Boundary lubricating is a condition when thin film of lubricant is insufficient and surfaces contact. This results in their wear and high friction value.
In engine bearings Boundary lubrication happens during start up, stopping, low RPM, high loads and when surfaces are not in perfect condition. Momentary loads become more severe such in case of rough surfaces and high contact pressures. Wear increases significantly in the system.
Below are few examples of engine parts affected by boundary lubrication:
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Piston rings and cylinder liner when piston is at TDC and BDC position, piston direction changes
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Guide and guide shoe in two stroke engine.
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Journal bearing
