Exam Question
Explain the difficulty faced in lubrication of crosshead bearings and how this is overcome by the engine makers of two stroke low speed main propulsion engines type with suitable sketches (16)
Class 4 (4th Engineer)
MEKM
📅 Nov 2022
Reference Answer
### 1. Fundamental Difficulty in Crosshead Bearing Lubrication (4 Marks)
The primary difficulty in lubricating a crosshead bearing arises from the unique combination of oscillating motion and a uni-directional load. Unlike crankshaft main and bottom end bearings which experience a full 360° rotation with reversing load directions, the crosshead bearing operates under different principles.
* **Uni-Directional Loading:** During the power stroke, the high-pressure combustion gas exerts a significant downward force on the piston, which is transmitted through the piston rod, crosshead pin, and onto the lower half of the crosshead bearing. During the compression and exhaust strokes, the upward inertial forces are insufficient to lift the pin off the lower bearing shell. This constant downward load makes it extremely difficult to maintain a lubricating oil film.
* **Oscillating Motion:** The connecting rod does not rotate fully but oscillates through a small arc (typically ±15-20 degrees) relative to the crosshead pin. This swinging motion, combined with the constant downward load, prevents the formation of a stable, classical hydrodynamic lubrication wedge.
The primary difficulty in lubricating a crosshead bearing arises from the unique combination of oscillating motion and a uni-directional load. Unlike crankshaft main and bottom end bearings which experience a full 360° rotation with reversing load directions, the crosshead bearing operates under different principles.
* **Uni-Directional Loading:** During the power stroke, the high-pressure combustion gas exerts a significant downward force on the piston, which is transmitted through the piston rod, crosshead pin, and onto the lower half of the crosshead bearing. During the compression and exhaust strokes, the upward inertial forces are insufficient to lift the pin off the lower bearing shell. This constant downward load makes it extremely difficult to maintain a lubricating oil film.
* **Oscillating Motion:** The connecting rod does not rotate fully but oscillates through a small arc (typically ±15-20 degrees) relative to the crosshead pin. This swinging motion, combined with the constant downward load, prevents the formation of a stable, classical hydrodynamic lubrication wedge.
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