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
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.
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
THE MAN B&W MC
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.
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
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.
arm also carries oil at system pressure (4 bar) for
Swinging Arm (Piston and Rod Removed)
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
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