WO2010092237A1

WO2010092237A1 - Method for using retainer, and retainer

Info

Publication number: WO2010092237A1
Application number: PCT/FI2010/050090
Authority: WO
Inventors: Juha Piipponen; Sami KERÄNEN
Original assignee: Sandvik Mining And Construction Oy
Priority date: 2009-02-12
Filing date: 2010-02-11
Publication date: 2010-08-19
Also published as: EP2396497B1; CA2734361A1; FI20095134A0; EP2396497A1; CN102216555A; CA2734361C; CN102216555B; JP2012504721A; FI20095134A; EP2396497A4; JP5487210B2; AU2010212736B2; FI121894B; AU2010212736A1

Abstract

The invention relates to a method for using a retainer, and to a retainer. The retainer(14) comprises a first clamping arm (18a) and a second clamping arm (18b) provided with clamping jaws (21a, 21b) for clamping a drill rod (9), a rock bolt or the like. The clamping arms are turned in relation to their pivot points (19a, 19b) by an actuator (20), whereby the clamping jaws move either towards or away from each other. On the portion between the pivot points of the clamping arms there are shape members that interconnect the movements of the clamping arms to make them interdependent and thus synchronize the movements of the clamping arms in both directions of movement of the clamping arms.

Description

METHOD FOR USING RETAINER, AND RETAINER

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method for moving clamping arms of a retainer belonging to an excavation apparatus in such a manner that clamping jaws provided in the clamping arms move simultaneously towards each other, clamping between them a drill rod, rock bolt or a similar excavation tool, which is located in the drilling centre of the rock drilling unit or another excavation apparatus. Mechanical synchronization is provided between the clamping arms to render their movements interdependent.

[0002] The invention further relates to a retainer adaptable to a feed beam of a rock drilling unit, bolting device or a similar excavation apparatus. The retainer comprises at least two clamping jaws that are moved by means of one or more actuators. Between the clamping arms there is a synchronization mechanism to render their movements interdependent.

[0003] The field of the invention is described in greater detail in the preambles of the independent claims.

[0004] Rock drilling is performed using a rock drilling unit provided with a rock drilling machine and a tool connected thereto to produce a drill hole into the material to be worked on. The tool may comprise a drill rod and a drill bit connected to the distal end thereof or, if long drill holes are to be drilled, a plurality of successive drill rods connected one after the other into an extension rod arrangement. When drilling is started, the drill rod needs to be centred by guiding it with the clamping jaws of the retainer in the drilling centre. Moreover, the retainer may be used for locking the drill rod or other drilling tools to prevent them from rotating for example when drill rods are attached to each other, to the drilling machine or the drill bit.

[0005] Publication US 4 438 984 discloses a retainer in which the clamping arms of the retainer are interconnected by a rod transmitting the forces from one arm to another. The disclosed structure occupies a lot of space on the feed beam.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to provide a novel and improved method for using a retainer, and a retainer.

[0007] The method of the invention is characterized by providing the clamping arms with shape members in the portion between their pivot points and synchronizing with the shape members the movement between the first clamping arm and the second clamping arm in both travel directions of the clamping arms.

[0008] The retainer of the invention is characterized in that the synchronizing means of the retainer are arranged on the portion between a first and a second pivot joint; and that the synchronizing means comprise shape members formed on the first and the second clamping arms and arranged to transmit the synchronization movements between the clamping arms in both travel directions thereof.

[0009] An idea of the invention is that the retainer comprises synchronization means with which the movements of the clamping arms are made mutually dependent. The synchronization means are separate from one or more actuators acting on the clamping arms and transmit the movements of the clamping arms in both travel directions thereof. The synchronization means are implemented by shape members arranged to the clamping arms and located on the portion between the pivot joints of the clamping arms.

[0010] An advantage of the invention is that since the synchronization means are arranged on the portion between the pivot joints, they do not add to the outer dimensions of the retainer in any way. A further advantage is that by utilizing shape surfaces formed to the clamping arms, synchronization may be implemented using a simple mechanical structure and a small number of components. Due to this synchronization does not increase significantly the weight of the retainer. Moreover, the retainer operates reliably.

[0011] According to an embodiment the shape members on the portion between the pivot points are in constant contact with each other, and thereby at all times ready to synchronize the movements of the clamping arms in both travel directions.

