GB2370558A

GB2370558A - Telescopic jib with locking element

Info

Publication number: GB2370558A
Application number: GB0126972A
Authority: GB
Inventors: Bernd Backes; Frank Conrad; Michael Seibert; Martin Bauer; Sascha Scholz
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2000-11-09
Filing date: 2001-11-09
Publication date: 2002-07-03
Also published as: DE10056649C1; GB0126972D0

Abstract

The invention relates to a telescopic jib, having a box frame (26, Fig 1) and one or more telescopic sections (27), each of which can be locked to the next, larger section or the box frame (26) of the telescopic jib in at least one extended position (B) and in a retracted position (A) by a locking element (1) which can be actuated by a securing and locking unit (29). The locking element 1 has in each case a bolt 2 which is mounted in a bolt guide 5, which is connected to an inner section (27), in such a manner that it can be displaced in the axial direction of the bolt 2 transversely with respect to the longitudinal axis of the telescopic jib (1) and the head region of which, as a result of a transverse displacement, can be introduced into a lock opening 19 on the next, larger section or the box frame of the telescopic jib and can be held in a locking position. In this jib, the bolt 2 has an axial recess, in which a control piston 3 is guided in such a manner that it can be displaced in the longitudinal direction relative to the bolt 2, between a first and a second fixed stop position. The control piston 3 has a profiling acting as a slotted guide, by means of which the at least one blocking body such as ball 7 can be pressed outwardly when the control piston 3 is in a position between the two fixed stop positions.

Description

TELESCOPIC JIB WITH LOCKING EIFM NT

The invention relates to a telescopic jib, having a box frame and one or more telescopic sections, each of which can be locked to the next, larger section or 5 the box frame of the telescopic jib in at least one extended position and in a retracted position by a locking element which can be actuated by a securing and locking unit.

A telescopic jib of such type is known, for 10 example, from EP 0 661 234 Al. This telescopic jib comprises a box frame and a plurality of telescopic sections, each of which can be locked to the next, larger section or the box frame of the jib in a retracted position (for transport) and an extended 15 position (working position) by a locking element which can be actuated by a securing and locking unit. The locking elements used for this purpose each comprise an axially displaceable bolt with a shank which is secured to the inner face of the bolt and in turn is provided 20 with a circular disc on its outer face. The disc can be coupled to a piston element of a securing and locking unit which is arranged towards the end of a cylinder and from which emerges a piston rod of a piston-

cylinder unit which extends and retracts the telescopic 25 sections. A compression spring surrounding the bolt is supported on the one hand on a plate, which is arranged in the rear bearing of the particular telescopic section and has a central opening for the passage of the bolt, and on the other hand is supported on a 30 collar arranged on the bolt. In this design, the compression spring is firstly responsible for assisting the displacement of the bolt into the locking position and secondly is responsible for holding the bolt in the locking position. To achieve additional securing, the

2 - front region of the bolt is designed as a bead which, in the locking position, engages a partial region of the edge of the recess of the surrounding section. For locking purposes, therefore, the bolt is displaced 5 transversely with respect to the longitudinal axis of the telescopic jib, in the axial direction of the bolt, and in the process is guided in a sliding fashion in a bolt guide which is connected to the inner section, which is in each case to be locked to the next, larger 10 section or to the box frame.

In another known design of a telescopic jib which is lockable by bolts (DE-A 26 56 186), a plurality of spring-loaded snap-action holders are arranged in the head region of the bolt and, after the locking position 15 of the bolt has been reached, engage behind the edge of the recess in the surrounding section in order to hold . the bolt in position. The latter design has the drawback that the recess in the surrounding section may only be slightly larger than the bolt engaging it so 20 that the snapaction holders can engage behind the edge. This requires very accurate movement of the bolts into the locking position. This takes no account whatsoever of the elastic deformation of the telescopic section when locked under load.

25 The first design has the drawback that the dimensions of the compression spring holding the bolt in position have to be relatively large, and consequently this spring takes up considerable space.

This means using a large-diameter bolt, and therefore a 30 considerable weight for the locking element. Moreover, the additional securing provided by the annular bead of the bolt is insufficient, since, if there is even a slight displacement of the bolt within the recess, the

À 3 - annular bead can no longer hold and, furthermore, this region is susceptible to wear.

It is an object of the invention to provide a locking element for locking a telescopic section which 5 is of small size and in which the locking position of the bolt is secured effectively with little wear.

