THIS INVENTION relates to 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 a tailing means is provided at the tail end of a rotatable drum. The tailing means is intended to receive only a single turn of rope or cable and comprises a circumferential groove 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 same is gripped by the tailing means. The tailing means holds the tail end of rope in position so that, on the one hand, an increase in the tension applied 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 become slack and, possibly, foul the line. U.S. Patent Specification. No. 4,272,606 stresses that the area of increased friction must only be provided at the tail end of the drum, because a number of turns of rope adjacent the tail end 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 that a large number of turns of cable are borne by the winch. However, because the driving capstans of the winch are disposed on more than one axis, it is difficult correctly to balance the torque applied by the load about the two axes.

It is an object of the present invention to enable the provision of a winch whereby the above disadvantages may be overcome or at least miligated.

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This invention provides α winch comprising α rotαtσble bearing surface for a cable, wherein the friction between the surface and a given cable material at a given cable tension varies continuously, or sfepwise in a plurality of steps, along a rotational axis.

Advantageously, the winch includes a capstan having a surface the coefficient of friction of which increases along the rotation axis of the capstan. 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.

Thus, 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 of friction 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 maintenance. 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 v/inch fn 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 contrarotatϊon, for example by means of an epicyclic gear train, and a brake ϊs 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 v/ay 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 made, by way of example, to the accompanying drawings, in which:

Figure I is α schematic side elevational view of a single-axis twin capstan winch 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 If I— II ! of Figure I .

Referring now to the drawings, Figure I shows a twin capstan winch comprising a first, single groove, capstan I and a second capstan 2, sharing a common rotation axis, and a plurality of sheaves or grooved pulleys 3. As can be seen in Figure 3, three sheaves 3 are rotatably mounted on a shaft 4 which is . parallel to the common rotation axis of the first and second capstans I, 2.

Also, a fourth sheave 3 is rotatably mounted on a shaft 5 which is slightly offset from that axis to facilitate passing cable over the fourth sheave 3 and the single groove capstan I. The capstans I, 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 on an integral shaft 9 of the second capstan 2 and an internal gear 10 of the first capstan I . A brake (not shown) is also provided for preventing relative rotation of the capstans I, 2 if desired.

The second capstan 2 is provided with three circumferential grooves 1 1 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 groove's width. The circumferential grooves 1 1 comprise a high friction material and the co¬ efficient of friction increases from the high tension (load) end to the low tension (tail) end of the second capstan 2.

Preferred high friction materials include Silumin, which is a high silicon content aluminium such as AL20 with more than 12% silicon content, and, for use in potentially corrosive environments, high friction polymers such as polyurethanes, polypropylenes and Kaυtex, which is a polyvinylchloride based material ^" . "Silumin" and "Kautex" are Trade Names. In a preferred arrange¬ ment, each groove comprises alternating areas of Kautex and Silumin, the former providing the high friction component and the latter serving to support the cable. The coefficient of friction of at least some 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 some 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, in one arrangement the density decreases exponentially from 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 the pressure between the cable and the surface of each groove at a given cable tension.

During use, tension in the cable guided by the winch is overcome by friction between the cable and the capstans, so that the cable at the tail end of the winch is almost completely slack. At least 40%, and as much as 50%, of the cable tension is taken up by the first capstan I , 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 1 1 and the cable increases from the load end to the tail end of the second capstan 2, the frictional force is spread evenly over the second capstan 2, as a result of which the cable, and the grooves N , are not subjected to unduly high frictional forces in any one place. At the same time, sufficient tension is maintained in the last few turns of cable adjacent the tail end to prevent the same from becoming slack.

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