Winches capable of selective raising and freefall lowering of an object are known, such as that disclosed in U. S. Patent No. 5,314,166. Such winches often include a gipsy for grasping a chain, rope or a combination thereof, coupled to, for example, an anchor.

This allows the anchor to be raised and lowered as desired. The winch typically includes a threaded winch shaft mounted in a winch housing and driven by an electric motor. A lower truncated winch cone is fixed to the shaft for rotation therewith, and an upper truncated threaded winch cone is mounted on the shaft for rotational and axial movement therealong. The gipsy is an annular member disposed on the shaft between the upper and lower cones and is rotatable about the shaft. A spring and brake pad may be provided to impart a force on a top surface of the upper cone such that when the shaft is rotated in a first direction, the upper cone is moved axially into abutment with the gipsy. The gipsy is then gripped between the upper and lower cones and is rotated with the winch shaft to pull in the rope or chain and raise the anchor. A shoulder on the upper cone may engage a key on the winch shaft to rotate the two cones together with the winch shaft and prevent further axial movement of the upper cone.

The winch of U. S. 5,314,166 includes a pawl which engages a detent in an upper surface of the upper cone when the shaft is rotated in the opposite direction, to restrain the upper cone from rotation with respect to the

winch housing. This causes the upper cone to move axially out of abutment with the gipsy, such that the gipsy is released, allowing freefall of the anchor.

However, such a winch requires an axial movement of the upper cone away from the gipsy in order to allow release thereof, and this requires the upper cone to be threaded. If the shaft is driven by the motor for too long in allowing freefall of the anchor, the upper cone may move too far axially and damage the winch by causing the winch to stall, or by driving the upper cone against the winch housing and locking or possibly cracking the upper cone and/or the housing.

It is amongst the objects of the present invention to obviate or mitigate at least one of the foregoing disadvantages.

According to a first aspect of the present invention, there is provided a winch for selectively allowing raising and lowering of an object, the winch comprising: a winch housing; a winch shaft capable of being driven, rotatably mounted in the winch housing and having an externally threaded portion; a first element coupled to the winch shaft for rotation therewith; a second element rotatably mounted on the winch shaft and axially spaced along said winch shaft from said first element; gripping means defining a channel for gripping a rope or chain connected to the object, the gripping means being rotatably mounted on the winch shaft between the first element and the second element; a threaded locking element mounted on the threaded portion of the winch shaft and having a detent on a peripheral portion thereof, and a spring loaded element disposed within the winch housing for engaging the detent.

According to a second aspect of the present invention, there is provided a winch for selectively raising and lowering an object, the winch comprising: a winch housing; a winch shaft capable of being driven, rotatably mounted in the winch housing and having an externally threaded portion; a first element coupled to the winch shaft for rotation therewith; a second element rotatably mounted on the winch shaft and axially spaced along said winch shaft from said first element; gripping means defining a channel for gripping a rope or chain connected to the object, the gripping means being rotatably mounted on the winch shaft between the first element and the second element; a threaded locking element mounted on the threaded portion of the winch shaft and having a detent on a peripheral portion thereof; a spring loaded element disposed within the winch housing for engaging the detent; whereby when the winch shaft is driven in a first direction, the spring loaded element abuts an outer surface of the threaded locking element to apply a force to the threaded locking element to cause it to move axially along the shaft into abutment with the second element, to squeeze the gripping means between the first element and the second element and, consequently, rotate the gripping means to raise the object; and when the winch shaft is driven in a second direction opposite to said first direction, the spring loaded element engages the detent to restrain the threaded locking element from rotation with respect to the winch housing, causing the threaded locking element to move out of abutment with the second element, to allow the object to freefall, and wherein continued rotation of the winch shaft in

said second direction after the threaded locking element has been caused to move out of abutment with the second element, causes the threaded locking element to move axially along the winch shaft away from the spring loaded element, until the spring loaded element disengages the detent, to allow the threaded locking element to rotate with the winch shaft.

Thus, a winch is provided having a first element and a second element which does not require one of said elements to be threaded to allow release of a rope or chain gripping means, and which does not require either the first element or the second element to move axially to allow said release. Also, a winch is provided having a spring loaded element for engaging a threaded locking element to allow release of the gripping means, wherein the spring loaded element is disengaged from a detent in the threaded locking element when it has been moved sufficiently axially for the gripping means to be released, thereby preventing damage to the winch.

