Note: Descriptions are shown in the official language in which they were submitted.
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THIS INVENTION relates fo improvements in or relating to winches.
Winches have been proposed, and are described and illustrated in U.S.
Patent Specification Nos. 4,274,606 and 4,230,306, in which cl tailing means is
5 provided at the tail end of a rotatable drum. The tailing means is intended toreceive only a single turn of rope or cable and comprises a circumferential
groové and/or a high friction material. An increase in tension in the turn of
rope received by the tailing means has the effect of increasing the force with
which the sarne is gripped by the tailing means. The tailing means holds the tail
10 end of rope in position so that, on the one hanci, an increase in the tensionapplied to the load end of the rope will not cause the turns oF rope to slip
around the drum and, on the other hand, the tail end of the rope will not
becosr,~e sl~,ck and, possibly, foul the line. U.S. Patent Specifica~ion No.
~i stresses that the area of increased friction must only be providecl at
15 the tail end of the drum, because a number of turns of rope adjacent the tailend might otherwise become slack. Thus, in these previously proposed winches,
the rope or cable and the cable bearing surface are subjected to uneven
frictional wear.
U.K. Patent Specification No. 1,599,521 describes a dual axis quadruple
capstan winch in which cable tension is overcome by providing a large number
of grooves so 1hat a large number of turns oF cable are borne by the winch.
However, L~ecause the driving capstans of 1he v~inch are disposed on more than
one axis, it is difficult correctly lo balance the Jorque applied by the load about
the two axes.
It is an object of the present invention to ellable the provision of a winch
whereby the o!~o\ e di~sadva,ltocJes may be overcome or at least mit i~ated.
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This invention provides a winch comprising a
rotatable bearing surface for a cable, wherein the
friction between the surface and a given cable material
at a given cable tension varies continuously, or
stepwise in a plurality of steps, along a rotational
axis.
Advan-tageously, the winch includes a capstan
having a surface the coefficient of friction of which
increases along the rotation axis or the capstan. This
may be achieved by known methods such as modification of
the surface finish and introduction of removal oE
surface films. Additionally, or alternatively, the
winch comprises a capstan provided with a plurality of
circumferential grooves, the cable bearing surface area
of each groove being different.
rhus, as the tension in the cable decreases,
the friction acting between the cable and the capstan
can be kept at substantially the same level by
increasing the coefficient oE Eriction and/or by
decreasing the effective contact area (and thus
increasing the pressure) between the cable and the
capstan. As a result, the circumferential grooves are
worn at substantially the same rate, facilitating
maintenence. Also, whilst in the absence of the
features of the present invention the cable tension
would decrease very gradually as the number of turns of
cable increases, the present invention enables the
provision of a winch in which cable tension is overcome
in relatively few turns of cable, so that a single axis
twin capstan winch, which has not been previously
proposed, becomes feasible.
Accordingly, the present invention also
provides a capstan winch in which the driving capstans
of the winch are disposed on a single axis and comprise
first and second capstans. Preferably, the first and
second capstans are drivable in contrarotation, for
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example by means of an epicyclic gear train, and a brake
is provided for securing the first and second capstans
against rotation with respect to each other, so that
they act as a single fixed capstan. When fixed in this
way the first and second capstans can continue to
support a load even whilst parts of the driving
mechanism are removed for inspection or repair.
For a better understanding of the present
invention, and to show how the same may be put into
effect, reference will now be made7 by way of example,
to the accompanying drawings, in which:
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Figure 1 is a schematic side elevational view of a single-axis
twin capstan wlnch in accordance with the invention,
Figure 2 illustrates the path followed by a cable guided by the
twin capstan winch of Figure 1, and
Figure 3 is a cross-sectional view taken along the line III-III of
Figure 1.
Referring now to the drawingsl Figure 1 shows a twin capstan winch
comprising a first, single groove, capstan 1 and a second capstan 2,
sharing a cammon rotation axis, and a plurality of sheaves or grooved
10 pulleys 3. As can be seen in Figure 3, three sheaves 3 are rotatably
mounted on a shaft 4 which is parallel to the cQ~mon rotation axis of the
first and second capstans 1, 2. Plso, a fourth sheave 3 is rotatably
mounted on a shaft 5 which is slightly offset frcm that axis to
facilitate passing cable over the fourth sheave 3 and the single groove
15 capstan 1. The capstans 1, 2 are adapted to be driven in rotation by
means of a motor 6, a sun gear 7 fixed to a drive shaft of the motor 6, a
planet gear 8 rotatably mounted o~ an integral shaft 9 of the second
capstan 2 and an internal gear 10 of the first capstan 1. A brake (not
shown) is also provided for preventing relative rotation of the capstans
20 1, 2 if desired.
The second capstan 2 is provided with three circumferential
grooves 11 which are offset along the capstan rotation axis relative to
the grooves of the three sheaves 3 on the parallel shaft 4 by half a
gro~ve's width~ The circumferential grooves 11 comprise a high friction
25 material. The coefficient of friction increases frcm the high tension
~load) end to the low tension (tail) end of the second capstan 2 by, for
exa~ple, varying the material in different grooves 11. Alternatively, an
~, ungrooved capstan (not shcwn) may have a surface texture which varies
from one end to the other.
~z~s~e~
Preferred high friction materials include Silumin, which is a
high silicon content aluminium such as AL20 with re than 12~ silicon
content, and, for use in potentially corrosive enviro~ments, high
friction polymers such as polyurethanes, polypropvlenes and Kautex,
5 which is a polyvinylchloride based material. "Silumin" and "Kautex" are
Trade Names. In a preferred arrangement, each gro~ve ccmprises equi-
sized patches of Kautex and Silumin alternating around the circumference
of the groove, the former providing the high friction ccmponent and the
latter serving to support the cable. The coefficient of friction of at
10 least scme of the surface of the second capstan 2 for a steel cable
should preferably be significantly greater than that of steel on steel.
It has been found that, for scme materials at least, such as
polyurethane, a relationship exists between the density of the material
and its coefficient of friction for a given cable material, Accordingly,
15 in one arrangement the density decreases exponentially Ercm the high
tension (load) end to the low tension (tail) end of the second capstan 2.
The widths of the grooves may also be varied (not shown) so as to vary
circumferential area of the cable bearing surface of each groove and
thereby the pressure between the cable and the surface of each groove at
20 a given cable tension.
During use, tension in the cable guided by the w mch is overccme
by friction between the cable and the capstans, so that the cable at the
tail end of the winch is almost ccmpletely slack. At least 40%, and as
much as 50~, of the cable tension is taken up by the first capstan 1,
25 about which the degree of wrap is between 250 and 280 usually about
270 The friction would normally decrease gradually from the load end
to the tail end of the second capstan 2. However, because -the
coefficient of friction between the grooves 11 and cable increases fron
the load end to the tail end of the second capstan 2, the frictional
30 force is spread evenly over the second capstan 2, as a result of which
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the cable, and the grooves 11, are not subiecte~ to unduly high
frictional forces in any aIle place. At the same time, sufficien~
tension is maintained in the last few turns of cable acljacent ~le tail
~ld to prevent the same from beccming slack.