[0012] According to an embodiment the synchronizing means comprise shape surfaces formed directly to the structure of the first and the second clamping arms, the surfaces being arranged against each other and transmitting synchronization movements between the clamping arms through their contact surfaces that set against each other.

[0013] According to an embodiment the first clamping arm and the second clamping arm are provided with mutually compatible teeth arranged against each other, movement between the clamping arms being transmitted by direct tooth contact. The teeth may be made directly to the structure of the clamping arms.

[0014] According to an embodiment the shape surfaces formed to the first and the second clamping arms are in constant contact with each other at least on some portion thereof.

[0015] According to an embodiment the synchronization means comprise shape surfaces formed to the first and the second clamping arm on a portion between the pivot joints. The shape surfaces are interconnected by a connecting piece separate from the clamping arms and arranged against the shape surfaces. However, the connecting piece is not attached to the clamping arms and the shape surfaces provided in them. During synchronization, movement may take place between the form surfaces and the connecting piece.

[0016] According to an embodiment the first clamping arm comprises at least two first connecting cams arranged at an axial distance from one another, when the retainer is observed in the direction of the centre. Correspondingly, the second clamping arm comprises at least two second connecting cams on a portion between pivot joints, the cams being arranged at an axial distance from one another. The first connecting cams and the second connecting cams are interleaved on the portion between the pivot joints. Further, the first and the second connecting cams are provided with connecting slots that open towards each other and are arranged to coincide when observed in the direction of the centre, the connecting slots being arranged to form a connecting channel when observed in the direction of the centre. This connecting channel is provided with a connecting pin that serves as a transmission member between the interleaved connecting cams. The connecting pin moves in a vertical direction when the clamping arms are moved. The vertical movement of the connecting pin is allowed through a longitudinal hole or the like provided in the body of the retainer. The connecting pin is a separate piece not attached to the clamping arms.

[0017] According to an embodiment the first clamping arm is provided with an immobile connecting pin, whose outer surface serves as a first shape surface. The second clamping arm comprises a longitudinal connecting opening that serves as a second shape surface. The connecting pin is arranged through the connecting opening, their shape surfaces transmitting synchronization movements between the clamping arms. [0018] According to an embodiment the retainer comprises only a single actuator arranged between the clamping arms and connected to act simultaneously on both clamping arms.

[0019] According to an embodiment the retainer comprises only a single actuator arranged to act on the first clamping arm only. In that case movement to the second clamping arm is transmitted by the synchronization means.

[0020] According to an embodiment the retainer comprises at least two actuators, at least a first actuator being connected to act on the first clamping arm only and at least one second actuator being arranged to act on the second clamping arm only.

[0021] According to an embodiment the actuator is a hydraulic cylinder connected to a clamping arm by a pivot.

[0022] According to an embodiment the actuator is arranged to direct the forces needed for moving the clamping arms directly to the clamping arms. This is advantageous for the durability of the construction.

[0023] According to an embodiment the actuator is connected to the clamping arm by a pivot. The pivoting makes the construction durable and, moreover, it may be used to reduce friction.

[0024] According to an embodiment the actuator is a pressure medium cylinder arranged transversally in relation to the longitudinal axis of the clamping arms. The linear travel direction of the actuator is thus arranged to substantially correspond to the travel directions of the clamping jaws.

[0025] According to an embodiment the actuator is placed outside the portion between the pivot points of the clamping arms. Hence a fixing point, where the actuator is connected to the clamping arm, may be located at a greater distance from a pivot point of the clamping arm, thus allowing a greater torque arm to be achieved and thereby the clamping force produced by the actuator to be increased.

[0026] According to an embodiment the clamping jaws comprise a slot of a substantially V-shaped cross-section on their surfaces facing each other, thus allowing the clamping jaws to centre a drill rod or the like pressed between them. Due to the shape of the jaws also drill rods and the like with differing diameters may be centred. [0027] According to an embodiment the clamping jaws are arranged to move at the same travel speed, for the same distance and symmetrically in relation to each other.

