RUNGS FOR INDIVIDUAL RUNG IADDERS AND A FABRICATION METHOD

FOR SUCH RUNGS

Technical Field

This invention relates to rungs for individual rung ladders (known as IRLs) , as well as a method for fabricating such rungs .

Background Art

Such IRLs are substantially U-shaped and have their ends cast into the walls of concrete vertical structures , and at vertically spaced apart intervals down the length of the structure, to provide foot supports for personnel as they descend down and ascend up the structure. Such structures may be vertical access shafts for underground tunnels, such as, main arterial tunnels for city or town sewage systems , and which need to be accessed from time to time for inspection and /or maintenance purposes. However, the IRLs of this invention are applicable to other concrete structures (walls) as well, and for which permanent ladders are required.

Unlike conventional ladders, where a plurality of horizontal rung members are provided between vertical rung support members, IRLs are individual rungs (steps) not having vertically extending rung supports to which they are attached, but instead are rungs which are individual in their own right with their only interconnection being the concrete structure to which they are attached by virtue of the ends of the horizontal support arms for the rungs being cast into the structure. However, although the IRLs of this invention are intended to have the ends of their horizontal support arms cast into the concrete structure, they may be otherwise attached to the structure, but with the structure still providing the

equivalent of the vertically extending rung supports of conventional ladders . The IRLs of this invention may also be attached to structures formed from other materials , such as, brick, timber, or other building materials.

Known IRLs are formed from steel rod, bent, or otherwise formed, to be of a U-shaped configuration, the legs of which provide the horizontal support arms, the ends of the legs being embedded in the concrete structure ( walls of an access shaft ) , with the intervening portion of the U providing the foot support rung for the IRL. The problem with steel IRLs is that , in some environments , such as when associated with sewage systems where the steel of the IRL is subject to corrosion , such as by Hydrogen Sulphide gas, and the IRL is weakened, breakage of the IRL under foot pressure resulting from body weight may occur.

It is also known to provide steel IRLs coated with a plastics material which acts as a barrier to corrosion of the steel substrate, but cracking or abrasion of the plastic coating, during continued use of the ladder, can expose the steel substrate to corrosion.

Disclosure of the Invention

One solution to the abovementioned problem with known IRLs is to provide an IRL formed solely from a high strength plastics material, such as, fibre reinforced Polybutylene Terephthalate (PBT) or various types of fibre reinforced Nylon. However, the moulding of a substantially U-shaped

IRL of such materials is difficult, if not impossible, and it is therefore an object of the present invention to provide an IRL formed from such plastics materials, whereby the vertical rung of the IRL, and the horizontal support arms , are moulded separately from each other ( and thus more easily moulded ) , and then subsequently attached to each other to provide a completed substantially U-

shaped IRL, and where movement of the rung member relative to its support arms is prevented, or at least minimized. Although the invention is particularly applied to components formed from high strength plastics materials, its principle can also be applied to rungs formed from other materials, such as metals, including steel.

In accordance with the present invention, there is envisaged a rung for an individual rung ladder, which includes a foot support rung member and two separately formed support arm members , the ends of the rung member and associated ends of the support arms being adapted to form joints therebetween, and such that, in use, the rung member will not move relative to its support arms .

The invention also envisages a method of fabricating a rung for an individual rung ladder, said method including the steps of, forming a foot support rung member and forming two separate support arm members , said forming including formation of means on the ends of said members whereby the rung member can be attached to associated ends of said support arm members , and such that, in use, the rung member will not move relative to its support arms , and subsequently attaching said support arm members to said rung member .

Preferably the rung member and the support arm members are formed by moulding them from high strength plastics material .

Brief Description of the Drawings

One preferred embodiment of the invention, and several variations thereof, as applied to a vertical concrete access shaft, will now be described with reference to the accompanying drawings , in which :

Figure 1 is a partial schematic view of a vertical access shaft incorporating rungs in accordance with a preferred embodiment of the present invention, to form an individual rung ladder,

Figure 2 is an enlarged view of part of the access shaft of Figure 1 ,

Figure 3 is an exploded perspective view of an individual rung assembly in accordance with the present invention, Figure 4 is a perspective view of the rung assembly of Figure 3 in the assembled condition,

Figure 5 is a perspective view of a mould for a vertical access shaft, incorporating the rung assembly of Figure 4,

Figure 6 is an enlarged view of part of the mould of Figure 5 ,

Figures 7 & 8 are side elevational cross-sectional views of an alternative way of attaching the rung assemblies of Figures 2 - 6 to a vertical access shaft.