Conveniently, the object is an anchor, such as a marine anchor.

Preferably, the first and second elements are generally truncated conical elements. The first and second generally truncated conical elements may have respectively inwardly facing converging surfaces.

Preferably, the spring loaded element is a pawl.

The winch housing conveniently includes a bearing and securing assembly for mounting the winch shaft therein. Conveniently also, the winch further comprises means for disengaging the rope or the chain from the gripping means. The disengaging means may be separate from or integral with the winch housing. The disengaging means may be a peeler for moving the rope or chain in a direction out of engagement with the gripping means when the rope or chain is rotated about the winch housing and comes into contact with the peeler.

Conveniently, the winch shaft is coupled to an

electric or hydraulic motor for driving the winch shaft.

The winch shaft may be coupled to the electric or hydraulic motor through a gear box. The threaded portion is conveniently provided on an upper end of the winch shaft.

Conveniently, the first element is formed on the winch shaft. The first and second generally truncated conical elements may be truncated cones, the converging surfaces of which are for abutting respective lower and upper conical surfaces of the gripping means.

Preferably, the gripping means is a gipsy which comprises upper and lower dished members which are generally circular in plan. Conveniently, the upper and lower dished members are interconnectable to form the gipsy. Preferably, the upper and lower dished members together define the channel for engaging the rope or chain, which channel extends circumferentially around the gipsy. Conveniently, the upper dished member is adapted to surround and engage a hub portion of the lower dished member, and the hub portion of the lower dished member is adapted to engage the winch shaft.

The upper and lower dished members may each include elements which are wedge-shaped in plan, for gripping links of the chain or for gripping the rope. The wedge- shaped elements may extend into the channel of the gipsy.

The depth of the channel may decrease in a radially inward direction. The conical surfaces of the gripping means may be formed on respective upper and lower surfaces of the upper and lower dished members.

Preferably, the threaded locking element is a hollow, generally cylindrical locking nut. The locking nut has a flat formed on the outer surface thereof which extends axially along at least part of the length of the locking nut. The detent may comprise a tooth having a substantially radially extending engagement surface for engaging the spring loaded element. The tooth may be formed at an axially upper end of the flat of the locking

nut. In this fashion, continued rotation of the winch shaft in said second direction may cause the locking nut to move axially away from the second element to a sufficient distance such that the pawl disengages the tooth, thereby allowing the rotation of the threaded locking element with the winch shaft in said second direction. Preferably, the locking nut comprises an upper, hollow, generally cylindrical portion on which the flat is formed, and a lower, hollow, generally cylindrical portion of a smaller outside diameter.

Conveniently, the second element is mounted on the lower, hollow, generally cylindrical portion of the locking nut, for mounting the second element on the winch shaft. A shoulder may be formed between the upper and lower portions of the locking nut for abutment with an upper face of the second element, for squeezing the gripping means between the first element and the second element when the winch shaft is rotated in said first direction.

Preferably, the pawl abuts the outer surface of the threaded locking element. Conveniently, the pawl is pivotably mounted within the winch housing by a central boss of the pawl, which engages a recess in the winch housing. The pawl has a tip which engages the detent when the winch shaft is rotated in said second direction, and an inner face which abuts the outer surface of the threaded locking element when the winch shaft is rotated in said first direction.

Conveniently, the first direction of rotation of the winch shaft is an anti-clockwise direction, when viewing the winch shaft in a direction A shown in Fig. 1 of the attached drawings, and the second, opposite direction of rotation of the winch shaft is a clockwise direction.

Alternatively, the first direction of rotation may be a clockwise direction and the second direction of rotation may be an anti-clockwise direction.

According to a third aspect of the present invention, there is provided a drive transmission

