Crosshead Lubrication

Crosshead Lubrication

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.

As engine powers and thus gas loads have increased, the difficulties with achieving effective lubrication have increased. Larger

pin diameters have helped by increasing the linear rubbing speeds, and the continuous lower bearing has reduced the loading/unit

area.

The older forked type crosshead as found in earlier engines (up to the mid 1980s) used various methods to improve the

lubrication of the crosshead. Oil grooves in the lower

bearings were used to distribute the oil. The grooves in some

cases extended to the edge of the bearing, although with a

reduced csa, to ensure a flow of oil through the bearing.

THE MAN B&W MC ENGINE CROSSHEAD

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.

Lower Bearing Shell Showing Arrangement of Grooves.

The lower bearing shell (tin aluminium with overlay) has oil grooves with machined wedges as shown in the diagram and photo.

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 SULZER RTA ENGINE CROSSHEAD

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.

Crosshead Bearing Shell - Old RTA

Con Rods - Modern RTA. Note Oil Supply Grooves in 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.

Swinging Arm (Piston and Rod

Removed)

OIL SUPPLY PRESSURE.

A Question sometimes asked is why do Sulzer need to boost their crosshead oil supply pressure to 12 bar whilst MAN B&W

supply oil to their crosshead at system pressure. The answer lies in the design of the bearing. More than 90% of the circulated oil has the sole purpose of cooling the bearings. If you study antique machines with open

crankcases, you will see that the amount of oil for lubrication is a few drops per minute. This is enough for maintaining the oil

film in the bearing and with an open crankcase the friction heat is removed by air-cooling. Modern engines have closed

crankcases and a much higher bearing load - hence the need for oil cooling.

In a main bearing, the oil is pumped into the upper shell and it will cool the upper part of the joumal. Since the shaft is rotating, it

is cooled on all sides and because the oil film thickness is very small in the loaded part, the shaft will cool the loaded bearing half

as well.

A crosshead bearing is only oscillating and the lower shell is always loaded. The cooling oil must be injected between shaft (crosshead pin) and lower bearing.

In MAN B&W engines, a set of channels have been machined in the lower crosshead bearing, in which the cooling oil can pass.

The geometry is designed in such a way that all the loaded square centimetres of the pin are flushed with cooling oil twice every

engine cycle. In contrast, the Sulzer crosshead has a plain lower bearing without channels. In order to inject oil between pin and bearing, they have to supply oil at a much higher pressure. The injection will take place at around 20 degrees crank angle before

TDC, where the cylinder pressure is still low and upward inertia forces on piston is still high. There is a short interval, in which

the bearing pressure is lower than the oil pressure.