BRIEF DISCLOSURE OF THE FIGURES

[0028] Some embodiments of the invention will be described in greater detail with reference to the accompanying drawings, in which

Figure 1 is a schematic side view of a rock drilling rig provided with booms and drilling units arranged thereto and a second excavation apparatus;

Figure 2 is a schematic perspective view of a drilling unit;

Figure 3 is a schematic view of a retainer of the invention, seen from the direction of the front end of the feed beam;

Figure 4 is a schematic perspective view of the retainer of Figure 3 without a body plate;

Figure 5 is a schematic cross-sectional view along line E - E of the retainer of Figure 2;

Figure 6 is a schematic cross-sectional view along line D - D of the retainer of Figure 2;

Figure 7 is a schematic cross-sectional view along line F - F of the retainer of Figure 2;

Figure 8 is a schematic view of a retainer of the invention seen from the direction of the front end of the feed beam, each clamping arm of the retainer being acted on by a separate actuator;

Figure 9 is a schematic view of a retainer of the invention seen from the direction of the front end of the feed beam, only one clamping arm of the retainer being acted on by an actuator;

Figure 10 is a schematic view of a second alternative solution for synchronizing the turning movement between the clamping arms by toothed peripheries;

Figure 11 is a schematic view of a third alternative solution for synchronizing the turning movement between the clamping arms by transverse longitudinal openings provided in the clamping arms and a connecting pin arranged through the openings;

Figure 12 is a further schematic view of a fourth alternative solution for synchronizing the turning movement between the clamping arms by a fixed connecting pin and a longitudinal opening. [0029] For the sake of clarity, some embodiments shown in the figures have been simplified. Like parts are referred to with like reference numbering throughout the figures.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

[0030] Figure 1 shows an excavation apparatus 1 comprising a movable carrier 2 provided with one or more booms 3. A first boom 3a is a drilling boom provided with a rock drilling unit 4. The drilling unit 4 comprises a rock drilling machine 5 that is movable on a feed beam 6 by a feed device 7. The rock drilling machine 5 may be provided with a tool 8 coupled thereto and comprising a plurality of successive drill rods 9, which are attached to each other by connecting means, such as a threaded joint. Further, at the distal end of the drill rod there is a drill bit 10 having drill buttons for making a drilling hole 12 into rock 11. The rock drilling machine 5 may comprise an impact device used for delivering impact pulses to the tool 8 that transmits them to the drill bit 10 and further to the rock 11 to be broken. The rock drilling machine 5 may also comprise a rotating device allowing the tool 8 to be rotated about its longitudinal axis during drilling. The drilling unit 4 may also be provided with a rod magazine 13 supported on the feed beam 6. The rod magazine 13 may be used for storing extension rods 9 and other drilling tools needed in long hole drilling. The rod magazine 13 is provided with transfer means for transferring drilling tools between the rod magazine 13 and the drilling axis, i.e. the centre K. The distal end portion of the feed beam 6 is provided with retainer 14 that has jaws for clamping the tool 9 and centring it in the drilling centre K. The jaws may take hold of the shaft or the connecting sleeve of the drill rod 9, thus allowing the rod to be held in place when the joints are screwed or unscrewed. Further, it is possible to grab the drill bit 10 with the retainer and to hold it in place when the drill bit 10 is to be changed. In addition to the rod magazine 13 the feed beam 6 may be provided with a drill bit magazine.

[0031] A second boom 3b of the excavation apparatus 1 of Figure 1 is provided with another excavation device 15 for handling excavation tools 16, such as rock bolts, injection tubes and the like. Also these may need to be held by the jaws or the tool may need to be centred to the centre K of the apparatus. In such cases also these excavation devices 15 may be provided with the retainer 14 of the invention. [0032] Figure 2 shows a rock drilling unit 4 in which the feed beam 6 is provided with a rod magazine 13 in which drill rods 9 needed in drilling are. The distal end of the feed beam 6 is provided with a retainer 14, the possible structural alternatives of which are shown in greater detail in Figures 3 to 12.

