GB2441108A

GB2441108A - A collar for attachment to a monitor support arm mounting post

Info

Publication number: GB2441108A
Application number: GB0616734A
Authority: GB
Inventors: Andrew Wills; Alex Lau
Original assignee: Colebrook Bosson Saunders Products Ltd
Current assignee: Colebrook Bosson Saunders Products Ltd
Priority date: 2006-08-23
Filing date: 2006-08-23
Publication date: 2008-02-27
Also published as: WO2008022873A3; WO2008022873A2; GB0616734D0

Abstract

A collar for attachment to a monitor support arm post 2 to enable fine adjustment of the height of a monitor support arm located on said support post at a selected height. The collar comprises first 3 and second 4 cooperating portions, the first portion being mountable to the support post and the second portion being rotatable relative to the first portion and the support post to cause it to move up or down the support post relative to the first portion to enable further fine adjustment of the height of a monitor support arm carried by the second portion. A resilient O-ring protruding from a groove on the support arm or a bolt can be used attach the first portion to the post. The collar can have co-operating helical treads having flattened portions. Also shown is a device for limiting the extent of rotation of a monitor support arm about the longitudinal axis of the post.

Description

A collar for attachment to a monitor support arm mounting post

Description

The present invention relates to collar for attachment to a monitor support arm mounting post. In particular, the invention relates to a collar having means to enable fine adjustment of the height of a monitor support arm that has already been located on said post at a selected height.

Flat screen computer monitors are now in widespread use and have the advantage of being relatively light and slim, which enables them to be mounted on one end of an elevated and adjustable support arm. It is conventional for the other end of the support arm to be attached to a support post such that the support arm extends therefrom. In certain environments, for example stock market trading offices, it is necessary to have a plurality of computer monitors disposed adjacent to each other so that they may be viewed simultaneously. With a conventional support arm and post arrangement, it is possible to mount a pair of monitors adjacent to each other by means of corresponding support arms mounted on the same support post or a pair of posts. In this arrangement it is desirable, in terms of appearance, for each computer monitor to be in precise vertical alignment with adjacent monitors so that all the computer screens are disposed at exactly the same height.

Although the support arms may be articulated so that, if a pair of support arms are mounted on the same support post, the height of the screen mounted on each arm may be adjusted, it can still be difficult to precisely position multiple monitors at precisely the same elevation and so there is often a small misalignment between adjacent computer monitors. Additionally, if a pair of monitors are mounted on separate support posts by means of support arms located at identical heights on each post, it may still not be possible to position each monitor at exactly the same height because, if the monitors are located at substantially different distances from t}eir respective posts, the moment of force caused by the weight of the monitor and the distance from its supporting post will be different and therefore cause the monitors to be at different heights. Furthermore, movement of the monitors can result in the need to adjust the height of the arms on the post on a periodic basis.

The present invention seeks to provide a mounting collar for attachment to a monitor support arm mounting post that overcomes or substantially alleviates the problems with the monitor support arm and corresponding post systems referred to above by enabling fine adjustment of the height of each monitor support arm attached to one or more support posts to precisely align adjacent monitors.

According to the present invention, there is provided a collar for attachment to a monitor support arm mounting post to enable fine adjustment of the height of a monitor support arm located on said support arm mounting post at a selected height, the collar comprising first and second cooperating portions, the first portion being mountable to the support post and the second portion being rotatable relative to the first portion and the support post to cause it to move up or down the support post relative to the first portion to enable fine adjustment of the height of a monitor support arm carried by the second portion.

In a preferred embodiment the collar comprises cooperating helical threads in said first and second portions so that rotation of said first and second portions relative to each other causes one portion to slide into, or out of, the other portion.

The cooperating helical threads may comprise a male part formed on the first portion that engages with a female part in the second portion.

The female part is conveniently a helically shaped channel.

In one embodiment, the male part is trapezoidal in shape and the female part is a correspondingly shaped channel.

In a preferred embodiment, an end stop is formed on the first portion that locates in a second helical channel in the second portion, the end stop abutting one end of the second channel to limit rotation of the second portion relative to the first portion in a first direction.

The first portion may comprise a shoulder so that the second portion abuts the shoulder to prevent the second portion rotating relative to the first portion to limit rotation of the second portion relative to the first portion in a second direction.

Preferably, the collar includes means for attaching the first portion of the collar to a monitor support arm mounting post.