mechanism for selectively transmitting a drive force, the drive transmission mechanism comprising: a drive transmission mechanism housing; a drive shaft capable of being driven, said drive shaft being rotatably mounted in the drive transmission mechanism housing and having an externally threaded portion; a first drive element coupled to the drive shaft for rotation therewith; a second drive element rotatably mounted on the drive shaft and axially spaced along said drive shaft from said first drive element; a drivable object rotatably mounted on the drive shaft between the first and second drive elements; a threaded locking element mounted on the threaded portion of the drive shaft and having a detent on a peripheral portion thereof; a spring loaded element disposed within the drive transmission mechanism housing for engaging the detent; whereby when the drive shaft is driven in a first direction, the spring loaded element abuts an outer surface of the threaded locking element to apply a force to the threaded locking element to cause it to move axially along the shaft into abutment with the second drive element, to squeeze the drivable object between the first and second drive elements and, consequently, drive the drivable object; and when the drive shaft is driven in a second direction opposite to said first direction, the spring loaded element engages the detent to restrain the threaded locking element from rotation with respect to the drive transmission mechanism housing, causing the threaded locking element to move out of abutment with the second drive element, to prevent the drivable object from being driven, and wherein continued rotation of the drive shaft in said second direction after the threaded locking element

has been caused to move out of abutment with the second drive element, causes the threaded locking element to move axially along the drive shaft away from the spring loaded element, until the spring loaded element disengages the detent, to allow the threaded locking element to rotate with the drive shaft.

The drive transmission mechanism may be a drive transmission mechanism of a winch. The winch may be for selectively pulling in and releasing a rope or chain coupled to the drivable object. The drivable object may be gipsy.

According to a fourth aspect of the present invention, there is provided a winch for selectively allowing raising and lowering of an object, the winch comprising: a winch housing; a winch shaft capable of being driven, rotatably mounted in the winch housing and having an externally threaded portion; a first element coupled to the winch shaft for rotation therewith; a second element rotatably mounted on the winch shaft and axially spaced along said winch shaft from said first element; gripping means defining a channel for gripping a rope or chain connected to the object, the gripping means being rotatably mounted on the winch shaft between the first element and the second element; a threaded locking element mounted on the threaded portion of the winch shaft; a spring loaded element for applying a force to the threaded locking element, to move it in a first axial direction along the winch shaft when the winch shaft is driven in a first direction; and a detent for engagement by the spring loaded element, for restraining the threaded locking element from rotation with respect to the winch housing and to

move the threaded locking element in a second axial direction along the winch shaft, when the winch shaft is driven in a second direction opposite to said first direction.

Preferably, the spring loaded element is disposed within the winch housing and the detent is formed on a peripheral portion of the threaded locking element.

Alternatively, the spring loaded element is disposed on the peripheral portion of the threaded locking element and the detent is formed in the winch housing.

Preferably, the spring loaded element is a pawl.

Alternatively, the spring loaded element may be a ball, catch, or any other suitable element. There may be a plurality of detents for engagement by the spring loaded element.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a partial cross-sectional side view of a winch in accordance with a preferred embodiment of the present invention, shown in a drive configuration where the winch is raising an anchor; Fig. 2 is an exploded view of a winch assembly including the winch of Fig. 1; Fig. 3 is a plan view of the winch of Fig. 1, partially sectioned to illustrate a threaded locking element and pawl, and partially cut-away to illustrate a gripping means for a rope or chain, each forming part of the winch of Fig. 1; Fig. 4 is a perspective view of the threaded locking element of Fig. 3; Fig. 5 is a view of the winch of Fig. 1, shown in a freefall configuration, where the winch is lowering the anchor, and Fig. 6 is a plan view of the winch of Fig. 1 in the freefall configuration of Fig. 5, similar to the view of Fig. 3.

Referring firstly to Fig. 1, there is shown a partial cross-sectional side view of a winch in accordance with a preferred embodiment of the present invention, indicated generally by reference numeral 10.

The winch 10 is shown in a drive configuration to raise an anchor (not shown) by paying-in a rope or chain (not shown-often referred to as a"rode") coupled to the anchor.

Referring also to Fig. 2, which is an exploded perspective view of a winch assembly 11 including the winch 10 of Fig. 1, the winch 10 includes the following principal parts: a winch housing 12; a winch shaft 14; a first element in the form of a lower truncated winch cone 16; a second element in the form of an upper truncated winch cone 18; a rode gripping means in the form of a gipsy 20; a threaded locking element in the form of a locking nut 22; a bearing and securing assembly, indicated generally by reference numeral 24, for mounting and centralising the winch shaft 14 in the winch housing 12; and a pawl assembly indicated generally by reference numeral 26. A main case 17 of the winch assembly is shown in Fig. 1 in broken outline.

The winch shaft 14 is cylindrical and has a lower end 28 coupled to an electric motor (not shown) disposed in a housing 15 of the winch assembly 11 via a gear box (not shown) disposed in the main case 17. An upper portion 30 of the winch shaft 14 extends through the gipsy 20, upper winch cone 18 and the locking nut 22.