[0033] The retainer 14 of Figure 3 comprises a body 17 allowing it to be attached to the feed beam. The retainer 14 also comprises a first clamping arm 18a connected by a first pivot joint 19a to the body 17 and a second clamping arm 18b connected by a second pivot joint 19b to the body 17. The clamping arms 18 may be moved in relation to the pivot joints 19 by an actuator 20 located between them, whereby clamping jaws 21 a, 21 b at the distal ends of the clamping arms 18 move either towards or away from each other, depending naturally on the direction of movement of the actuator 20. The actuator 20 may be a pressure medium cylinder, such as a hydraulic cylinder, that may be connected to the clamping arm 18 by joints 22. The actuator 20 then transmits the forces directly to the clamping arms. The actuator 20 is placed outside the portion between the pivot joints 19a and 19b. The point of connection of the actuator 20, i.e. the joint 22, is located at a distance from the pivot joint 19a, 19b, whereby a torque arm is generated between the joints 19 and 22. The clamping jaws 21 may comprise centring surfaces on their surfaces facing each other and thus, when they move towards each other, they may centre the drill rod 9 or other tool between them precisely into the centre K. The centring surfaces 23 may have a substantially V-shaped cross-section and they naturally open towards the centre K. Of course other inclined or conical centring surfaces 23 may also be used in the clamping jaws 21. The clamping arms 18 are made to move interdependently by synchronization means 24 provided on the portion between the pivot joints 19. In other words, the synchronization means 24 are separate from the actuator 20. Figure 3 shows a connecting pin 25 which is parallel with the centre K and may belong to the synchronization means 24. As is seen, the body 17 is provided with a longitudinal opening 26 in the vertical direction, the opening allowing the connecting pin 25 to move in vertical direction V, when the clamping arms 18 are moved.

[0034] In Figure 4 part of the body 17 has been removed for better visibility of the synchronization means 24. On the portion between the pivot joints 19 the first clamping arm 18a may comprise two or more first connecting cams 27, which are at an axial distance from one another when the retainer 14 is observed in the direction of the centre K. Correspondingly, the second clamping arm 18b may comprise two or more second connecting cams 28. The connecting cams 27, 28 are interleaved on the portion between the pivot joints 19, as shown in Figure 4. Further, the connecting cams 27, 28 are provided with connecting slots 29 that may open towards each other and are arranged to coincide in the axial direction so that together they form a kind of a connecting channel 30. This axial connecting channel 30 is provided with a connecting pin 25 that may serve as a transmission member between the connecting cams 27, 28. When the clamping arms 18 are moved, movement takes place in the contact surfaces of the connecting slots 29 and the connecting pin 25, and in addition, the connecting pin 25 moves linearly in a vertical direction, i.e. it moves either towards the centre K or away from it. In this application the shape surfaces belonging to the synchronization means 24 are implemented by surfaces of the connecting channel 30, which are in contact with the connecting pin 25.

[0035] The cross-section of Figure 5 further clearly shows the formation of a connecting channel 30 between the connecting cams 27, 28. For vertical movement of the connecting pin 25 the body 17 is provided with longitudinal openings 26a, 26b. The cross-sectional Figures 6 and 7 further clarify the construction of this embodiment. Unlike in Figures 4 to 7, it may be possible to provide connecting slots 29 with a closed shape. The cross-section of the connecting pin 25 is preferably circular.

[0036] In the embodiment of Figure 8 a first actuator 20a transmits force to a first clamping arm 18a only and a second actuator 20b transmits force to a second clamping arm 18b only. The synchronization means 24 ensure the simultaneity of the movement of the clamping arms 18.

[0037] In the embodiment of Figure 9 the force from the actuator 20 is transmitted only to the first clamping arm 18a and the synchronization means 24 transmit the movement to the second clamping arm 18b.

[0038] Figure 10 shows a strongly simplified embodiment of synchronization means 24 including teeth 31 a, 31 b formed to the clamping arms

18. The teeth are formed to the curved transmission surfaces of the pivot joints

19. In this application the synchronization movements are transmitted directly through contact with the teeth, therefore no connecting pins or a similar connecting members are needed. The teeth 31 serve as the necessary shape members between the clamping arms 18. [0039] Figure 11 shows a strongly simplified embodiment of synchronization means 24 provided with a first longitudinal connecting opening 32a in the first clamping arm 18a and a second longitudinal connecting opening 32b in the second clamping arm 18b. At least on the portions of the connecting openings 32 the clamping arms 18 are interleaved in relation to each other. The clamping arms 18 may be provided with connecting cams 27, 28 or corresponding projecting portions on these interleaved portions. The connecting openings 32 are arranged transversally with respect to each other, whereby they together define a connecting channel 30 with a connecting pin 25 arranged therein. The connecting pin 25 is not attached to the connecting openings 32 but it is allowed to move in the longitudinal direction of the connecting openings 32 and, at the same time, also in a vertical direction with respect to the centre K, when the clamping arms 18 are moved by applying a turning force to the force transmission points 22.