The attachment means advantageously comprises a recess in the first portion that receives a portion of a resilient 0-ring seated in, and protruding from, one of a plurality of grooves on the support post.

In an alternative embodiment, the attachment means may comprise a bolt rotatably mounted to the first portion and operable to clamp the first portion to the support post.

A retaining disc may be locatable between the second portion of the collar and the support arm.

The retaining disc includes retaining elements that locate in longitudinally extending channels in a support post to prevent rotation of the disc.

Preferably, the retaining disc comprises an upstanding guide element that cooperates with a groove formed in a bush located in the monitor support arm, the guide element sliding along said groove when the support arm is rotated relative to the retaining member.

In one embodiment, the ends of the groove and the guide element engage to limit rotation of the support arm to a predetermined angle dependent on the length of the groove.

According to another aspect of the invention, there is provided a method of enabling fine adjustment of the height of a monitor support arm located on a support post at a selected height, comprising a collar including first and second cooperating portions, the method comprising the steps of mounting the first portion to the support post and rotating the second portion relative to the first portion and the support post to cause it to move up or down the support post relative to the first portion to enable fine adjustment of the height of a monitor support arm carried by the second portion.

In a further aspect of the invention, there is provided a device for limiting the extent of rotation of a monitor support arm about the longitudinal axis of a monitor support arm mounting post to which it is attached, the device comprising co-operating portions mountable to a support post, and to a monitor support arm, respectively and including co-operating means that engage to limit rotation of said portions relative to each other.

Preferably, the co-operating means comprises a guide element and an arcuate guide recess to slidably receive the guide element, which slides in the guide recess until it abuts an end of the guide recess to prevent further rotation of the support arm relative to the support post about said longitudinal axis of the post.

The co-operating portions may comprise a first portion mountable to the support post and a second portion mountable to the support arm.

In one embodiment, the first portion is an annular ring, the guide element upstanding from said ring.

Advantageously, the ring includes at least one notch extending in a radial inward direction and which locates in a longitudinally extending channel in the support post to prevent rotation of the ring relative to the post.

Preferably, the support arm includes a sleeve to couple the arm to the support post, the second portion comprising a bush located in said sleeve to allow rotation of said sleeve about said longitudinal axis of the post.

The guide recess may be formed in said bush.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an elevated perspective view of a collar mounted to a first type of post in accordance with an embodiment of the present invention; Figure 2 is an elevated perspective view of the collar shown in Figure 1 mounted to a second type of post; Figure 3 is an exploded perspective view of first and second portions of the collar shown in Figures 1 and 2; Figure 4 is a cross-sectional view of the collar attached to the first type of post, as shown in Figure 1; Figure 5 is an exploded perspective view of the collar, as shown in Figures 1 to 4; Figure 6 is an elevated perspective view of a collar in accordance with a second 13 embodiment of the present invention; Figure 7 is an exploded perspective view of the second embodiment of the collar, as shown in Figure 6; Figure 8 is a cross-sectional view of the second embodiment of the collar attached to the first type of post, as shown in Figure 6, and Figure 9 is a bottom plan view of a bush according to the second embodiment of the collar, as shown in Figure 6.

Referring now to the drawings, there is shown in Figure 1 a collar I according to an embodiment of the present invention, mounted to a, cylindrically shaped, vertical support post 2 which may be upstanding from a desk or other horizontal surface (not shown), or mounted to a wall or the like by means of a mounting bracket (not shown). The collar 1 comprises a first portion 3 and a second portion 4, wherein the first portion 3 extends substantially into a tubular opening 5 in the second portion 4. The first portion 3 has a tubular opening 6 (see Figure 3) extending therethrough to enable it to slide onto the support post 2 to which it is subsequently attached. It will be appreciated that one or more collars 1 may be attached to the same post 2.

The support post 2 shown in Figure 1 is conventional and comprises a cylindrical tube having a plurality of longitudinal channels 7 formed in its outer surface 8, the channels 7 being spaced diametrically opposite each other and extending along the length of the post 2. Although four channels are shown formed in the post 2, it will c be appreciated that any number of channels 7 may be formed therein.

In an alternative embodiment of the support post 2, shown in Figure 2, the support post 2 has a plurality of circumferential recesses 9 formed in its outer surface 10, the recesses 9 being spaced along the length of the post 2 and being parallel to each other.