Best Modes for Carrying Out the Invention

Referring to Figures 1 & 2 of the drawings, in this preferred embodiment of the invention, as applied to a vertical concrete access shaft, the shaft 10 consists of a cylindrical concrete casting formed in accordance with any one of a number of conventional casting techniques , such as, vertical moulding or centrifugal moulding. The shaft 10 as shown is either cast insitu using a vertical moulding technique, or cast off-site by the vertical moulding or centrifugal moulding technique, and delivered to the site where it is dropped into a pre-prepared hole communicating with a tunnel beneath the surface of the ground 11. The top of the shaft at ground level is formed

-to provide a ledge 12 adapted to receive and seat an access shaft lid (not shown) . A plurality of rungs 13 to provide an individual rung ladder are moulded into the wall 14 of the access shaft during the casting of the shaft, and are situated at spaced apart intervals down the length of the shaft to form the individual rung ladder.

Turning to Figures 3 & 4 of the drawings, the rungs 13 of this preferred embodiment consist of an elongated foot support rung member 15 and two elongated support arm members 16.

The rung member 15 is of a substantially square cross- section for most of its length 15a, as shown, and terminates at either end with a cylindrical section 15b stepped at 15c to provide smaller and larger diameter sections 15d and 15e. The smaller diameter section 15d carries a pair of radially outwardly directed lugs 15f on diametrically opposite sides thereof. The sides of the lugs facing the step 15c are tapered in their circumferential directions, with the direction of the taper of the lugs at one end of the rung member being opposite to that of the lugs at the other end of the member. The ends of the support arm members 16 have holes 16a formed transversely through the member and of a diameter sufficient to snugly receive the larger diameter sections 15e on the associated ends on the rung member. The holes 16a have radially, inwardly directed lugs 16b formed on diametrically opposite sides of the holes , and each lug is also tapered on its outer sides to match the taper of the lugs 15f on the ends of the rung member. The ends of the arm members are moved axialIy onto the ends of the rung member with the lugs 15f passing through the gaps between the lugs 16b to the other side thereof, whereafter by swinging the respective arm members through 90 degrees , from their positions shown in Figure 3 to their positions shown in Figure 4, the tapering sides of

the lugs move into engagement with each other in wedge- like fashion to lock the arm members onto the arms of the rung member . This serves to prevent removal of the arm members from the rung member, whilst as a result of the wedging action between the lugs continued rotation of the arm members about the ends of the rung member beyond 90 degrees is prevented. As the tapering of the facing sides of the lugs at one end of the rung member is in the opposite circumferential direction to that of the tapering of the facing sides of the lugs at the other end, when the arm members are attached to the rung member, and swung or rotated through 90 degrees in opposite directions as shown, rotation of the rung member about its longitudinal axis in one direction within the arm member at one end is prevented and rotation of the rung member in the opposite direction within the other arm member is also prevented, thus effectively locking the rung member against rotation about its axis in either direction once the arm members are restrained against rotation about the ends of the rung member. Such restraint occurs when the free ends 23 of the arm members are embedded within the concrete mass of the access shaft after it is cast and cured, and to be later described.

In order to further restrain the rung member against rotation about its longitudinal access, and within the holes 16a through the ends of the associated arm members 16, the ends of the smaller diameter sections 15d of the rung member have flats 15g formed thereon cooperating with flats 18 formed within blind bores 19 in square cross- sectional button-like locking members 20, which in turn are received within similarly dimensioned square cross- sectional recesses 21 in the outer ends of the holes 16a through the ends of the support arms 16.

Turning to Figures 5 & 6 of the drawings, the vertical access shaft is cast with a conventional steel reinforcing

cage 22 embedded therein, whilst the free ends 23 of the support arms 16 are of cylindrical configuration as shown and have transverse holes 24 formed diametrically therethrough to receive steel retaining bars 25, which are subsequently spot-welded to the steel reinforcing cage 22. Flexible spring steel clips 26 are fitted to clamp around the ends of the support arms and have legs 26a surrounding the steel retaining bars 25 and such as to be sprung into position around the bars thus holding the ends of the rungs in position on the bars during casting of the concrete access shaft. A jig (not shown) engages the rungs and retains them in the correct orientation inwardly of the access shaft during its casting.

The access shaft maybe centrifugally spun in a known fashion, with the concrete being flung outwardly against an outer cylindrical mould wall 27 to a thickness whereby its inner diameter corresponds with matching curved faces 28 at the transition between the cylindrical ends 23, and the remainder of the lengths, of the support arms 16.

Figures 7 & 8 show an alternative way of attaching the rungs 13 to the vertical access shaft, and involve the ends 23 of the support arms 16 having threads 29 formed thereon, with the ends 23 being received through preformed holes 30 through the concrete wall 31 of the access shaft and held in position by nuts 32 tightened onto the threaded ends and against the outer wall of the access shaft, with intervening washers 33.

Other forms of attachment of the rungs to the access shaft may be utilized, such as casting ferruls in holes through the wall of the access shaft and driving the ends of the support arms of the rung into tight fitting engagement within the ferruls.

Other modifications, employing the spirit of the invention, will be evident to persons skilled in the art.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as

"comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.