Not too long ago, most vessels were sailing around with lignum vitae water-lubricated stern tube bearings. The concept still holds good now, using rubber rather than wood
When oil-lubricated and cooled white-metal
stern tube bearings became popular, they were definitely a great
technological advance. No more water leaking
into the engine room aft seal, changing packing
or pumping bilges. But with the ever-present
threat of seal failure and pollution risk, water-lubricated
bearings are regaining popularity.
There are a many different types of water-lubricated
bearings on the market at the
moment, mostly based on various woven
composites or elastomer-based materials, but
Ohio-based Duramax Marine is convinced that
nitrile rubber is the best all-round material for
this kind of application.
"We have over 80 years of experience in
producing rubber products and since 1960 we
have been producing rubber water-lubricated
stern bearings," said Michael Schonauer, vice
president for marketing at Duramax Marine.
Talking to Marine Propulsion he explained the
history behind using rubber as a bearing material.
It was first developed by mining engineer Charles
Sherwood on water pumps back in the I920s. He
quickly realized that the rubber handled the very
high abrasive mine water very well and that the
rubber bearing material basically deflected and
allowed the abrasives to pass through without
damaging the pump shaft or the rubber itself. it
was a ground breaking discovery
that technology was bought and licensed by
the BF Goodrich Company in the 1922, selling the
bearings under the 'Cutless' name. That company
grew the product and invested vast sums on
money in R&D until, by the 1960s, a lot of vessels
were operating with water-lubricated bearings
made of rubber. In l9ó0, Duramax Marine's
then parent company, Johnson Rubber, decided
to compete with BF Goodrich and entered the
water-lubricated bearing market by introducing
the Johnson demountable rubber stave bearing.
Through the years both companies developed
and innovated newer rubber polymer bearing
technologies. Then in the 1996, Duramax Marine
acquired the bearing division of BF Goodrich. All
of Goodrich's bearing technology and a senior
engineer went to Duramax and it continued
making bearings under the name Johnson Cutless.
From a lubrication and friction point of view
rubber is very different from other water-lubricated
bearing materials. There are complicated theories
about contact mechanics and other related
subjects but at its most basic, nitrile rubber has
the advantage that it can elastically deform and,
while it is dry has a high coefficient of friction.
When wet, it is extremely slippery with a dynamic
coefficient of friction that can be as low as 0.0001.
When the propeller shaft starts to rotate, the
water already present immediately forces the fluid
film underneath the load of the shaft and creates
a water-trapping hydrodynamic pocket, creating
pressures in the pocket significantly higher than
in the other areas around the bearing. Although
this film or wedge (similar in principle to an oil
wedge acting on the main bearings of an engine)
is ultra thin, it is enough to stop any direct
contact between shaft and rubber. The pressure in
the pocket has an added benefit of creating a grit
rejecting action and forcing abrasives out of the
bearing through the water grooves.
“Due to the low viscosity of water as a
lubricant, you need the elastic resiliency and
durability of rubber to create this pocket
and no other material can offer that”, said
Mr. Schonauer "The composites do become
hydrodynamic, but they don't have the same
resiliency as rubber, so when you get into an
abrasive environment, rubber outperforms those
other materials," he explained.
Rubber has an almost perfect Poisson’s ratio
(the ratio between transverse and axial strain
when a material is compressed or stretched)
of about 0.5, which means that rubber is
incompressible. This property has been exploited
by Duramax by designing bearings that it says
have their load-carrying and pocket-forming
In terms of alignment, rubber bearings are the
most forgiving stern tube bearings available, Mr.
Schonauer assured Marine Propulsion, reporting
that they can tolerate a higher degree of
misalignment than any other bearing materials.
Rubber never becomes edge loaded because it
elastically conforms to accommodate irregular
loads. Rubber bearings are particularly effective,
he said, because they allow a shaft to turn on its
centre of gyration even if this does not correspond
exactly with the geometric center. This reduces
dynamic stresses and vibration in the supporting
structure, outboard struts and stern tube.
Another unique property of rubber bearings
is the ability to absorb and reduce shipboard
vibrations and resonance originating from the
propulsion shaft. While this is particularly
important to designers of military vessels such
as submarines, he said, it is also becoming a
very important subject for environmentalists
who want to see a reduction in noises resonating
from ships for ecological reasons.
