The crosshead on a slow speed 2 stroke is a difficult bearing to lubricate effectively. The load is continually downward and because the con rod swings about the pin, changing direction each stroke, true hydrodynamic lubrication cannot take place. Instead the lubrication starts as boundary, and as the rubbing speed increases, a hydrodynamic film is built up. As the rubbing speed decreases the lubrication becomes boundary once again.

The lower half of the bearing housing is formed by the top end of the connecting rod. It supports the crosshead pin over its entire length, the piston rod being bolted to the top half of the crosshead pin through a cut out in the bearing top half. Oil supply to the crosshead is via a telescopic pipe from the main LO supply at a pressure of about 2.5 bar.

The lower  bearing shell (tin aluminium with overlay) has oil grooves with machined wedges . The oil enters via the cut out channel in the centre. The grooves extend right to the edges of the bearing to ensure a flow of oil, thus cooling the bearing.

The early RTA had a forked crosshead with the piston rod passing through a hole in the crosshead pin and secured underneath with a nut. Oil entered the bearing through holes in the shell. via a groove machined in the the lower bearing housing.

The Modern RTA has a continuous lower bearing, the housing formed by the top of the conrod. Only a lower bearing shell is fitted, the top bearing housing being lined with white metal. Oil boosted in pressure to 10 - 12 bar is supplied via a swinging arm.The swinging arm also carries oil at system pressure (4 bar) for piston cooling.


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  3. There was also another patented solution: that of GMT (ex. Fiat Grandi Motori Trieste).
    An eccentric geometry of the pin allowed the pin to move away from the bearing seat, allowing the lubricant to penetrate the gap. A brilliant idea. With the increase in engine performance (i.e. the effective medium pressure of the thermodynamic cycle) in the 1980s, it was only surpassed by Sulzer hydrostatic bearings. When I was young I designed those bearings, but it's just history in development now. of modern two-stroke engines for marine propulsion.

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