Referring now to Figures 3 to 5, it can be seen that the first portion 3 has a tubular body 11 wherein an inner surface 12 of the first portion 3 is slidable along the length of the post 2, the inside diameter of the first portion 3 being slightly greater than the outside diameter of the support post 2 for this purpose. The lower inside surface 12 of the tubular body 11 has a chamfered or recessed surface 13 for reasons which will become apparent. The first portion 3 has a shoulder 14 around its lower end and a recess 15 is formed in a lower surface 16 of the shoulder 14. The recess 15 extends circumferentially around the lower surface 16 of the collar 14 to form an annular shape. A hole 17 is formed in the shoulder 14 and extends radially therethrough such that it intersects the recess 15 and communicates between the inner surface 12 of the first portion 3 and an outer surface 18 of the shoulder 14.

A helical thread 19 protrudes from an outer surface 20 of the tubular body 11 of the first portion 3 and extends around the outer surface 20 of the tubular body 11 for approximately one and three quarter turns over substantially the height of the tubular body 11, from proximate an upper end 21 of the shoulder 14 to proximate an upper end 22 of the tubular body 11. It will be appreciated that the pitch and length of the helical thread 19 may vary and may be constant or, vary along its length. The helical thread 19 is trapezoidal in cross-section with the long end of the trapezoid forming a base of the thread 19 and with upper and lower side portions 19a, 19b. Flattened portions 23 of the helical thread 19 are formed on diametrically opposite sides of the tubular body 11, such that the flattened sections form a tangent with the outer surface 20 thereof. Although two flattened portions are formed on each side it will be understood that the number of flattened portions 23 is not limited thereto.

Located proximate to the upper end 21 of the tubular body ills a stop 24 which extends in a small arc around it and has the same pitch as the helical thread 19. The stop 24 forms a wedge shape as it extends around the tubular body 11 with an inclined surface 25 extending from the outer surface 20 of the tubular body 11 to a flattened raised surface 26 which forms the outermost point of the stop 24. The stop 24 is orientated such that the inclined surface 25 is formed to increase in thickness in the same direction as the direction of screw of the helical thread 19.

Slots 27, 28 are formed through the upper end of the tubular body 11 and extend longitudinally down each side of the stop 24 to a depth just below the stop 24 such that a flexible portion 29 of the tubular body 11 is formed between the two slots 27,28. This enables the flexible portion 29 to flex along a line between the two lower ends of the slots 27,28.

A locking mechanism 30 for clamping the first portion 3 to the support post 2 is shown in Figures 4 and 5 and comprises a locking bolt 31 and a captive nut 32. The locking bolt 31 is screw threaded along its length and has a tool engaging face 33, such as an Allen key slot, provided at one end to facilitate rotation of the bolt 31.

The captive nut 32 is rectangular such that it has four flat side faces 34 and the locking bolt 31 is threadable therethrough.

The locking mechanism 30 is received in the first portion 3 of the collar I with the locking bolt 31 extending through the hole 17 such that the tool engaging face 33 is exposed radially outwards and the other end of the locking bolt 31 extends from the inner surface 12 of the first portion 3. The captive nut 32 is disposed in the recess such that the locking bolt 31 is threaded therethrough. One face 34 of the captive nut 32 locates against the base of the recess 15 to stop the nut from rotating, as will be explained hereinafter.

The second portion 4 for engagement with the first portion 3 is shown in figures 3 to 5 and comprises a second portion tubular body 37. A plurality of parallel grooves 39 are formed on an outer surface 40 of the second portion 4 and run along the axial height of the second portion 4 to facilitate gripping by a user. A pair of parallel helical channels 41, 42 are formed on an inner surface 38 of the tubular body 37.

Both the first and second helical channels 41, 42 extend around the inner surface 38 of the tubular body 37 for approximately one and three quarter turns over substantially the height of the second portion tubular body 37. The cross-section of each channel 41,42 is formed with a base 41a, 42a and an upper and a lower side 41b, 41c, 42b, 42c which are inclined inwards from the channel opening to the base 41a, 42a. A first end (not shown) of the first helical channel 41 is formed such that it is open at the lower end 43 of the second portion tubular body 37 for reasons which will become apparent shortly. A first end 44 of the second helical track 42 terminates in the second portion tubular body 37 such that there is a dead end to the helical track 42. Second ends 45, 46 of each track 41, 42 extend substantially to an upper surface 47 of the second portion tubular body 37 such that an opening to each channel is formed therewith. The upper surface 47 of the second portion tubular body 37 is substantially flat and uniform.

