Sketch arrangement:

In a large two-stroke engine, the crosshead is lubricated by the main lubricating oil system. Oil is fed through the crosshead pin (which is hollow/ has axial and radial drillings) to the top-end (crosshead) bearing, the slippers (guide shoes) and the guide faces.

Arrangement:

• The main lube oil is supplied to the crosshead via a feed from the engine's lubricating oil system. In many designs, oil is fed through the connecting rod (a drilling in the rod) or through a separate pipe/ telescopic arrangement to the crosshead pin.
• The crosshead pin has axial and radial oil drillings that distribute oil to the top-end bearing (the bearing between the crosshead pin and the connecting rod top end).
• Oil also feeds to the slippers (guide shoes) through drillings in the crosshead, and to the guide faces.
• A booster pump (see part c) may be used to raise the oil pressure for the crosshead/ top-end bearing supply.

• The top-end (crosshead) bearing clearance determines the oil film thickness and the load-carrying capacity of the bearing. If the clearance is too small, the oil film cannot form properly, causing metal-to-metal contact, overheating, wiping and possible seizure.
• If the clearance is too large, the bearing has excessive play, causing hammering/ knocking, uneven load distribution, and accelerated wear; the oil film may break down.
• The correct clearance ensures proper hydrodynamic lubrication, even load distribution, and long bearing life.
• Because the top-end bearing carries the full combustion load and is subject to reversing loads, the clearance must be set precisely to the maker's specification to avoid failure.
• Incorrect clearance in one top-end bearing can cause misalignment and uneven loading of the adjacent bearings and the crosshead.

• The top-end (crosshead) bearing is subject to high, reversing loads and requires a reliable, high-pressure oil supply to maintain the oil film.
• The main lubricating oil system pressure may be insufficient to overcome the high load and to feed oil through the long, small-bore drillings in the connecting rod/ crosshead to the bearing.
• A booster pump raises the oil pressure (e.g. to 5-10 bar or higher) specifically for the crosshead/ top-end bearing supply, ensuring an adequate oil film is maintained under all load conditions.
• The booster pump also ensures a continuous, positive oil supply to the top-end bearing even when the main system pressure fluctuates, improving reliability and preventing bearing failure.