Chain stopper

An improvement in the class of chain stopper comprising a framework having a chain track extending through it for receiving the chain and a pawl which is pivotably attached at one of its ends to the framework and comprising two parts pivotably connected to each other in the fashion of a toggle link which remains substantially unbent in its chain-stopping position and is actuated to a bent position when the chain is released; this class of chain stopper also comprises an arm pivotably attached at one of its ends to the other end of the pawl and pivotably attached at its other end to the framework, this arm having a bearing portion resting against, and substantially conforming to, the surface of the chain when the pawl is in its chain stopping position. According to this improvement, the pivot axis of the pivotable attachment of the pawl to the framework and the pivot axis of pivotable attachment of the arm to the framework lie on opposite sides of a plane perpendicular to the chain track and which extends through the axis of pivotable attachment of the arm to the pawl, with advantages in reducing stress and frictional forces.

The invention relates to an arrangement in or relating to a chain stopper, 
comprising a framework, a track extending through the framework for the 
chain and a pawl which at one end is pivotably mounted in the framework 
and comprises two parts connected with each other after the fashion of a 
toggle link which is kept substantially unbent in the locking position of 
the pawl. 
Chain stoppers of the kind specified are used inter alia for the anchoring 
of floating drilling rigs at sea and for mooring of tankers to loading 
buoys for oil. In the last-mentioned case a mooring cable of synthetic 
fibre is preferably used with a stretch of chain at the end attached on 
board the tanker, to reduce wear in the fairlead or the like. The 
articulated construction of the pawl enables the chain to be released 
quickly without being further hauled in when this is desirable, for 
instance, in an emergency. 
In the prior art installations, for instance, as disclosed in Norwegian 
Pat. No. 134 978, one of the two parts of the pawl, connected after the 
fashion of a toggle link, rests directly against one of the links of the 
chain. 
When the pawl is released--i.e., when the toggle links bends--, the 
aforementioned part of the pawl must slide against the chain link. The 
sliding takes place while very considerable forces are still transmitted 
from the chain to the pawl, and since at least the contact surface of the 
chain link is unmachined, considerable frictional forces will be produced. 
They can easily damage both the chain link and the pawl part, and moreover 
mean that considerable forces are required for the release (bending) of 
the pawl. 
It is difficult to calculate the frictional forces with any particular 
degree of accuracy, so that the release system of the pawl must be 
overdimensioned to ensure an adequate release force under the most 
unfavourable conditions possible. 
It is an object of the invention to reduce the aforementioned disadvantage 
and defects to a considerable extent. This is achieved according to the 
invention by an arrangement of the kind specified whose novel and 
characteristic feature is that the chain end of the pawl has a pivotably 
attached bearing part which rests against the chain in the locking 
position of the pawl. 
Other advantageous features of the invention can be gathered from the 
sub-claims. The pivotable connection between the pawl and the bearing part 
is advantageously constructed in the form of a bearing with accurately 
machined surfaces. These, if necessary assisted by suitable lubricants, 
will produce low and readily predictable friction, a feature which enables 
the pawl to be released without using considerable forces. 
Moreover, the bearing part will remain lying in the space against the chain 
during the initial release of the pawl, thus obviating harmful friction 
and sliding between the pawl and the chain.

The drawings show a framework 1 mainly formed by two side walls 2 and a 
base part 3 connecting the latter. Disposed in the base part is a track 4 
for a chain. Its horizontally and vertically oriented links, having the 
references 5 and 6 respectively, are shown in fine lines on certain of the 
drawings. 
The track affords a location for the lower part of the vertical links 6 and 
continues at the top into horizontal surfaces 7 forming a support for the 
horizontal links. 
The pawl of the chain stopper comprises two parts 8, 9 which are connected 
to one another after the fashion of a toggle link by means of a spindle 10 
whose ends extend out of cut-away portions 11, in the form of arcs of 
circles, in the side walls 2. In the locking position of the pawl the ends 
of the spindle 10, rest in saddle members 12 which are adjustably 
supported by brackets 13 attached to the walls 2. 
At the opposite end of the toggle link the pawl part 8 is pivotably 
attached between the walls 2 by means of a spindle 14. At this end the 
pawl part 8 has a forked prolongation 15, and disposed between the two 
prongs of the fork is a pin 16 extending through a slot 17 in a piston rod 
18 associated with a hydraulic cylinder 19. The cylinder 19 is pivotably 
attached by means of pivot pins 20 mounted in the side walls 2. 
At the opposite end of the toggle link the pawl part 9 is pivotably 
attached to a bearing part 21 by means of a spindle 22. The bearing part 
is given a surface shape which causes it to follow the outline of the 
chain and to contact the chain over a relatively large area in the locking 
position of the pawl. The result is a relatively low surface pressure on 
the chain, and therefore a reduced likelihood of its becoming damaged and 
deformed. Thus as can be seen in FIG. 3, the bearing part 21 rests by the 
surface 23 against the upper part of the vertical link 6. The lower part 
of the link advantageously rests against a ridge 24 in the track 4. The 
bearing part also rests against the horizontal link 5 by surfaces 25 and 
presses it into positive contact with the surfaces 7 in the base part 3 
(FIG. 2). This retention of the link 5 contributes towards retaining the 
link 6 in the correct position, so that it is subjected to the minimum 
possible stressing in the locked condition. 
In the example illustrated the bearing part 21 takes the form of an arm 
which at its end remote from the spindle 22 is pivotably attached to the 
side walls 2 of the framework 1 via a spindle 26. This construction means 
that the pawl is controlled during its release and also ensures the 
movement of the bearing part out of the path of the chain. It also ensures 
that the bearing part assumes the correct position during the locking of 
the chain and that the pawl remains in the loaded position until the 
intended release takes place. 
When the chain stopper illustrated is in the locked condition, its 
individual parts will be in the position shown in continuous lines in FIG. 
3. The spindle 10 connecting the two pawl parts 8, 9 rests against the 
saddle members 12, which are so adjusted that the centre line of the 
spindle 10 has a slightly undercentred eccentricity in relation to the 
plane through the centre lines of the spindles 14 and 22. The threelink 
mechanism formed by the two pawl parts 8 and 9 and the arm 21 is therefore 
locked for any force which can be transmitted from the chain when it is 
streched (to the right as viewed in FIGS. 1 and 3). 
In the release of the chain stopper the hydraulic cylinder 19 can be used. 
The piston rod 18 of the cylinder forces the pin 16 on the prolongation 15 
of the pawl part 8 downwards, so that the toggle link spindle 10 is moved 
from the undercentred into the overcentred position. Sliding takes place 
without appreciable friction in the bearings for the spindles 10, 14 and 
22. There will be no sliding between the bearing part 21 and the chain 
links 5 and 6. 
As soon as the toggle link is overcentred, the force from the chain will 
contribute towards a further bending effect, and the pawl parts 8, 9 and 
the bearing part 21 can, if necessary assisted by the cylinder 19, take up 
the position shown in broken lines in FIG. 3, so that the chain can pass 
unimpeded through the chain stopper.