Wear pad assembly

Abstract

The present disclosure relates to a wear pad assembly (20) for an extensible boom having a first boom section (21) and a second boom section. The wear pad assembly (20) comprises a base plate (30) for mounting to the first boom section (21), the base plate (30) comprising one or more positioning protrusions (35). The wear pad assembly (20) also includes a wear pad (40) having a thickness (41) extending between the coupling surface (42) and the opposing sliding surface (43). The coupling surface includes a recess (44) for engagement with the one or more positioning protrusions (35) of the base plate (30). The recess (44) is configured to provide a clearance fit with the one or more positioning protrusions (35) of the base plate (30).

Description

Technical Field

[0001] The present disclosure relates to a wear pad assembly for an extendable boom, and a machine or telehandler including such a wear pad. Background Art

[0002] Machines such as telehandlers (also known as telehandlers or rotary telehandlers) typically include a lifting arm. These lifting arms are used to lift, carry and place materials around construction sites, agricultural sites and the like. The lifting arm typically includes an extendable boom or a telescopic cylinder to which a work tool (such as a fork or a bucket) can be attached. The extendable boom includes a main boom section that is attached to the telehandler body via a hinge and a telescopic boom section mounted in the main boom section. The telescopic boom section is able to slide along the main boom section between a retracted direction and an extended direction. The telehandler may have additional telescopic boom sections mounted within the telescopic boom section.

[0003] One or more consumable replaceable wear pads are usually located between two sliding telescoping sections to prevent wear of the telescoping sections, reduce friction, and occupy the gap between the telescoping sections. Due to variations in manufacturing tolerances, shims are usually used to achieve the required working gap of the wear pads between the sliding telescoping sections. Shims can also be used to maintain the desired working gap during the service life of the wear pads, thereby extending the service life of the wear pads. JP-Y2-H0649581 discloses a wear pad assembly that uses shims to adjust the gap between telescoping sections.

[0004] There is a continuing need to improve the structure of wear pad assemblies for telescopic sections so that adjustment of the wear pad assemblies is more convenient. A particular need is to reduce maintenance costs, which can be relatively high due to the cost of skilled labor required to repair telescopic boom handlers. In addition, many telescopic boom handlers are leased to operators, so reducing maintenance or downtime would be particularly beneficial. Summary of the invention

[0005] The present disclosure provides a wear pad assembly for an extendable boom having a first section and a second section, the wear pad assembly comprising: a base plate for mounting to the first section of the extendable boom, the base plate comprising one or more positioning protrusions; and a wear pad having a thickness extending between a connecting surface and an opposing sliding surface, the connecting surface comprising a recess for engaging with the one or more positioning protrusions of the base plate; wherein the recess is constructed to provide a clearance fit with the one or more positioning protrusions of the base plate.

[0006] Optionally, the recess engages with the one or more locating projections such that the wear plate is engaged with but separate from the base plate.

[0007] Optionally, the recess is sized and / or shaped to provide the clearance fit with the one or more locating projections of the base plate.

[0008] Optionally, one or more edges of the perimeter of the sliding surface of the wear pad are provided with a chamfer. Optionally, the leading edge of the wear pad is provided with a chamfer relative to the sliding direction of the wear pad.

[0009] Optionally, the base plate further comprises a base plate tab extending from a side of the base plate in the plane of the base plate.

[0010] Optionally, the base plate further comprises at least one base plate hole. Optionally, each base plate hole extends through a locating protrusion of the one or more locating protrusions.

[0011] Optionally, the wear pad assembly further comprises one or more shims for positioning between the base plate and the wear pad, each of the shims comprising a shim hole for engaging with the one or more positioning projections of the base plate.

[0012] Optionally, the shim hole is configured to provide a clearance fit with the one or more locating projections of the base plate. Optionally, the shim hole is sized and / or shaped to provide a clearance fit with the one or more locating projections of the base plate.

[0013] Optionally, each gasket comprises a gasket tab extending from one side of the gasket in the plane of the gasket. Optionally, when the gasket is located on the base plate, the gasket tab is aligned with the base plate tab.

[0014] The present disclosure also provides an extendable boom having a first section and a second section, the extendable boom including the aforementioned wear pad assembly.