The first portion 3 is received within the second portion 4 with the tubular body 11 extending therein and the shoulder 14 is disposed proximate to the lower end 43 of the second portion 4. The outer diameter of the shoulder 14 is substantially the same as the outer diameter of the second portion 4. Referring to Figure 4, it can be seen that the inner surface 38 of the second portion 4 is slidable along the outer surface 20 of the first portion 3, the inside diameter of the second portion 4 being substantially equal to the outside diameter of the tubular body 11. The pitch and cross-section of the helical thread 19 formed on the outer surface 20 of the tubular body 11 engagably corresponds to the pitch and cross-section of the first helical channel 41 such that the helical thread 19 cooperates therewith and is slidable therealong. Similarly, the stop 24 engages with the second helical channel 42 such that it is slidable therealong. It is noted that the first end (not shown) of the first helical channel 41 is open to allow a first end 48 of the helical thread 19 to be slid into it and the stop 24 is on a flexible portion 29 of the tubular body 11 to allow it to be resiliently deformed for insertion into the second helical channel 42.

Referring further to Figure 4, a lower surface 50 of a bush 49 abuts the upper surface 47 of the second portion 4. The bush 49 is tubular and the inside diameter of the bush 49 is substantially the same as the outside diameter of the support post 2 such that it is slidable therealong. The outer surface of the bush 49 has an inclined portion 51 which seats the bush 49 within a sleeve of a mounting arm (not shown) that slides over a support post 2.

Operation of the first embodiment of the mounting collar I will now be described.

The collar 1 is fitted over the support post 2 and is slid therealong. Dependent on the type of support post 2, one of two attaching means is used to attach the collar tat a selected height and prevent it from sliding down the support post 2. When the support post 2 with circumferential recesses 9 as illustrated in Figure 2 is used, a resilient 0-ring (not shown) is slid over the support post 2 and fitted into one of the circumferential recesses 9 such that a portion of the 0-ring (not shown) extends therefrom. The collar I is then slid along the length of the post 2 until the chamfered surface or recess 13 of the first portion 3 seats on the 0-ring (not shown) thereby fixing the position of the collar 1 relative to the support post 2. In use, the first portion 3 cannot rotate relative to the post 2 because of the friction between the first portion 3 and the 0-ring and so it is held in position. However, when the support post 2 with the longitudinal channels 7 is used, the collar 1 is fixed to the support post 2 by means of the locking mechanism 30. The locking bolt 31 is rotated by means of an Allen key, or the like, engaging with the tool receiving end 33. Therefore, because the locking bolt 31 is threadingly engaged with the captive nut 32, which is prevented from rotating as it is seated in the recess 15 with one face 34 located against the base of the recess 15, the locking bolt 31 moves axially into or out of the tubular opening 6 of the first portion 3 thereby securely mounting the support post 2 between the locking bolt 31 and the diametrically opposite inner surface 12 of the first portion 3.

-10 -It is understood that the chamfer 13 on the inside surface 12 of the first portion 3 and the locking mechanism 27 are used to securely fasten the mounting collar to the "circumferential recess" and "longitudinal channel" posts respectively and so it will be appreciated that only one of the above means is required if it is envisaged that only one type of post is to be used, or an alternative means of mounting the collar I to the support post 2 may be used, such as a separate friction band which locks onto the post. The collar I may then be slid over the post until it is seated on top of the friction band which prevents it from sliding further down the post. If separate mounting means are used, it is important to provide some means of attaching the first portion 3 of the collar I to the post so that it cannot rotate about the axis of the post 1.

When the collar I is fixed to the support post 2 by one of the means described above, the sleeve of a monitor support arm, in which a bush 49 is received, is slid onto the support post 2. The monitor support arm (not shown) is carried by the second portion 4 of the collar 1 because the lower surface 50 of the bush 49 is lowered onto and abuts the upper surface 47 of the second portion 4. As discussed above, the second portion 4 of the collar I is rotatably received on the first portion 3 and the helical thread 19 formed on the outer surface 20 of the first portion 3 is threadingly engaged in the first helical channel 41 of the second portion 4.

Furthermore, because the first portion 3 is fixedly disposed on the post 2 such that it cannot rotate, if the second portion 4 is rotated by means of a user applying a rotational force to the outer surface 40 of the second portion 4, the helical thread 19 slides along the first helical channel 41 thereby drawing the first and second portions 3, 4 towards or away from each other depending on which direction the second portion 4 is rotated. The flattened portions 23 of the helical thread 19 help to ensure smooth operation of the collar I by providing a gap for any dust to collect as the first helical channel 41 rotates relative to the helical thread 19.