[0040] Figure 12 shows yet another strongly simplified embodiment of the synchronization means 24 having a longitudinal connecting opening 32 formed to the first clamping arm 18a and an immovably fixed connecting pin 25 provided on the second clamping arm 18b.The clamping arms 18 are interleaved with respect to each other on the portion of the connecting opening 32 and the connecting pin 25. The clamping arms 18 may be provided with connecting cams 27, 28 or similar protruding portions on these interleaved portions. When the actuator 20 turns the clamping arms 18 with respect to their pivots 19, the connecting pin 25 follows a curved path and, at the same time, its position in the connecting opening 32 changes. Further, it is possible that the connecting cam 27 comprises a slot-like connecting opening 32 that opens towards the second clamping arm 18b. In this application the outer surface of the connecting pin 25 and the edges of the connecting opening 32 serve as shape surfaces transmitting the synchronization movements.

[0041] In the embodiments of Figures 3 to 7, 11 and 12 the connecting pin 25 is arranged between the interleaved portions and is to be subjected substantially to shearing forces only. In that case the connecting pin 25 may be relatively thin, yet it is able to transmit even strong forces between the clamping arms 18. In the embodiments of Figures 3 to 7 and 11 there is a separate connecting pin 25 that is easy to detach and replace.

[0042] It should be noted that the opening movement of the clamping jaws 21 of the retainer 14 may be arranged to be sufficiently large to allow it to be used for clamping both the shaft of the drill rod 9 and the connecting sleeve between successive drill rods 9. Also the drill bit 10 or any other tool used in excavation may be gripped with it.

[0043] When in this application reference is made to a vertical direction or a vertical movement, this means a vertical movement in relation to the basic position of the feed beam and the retainer, the feed beam being horizontal in that position. When the boom and the feed beam are turned during operation, this naturally changes also the position of the retainer. Although in absolute terms the direction changes, the retainer nevertheless maintains its relative position with respect to the feed beam.

[0044] In some cases the characteristics presented in this application may be used as such, independently of other features. However, the characteristics presented in this application may also be combined, when necessary, to provide different combinations.

[0045] The drawings and the related specification are only meant to illustrate the inventive idea. The details of the invention may vary within the scope of the claims.

Claims

1. A method for using a retainer belonging to an excavation apparatus, the retainer (14) comprising: a body (17), a first clamping arm (18a) and a second clamping arm (18b), which are pivoted to the body (17), clamping jaws (21a, 21b) at the distal ends of the clamping arms (18a, 18b), and at least one actuator (20); and the method comprising the steps of turning with the actuator (20) the clamping arms (18a, 18b) with respect to their pivot points (19a, 19b), whereby the clamping jaws (21a, 21b) move towards each other and, correspondingly, away from each other depending on the direction of movement of the actuator (20); clamping a tool (9) in the centre (K) of the excavation apparatus (4, 15) with the clamping jaws (21a, 21b) to centre and gripping it while positioned in the centre (K); connecting the clamping arms (18a, 18b) with each other by separate mechanical synchronization means (24), the movements of the clamping arms (18a, 18b) being thus interdependent and the clamping jaws (21a, 21b) moving simultaneously and symmetrically, characterized by arranging shape members on the portion between the pivot points (19a, 19b) of the clamping arms (18a, 18b) and synchronizing the movement between the first clamping arm (18a) and the second clamping arm (18b) with the shape members in both travel directions of the clamping arms.

2. A method according to claim ^ characterized by arranging mutually compatible teeth (31a, 31b) to the first clamping arm (18a) and the second clamping arm (18b) and arranging them against one another; and transmitting movement between the clamping arms (18a, 18b) by direct tooth contact.

3. A method according to claim 1 or 2, characterized by arranging at least one portion of both the first clamping arm (18a) and the second clamping arm (18b) in an interleaved manner and arranging shape surfaces (29, 32a, 32b) provided in the form of openings in the interleaved portions to coincide; and transmitting movement between the clamping arms (18a, 18b) by means of a connecting pin (25) parallel with the centre (K), the pin being in contact with the shape surfaces of the clamping arms (18a, 18b).

4. A method according to claim 3, characterized by allowing a transverse movement of the connecting pin (25) in a vertical direction when the clamping arms (18a, 18b) are moving.