[0015] Optionally, the wear pad assembly is mounted to the first boom section, and the second boom section is configured to slide relative to the first boom section along the sliding surface of the wear pad.

[0016] Optionally, the entire wear pad assembly is positioned between the first boom section and the second boom section.

[0017] Optionally, the wear pad and the base plate are constrained by the first boom section and the second boom section in a direction perpendicular to the plane of the planar element.

[0018] The present disclosure also provides a telescopic boom loader including the aforementioned extendable boom. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0020] Figure 1shows a schematic side view of a telehandler including a wear pad assembly according to the present disclosure;

[0021] Figure 2 shows an exploded perspective view of a wear pad assembly according to the present disclosure;

[0022] Figure 3 The first boom section is shown mounted on Figure 2 a perspective cross-sectional view of a wear pad assembly; and

[0023] Figure 4 The diagram shows a first boom section mounted to the first boom section and facing the second boom section. Figure 2 and Figure 3 Side view of the wear pad assembly. DETAILED DESCRIPTION

[0024] Figure 1 An example of a wear pad assembly 20 (in Figure 1 The telescopic boom loader 10 is also referred to as a telescopic boom loader and can be any type of telescopic boom loader, such as a rotary telescopic boom loader or a rotating telescopic boom loader. The telescopic boom loader 10 may include a chassis or body 11 and a boom 12 mounted on the chassis or body at a boom hinge 13. The boom 12 may include a main boom section 14 and a telescopic boom section 15 mounted to the main boom section 14. The telescopic boom section 15 may be retractably mounted within the main boom section 14 so that the telescopic boom section 15 can slide relative to the main boom section 14 along a sliding direction 80. Although in Figure 1 Not shown, but a further telescopic boom section may be retractably mounted within the telescopic boom section 15 such that it is slidable relative to the telescopic boom section 15 along a sliding direction 80. A work implement 16, in this case a fork, may be attached to the telescopic boom section 15 to perform work.

[0025] At least one wear pad assembly 20 may be located between the main boom section 14 and the telescopic boom section 15 so that the boom sections 14, 15 may slide relative to each other along the wear pads 40 of the wear pad assembly 20. The at least one wear pad assembly 20 may prevent wear of the boom sections 14, 15 due to sliding motion, reduce friction, and increase the stability of the boom 12 by occupying the gap between the boom sections 14, 15. Generally, a plurality of wear pad assemblies 20 may be disposed between the boom sections 14, 15. For example, for a typical square boom section 14, 15, one or more wear pad assemblies 20 may be disposed on each of two opposite sides of the main boom section 14 or the telescopic boom section 15, or even on all four sides. In addition, the wear pad assemblies 20 may be disposed at both the upper and lower ends of the boom sections 14, 15. The following description of a single wear pad assembly 20 may also apply to some or all of the other wear pad assemblies 20.

[0026] The wear pad assembly 20 may be mounted to the main boom section 14 so that the telescoping boom section 15 slides along the wear pad assembly, or the wear pad assembly 20 may be mounted to the telescoping boom section 15 so that the main boom section 14 slides along the wear pad assembly. The boom sections 14, 15 to which the wear pad assembly 20 is mounted will be referred to herein as first boom sections 21, and the boom sections 14, 15 that slide along the wear pad assembly 20 will be referred to herein as second boom sections 22. There may be no contact between the boom sections 14, 15 during extension and retraction except via the wear pad assembly 20.

[0027] like Figures 2 to 4 As shown, the wear pad assembly 20 generally includes a base plate 30 and a wear pad 40. The wear pad assembly 20 may also include one or more shims 50.

[0028] The bottom plate 30 may include a planar element 31 having a first surface 32 and an opposite second surface 33, the planar element 31 having a length and a width substantially greater than its thickness. As illustrated, the planar element 31 may have a substantially rectangular shape. Alternatively, the planar element 31 may have a circular shape, a polygonal shape, or an irregular shape. The planar element 31 may be provided with a bottom plate tab 34 that may extend from one side of the planar element 31 in the plane of the planar element 31.