Furthermore, because of the small pitch angle of the helical thread 19 and the first helical channel as well as the friction between the sides of the helical thread 19a, 19b and the sides of the first helical channel 41b, 41c, the second portion 4 retains it's position relative to the second portion when there is no external rotational force -11 -acting on the second portion, despite the weight of the monitor support arm and monitor acting thereon.

If a user rotates the second portion 4 in the direction such that the first and second portions 3, 4 are drawn towards each other, then the movement is restricted by the lower surface 43 of the second portion 4 abutting the upper surface 21 of the shoulder 14 of the first portion 3. Similarly, if a user rotates the second portion 4 in the direction such that the first and second portions 3, 4 are drawn away from each other, then rotation is restricted by the stop 24, which slides in the second helical channel 42, abutting the first end 44 of the second helical channel 42. This prevents the second portion 4 from sliding off the first portion 3.

A second embodiment of the invention is shown in Figures 6, 7 and 8. It is noted that this embodiment of the invention is only used with the "longitudinal channel" support post, as shown in Figure 1. The bush 52 is generally the same as the bush 49 described above, however, in this embodiment an arc-shaped recess 53 is formed along a portion of the lower surface 54 of the bush 52.

A retaining disc 55 is shown in figures 6 to 9. The retaining disc 55 has lower and upper surfaces 56, 57 and a circular aperture 58 is formed therethrough. Four retaining elements 59 extend radially inwards from an inside surface 60 of the circular aperture 58. One of the elements 59a has an extended portion which extends above the upper surface 57 and outwardly thereupon to form a notch portion 61.

In the previous embodiment the bush 51 is mounted directly upon the upper surface 47 of the second portion 4 when each component is mounted on the support post 2. However, in the present embodiment, the retaining disc 55 is located between the bush 52 and the second portion 4 such that the lower surface 56 of the retaining disc 55 abuts the upper surface 47 of the second portion 4 and the upper surface 57 of the retaining disc 55 abuts the lower surface 56 of the bush 52. The retaining elements 59 are arranged such that they are disposed in -12 - corresponding longitudinal channels 7 of the support post 2 and notch portion 61 locates in the arc-shaped recess 53 formed in the bush 52.

Operation of the second embodiment of the mounting collar I will now be described. The relative movement of the first and second portions 3,4 of the collar 1, wherein the helical thread 19 slides along the helical channel 41 thereby drawing the first and second portions 3, 4 towards or away from each other depending on which direction the second portion 4 is rotated, operates identically to the first embodiment. However, in this embodiment the retaining disc 55 is disposed above the upper surface 47 of the second portion 4 such that the lower surface 56 of the retaining disc 55 abuts therewith. When the retaining disc 55 is slid onto the support post 2, the retaining elements 58 are located to slide in the longitudinal channels 7 to prevent rotation of the retaining disc 55 relative to the support post 2. When a sleeve of a support arm in which bush 52 is located is slid onto the support post 2, rotation of the sleeve and bush 52 is prevented as the second portion 4 is rotated because the retaining disc 55 is prevented from rotating by the retaining elements 59.

Furthermore, rotation of the retaining disc 55 relative to the bush 52 is restricted because the notch portion 59 slides in the in the arc-shaped recess 53 of the bush 52. Although the bush 52 can rotate about the post 2, the notch portion 59 contacting end portions 53a, 53b of the arc-shaped recess 53, limits the range of movement of the bush 52, thereby limiting the angle through which the monitor support arm may rotate about the post 2. In the embodiment shown in Figure 9, the angle through which the monitor arm may rotate about the support post 2 is approximately 180 degrees, although it will be understood that the invention is not limited thereto.

In the illustrated embodiments, the retaining disc 55 and bush 52 are used in conjunction with the collar 1. However, it is envisaged that the retaining disc 55 and bush 52 could be used independently from the collar 1, in which case the retaining disc 55 is fixedly mounted to the support post 2 by use of it's own locking mechanism, or the retaining disc 55 may be carried on an alternative component -13 -locked to the post 2, such as the separate friction band described above. Such a configuration provides limited rotation of a support arm mounted to a support post but without the fine-adjustment capability provided by the collar.

It will also be appreciated that, although in the illustrated embodiments, the helical thread is formed on the first portion and the helical channel is formed in the second portion, in an alternative embodiment the helical thread may be formed in the second portion.

