WO1990015915A1

WO1990015915A1 - A collapsible ladder

Info

Publication number: WO1990015915A1
Application number: PCT/CA1990/000083
Authority: WO
Inventors: L. J. Gosling
Original assignee: Gosling L J
Priority date: 1989-06-15
Filing date: 1990-03-12
Publication date: 1990-12-27
Also published as: AU6895591A

Abstract

A collapsible ladder is disclosed wherein adjacent side pieces of the ladder are straps (21, 22) pivotally secured to each other and lockable in an extended position in inwardly spring-urged bolts (51) the release of which bolts can only be achieved by dexterous manipulation of spring means (49) which urge the bolts into locking engagement. The locking engagement occurs automatically when the two adjacent straps are aligned in a coextensive position. The advance in the art is in a structural simplicity combined with making it virtually impossible to inadvertantly release the locked connection between adjacent links of the ladder even if the ladder is subjected to extreme forces or impact.

Description

A COLLAPSIBLE LADDER

The present invention relates to ladders and in particular to collapsible ladders of the type wherein the ladder is comprised of sections which are disposed relative to each other much in the fashion of a chain link.

The ladders of the above type have been known for many decades. Attempts have been known to be made to improve one aspect of the collapsible ladder or the other, but such modifications usually had to be made at the expense of another desired feature of a collapsible ladder. In particular, foldable ladders of the known type have the disadvantage of being relatively difficult to quickly and reliably place into a straight, generally rigid state. Another disadvantage is that the mechanisms holding the link sections of a folding ladder in a straight state are usually inoperative when the ladder is placed in an upside-down position which limits the field of use to a very narrow scope. Another disadvantage is the relative ease at which an assembled, straight ladder can be unlocked at the pivotal joints with serious consequences, particularly when the ladder serves as an emergency fire escape device. Accordingly collapsible ladders of the above type have not found general acceptance in the art of fire escape ladders or in other fields of potential use of same.

Prior art to which the present invention pertains is represented by U.K. Patent Application 2,123,470 A, published February 1,1984 (H. A. Collins), by U.K. Patent Application 2,137,681 A, published October 10, 1984 (F. Brown) and by U.K. Patent Specification No. 1,518,985 published July 26, 1978 (H. Olsen). It is an object of the present invention to provide an improvement in the aforesaid type of ladders and in particular to provide an arrangement wherein the ladder is locked in an extended position such that inadvertent collapse of one or more joints during the use of the ladder is virtually eliminated.

In general terms, the present invention provides a collapsible ladder of the type comprising a plurality of subsections pivotally linked to each other, each subsection comprising: a) a generally rectangular, rigid member which includes a pair of generally parallel, opposed inside straps, and a pair of rungs generally perpendicular to and integral with the inside straps and extending therebetween, one rung near each end of the rigid member; b) a pair of generally parallel, opposed outside straps each having a first end portion partially overlapping an outside surface position of the adjacent inside strap, both outside straps being pivotally secured to said rigid member near one end and at opposed sides thereof; c) pivot means providing pivotal securement of said outside straps to said frame member, said pivot means comprising two pivot assemblies coaxial with each other about a transverse pivot axis and disposed at said one end of the rigid member; d) each said pivot assembly comprising a support pin and a complementary bearing means, said support pin and said bearing means being each formed in one of the inside and outside straps, at the respective side of the rigid members, said pivot assembly further including axial displacement limiting means preventing displacement of the respective outside strap relative to the adjacent inside strap in both directions of said axis only to a minute axial spacing required for allowing pivotal movement between the straps about said axis; e) lock means for generally rigidly but releasably connecting the outside straps to the rigid member to maintain the outside straps generally coextensive with the respective inside straps when the ladder is in an extended state, by preventing the outside straps to pivot relative to the respective inside straps about the pivot axis; f) locking bolt means disposed at one end of spring means whose other end is fixedly secured to the surface of one of said straps, said spring means being adapted to resiliently urge the bolt means in a direction generally parallel with the direction of the pivot axis, to assume a locked position, wherein the locking bolt means interengages a first engagement portion which forms a part of said sping and a second engagement portion which forms a part of one of said outside straps such that the frame member is maintained in a generally rigid, coextensive position relative to the associated outside strap, said spring means allowing displacement of said bolt means to a released position, wherein the bolt is removed from at least one of said engagement portions to render said outside strap free to pivot relative to the frame member about the pivot axis.