[0029] At least one locating protrusion 35 may extend from the first surface 32 of the planar element 31 for locating the wear pad 40 and the shim 50 on the base plate 30. The locating protrusion 35 may extend substantially perpendicularly from the first surface 32. Any feasible cross-sectional shape may be used for the locating protrusion 35. For example, the cross-section of the locating protrusion 35 may have a circular, square, rectangular, triangular, other polygonal or irregular cross-sectional shape. The locating protrusion may be positioned anywhere on the first surface 32 of the planar element 31, optionally positioned approximately in the center. Figures 2 to 4 In the particular embodiment shown, two spaced apart, generally circular locating projections 35 may be provided.

[0030] The base plate 30 may be configured to be releasably mounted to the first boom section 21. For example, Figure 2 and Figure 3 As shown, the planar element 31 may include at least one floor hole 36 for receiving a releasable fastener 61. The at least one floor hole 36 may be located on the planar element 31 in the area of ​​the at least one locating protrusion 35, such that the floor hole 36 may extend through the locating protrusion 35. Figures 2 to 4 In the particular embodiment shown, the bottom plate hole 36 can be concentric with the corresponding locating protrusion 35. The bottom plate hole 36 can be threaded. One or more boom holes 60 can be provided in the first boom section 21, corresponding to at least one bottom plate hole 36. One or more boom holes 60 can be threaded.

[0031] The base plate 30 may be formed from any suitable material, such as structural steel.

[0032] The wear pad 40 may extend across the thickness 41 between a coupling surface 42, which may face and be coupled to the base plate 30, and an opposing sliding surface 43, which may be configured for the second boom segment 22 to slide along the sliding surface. Thus, the wear pad 40 may be coupled to the base plate 30, and the second boom segment 22 may be configured to slide along the sliding surface 43 of the wear pad 40 relative to the first boom segment 21, particularly in the sliding direction 80. In the present disclosure, the term "coupled" refers to two parts contacting each other, such as one part stacked on another, and does not mean or imply any form of attachment.

[0033] The wear pad 40 may be a plate, and the dimension across the coupling and / or sliding surfaces 42, 43 may be substantially greater than the thickness 41. As illustrated, the wear pad 40 may be elongated along the sliding direction 80. The coupling and / or sliding surfaces 42, 43 may be substantially rectangular, and the wear pad 40 may be substantially a cuboid, but other shapes are possible. The shape and size of the wear pad 40 may generally correspond to the shape and size of the base plate 30, but this is not required.

[0034] The coupling surface 42 of the wear pad 40 may be provided with at least one recess 44 for engaging with one or more positioning protrusions 35 of the base plate 30. That is, the shape and size of the at least one recess 44 may be set to fit around the one or more positioning protrusions 35. Figures 2 to 4 In the particular embodiment shown, a single generally oval recess 44 can be provided that fits around two spaced-apart generally circular locating projections 35. Alternatively, a single recess 44 can be shaped to correspond to a single locating projection 35, two or more recesses 44 can be individually shaped to correspond to two or more locating projections 35, or one or more recesses 44 can each be shaped to fit around two or more locating projections 35. At least one recess 44 can be a blind recess such that it does not penetrate the thickness of the wear pad 40, as shown in FIG. Figures 2 to 4 Alternatively, at least one recess 44 may be a through recess such that it penetrates the thickness of the wear pad 40 .

[0035] The at least one recess 44 can be configured to provide a clearance fit with one or more locating projections 35 of the base plate 30, so that the wear pad 40 can be assembled with the base plate 30 (and subsequently disassembled) without the need for force or the use of tools. In particular, the shape and / or size of the at least one recess 44 can be set to provide a clearance fit with one or more locating projections 35 of the base plate 30. The term "clearance fit" herein refers to the size of the hole and the shaft so that there is a gap or space between the hole and the shaft when the hole and the shaft are assembled; that is, the hole is larger than the shaft. Therefore, the at least one recess 44 can be larger than the one or more locating projections 35 of the base plate 30.

[0036] One or more edges of the perimeter of the sliding surface 43 of the wear pad 40 may be provided with chamfers 45. In particular, two leading edges (relative to the sliding direction 80) of the perimeter of the sliding surface 43 of the wear pad 40 may be provided with chamfers 45, such as Figures 2 to 4 However, any number or all of the edges of the perimeter of the sliding surface 43 of the wear pad 40 may be provided with chamfers 45. The chamfers 45 may help reduce friction by providing a transition edge to the wear pad 40. The chamfers 45 may also be used as indicators for monitoring the wear of the wear pad 40. For example, when the sliding surface 43 of the wear pad 40 has been worn so that the chamfers 45 are no longer visible, the wear pad 40 may be replaced.

