Note: Descriptions are shown in the official language in which they were submitted.
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Thl~ lnvention relates to ~tern ~eals for the
propeller shafts of ships. Not only ls it nece~sary to prevent
sea-water leaking into the hull of the ship but it is also
necessary, and indeed becoming a legal requirement, to prevent
oil from the stern bearing of the shaft from escaping into the
sea and causing pollution.
It has been proposed to provide an annular air space
between, on the one hand, a stern seal that keeps out the sea-
water and, axially spaced from it, an oil seal at the outboard
end of the bearing, so that any oil leaking past the oil seal
and any sea-water leaking past the oil seal both only reach
tbis air space and can be conducted away to a tank for
separation.
This is fine as far as it goes, but in the event
of gradual wear or failure, leading to severe or even
catastrophic leakage there is nothing to warn of the dsnger,
apart from periodic checks; moreover there is little that can
- be done to remedy the situation without dry-docking.
Moreover the presence of a substantial air space
between the sea-water seal and the stern bearing is in conflict
~lth the need to keep the stern bear$ng as close to the
propeller as possible, 80 as to keep the overhang to a minimum.
The aim of the present invention is to provide a
compact stern seal and bearing assembly that not only avoids
pollution of the surrounding sea but moreover responds to
fallure and allows costlnued use ln an emergency while action
18 taken to remedy the fault, all thls being without the
complicatlons involvet in having to provide external hi8h
pressure supplles of oil and/or water.
According to the invention there is now proposed
a stern ~eal and bearing assembly for ships comprising an
oll-lubrlcated shaft bearing having a~ least at its outboard
end two llp seals directed towards one another, a sea-water
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8eal po~tloned outboard from these llp ~eal~, a flrst vold
space deflned between the llp seals and the outboard seal, and
a pa~sage allowing llqu~d to escape from the flrst void space,
a 6econd vold space defined between the two lip seals, and a
8econd passage allowing liquid to e~cape from the second vold
space.
In the event of failure of the lip seals, oil will
flow into the air space and the presence of oil in the drain
from that space can be observed. As the oil is at low pressure,
generally lower than the hydrostatic head of water in the
surrounding sea at the level of the stern shaft, there is no
possibility of this oil leaking past the sea-water seal to
pollute the surrounding sea.
In the event of failure of the sea~water seal, the
draln from the air space will normally cope with thi~ but if,
for any reason, the drain is blocked or cannot cope, or is
deliberately shut off, and if therefore the sea-~ater gets
pa8t the outboard lip seal, the pressure in the region between
the lip seals will rise and the sea-water wlll be able to
escape through the second drain. Flow through this drain can
be caused to signal an alarm.
There may be flow-restricting means in ehe second
draln, formed simply by a goose neck ri8ing to a higher level
than the level of oil in a 8tatic reservoir supplying the
bearing.
The invention will now be further described by way
of sample with reference to the accompanying drawing which
i- an axlal section through a bearlng ant seal a-8embly ;i~
according to the lnvention.
~ propeller bo88 1 18 carrled by a 8tern 8haft 2
pa88lng through the 8tern frame 3 of a 8hip. The 8ea-wa~er
i8 escluded by a radial face seal compri81ng a rotatlng wearlng
element 4 8ecuret to the ~ opeller bo8s 2 and runnlng agalnst
r~ 3 ~
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a statlonary but flexlbly mounted se~l face member 5. A f~ce
seal 1B the preferred
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type to use at this point as it seals well against
water without the need for any other lubricant. The
flexibility can be imparted by mounting the member 5 on
a single convolution bellows structure 6 which, in the
example shown is built up in the manner that forms the
subject of our sritish Patent No. 986 217 and its
Patent of Addition No. 1 099 688. The bellows structure
is secured to the stern frame 3 by a mounting ring 7
within which are secured two lip seals 8 and 9, the
seal 8 facing inboard and the seal 9 facing outboard.
