The crosshead pin connects the piston rod to the connecting rod. On either side of the crosshead pin are mounted the crosshead slippers. The slippers run up and down in the crosshead guides as the piston and rod are reciprocating and prevent the top of the connecting rod from moving sideways.
The crosshead bearing is difficult to lubricate effectively. Because the top of the connecting rod swings about the pin and changes direction each time the piston reaches mid stroke, the relative speed between bearing and pin at mid stroke is zero, accelerates to a maximum as the piston approaches top or bottom dead centre and then decelerates back to zero again as the piston approaches mid stroke and the con rod changes direction. This means that hydrodynamic lubrication, where the pin is separated from the bearing by a wedge of oil only occurs over part of the swing; i.e when the relative speed between the two components is high enough.
The crosshead slippers are mounted on stepped journals machined either end of the crosshead pin, secured in place by end plates. The slippers float on the journals, to allow for any slight misalignment in the guides. The rubbing surfaces are white metal lined. Oil is supplied to the slipper rubbing faces from the crosshead oil supply. The slippers have gutters machined in them to assist the spread of the lube oil.
The guide surfaces are either machined into the A frames or are separately cast and machined, then bolted to the A frames. The alignment of the guides is very important as is the clearance between the guides and slippers. If this alignment is out of true or the clearances excessive then excessive wear will occur between piston rod and stuffing box and piston and cylinder liner.