[0037] The wear pad 40 may comprise a suitable pad material for wearing during use. The pad material may have a lower hardness than the boom sections 21, 22 (14, 15) so that the wear pad is consumably worn rather than the boom sections 21, 22 (14, 15). The pad material may comprise nylon, for example.

[0038] One or more shims 50 may be provided for positioning between the base plate 30 and the wear pad 40. The one or more shims 50 may be identical to each other in all aspects except thickness. Each shim 50 may be planar, having a length and width substantially greater than its thickness. As illustrated, the shim 50 may have a substantially rectangular shape. Alternatively, the shim 50 may have a circular shape, a regular polygonal shape, or an irregular shape. The shape and size of the shim 50 may correspond to the shape and size of the base plate 30 and / or the wear pad 40, but this is not required. The shim 50 may be provided with a shim tab 52, which may extend from one side of the shim 50 in the plane of the shim 50. The size, shape, and / or position of the shim tab 52 on the shim 50 may correspond to the size, shape, and / or position of the base plate tab 34 on the base plate 30, so that the shim tab 52 may be aligned with the base plate tab 34 when the shim 50 is stacked on top of the base plate 30.

[0039] The gasket 50 may include at least one gasket hole 51 for engaging with one or more positioning protrusions 35 of the bottom plate 30. That is, the shape and size of the at least one gasket hole 51 may be set to fit around the one or more positioning protrusions 35. Figures 2 to 4 As shown in the particular embodiment of , a single shim hole 51 can be generally oval and can fit around two spaced-apart, generally circular locating protrusions 35. Alternatively, a single shim hole 51 can be shaped to correspond to a single locating protrusion 35, two or more shim holes 51 can be individually shaped to correspond to two or more locating protrusions 35, or one or more shim holes 51 can each be shaped to fit around two or more locating protrusions 35. The configuration of at least one shim hole 51 in the shim 50 can correspond to the configuration of at least one recess 44 in the coupling surface 42 of the wear pad 40, so that when the wear pad 40 is stacked on top of the shim 50, the shim hole 51 can be aligned with the recess 44 of the wear pad 40.

[0040] The at least one shim hole 51 can be configured to provide a clearance fit with one or more locating protrusions 35 of the bottom plate 30, so that the shim 50 can be assembled with the bottom plate 30 (and subsequently disassembled) without the need for applying force or using tools. In particular, the shape and / or size of the at least one shim hole 51 can be set to provide a clearance fit with one or more locating protrusions 35 of the bottom plate 30. Therefore, the shim hole 51 can be larger than the one or more locating protrusions 35 of the bottom plate 30.

[0041] One or more shims 50 may have thicknesses equal to each other. Alternatively, shims 50 of different thicknesses may be provided.

[0042] The one or more shims 50 can be made of any suitable material. For example, the one or more shims 50 can be made of "shim steel", which is a cold rolled carbon steel tempered to a specified hardness range.

[0043] The wear pad assembly 20 may be assembled by positioning the wear pad 40 on the base plate 30 such that the coupling surface 42 of the wear pad 40 contacts the first surface 32 of the planar element 31 of the base plate 30. The wear pad 40 is positioned on the base plate 30 by engaging at least one recess 44 of the wear pad 40 with one or more positioning protrusions 35 of the base plate 30. If one or more shims 50 are required, one or more shims 50 are positioned between the wear pad 40 and the base plate 30 such that the coupling surface 42 of the wear pad 40 and the first surface 32 of the planar element 31 of the base plate 30 each contact a surface of the shim 50. Each shim 50 may be oriented such that the shim tab 52 is aligned with the base plate tab 34.

[0044] The technician may measure the wear pad assembly 20 against the gap 23 between the first boom section 21 and the second boom section 22. If the wear pad assembly 20 does not substantially fill the gap 23, one or more shims 50 may be added between the base plate 30 and the wear pad 40. The optimal thickness of the wear pad assembly 20 may be such that when the second surface 33 of the base plate 30 contacts the first boom section 21, there is a working gap 53 between the sliding surface 43 of the wear pad 40 and the second boom section 22. The working gap 53 may be set to avoid constraining the first boom section 21 and the second boom section 22 due to changes in the first boom section 21 and the second boom section 22 along the travel path (i.e., in the sliding direction 80). For example, if the working surface of the first boom section 21 or the second boom section 22 has a flatness of 2 mm, the working gap may be set to 3 mm.