According to this invention, there is provided a telescopic jib, having a box frame and one or more telescopic sections, each of which can be locked to the 10 next, larger section or the box frame of the telescopic jib in at least one extended position and in a retracted position by a locking element which can be actuated by a securing and locking unit, the locking element having in each case a bolt which is mounted in 15 a bolt guide, which is connected to an inner section, in such a manner that it can be displaced in the axial direction of the bolt, transversely with respect to the longitudinal axis of the telescopic jib and the head region of which, as a result of a transverse 20 displacement, can be introduced into a lock opening on the next, larger section or the box frame of the telescopic jib and can be held in the locking position, and the locking element having a spring which acts in the axial direction of the bolt and a coupling device 25 for connection to the securing and locking unit, in which jib; the bolt has an axial recess; a control piston is guided in the recess in such a manner that it can be displaced in the longitudinal 30 direction relative to the bolt between a first and a second fixed stop position; the control piston, at the end which is remote from the head region, has a part which includes the coupling device which projects out of the bolt;

- 4 - the bolt has at least one through passage which runs radially from the outside, transversely with respect to the longitudinal axis of the bolt, into the axial recess and in which at least one blocking body is 5 arranged displaceable; the control piston has a profiling acting as a slotted guide, by means of which the at least one blocking body can be pressed outward when the control piston is in a position between the two fixed stop 10 positions; the bolt guide on the inner section in the unlocked position of the locking element covers at least one passage hole; in each case, an opening corresponding to the 15 through passage and into which the blocking body can be latched under the compressive action of the control piston when the bolt is pushed into the respective locking opening on the next, larger section or the box frame is arranged in the region of the bolt guide, and 20 the spring applying the spring force is arranged in such a way between the bolt and the control piston that the control piston is pressed or pulled toward the second, fixed stop, where the profiling of the control piston presses outward the at least one blocking body.

25 According to the teaching of the patent, the telescopic sections of the telescopic jib are secured against undesirable longitudinal displacement with respect to one another by locking elements which, in a manner known per se, have a bolt which is guided 30 displaceable in each case in the inner section, transversely with respect to the longitudinal axis of the telescopic jib, and with it being possible for the bolts to can be introduced into a bolt receiving opening in the next, larger section or in the box

frame. The bolt is prevented from unintentionally slipping out of the locking position by a special positively-locking retaining device. For this purpose, the principle of a blocking device which has been known 5 for many years, for example in so-called ball blocking devices, is employed.

In this context, reference is made, for example, to US 5,394,594. A rod which is under spring force and can also be referred to as a control piston is guided 10 displaceable centrally in an elongate insertion bolt.

In the front part, the rod is provided with an annular groove in which two balls can be mounted. Two openings, which lie opposite one another and into which the balls can be pushed, are arranged in corresponding positions 15 in the elongate bolt. The compression spring arranged in the rear part holds the rod in the blocking position, i.e. the front, cylindrical portion of the rod presses the balls into the openings of the bolt. If the compression spring is compressed by means of a 20 handle which can be attached, the rod moves relative to the bolt and the groove moves into a position in which it is aligned with the openings. Consequently, the balls can drop into the groove, so that the device is unblocked. 25 In the present invention, the operating principle of this locking unit is applied as follows. The bolt which effects the actual locking of the telescopic sections which are to be locked to one another has an axial recess, which is expediently designed as a 30 central, stepped bore running in the longitudinal direction. This stepped bore is used to receive a control piston which is guided in such a manner that it can be displaced in the longitudinal direction relative to the bolt between a first and a second fixed stop

position. The cross-sectional shape of both the axial recess of the bolt and the control piston is expediently round. However, it is also quite possible to use other cross-sectional shapes (e.g. square or 5 other polygonal shapes). However, to facilitate production, round cross sections are preferred. On the front end, which is remote from the head region of the bolt which effects the locking action, part of the length of the control piston projects out of the bolt.

10 A coupling device, which allows mechanical connection to form the securing and locking unit used for the actuation of the locking, is arranged on this projecting part. The bolt has at least one through passage which runs radially from the outside, 15 transversely with respect to the longitudinal axis of the bolt, into the axial recess and in which at least one blocking body is arranged displaceable. This through passage is expediently a radial bore.

