Bearings

Guide shoes

The guide shoes mounted on the fore and aft ends of the crosshead reciprocate in the guides and transfer the side component of thrust into the engine frame. The guide shoe has a screwed locating pin, which protrudes into a hole in the crosshead pin, restricting the rotational movement of the crosshead pin. The inner side of the guide shoes has the guide bolted guide strips to ensure that the piston rod is in the correct position in the fore-and-aft direction. with the shims, this alignment and the clearance between the guide & guide strips are adjusted. White metal with machined oil supply grooves is used on the sliding surfaces of the guide shoes and the guide strips.

 

Cross-head Bearing (or, Top end Bearing)

Due to the swinging movement of the crosshead, it is difficult to lubricate and maintain true hydrodynamic lubrication. Hence maintaining an oil film to separate the metal layers has always been difficult. Previously an oil pump is driven by the crosshead to inject high-pressure oil in the lower bearing with some success.

 

Due to the oscillating motion, only the lower half of the bearing takes the load. The cooling oil must be injected between the shaft (crosshead pin) and lower bearing at the correct time when the load on the bearing is low; approximately 20 degrees before the TOC of the crank angle. To help with the distribution of oil over the loaded half of the bearing, grooves are machined in the shell which also assists with the build-up of a hydrodynamic film.

 

Bottom end bearing: MAN B&W

The Oil supply to the bottom end bearing comes down drilling in the connecting rod from the cross-head oil supply. It is led into the bearing via drilled passages.

 

The engine uses thin wall-bearing shells lined with white metal or shells lined with tin aluminum and a fresh layer of tin for upper and lower bearing halves.

 

The shells are manufactured with a circumferential overlength (crush/nip), and when the shells are mounted and tightened up, will produce the required radial pressure between the shell and the bearing housing. The crankpin bearing shells are retained in position utilizing screws fitted in the bearing housings.

 

Main Bearings for MAN B&W Crosshead Engines

Thin shell bearings have a wall thickness between 2% and 2.5% of the journal diameter. The steel back does not have sufficient stiffness to support the cast-on white metal alone.

 

The bearing must therefore be supported rigidly over its full length to prevent fretting between the shell and housing and failure of the bearing material.

 

The bearing surface is furnished with a centrally placed oil supply groove and other design features such as tangential run-outs and bore reliefs.

 

The tangential runout prevents an oil-scraping effect and reduces the resistance to the flow of oil towards the loaded area of the bearing.

 

 

The bore relief is to compensate for misalignment which could result in a protruding edge of the lower shell’s mating face to that of the upper shell. Such a protruding edge can act as an oil scraper and cause oil starvation.