[0045] When the desired thickness of the wear pad assembly 20 is achieved, the wear pad assembly 20 may be slid along the first side 25 of the first boom section 21 between the first boom section 21 and the second boom section 22. The wear pad assembly 20 may be slid along the first side 25 of the first boom section 21 with the base plate tab 34 (and shim tab 52, if present) proximate the proximal ends of the boom sections 21, 22 such that the base plate tab 34 (and shim tab 52, if present) may be used to facilitate adjusting the position of the wear pad assembly 20 between the boom sections 21, 22.

[0046] A technician may adjust the position of the wear pad assembly 20 along the first side 25 of the first boom section 21 so that the boom holes 60 are visually aligned with the base plate holes 36. The wear pad assembly 20 may then be secured to the first boom section 21 by inserting a releasable fastener 61 through each boom hole 60 and into the aligned corresponding base plate hole 36 in a direction from the second side 24 of the first boom section 21 (the second side 24 of the first boom section 21 being opposite the first side 25 of the boom section 21). Any suitable releasable fastener may be used. Figures 2 to 4 In the particular embodiment shown, the fastener 61 can be, for example, a threaded bolt.

[0047] Thus, the base plate 30, wear pads 40, and any shims 50 (if present) are located on the first side 25 of the first boom section 21 between the first boom section 21 and the second boom section 22. That is, the entire wear pad assembly 20 is located between the first boom section 21 and the second boom section 22. This does not include the fasteners 61, which are auxiliary to the wear pad assembly 20.

[0048] In the assembled wear pad assembly 20, the wear pad 40, the base plate 30 and any shims 50 (if present) are not fixed or attached to each other. The wear pad 40, the base plate 30 and any shims 50 (if present) are in contact and engaged with each other, but remain separated.

[0049] Although the wear pad 40, the base plate 30 and any shims 50 (if present) are not attached to each other, they are, in use, aligned perpendicular to the plane of the planar element 31 (e.g. Figure 4 The wear pad assembly 20 (as shown in the figure) is constrained in the Z direction by the first boom section 21 and the second boom section 22. That is, the wear pad 40, the base plate 30 and any shims 50 (if present) are clamped between the first boom section 21 and the second boom section 22 in the Z direction. At least one locating protrusion 35 and corresponding recess 44 and shim hole 51 (and releasable fastener 61) constrain the wear pad assembly 20 (as shown in the figure) in the plane of the planar element 31 along the X direction and the Y direction. Figure 4 as indicated).

[0050] A reinforcement plate (not shown) may be provided on the second side 24 of the first boom section 21 to reinforce the first boom section 21 in the region of the boom hole 60, the reinforcement plate being provided with a corresponding hole. The reinforcement plate may be non-releasably attached to the second side 24 of the first boom section 21, for example by welding.

[0051] In use, the thickness 41 of the wear pad 40 is reduced due to the sliding contact of the second boom section 22 thereon. The wear pad assembly 20 may be periodically inspected to monitor the wear of the wear pad 40. In this disclosure, the term "wear" refers to the reduction in thickness from an original predetermined pre-installed thickness due to the consumable removal of wear pad 40 material caused by the sliding contact of the second boom section 22 thereon.

[0052] If the gap between the installed wear pad 40 and the second boom section 22 increases beyond a predetermined dimension, the wear pad assembly 20 may be adjusted by adding one or more shims 50 to increase its thickness. First, the wear pad assembly 20 may be removed from the first boom section 21 by removing the fasteners 61. The wear pad assembly 20 may then be slid out from between the first boom section 21 and the second boom section 22. The base plate tabs 34 (and shim tabs 52, if present) may assist in removing the wear pad assembly 20 from between the boom sections 21, 22. The wear pad 40 may be lifted from the base plate 30, and one or more shims 50 may be added between the wear pad 40 and the base plate 30 to achieve the optimal thickness of the wear pad assembly 20. The adjusted wear pad assembly 20 may then be reinserted between the first boom section 21 and the second boom section 22 and secured by the fasteners 61. When the wear pad 40 reaches the end of its useful life, it may be replaced with a new wear pad 40 having a pre-installed thickness.