Preferably, a plurality of through passages are 20 provided, for example two through passages which lie diametrically opposite one another. The control piston is provided with a profiling acting as a slotted guide, by means of which, when the control piston is in a position between the two fixed stop positions, the at 25 least one blocking body can be pressed outward or is kept pressed outward. The locking bolt is guided via a bolt guide which is, in each case, attached to the inner section in such a manner that the through passage(s), in the unlocked position of the locking 30 element, is or are covered by the bolt guide, so that the blocking body or bodies cannot extend out of the corresponding passage hole even if they are pressed in a corresponding way by the control piston. However, in each case there is arranged in the region of the bolt

- 7 guide, an opening which corresponds to the through passage or passages and in or into which blocking body or bodies can be latched under the compressive action of the control piston when the bolt is pushed into the 5 respective lock opening in the next, larger section.

Therefore, there is a position on the displacement path of the bolt in which the respective blocking body can penetrate beyond the contour of the bolt and engage in the opening in the bolt guide, so that, in this way, 10 the bolt is mechanically locked so that it cannot unintentionally move in the axial direction.

Furthermore, there is a spring, which is expediently designed as a coil spring and which, in terms of the spring force, is arranged in such a way between the 15 bolt and the control piston that the control piston is pressed or pulled towards a second, fixed stop, where the profiling of the control piston presses outwardly the at least one blocking body. The spring force therefore holds the control piston in a position in 20 which the latched-in blocking bodies cannot escape from their latched position. As long as this spring force is not relieved, therefore, there is a positively-locking, mechanical block for longitudinal displacement of the bolt. 25 Preferably, the control piston is substantially composed of cylindrical sections, the diameters of two adjacent sections being dimensioned in such a way that one section holds the at least one blocking body in the pressed-out position, and the other section allows the 30 at least one blocking body to slide back fully into the associated through passage in the bolt. The transition between these two sections may, in principle, be abrupt. However, it is recommended to design the slotted guide for the movement of the blocking body or

bodies as a slope, i.e., given a substantially cylindrical form of the control piston, to provide a frustoconical portion between the two cylindrical sections. The cylindrical section with the larger 5 diameter is also expediently adjoined by a third cylindrical section, which, at the front end of the bolt projects therefrom and is equipped with the coupling device for the coupling of a securing and locking unit.

10 The blocking bodies are expediently designed as balls, in which case preferably even two or more balls are arranged within one through passage. Naturally, it would also be possible for cylindrical blocking bodies or prism-shaped blocking bodies to be used. However, 15 balls have the considerable advantage that they are less likely to become jammed in the through passage.

The spring which interacts with the blocking piston and holds it in a position in which the blocking bodies are in their locking position is expediently 20 designed as a compression spring. In principle it is, of course, also possible for the spring to be used as a tension spring, if the arrangement is reversed appropriately. The compression spring is preferably designed as a coil spring which winds coaxially around 25 the third cylindrical portion. It is expediently supported on an annular bearing surface which is formed by a step-like transition between the second cylindrical portion and the third cylindrical portion of the control piston. On the other hand, it is 30 supported against the bolt, on an annular bearing surface in the region of the front end of the bolt, which lies opposite the head region of the bolt which effects the locking of the telescopic sections with respect to one another. For assembly reasons, it is

- 9 recommended to provide the first part of the axial recess in the bolt with an internal screw thread in which is placed a screw-in sleeve, which surrounds the outside of the compression spring. The front end side 5 of this screw-in sleeve can then in fact serve as one of the two fixed stops (first stop position) for the control piston. To fix the compression spring with respect to the bolt, there is preferably a disc-like cover which has a central passage through which the 10 control piston extends and the internal diameter of which is smaller than the external diameter of the compression spring. This cover is fixedly connected to the bolt. It is particularly advantageous to form the disc-like cover and the screwin sleeve integrally, 15 i.e. to provide the bearing surface for the compression spring in the region of the external end side of the screw-in sleeve. Therefore, the screw-in sleeve does not have a simple cylindrical central passage, but rather a stepped passage composed of two cylindrical 20 sections.

To limit a defined insertion depth of the bolt to a defined level in the locking opening on the next, larger telescopic section, it is expedient to provide a stop, for example in the head region of the bolt, which 25 stop comes into contact with the edge of the locking opening. A stop of this type may, for example, be formed by a cylindrical shoulder in the head region of the bolt. Naturally, other embodiments are also possible, such as for example a transverse bolt which 30 protrudes radially from the surface of the head region.