Duramax 'Romor' rubber bearings are used
extensively by navies throughout the world. One
reason why naval vessels favor water-lubricated
bearings is because of their extreme robustness
and reliability, he suggested. "Naval Engineers
don't have to worry about catastrophic seal
failures during complex military maneuvers. In
white-metal bearings, if an oil seal fails it will be
a very short time before there is a complete loss
of propulsion," Mr. Schonauer added.
"Most naval and commercial vessels use
rubber bearings in their aft struts with a 4:1
length of shaft to diameter ratio but the need
to do so is a common misconception," said Mr.
Schonauer. "In reality, the length-to-diameter
ratio is irrelevant when it comes to rubber
bearing design." Instead, the company designs
its bearings around shaft load and maximum
permissible shaft deflection with a maximum
typical design load of 40 psi, which is typically
the load limit set by most major class societies
for rubber bearings, he said.
That general rule was developed by class
societies back in the I940s and was based on the
limits of natural rubber, he recalled. "In reality they
haven't been reviewed or updated to keep pace
with the load-handling capacity of today's highly
engineered synthetic rubber polymers," he said
and mentioned that Duramax Marine has rubber-based
bearings that are capable of handling loads
greater than 80psi with the properly engineered
polymer and geometric bearing design.
To address higher load requirements of some
applications, Duramax uses a compound called
DMX which is a hybrid of rubber, ultra high
molecular weight polyethylene and a few other
secret ingredients. "It still has the unique water-lubricated
pocket forming characteristics of rubber
but it controls shaft deflection and is class approved
for use in applications with a 2:1 or less length-to-
diameter ratio," Mr. Schonauer said. DMX can
handle shaft loads in excess of 80-100 psi and it
has been approved by Lloyd's Register and has been
fully tested by the UK s Ministry of Defense, he said.
Under certain conditions, composites may
perform as well as rubber bearings but they don’t
have the material properties that make rubber far
more resilient, Duramax believes. Composites and
polyurethanes tend to fail more from hysteretic
failure, caused by vibration and cyclic stresses.
Mr. Schonauer agreed that "there will always be
debate over which is best" and there are applications
such as rudder bushings where composites and
polyurethane bearings can perform well, “but for
all-round performance in stern tube applications,
particularly in highly abrasive conditions, rubber
will outperform the others."
There are two basic concepts for the rubber
bearings. First, there are the fully molded types
that have rubber molded and vulcanized into a
bronze sleeve. As the rubber inside diameter is very
difficult to machine in the field, these are typically
ordered from the factory to the correct operating
size. As a cost-saving measure, the Johnson Cutless
bearings can be sent back to the factory where the
old rubber will be removed and relined to its original
specifications, Mr. Schonauer said.
Second, there is the stave type, in which
individual 'staves' of rubber, mounted in a very
similar way to how the old lignum vitae strips
were fitted to stern tube many years ago.
In this design the staves are manufactured
to fit in a dovetail housing and are locked into
a split housing that is machined to accept each
individual stave. This allows easier removal
of the staves without having to remove the
shaft and, once fitted, the external locking
ring will keep the staves in the housing during
operation. As there is a lot of machining of
brass housìngs to be done on this system, it is
For vessels that want the serviceability of the
stave design without the cost of the dovetailed
housing, staves can be manufactured with
a rounded outer diameter. They are produced
to fit exactly into the stern tube with the right
compression so that the internal diameter ID is
correct for the shaft. A compression collar holds
them in place and compresses them longitudinally
which locks the staves into the housing and
adjusts them to the proper internal diameter.
When the shaft liner starts to wear clearances
can be maintained by simply exchanging the rubber
staves for a thicker stave to accommodate liner
wear. The shaft does not need to be pulled and the
original clearances are restored, Mr. Schonauer said.
Once bearings need refurbishment they can
be returned to Duramax for a full overhaul and
rubber replacement. "People say the rubber is too
soft, but I have never seen any abrasives embedded
in the rubber, so that's just not the case," he said.
Given the penalties that can arise from
oil pollution, including fines and possibly
imprisonment in some places, knowing that the
risk of oil leaking from the stern tube has been
eliminated would bring peace of mind to the
master and chief engineer.