The lips run not in direct contact with the shaft 2 but
on a sleeve 10 which can be replaced in the event of
wear and which is sealed to the propeller boss 1 by a
sealing ring 11.
The shaft 2 runs in a journal bearing 12
mounted in the stern frame 3 and at the inboard end of
the bearing is a lip seal 13 facing outboard and running
on a wearing sleeve 14 secured to the shaft. In view
of the low pressure under which the bearing operates,
it would be possible to use a labyrinth seal at this
point instead of a lip seal.
Within the stern frame 3 are two drain passages,
namely a first passage lS at the 'six o'clock' or
lowest position, draining to the bilge of the ship the
air space between the sea-water seal 4,5 and the first
lip seal 8, and a second passage 16 a' the 'twelve
o'clock' or highest position, leading from the annular
space between the two lip seals 8 and 9. Both passages
carry control cocks, as indicated at 15' and l6' res-
pectively.
The assembly is completed by the provision of
a low level header tank 17 at a level slightly above
the bearing 12, carrying oil and connected to the
inboard end of the bearing through a pipe 18. The tank
is not under pressure and so the pressure at the bearing 12
is simply that due to the static head of oil in the
tank 17, which is small in comparison, for example,
with the draught of the ship, and bears no relation to
the depth of the shaft 2 below the sea-water surface.
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It will be understood that, if desired, the
pipe 18 may be connected to the midpoint of the bearing 12
through a passage in the stern frame 2, or there could
be connections from the tank 17 to both ends of the
bearing.
The passage 16 from the space between the
seals 8 and 9 contains a swan-neck 19 that is above the
normal level of the surface of the oil in the tank 17
so that liquid can only flow from the space in question
when the pressure in that space is higher than the
static oil pressure.
Under normal operating conditions the sea-
water is excluded by the face seal 4,5 and any slight
leakage across its face is collected in the free air
space between this seal and the lip seal 8. The cock lS'
is normally open so this leakage passes to the bilge
through the pipe 15.
In the event of failure of the face seal 4,5
there will be an excessive flow through the pipe 15. A
sight glass (not shown) may be provided, if desired, to
make this apparent. The controlling personnel will
then close off the cock 15', the space between the
seals 4,5 and 8 will fill up and the sea-water will
penetrate past the inboard-facing lip seal 8. The
resulting pressure on the outwardly facing lip seal 9
will help to make that seal water-tight and so no sea
water will reach the bearing 12. Instead the pressure
in the space between the seals 8 and 9 wlll rise and
there will be a controlled flow of sea-water to the
bilge through the pipe 16 and the swan-neck 19. By
partly closing the cock 16' the personnel can keep up
the pressure in the space between the seals to ensure a
good sealing action by the seal 9. Under these emergency
conditions the shaft 2 can be kept running under power
and the ship is able to reach dock for repair, relying
simply on the outboard-facing seal 9 to keep the oil
and the sea-water apar~. Even under these conditions
there can be no danger of pollution of the sea by oil
as the oil is at a much lower static head than the sea.
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In the event, on the other hand, of the main
oil-retaining lip seal failing there will be no problem
at all as the oil pressure will simply be applied then
to the inboard-facing lip seal 8 and will act to urge
that seal into contact with the sleeve 10. If both the
lip seals 8 and 9 were to fail, the only result would
be a loss of oil to the bilge through the drain passage 15
and this could be observed either by seeing the flow to
the bilge or observing the fall in the level of the
tank 17. At all events, the oil would still be fully
lubricating the bearing 12.
It will thus be seen that the assembly described
allows continued operation without danger of pollution
even in the event of seal failures; this is largely
attributable to the fact the oil is only at a low
static head, lower than the sea-water outside. Moreover,
as the lip seals 8 and 9 can be mounted, as shown,
within the mounting ring of the face seal member 5 the
overall seal assembly takes up little axial space and
so the propeller boss 1 has only a small overhang
beyond the bearing 12.
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