The stop could also be arranged elsewhere on the bolt and could, for example, interact with a corresponding mating surface on the bolt guide. A stop of this type could simultaneously also act to prevent rotation of

the bolt with respect to the bolt guide (e.g. by being in the form of a radial stop pin) r which is guided in a slot in the bolt guide. An antirotation means which ensures that the openings in the bolt guide for the 5 blocking bodies are always precisely aligned with the corresponding through passages in the bolt during displacement of the bolt can, however, also be provided by other means. For example, guide faces which are planar in the longitudinal direction and are supported 10 on corresponding surfaces of the bolt guide may be created on the outer contour of the bolt. It should be noted that the bolt does not necessarily have to be substantially cylindrical, but rather may, for example, be in the basic shape of a prism.

15 The coupling device, which can be used to connect the control piston to a securing and locking unit of the telescopic jib, may be designed differently. It is recommended to design this coupling device, in a form which is known per se' as a circular disc which is 20 attached to the end side of the third cylindrical section of the control piston which projects out of the bolt and is preferably selected to be mushroom-shaped on one side. The gripper of a securing and locking unit which is designed, for example, in the form of a box 25 section which is open on one side, can then very easily slide over this disc and engage behind it. This ensures a positive lock in both directions with respect to the longitudinal axis of the bolt.

For a better understanding of the present 30 invention, and to show how the same may be carried into effect, reference will now be made, by way of example only to the accompanying drawings r wherein:

- 11 -

Fig. 1 diagrammatically depicts a telescopic jib with an extended telescopic section in the locking position, Fig. 2 shows a locking element in the unlocked 5 position, and Fig. 3 shows the locking element illustrated in Fig. 1 in the locked position.

Figure 1 diagrammatically depicts a telescopic jib according to the invention, having a box frame 26 and 10 an extended telescopic section 27. The telescopic section 27 can be moved, by means of a hydraulic pistoncylinder system 28 mounted on the box frame 26, over a securing and locking unit 29 attached to the piston rod of this system. The telescopic section 27 15 can be positively locked to the box frame 26 in the position B by means of a locking element 1 via a gripper 30 of the securing and locking unit 29. The telescopic section 27 may also be positively locked to the box frame 26, the retracted position, with the 20 locking element 1 being inserted at the location indicated by A. The design and function of this locking element are explained in more detail below on the basis of the two sectional illustrations shown in Figs. 2 and 3, which show an unlocked and a locked position, 25 respectively, of the locking element 1, illustrated in longitudinal cross section.

An essential part of the locking element 1 is a bolt 2 which has an axial recess which, in the embodiment illustrated, is designed as a stepped, 30 cylindrical longitudinal bore. This stepped longitudinal bore has three main portions, which are denoted by reference numerals 22, 23-and 25 and have diameters which increase in this order, resulting in the stepped arrangement of the axial recess. The bolt 2

also has two through passages 6, 6' which are designed as radial transverse bores which extend from the outside into the region of the bore portion 23. A control piston 3, which is substantially composed of 5 cylindrical portions arranged one behind the other, is mounted in an axially displaceable manner in the axial recess in the bolt 2. A first cylindrical portion on the right-hand side of the control piston 3 is denoted by 9 and has a diameter which corresponds to the 10 internal diameter of the bore portion 22. To the left, this is adjoined by a second cylindrical portion 11, the external diameter of which corresponds to the internal diameter of the second bore portion 23. A short frustoconical portion 10 is arranged in the 15 region of the transition between the cylindrical portions 9 and 11, in order to make this transition more gentle. In principle, it would also be possible to use an abrupt, stepped transition at this location. To the left, the second cylindrical portion 11 is adjoined 20 by a third cylindrical portion 13, which projects out of the end side of the bolt 2 and has a smaller diameter than the second cylindrical portion 11. A compression spring 16 is arranged coaxially around the third cylindrical portion 13; on the right-hand side, 25 this spring is supported on the annular end face 12 (Fig. 3) which is formed in the transition region from the second cylindrical portion 11 to the third cylindrical portion 13. On the left, the compression spring 16 is supported on a cover 17 which is attached 30 to the left-hand end of the bolt 2 and is of disc-like design, with a central passage for the control piston 3, the central passage being slightly larger than the external diameter of the cylindrical portion 13. As a result, a corresponding annular bearing surface for the