The upper portion 30 is threaded with right-hand threads 32 (viewing in the direction of the arrow A in Fig. 1) along part of the length of the upper portion 30, which engage corresponding threads 34 on the locking nut 22.

An upper end 36 of the winch shaft 14 engages a bore 38 in the winch housing 12 and is retained by the bearing and securing assembly 24. The bearing and securing assembly 24 comprises a threaded bolt 40 which couples an upper shaft washer 42 to the upper end 36 of winch shaft

14, a lower shaft washer 44 which abuts a shoulder 46 of the upper portion 30, and a circlip 48 for securing the washer 44 in position. Thus, the winch shaft 14 is coupled to the winch housing 12 and is rotatable in the bore 38 of the housing 12. The bearing and securing assembly 24 and the volume between the locking nut 22 and winch housing 12 is packed with grease for lubrication.

The lower winch cone 16 is formed on the winch shaft 14 and has an upper conical surface 50 which converges towards the gipsy 20 and rotates in unison with the winch shaft 14 when it is driven by the motor. The surface 50 abuts a corresponding conical surface 52 of the gipsy 20, as will be described in more detail below.

The gipsy 20 generally comprises a lower dished portion 54 and an upper dished portion 56 which are coupled together with the dished portions 54 and 56 arranged to define the generally conical outer surfaces 52 and a similar surface 74, and to define a generally V- shaped channel 97 which converges towards the shaft 14.

Each dished portion 54 and 56 has a complex inner profile. Only the lower dished portion 54 will be described here in detail; like components of the upper dished portion 56 share the same reference numerals as the lower dished portion 54, with the addition of the letter'a'. Lower dished portion 54 comprises a rim 58, shoulder 60 and a tubular body portion 62 by which portion 54, and thus the gipsy 20, is rotatably mounted on the winch shaft 14. Generally wedge-shaped elements 102 and fingers 104 (shown more clearly in Fig. 3) extend from the rim 58 and shoulder 60 into the channel 97, to grip a link of the chain or to grip the rope. The upper dished portion 56 differs in that the tubular body portion 62a allows the portion 56 to engage the lower dished portion 54 to form the gipsy 20, as shown in Fig.

1. Also, the upper and lower dished portions 56 and 54 define a tongue and groove assembly 70, to allow the portions 54 and 56 to be connected to form the gipsy 20,

which rotates as a unitary item with respect to the winch shaft 14.

The upper winch cone 18 includes a surface 72 for abutting the corresponding conical surface 74 in the upper dished portion 56 of the gipsy 20. Also, the upper winch cone 18 defines a through-bore 76 which is engaged by a lower portion 78 of the locking nut 22, to mount the upper winch cone 18 to the winch shaft 14.

The through-bore 76 is unthreaded, allowing the upper winch cone 18 to freely rotate about the locking nut 22, and thus the winch shaft 14, in the absence of a mating force between the upper cone 18 and the locking nut 22, as will be described in more detail below.

The locking nut 22 comprises the lower, hollow, generally cylindrical portion 78, and an upper, hollow, generally cylindrical portion 80 which includes a flat 92, best shown in Figs. 3 to 4 and described below, formed on a periphery of the upper portion 80. The lower portion 78 of the locking nut 22 has a smaller outside diameter than that of the upper portion 80, and a shoulder 82 is defined therebetween. This shoulder 82 acts upon an upper face 84 of the upper winch cone 18, to squeeze the gipsy 20 between the upper and lower winch cones 18 and 16 respectively, when the winch shaft 14 is rotated in an anti-clockwise direction, as will be described in more detail below.

Referring now to Fig. 3, there is shown a plan view of the winch 10 of Fig. 1, partially sectioned to show the locking nut 22 and pawl assembly 26 in more detail, and partially cut-away to illustrate the gipsy 20 in more detail. The pawl assembly 26 includes a pawl 86, mounted in a recess of the winch housing 12 by a central boss 88. A torsion spring 90 is located on the boss 88, and acts between an inner surface of the winch housing 12 and the pawl 86, to exert a force on the pawl 86 to bring it into engagement with the locking nut 22.

