DieselShip

Crosshead

The purpose of the crosshead is to translate reciprocating motion of the piston into the semi rotary motion of the con rod and so bearings are required. It is also necessary to provide guides in order to ensure that the side thrust due to the conrod is not transmitted to the piston. This also ensure the piston remains central in the cylinder thus limiting wear in the liner.

Two faces are required as the thrust acts in opposite directions during power and compression stroke. Guide shoes positioned at the extreme ends of the crosshead pin provided a large area and minimise risk of twisting. The doxford engine uses a centrally positioned shoe because there is no room at the ends of the pin due to the side rod crossheads.

The usual way of checking guide clearance is by means of a feeler gauge with the piston forced hard against one face and the total clearance taken at the other face. This gives a reasonable estimation as wear should be approximately the same in the ahead and astern faces. A more accurate idea can be gained by chocking the piston centrally in its bore than measuring the clearance at each face. This will also give the athwartships alignment. The edges of the guide shoes are also white metal faced and these run against rubbing strips. Clearance at these faces can be checked with feelers and this gives the fore and aft alignment.

Guide clearances are usually adjusted by means of shims between the hardened steel guide bars and the mounting points. Bolts are slackened off allowing slotted shims to be inserted or removed. Note, care must be taken when handling these shims.

Crosshead pins are supported in bearings and the traditional way has been to mount the piston rod at the centre of the pin with a large nut and having two bearings alongside. This arrangement is like a simply supported beam and the pin will bend when under load. This gives rise to edge pressures which break through the oil film resulting in bearing failure. The Sulzer solution is to mount the bearings on flexible supports. When the pin bends the supports flex allowing normal bearing contact to be maintained.

In order to minimise the risk of bearing failure the actual force on the oil within the bearing should be kept within reasonable limits this can be achieved by having as large a bearing area as possible. Increasing the diameter of the pin and hence the bearing will minimise the problems as this not only allows for a large bearing area but it also avoids the problem of pin bending. Pin bending is further prevented by means of a continuous bearing. This also avoids the loss of oil which can take place with short bearings. Most modern engines tend to have single continuous bearings. Oil loss from the ends of bearings is prevented by means of restrictor plates. Some engine builders provide booster pumps which increase the oil pressure to the crosshead during the critical firing period. Cross heads do not have complete rotary motion and so a complete oil wedge does not form. The use of means for preventing oil loss are therefore useful in maintaining an oil film between pin and bearings.

The crosshead pin is fitted with a loose fitting pin. This pin allows a small degree of movement (about 1mm) between the guide shoe and the pin giving better alignment.

Types of damage associated with the crosshead bearing

There are two possible types of damage which may be sustained;

1. wiping-this is where part of the white metal contact faces are wiped out so that machining marks and oil grooves disappear, the material is displaced into the lubrication grooves where it forms 'stubble' or may fill them completely. Providing adequate lubrication this may be caused by two high a degree of roughness of the crosshead journal. Possibly due, if occurring after trouble free operation, to particles in the lubricating oil. Roughness may also occur due to corrosion by weak acids forming in the lubricating oil. A water content above 1% can attack the white metal and cause formation of SnO which has the appearance of dark smudges on the surface. This must be removed whenever possible as the tin oxide can become harder than the metal of the journal causing obvious distruction of surface finish.
2. cracking- these may appear as individual cracks, hair line cracks, or densely cracked or crackled areas.
   The latter may be so dense as to give the appearance of segregated grains. This can lead to scratching on the journal. The reasons for cracking may be insufficient bonding of white metal to the steel.
   Densely nested networks of cracks is due to fatigue fractures.