- 13 compression spring 16 is formed on the right-hand flat side of the cover 17. The compression spring 16 is directly surrounded by a screw-in sleeve 31, which is screwed into the passage portion 25 of the axial recess 5 in the bolt 2. The internal diameter of this screw-in sleeve 31 corresponds to the external diameter of the compression spring 16 and is smaller than the diameter of the bore portion 23 or of the second cylindrical portion 11 of the control piston 3. Therefore, the 10 righthand end side 24 of this screw-in sleeve 31 forms a fixed stop for the control piston 3 on the left. To the right, the possible displacement of the control piston 3 is limited, as can be seen from Fig. 3, by the step between the bore portions 22 and 23, which the 15 transition 10 on the control piston 3 comes into contact with. However, this stop position could also be produced, for example, by the right-hand end side of the control piston 3 in combination with the right-hand end, i.e. the end face of the bore portion 22. A disc 20 4, which is preferably circular, is curved in the shape of a mushroom having a flat side and is used for coupling to the securing and locking unit 29 in accordance with Fig. 1, being attached to the left-hand part of the control piston 3, which projects out of the 25 bolt 2, via a pin 14 which is inserted into a blind bore 15 in the control piston 3. Two balls 7 - 7"' as blocking bodies are arranged displaceable in the two through passages 6, 6' in the bolt 2. The diameter of the balls 7 - 7"' approximately corresponds to the 30 diameter of the through passages 6, 6' and is half as large as the difference between the radii of the external diameter of the bolt 2 and the diameter of the bore portion 22. This means that, in the position of the control piston 3 illustrated in Fig. 2, the two

- 14 inner balls 7', 7" rest precisely on the first cylindrical portion 9 of the control piston 3, and the two outer balls 7, 7"' end precisely flush with the outer contour of the bolt 2. The balls 7 - 7"' are 5 forcibly held in this position by a bolt guide 5 formed in the manner of a housing (e.g. a cylindrical sleeve).

In this bolt guide 5 there are two openings 8, 8' which correspond to the through passages 6, 6' in the bolt, i.e. have a corresponding diameter and can be brought 10 into alignment with these through passages through displacement of the bolt 2 to the right, as illustrated in Fig. 3. During the displacement to the right, which is indicated by the arrow 20 in the drawing, the balls 7 - 7"' form, as it were, a temporary stop for the 15 control piston 3, preventing relative displacement of the control piston 3 to the right with respect to the bolt 2, since the frustoconical transition 10 is bearing against the two inner balls 7' and 7".

Therefore, a displacement of the control piston 3 to 20 the right, out of the position illustrated in Fig. 2, will directly lead to a similar displacement of the bolt 2 to the right. However, as soon as the passage holes 6, 6' are approximately congruent with the two openings 8, 8' in the bolt guide 5, which is attached 25 to an inner telescopic section (not shown in more detail), the balls can be moved radially outward under the pressure of the control piston 3 in the region of the transition 10 and penetrate into the openings 8, 8'. The movement of the balls 7 - 7"' is associated with 30 a relative displacement of the control piston 3 within the axial recess in the bolt 2. This relative displacement is assisted by the compressive force of the spring 16. The temporary fixed stop which has previously been provided by the balls 7 - 7"', i.e. the

- 15 rigid locking between the bolt 2 and the control piston 3, is therefore removed. Fig. 3 shows the limit position of the locking element 1 which has been reached, in which the balls 7 and 7"' which function as 5 blocking bodies are latched into the openings 8, 8' and the inner balls 7' and 7" bear directly against the second (largest) cylindrical portion 11 of the control piston 3 and are therefore inevitably held in a blocking position. The balls 7 and 7"' form a mechanical 10 stop which ensures that the bolt 2 cannot be displaced either to the left or to the right while the balls 7, 7"' are in the latched-in position. They can only leave this position when the second cylindrical portion 11 is pulled out of the contact region of the inner balls 7', 15 7", to the left counter to the force of the compression spring 16. The position of the openings 8, 8' on the bolt guide 5 in the direction of displacement 20 is selected in such a way that the head region of the bolt 2 engages securely in a locking opening 19 on the next, 20 larger telescopic section 18, which is only indicated by means of an outer wall. To limit the depth to which the bolt 2 can be pushed into the wall of the telescopic section 18, a shoulder 21 may be provided in the head region of the bolt 2. Since the control piston 25 3 is forced, under the action of the compression spring 16, to remain in the position illustrated in Fig. 2, in which the bolt 2 is locked against longitudinal displacement relative to the bolt guide 5 by the balls 7 - 7"', the bolt 2 cannot unintentionally slip out of 30 the locking opening 19. The bolt 2 can only be unlocked when the control piston 3 is pulled back to the left, counter to the spring force of the spring 16, by means of the securing and locking unit 29 illustrated in Fig. 1, so that the balls 7 _ 7U, which act as blocking

bodies are pressed back into the associated through passages 6, 6'.