Referring also now to Fig. 4, which is a perspective

view of the locking nut 22, the upper portion 80 of the locking nut 22 includes the flat 92 extending along the length thereof, with a detent 94 at the top of the flat 92 level with an upper end of the upper portion 80. The detent 94 comprises a tooth having a radially extending abutment surface 95 for engaging an end 87 of the pawl 86. The detent 94 extends only along part of the axial length of the flat 92, so that when the locking nut 22 has travelled sufficiently axially away from the upper winch cone 18, the pawl 86 will disengage the detent 94 (during rotation of the winch shaft 14 in a clockwise direction, as will be described with reference to Fig. 6 below). Also, an outer surface 91 of the detent 94 forms part of the circumference of the upper portion 80.

Thus, when the winch shaft 14 is rotated in an anti- clockwise direction, the pawl 86 moves over the outer surface 91 of detent 94 during rotation of nut 22. The pawl 86 does not therefore enter into abutment with the surface 95.

When it is desired to raise the anchor, the motor is activated and the winch shaft 14 is rotated in the anti- clockwise direction when viewing the winch 10 in the direction of the arrow A of Fig. 1. This in turn rotates the locking nut 22 in an anti-clockwise direction with the winch shaft 14. However, an inner surface 89 of the pawl 86 abuts the outer surface of the nut 22, creating a drag force on the locking nut 22, causing it to be partially restrained from rotating with the winch shaft 14. This causes the locking nut 22 to move axially towards the upper winch cone 18. Continued rotation of the winch shaft 14 in the anti-clockwise direction brings the locking nut 22 into abutment with the upper winch cone 18, such that the shoulder 82 of the upper portion 80 abuts the upper face 84 of the upper winch cone 18.

This causes the upper winch cone 18 to exert an axial force upon the gipsy 20, squeezing the gipsy 20 between the upper and lower cones 18 and 16. The gipsy 20 is

therefore firmly gripped between cones 16 and 18 and rotates in an anti-clockwise direction with the locking nut 22, upper and lower winch cones 18 and 16 and the winch shaft 14. A rode connected to the anchor and threaded into the channel 97 of the gipsy 20 is therefore gripped and pulled in/recovered, raising the anchor.

The winch housing 12 is prevented from rotation with the winch shaft 14 by means of a bolt 98, which secures the winch housing 12 and a stripper or peeler 100, shown in Fig. 1, to a boss of a lower portion of the winch assembly. The peeler 100 extends around the gipsy 20 to peel rope or chain out of the channel 97 when it is being pulled-in/recovered.

Referring now to Fig. 5, there is shown a view of the winch 10 of Fig. 1 in a freefall configuration, where the winch 10 is lowering the anchor. In the configuration of Fig. 5, the winch shaft 14 has been rotated in a clockwise direction (when viewing the winch 10 in the direction of the arrow A of Fig. 1), as shown in Fig. 6, which is a plan view of the winch 10 in the freefall configuration of Fig. 5. This causes the locking nut 22, upper and lower winch cones 18 and 16 and the gipsy 20 to be rotated in the clockwise direction with the winch shaft 14, thereby lowering the anchor.

This continues until the end 87 of the pawl 86 engages the abutment surface 95 of the detent 94, as shown in Fig. 6. It will be understood that when the pawl 86 has engaged the detent 94, the winch shaft 14 continues to rotate in the clockwise direction. The locking nut 22 is restrained from any further rotation in the clockwise direction by the pawl 86. This causes the locking nut 22 to move in an axial direction along the winch shaft 14 away from the upper winch cone 18 by an interaction between the threads 32 and 34.

When the force exerted upon the upper winch cone 18 by the shoulder 82 of locking nut 22 is reduced, the force imparted on the gipsy 20 is also reduced.

Eventually, the gipsy 20 slips under the weight of the anchor, and the anchor begins to freefall under gravity.

Meanwhile, the locking nut 22 continues to move axially away from the upper winch cone 18. Continued rotation of the winch shaft 14 therefore causes the locking nut 22 to move to a position where the pawl 86 disengages the detent 94. This is because the detent 94 only extends along a short axial length of the flat 92. Thus, when the locking nut 22 has moved sufficiently axially, the detent 94 disengages from the pawl 86. The only force then imparted upon the locking nut 22 is a relatively small frictional force imparted by the pawl 86. This may allow some further axial movement of the locking nut 22. However, the locking nut 22 is restrained from moving too far and damaging the winch 10 by the lower shaft washer 44, which is restrained by the circlip 48.