5. A retainer comprising a body (17) allowing the retainer (14) to be attached to a feed beam (6) of an excavation apparatus (4, 15); a first clamping arm (18a) connected by a first pivot joint (19a) to the body (17) and a second clamping arm (18b) connected by a second pivot joint (19b) to the body (17); clamping jaws (21a, 21 b) arranged on the portions of the distal ends of the clamping arms (18a, 18b) and facing each other; at least one actuator (20) for moving the clamping arms (18a, 18b) in relation to the pivot joints (19a, 19b), the clamping jaws (21a, 21b) being movable towards and away from each other; and mechanical synchronization means (24) separate from the actuator (20) for connecting the movement of the first clamping arm (18a) and that of the second clamping arm (18b) to be interdependent, characterized in that the synchronization means (24) of the retainer (14) are arranged on the portion between the first and the second pivot joint (19a, 19b); and the synchronization means (24) comprise shape members formed to the first and the second clamping arm (18a, 18b) and arranged to transmit synchronization movements between the clamping arms (18a, 18b) in both travel directions of the clamping arms.

6. A retainer according to claim 5, characterized in that the synchronization means (24) comprise shape surfaces formed directly to the structure of the first and the second clamping arm (18a, 18b), the surfaces being arranged against each other and arranged to transmit synchronization movements between the clamping arms (18a, 18b) through their contact surfaces facing each other.

7. A retainer according to claim 6, characterized in that the first clamping arm (18a) and the second clamping arm (18b) are provided with curved transmission surfaces facing each other on the portion between the pivot joints (19a, 19b); and the transmission surfaces are provided with mutually compatible teeth (31a, 31b), the synchronization movement of the clamping arms (18a, 18b) being arranged to be transmitted directly by the tooth contact.

8. A retainer according to claim 6 or 7, characterized in that the shape surfaces of the clamping arms (18a, 18b) are in constant contact with each other.

9. A retainer according to claim 5, characterized in that the synchronization means (24) comprise shape surfaces formed to the first and the second clamping arm (18a, 18b) on a portion between the pivot joints (19a, 19b) ; and the shape surfaces are interconnected by a separate connecting piece (25) arranged against the shape surfaces but without attachment thereto.

10. A retainer according to claim 9, characterized in that the first clamping arm (18a) comprises at least two first connecting cams (27) on the portion between the pivot joints (19a, 19b), the cams being arranged at an axial distance from one another; the second clamping arm (18b) comprises at least two second connecting cams (28) on the portion between the pivot joints (19a, 19b), the cams being arranged at an axial distance from one another; the first connecting cams (27) and the second connecting cams (28) are interleaved; the first connecting cams (27) and the second connecting cams (28) are provided with connecting slots (29) opening towards each other and arranged to coincide in such a way that connecting slots (29) on opposite sides are arranged to form a connecting channel (30) in an axial direction; a connecting pin (25) is arranged to the connecting channel (30) and arranged to serve as a transmission member between the first and the second connecting cam (27, 28); and the connecting pin (25) is free to move in a vertical direction when the clamping arms (18a, 18b) are moved.

11. A retainer according to claim 5, characterized in that the first clamping arm (18a) comprises a connecting pin (25) parallel to the centre (K) on a portion between the pivot joints (19a, 19b), the pin being immovably fixed to the first clamping arm (18a); the second clamping arm (18b) comprises a longitudinal connecting opening (32) on the portion between the pivot joints (19a, 19b); and the connecting pin (25) is arranged through the connecting opening (32), whereby they act as shape members and transmit synchronization movements between the clamping arms (18a, 18b).

12. A retainer according to any one of the preceding claims 5 to 11 , characterized in that the retainer (14) comprises only one actuator (20), which is arranged between the clamping arms (18a, 18b) and connected to act directly on both clamping arms (18a, 18b).

13. A retainer according to any one of the preceding claims 5 to 11 , characterized in that the retainer (14) comprises only one actuator (20), which is arranged to act only on the first clamping arm (18a), the second clamping arm (18b) being arranged to move under the influence of the synchronization means.

14. A retainer according to any one of the preceding claims 5 to 11, characterized in that the retainer (14) comprises at least two actuators (20a, 20b), at least one first actuator (20a) being connected to act directly on the first clamping arm (18a) only and at least one second actuator (20b) being arranged to act directly on the second clamping arm (18b) only.