Only very slight spring forces are required to maintain the position of the control piston 3 relative 5 to the bolt 2. Therefore, this spring 16 can be of relatively small design. Unlike in the generic prior art, the bolt 2 is not held in the locked position directly by the force of this spring, but rather by the blocking bodies, i.e. by a positively locking 10 engagement. Since, in the solution according to the invention, there is no need for strong spring forces either to reach or to hold the locking position, the locking element according to the invention can be of relatively small design and therefore only requires a 15 small amount of space. At the same time, complete security against the locking position of the bolt 2 being unintentionally released is ensured. Wear is low.

Claims

- 17 CLAIMS:

1. Telescopic jib, having a box frame and one or more telescopic sections, each of which can be locked to the next, larger section or the box frame of the telescopic jib in at least one extended position (B) 5 and in a retracted position (A) by a locking element which can be actuated by a securing and locking unit, the locking element having in each case a bolt which is mounted in a bolt guide, which is connected to an inner section, in such a manner that it can be displaced in 10 the axial direction of the bolt, transversely with respect to the longitudinal axis of the telescopic jib and the head region of which, as a result of a transverse displacement, can be introduced into a lock opening on the next, larger section or the box frame of 15 the telescopic jib and can be held in the locking position, and the locking element having a spring which acts in the axial direction of the bolt and a coupling device for connection to the securing and locking unit, in which jib; 20 the bolt has an axial recess; a control piston is guided in the recess in such a manner that it can be displaced in the longitudinal direction relative to the bolt between a first and a second fixed stop position) 25 the control piston, at the end which is remote from the head region, has a part which includes the coupling device and which projects out of the bolt; the bolt has at least one through passage which runs radially from the outside, transversely with 30 respect to the longitudinal axis of the bolt, into the axial recess and in which at least one blocking body is arranged displaceablyi

the control piston has a profiling acting as a slotted guide, by means of which the at least one blocking body can be pressed outward when the control piston is in a position between the two fixed stop 5 positions; the bolt guide on the inner section in the unlocked position of the locking element covers at least one passage hole; in each case, an opening corresponding to the 10 through passage and into which the blocking body can be latched under the compressive action of the control piston when the bolt is pushed into the respective locking opening on the next, larger section or the box frame is arranged in the region of the bolt guide, and 15 the spring applying the spring force, is arranged in such a way between the bolt and the control piston that the control piston is pressed or pulled toward the second, fixed stop, where the profiling of the control piston presses outward the at least one blocking body.

20

2. Telescopic jib according to Claim 1, wherein

the control piston is essentially composed of cylindrical portions, the diameters of two adjacent portions being dimensioned in such a way that one portion holds the at least one blocking body in the 25 outwardly pressed position, and the other portion allows the at least one blocking body to slide back fully into the through passage.

3. Telescopic jib according to Claim 1 or 2, wherein at least one through passage is designed as a 30 transverse bore.

4. Telescopic jib according to any of Claims 1 to 3, wherein the bolt comprises two-through passages which lie diametrically opposite one another.

- 19

5. Telescopic jib according to any of Claims 1 to 4, wherein each blocking body is a ball.

6. Telescopic jib according to Claim 5, wherein two or more balls are arranged within one through 5 passage hole.

7. Telescopic jib according to any of Claims 1 to 6, wherein the spring is designed as a compression spring and on one hand is supported against an annular bearing surface of the control piston and on the other 10 hand is supported against an annular bearing surface in the region of that side of the bolt which is remote from its head region.

8. Telescopic jib according to Claim 7, wherein the spring engages coaxially around a cylindrical 15 portion of the control piston and is arranged inside a screw-in sleeve, one end face of which forms the first stop position for the control piston, and wherein a disc-like cover having a central passage through which the control piston extends and the internal diameter of 20 which is smaller than the external diameter of the spring is fixedly connected to the bolt.

9. Telescopic jib according to Claim 8, wherein the disc-like cover and the screw-in sleeve are of integral design.

25

10. Telescopic jib according to any of Claims 1 to 9, wherein the bolt has, in the head region, a stop for limiting the insertion depth into the locking opening.

11. Telescopic jib according to one of Claims 1 30 to 10, wherein the coupling device is formed on the control piston as a disc which is attached to the front end of the part which projects out of the bolt.

À 20

12. Telescopic jib according to claim 11, wherein said disc has a mushroom shape.

13. A telescopic jib substantially as hereinbefore described with reference to, and as shown 5 in, Figs. 1 to 3 of the accompanying drawings.