictate the health of the engine and its avoidance of breakdowns. As the clearances of bearings will often denote the amount of wear that such components have undergone, it is important to understand how to conduct such measurements as well as what they mean.
When a bearing is placed within a marine engine and is in contact with its crankshaft, that bearing is known as the main bearing. The main bearing is tasked with supporting the length of the crankshaft as it passes through the length of the engine, and as such it is crucial that clearances are checked on a regular interval. Bearing internal clearances are considered to be the distance that a bearing ring may move in relation to another in a radial or axial direction, and their measurements are regularly used to determine wear and damage.
While breaking clearances will often depend upon the size of marine engine parts and assemblies, there are some general standards that are seen across systems. For a 900 mm bore engine, for example, the clearance range will often sit between 0.40 and 0.70 mm and should never exceed 0.90 mm. As many modern bearings feature thin walls and non-adjustable clearances, surpassing maximum clearance limits means that the bearing has reached a significant level of damage that warrants replacement.
The wear and tear that results in an increase of clearances can originate from many different issues, the primary causes being journal or bearing defects. The journal of the crankshaft is what accommodates bearings, and defects may cause quicker clearance growth. Issues with the journal can often lead to overheating, that of which brings about an increase in bearing hardness and cracks. Localized journal stress can also be detrimental, posing the risk of crack formations near fillet radii and oil grooves. Lastly, journal defects may cause metal contact which will leave abrasive damage on surfaces and components.
Bearing defects are when the bearing itself is having issues, and these tend to originate from temperature issues, oil issues, and spark erosion. If lubricating oil temperature is not managed, oil thickness may decrease which can lead to metal contact. Cold ambient engine temperatures are also risky, posing the chance of low oil supply. If the oil that is used for lubricating bearings presents insufficient viscosity or carrying capacity, the assembly may quickly fail. Lastly, spark erosion is often the result of improper lubrication due to cavities, affecting health as overheating ensues.
To determine the clearance of the main bearing of a marine engine, there are a few common methods that one may use. A depth gauge is one option in which the bearing shell and keep are removed, allowing for the gauge bridge to fit over the top of the journal pin. Inserting the depth gauge in the hole of the bridge, a measurement can be made and compared with previous readings. In some instances, a feeler gauge may be used in lieu of a depth gauge, providing a reading of the clearance between the top of the journal pin and the bottom of the bridge. The use of lead wire tools is the most traditional method, carried out through the insertion of a lead wire at varying positions of the bearing and pin. If the clearance is outside of acceptable limitations, then a replacement must be conducted. Alongside the aforementioned methods, other various tools may be used to determine the health of the marine engine main bearing.
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