[0053] Industrial Applicability

[0054] By being able to adjust the thickness of the wear pad assembly 20 using one or more shims 50, the wear pad assembly 20 can be customized to a desired thickness to achieve an optimal working gap between the first boom section 21 and the second boom section 22 for a particular telehandler. Additionally, as the thickness 41 of the wear pad 40 decreases during the useful life of the wear pad 40, the thickness of the wear pad assembly 20 can be maintained by adding one or more shims 50, thereby extending the useful life of the wear pad assembly 20 and, therefore, reducing cost and waste.

[0055] Providing a clearance fit between the wear pad 40 and the base plate 30 and between the shim 50 and the base plate 30 reduces the complexity of assembling and disassembling the wear pad assembly 20. This in turn can reduce maintenance time and, therefore, maintenance costs.

[0056] Positioning the fastener 61 outside the load path can make the joint stronger and less prone to loosening. Likewise, this can reduce maintenance requirements and therefore reduce costs.

Claims

1. A wear pad assembly (20) for an extendable boom (12) having a first boom section (21) and a second boom section (22), the wear pad assembly (20) comprising: a base plate (30) for mounting to the first boom section (21), the base plate (30) comprising one or more locating protrusions (35); and a wear pad (40) having a thickness (41) extending between a coupling surface (42) and an opposing sliding surface (43), the coupling surface (42) including a recess (44) for engaging the one or more locating projections (35) of the base plate (30); wherein The recess (44) is configured to provide a clearance fit with the one or more locating projections (35) of the base plate (30).

2. The wear pad assembly (20) of claim 1, wherein the recess (44) is sized and / or shaped to provide the clearance fit with the one or more locating projections (35) of the base plate (30).

3. A wear pad assembly (20) according to claim 1 or claim 2, wherein one or more edges of the perimeter of the sliding surface (43) of the wear pad (40) include a chamfer (45).

4. The wear pad assembly (20) of any one of the preceding claims, wherein the base plate (30) further comprises a base plate tab (34) extending from a side of the base plate (30) within the plane of the base plate (30).

5. The wear pad assembly (20) of any one of the preceding claims, wherein the base plate (30) further comprises at least one base plate hole (36).

6. The wear pad assembly (20) of claim 5, wherein each base plate hole (36) extends through a locating projection (35) of the one or more locating projections (35).

7. A wear pad assembly (20) according to any one of the preceding claims, further comprising one or more shims (50) for positioning between the base plate (30) and the wear pad (40), each of the shims comprising a shim hole (51) for engaging with the one or more positioning protrusions (35) of the base plate (30).

8. The wear pad assembly (20) of claim 7, wherein the shim hole (51) is configured to provide a clearance fit with the one or more locating projections (35) of the base plate (30).

9. A wear pad assembly (20) according to claim 7 or claim 8, wherein the shim hole (51) is sized and / or shaped to provide a clearance fit with the one or more locating projections (35) of the base plate (30).

10. The wear pad assembly (20) according to any one of claims 7 to 9, wherein each shim (50) includes a shim tab (52) extending from a side of the shim (50) in the plane of the shim (50).

11. A wear pad assembly (20) according to any one of claims 7 to 10 when appended to claim 4, wherein the shim tabs (52) are aligned with the base plate tabs (34) when the shim is located on the base plate (30).

12. An extendable boom (12) having a first boom section (21) and a second boom section (22), the extendable boom (12) comprising a wear pad assembly (20) according to any one of the preceding claims.

13. The extendable boom (12) according to claim 12, wherein the wear pad assembly (20) is mounted to the first boom section (21), and the second boom section (22) is configured to slide relative to the first boom section (21) along the sliding surface (43) of the wear pad (40).

14. An extendable boom (12) according to claim 12 or claim 13, wherein the entire wear pad assembly (20) is located between the first boom section (21) and the second boom section (21).

15. The extendable boom (12) according to any one of claims 12 to 14, wherein the wear pad (40) and the base plate (30) are constrained by the first boom section (21) and the second boom section (22) in a direction perpendicular to the plane of the first boom section (21).

16. A telescopic handler (10) comprising an extendable boom (12) according to any one of claims 12 to 15.

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