When the locking nut 22 comes into contact with the washer 44, the washer 44 engages a recess 23 in the top of the upper portion 80 of the nut 22. The shaft 14, nut 22 and washer 44 then rotate together in the clockwise direction until drive ceases.

Freefall of the anchor is stopped either when the anchor comes into contact with the seabed, or by driving the winch shaft 14 in the anti-clockwise direction, as shown in Figs. 1 and 3 and described above, to bring the locking nut 22 into contact with the upper winch cone 18 and impart a force on the gipsy 20. This brings the anchor to a controlled stop as the force upon the gipsy 20 slowly increases. To prevent drift after lowering of the anchor, the winch shaft 14 is rotated anti-clockwise when viewing in the direction of arrow A of Fig. 1 after freefall has ended, to prevent further paying-out of the rode.

If the winch shaft 14 is rotated in the anti- clockwise direction of Figs. 1 and 3 whilst the pawl 86 is in engagement with the detent 94, the pawl 86 rotates out of engagement with the detent 94. On further

rotation, the pawl 86 is simply pushed back against the force of the torsion spring 90 by the outer surface 91 of detent 94 and the frictional force between the pawl 86 and the locking nut 22 brings the locking nut 22 axially towards the upper winch cone 18.

As will be appreciated by persons skilled in the art, the mechanism may be used for selective transmission of a drive force to a wheel, axle, shaft, gearbox, differential, any other winch, or any rotatably drivable object. This is achieved by providing a drive transmission mechanism including a drive transmission mechanism housing; a drive shaft rotatably mounted in the housing; a first drive element coupled to the shaft; a second drive element rotatably mounted on the shaft and axially spaced along the drive shaft from the first drive element; a threaded locking element mounted on a threaded portion of the winch shaft, and a spring loaded element in the form of a pawl disposed within the housing. The drivable object which may, for example, be a gear would be disposed between the drive elements for being selectively driven in a similar fashion to the gipsy 20 of the winch 10 described above with reference to Figs. 1 to 6. It will be understood that the components of the drive mechanism function in a fashion similar to the corresponding components of the winch 10, namely the winch housing 12; winch shaft 14; lower cone 16; upper cone 18; gipsy 20; threaded locking nut 22, and the pawl assembly 26. In particular, the drive transmission mechanism may be a drive transmission mechanism of a winch. The winch may be for selectively pulling-in and releasing a rope or chain coupled to the drivable object, which may be a gipsy. A typical such winch may be a winch mounted on a vehicle for use in hauling the vehicle or another object, where the rope or chain requires to be quickly paid-out prior to being pulled-in.

Various modifications may be made to the foregoing within the scope of the present invention. For example,

the peeler 100 may be an integral part of the winch housing 12. The lower cone 16 may be a separate component which engages the winch shaft 14. The lower cone 16 may thus be secured and driven by the winch shaft 14. The locking nut 22 may be of any desired shape which permits movement of the pawl 86 over the outer surface 96 of the nut 22. The detent 94 may be provided at any suitable location on the flat 92. The threaded portion of the winch shaft 14 may be threaded with a left-hand thread 32 and the detent 94 and pawl 86 may engage when the winch shaft 14 is rotated in an anti-clockwise direction. The spring loaded element may be a spring loaded ball, catch, or any other suitable element. For example, the spring loaded element may comprise a resilient member, such as a spring steel element having one secured end. A plurality of detents 94 may be provided spaced around the periphery of the locking nut 22. A braking device may be provided for selectively applying a brake force to the gipsy 20 during freefall, to allow a controlled fall of the object (such as the anchor).

As will be appreciated by persons skilled in the art, the spring loaded element may be disposed on a peripheral portion of the threaded locking element (locking nut 22) and the detent 94 may be formed in the winch housing 12. However, such an arrangement is more difficult and expensive to manufacture than the structure described in the main embodiment.

Although the first and second elements 16 and 18 are shown as generally truncated cones in a preferred embodiment of the invention, it will be appreciated that the angle of the elements may be varied to provide a suitable performance in gripping the gipsy 20. This angle may be substantially as shown in the accompanying figures, or may be steeper or shallower as desired, so that the shape of the elements may be, for example, cylindrical, although the exact shape may depend on particular parameters which may include material friction coefficients and gipsy shape.