Although embodiments of the invention have been shown and described, it will be appreciated by those skilled in the art that these are preferred embodiments only and that changes may be made to these embodiments, or alternative embodiments are included, within the scope of the invention which is defined in the claims hereafter.

Claims

-14 -Claims 1. A collar for attachment to a monitor support arm

mounting post to enable fine adjustment of the height of a monitor support arm located on said support arm mounting post at a selected height, the collar comprising first and second cooperating portions, the first portion being mountable to the support post and the second portion being rotatable relative to the first portion and the support post to cause it to move up or down the support post relative to the first portion to enable fine adjustment of the height of a monitor support arm carried by the second portion.

2. A collar according to claim 1, comprising cooperating helical threads in said first and second portions so that rotation of said first and second portions relative to each other causes one portion to slide into, or out of, the other portion.

3. A collar according to claim 2, wherein said cooperating helical threads comprises a male part formed on the first portion that engages with a female part in the second portion.

4. A collar according to claim 3, wherein the female part is a helically shaped channel.

5. A collar according to claim 3 or 4, wherein the male part has flattened portions.

6. A collar according to claims 4 or 5, wherein the male part is trapezoidal in shape and the female part is a correspondingly shaped channel.

7. A collar according to any of claims 4 to 6, comprising an end stop on the first portion that locates in a second helical channel in the second portion, the end stop abutting one end of the second channel to limit rotation of the second portion relative to the first portion in a first direction.

-15 - 8. A collar according to claim 7, wherein the first portion comprises a shoulder so that the second portion abuts the shoulder to prevent the second portion rotating relative to the first portion to limit rotation of the second portion relative to the first portion in a second direction.

9. A collar according to any preceding claim, comprising means for attaching the first portion of the collar to a monitor support arm mounting post.

10. A collar according to claim 9, wherein said attachment means comprises a recess in the first portion that receives a portion of a resilient 0-ring protruding from a groove on the support post.

11. A collar according to claim 8, wherein said attachment means comprises a bolt rotatably mounted to the first portion and operable to clamp the first portion to the support post.

12. A collar according to any preceding claim, comprising a retaining disc locatable between the second portion of the collar and the support arm.

13. A collar according to claim 12, wherein the retaining disc includes retaining elements that locate in longitudinally extending channels in a support post to prevent rotation of the disc.

14. A collar according to claim 13, wherein the retaining disc comprises an upstanding guide element that coop crates with a groove formed in a bush located in the monitor support arm, the guide element sliding along said groove when the support arm is rotated relative to the retaining member.

15. A collar according to claim 14, wherein the ends of the groove and the guide element engage to limit rotation of the support arm to a predetermined angle dependent on the length of the groove.

-16 - 16. A collar substantially as hereinbefore described, with reference to the accompanying drawings.

17. A method of enabling fine adjustment of the height of a monitor support arm located on a support post at a selected height, comprising a collar including first and second cooperating portions, the method comprising the steps of mounting the first portion to the support post and rotating the second portion relative to the first portion and the support post to cause itto move up or down the support post relative to the first portion to enable fine adjustment of the height of a monitor support arm carried by the second portion.

18. A device for limiting the extent of rotation of a monitor support arm about the longitudinal axis of a monitor support arm mounting post to which it is attached, the device comprising co-operating portions mountable to a support post, and to a monitor support arm, respectively and including co-operating means that engage to limit rotation of said portions relative to each other.

19. A device according to claim 18, wherein the co-operating means comprises a guide element and an arcuate guide recess to slidably receive the guide element, which slides in the guide recess until it abuts an end of the guide recess to prevent further rotation of the support arm relative to the support post about said longitudinal axis of the post.

20. A device according to claim 19, wherein the co-operating portions comprise a first portion mountable to the support post and a second portion mountable to the support arm.

21. A device according to claim 20, wherein the first portion is an annular ring, the guide element upstanding from said ring.

22. A device according to claim 21, wherein the ring includes at least one notch extending in a radial inward direction and which locates in a longitudinally -17 -extending channel in the support post to prevent rotation of the ring relative to the post.

23. A device according to any of claims 20 to 22, wherein the support arm includes a sleeve to couple the arm to the support post, the second portion comprising a bush located in said sleeve to allow rotation of said sleeve about said longitudinal axis of the post.

24. A device according to claim 23, wherein the guide recess is formed in said bush.