Crosshead bearings are very difficult to lubricate and they run under the most arduous of conditions.
Throughout the full cycle, 2-stroke crossheads are subjected to a vertical downward loading that is never reversed, whereas in the 4-stroke engine, the induction stroke reverses the loading on the gudgeon pin. The magnitude of the load varies throughout the stroke, being a maximum around TDC and gradually reducing as the cylinder pressures drop during the expansion stroke.
The lowest loading on the crosshead occurs around the opening of the exhaust ports when the cylinder pressure drops to below scavenge pressure. It is at this point that the high pressure lubricating oil has the best chance of lifting the pin and forming a film under it in preparation for the next cycle. The crosshead bearings oscillates about the pin, so that it is difficult, if not impossible, to generate the hydrodynamic wedge of oil much more readily achieved in bottom end and main bearings, or indeed in any bearing/shaft system in which continuous rotation occurs. Even so, a film of oil set up whilst the con rod is swinging at its fastest across the pin provides, to a large extent, hydrodynamic lubrication. At the extremities of the swing of the con rod, as it passes over mid stroke, the movement of the bearing must slow down, stop and reverse in direction. Fortunately the loading at this point is much reduced so any boundary lubrication conditions that occur are more easily accepted.
On of the greatest problems associated with crossheads is the need for them to absorb very high and almost instantaneous loading just after TDC; this point the piston rod, connecting rod and crankshaft webs are in virtual alignment so that the full force of the piston load is directed into the crosshead bearing.
Because of the ever increasing mep, and the difficult circumstances in which a crosshead bearing works, lubrication is now taken to it directly. The oil feed, usually from some supply higher in pressure than is required for main bearings, is supplied via ‘swinging’ links or though ‘telescopics’. The residue of this supply the bottom end bearing. Some of the crosshead oil supply is taken on to lubricate the guide shoes.
There are many forms of crosshead. One employs jacking pumps to increase the oil feed pressure (using the arc of the connecting rod to develop a pumping motion). Another has bearing pins which are divided into two slightly eccentric sheaves, so that the oil film developed as the rod swings over them is extended beyond that formed on a simple single pin. Each of these, as well as the above methods, has been designed to maximise the effective lubrication of the crosshead so that it will operate successfully for long periods.
In all cases attention should be paid to the correct adjustment of the working clearances. Large clearances should be avoided, as the oil will be squeezed out more readily. A rough indication of the state of a bearing can be obtained by checking the oil flow from the bearing, particularly when the oil is hot (thin); after the engine has been running, for example. Any unduly large flows from the crosshead will become obvious as the engine is turned over. It would be sensible to compare the flows between each crosshead so that a pattern can be provided against which the suspect bearing can be compared.
It should be noted that oscillatory bearings (crossheads included) are the only ones in which ‘axial’ oil distribution grooves are acceptable. Even so, the contouring of these grooves should be carefully regulated so that adequate radii are given to their edges to encourage oil flow and prevent them from becoming oil scrapers.