The invention will now be described by way of two exemplary embodiments, with reference to the accompanying simplified diagrammic drawings, in which certain parts have been omitted from some of the views for the sake of clarity. In the drawings:

Fig. 1 is a perspective view of an embodiment of the ladder according to the present invention;

Fig. 2 is an enlarged, partially perspective view of one embodiment of the connecting mechanism used in the ladder and shown in the extended state; Fig. 3 is a view similar to that of Fig. 2 but showing the ladder in a partly collapsed state;

Fig. 4 is an exploded perspective view of the embodiment of the preceding figures showing one way of securement of the support pin to its rigid member;

Fig. 5 shows a modification of the securement of Fig. 4 in an exploded perspective view;

Fig. 6 is another exploded perspective view showing a modified version of releasable locking arrangement between the rigid member and one outside strap associated therewith; Fig. 7 (on the sheet of Fig. 5) is a diagrammic side view showing the ladder in a folded state.

It should be understood that the same reference numerals designate the same parts or their equivalents. Where there is a plurality of identically shaped parts, index letters have been used to distinguish the respective parts. Turning firstly to the representation in Fig. 1, the illustration shows a collapsible ladder whose collapsed state is shown in Fig. 7.

Reference number 10 designates the top end of the ladder, in the embodiment shown, provided with a pair of hooks 11 by which the ladder, normally stored in a storage box 12 shown in Fig. 7, can be secured to an element such as a window sill 13 (diagrammically indicated in Fig. 1) to provide an emergency egress in case of fire or the like. Reference number 14 designates the bottom end of the ladder which may be either suspended or may rest on the ground. It is to be noted in this context that the representation of Figure 1 shows the bottom end 14 resting on the ground. However, this is only intended to show that the bottom end 14 can rest on the ground to become a supporting element of the ladder. It will be readily appreciated that if the ladder were used with hooks such as hook 11, then its length would be designed such as to show that the ladder is suspended. In such case, the bottom end 14 would be spaced above the ground because gravity would be required to bring the ladder from a folded state to a locked, extended state as will be explained hereinafter.

The ladder is comprised of a number of subsections such as subsection 15 which are pivotally linked together, much in the fashion of a drive chain of a bicycle or the like. The subsection 15 marked in Fig. 1 forms a portion of the upper part of the ladder. The infinite number of the rigid sections referred to above and their outside straps is indicated by the interruption of the connecting links or straps just above the lowermost element of the ladder as shown.

The structural features of each subsection 15 will become apparent on review of Figs. 2 and 3, which show a typical arrangement of the subsection 15, it being understood that the remaining subsections are of a generally identical structure.

The subsection 15 is composed of two members: rectangular, rigid member 19 and section member 20. The rectangular, rigid member 19 includes two parallel side pieces formed by a pair of opposed rectangular straps 21 and 22, each formed by a rectangular web. For convenience, the straps 21, 22 are referred to as "inside" straps to distinguish same from straps of the second member 20 which will be referred to later. In the embodiment shown, the two straps 21, 22 are integral with two rung members 23, 24 which, in the embodiments shown, are of the type of aluminum tubes fixedly secured to the straps 21, 22 at the respective ends of the rectangular, rigid member 19.

One of the available modes of fixedly securing the rung 23 to the straps 21, 22 is apparent from Fig. 4, wherein the aluminum tube forming the rung 23 is adapted to fit in a circular bore 26 machined in one end of the inner strap 21. In the free end of rung 23 is inserted and secured by welding a support pin 27. The rung 23 has a diameter which allows it to pass through the bore 26 to become flush with the outside face 34 of the strap 21, to be then welded thereto. The length of the pin 27 received within the end of the rung 23 presents an entirely cylindric section 28 which is distinct from the outer surface portion of the pin 27 which begins at a shoulder 29 and is comprised of a first outer surface section 30 forming a continuation of the cylindric surface of section 28. The cylindric surface of the first outer surface section 30 is interrupted at a generally planar second outer surface section 31 extending all the way to the free end of the pin 27 and changing the shape of the section 30 to that of a cylindric segment between whose peripheral end portions extends the flat portion 31 in the general direction of periphery of the pin 27. The cross-sectional contour of the second outer surface section 31 is that of the letter "D".

With the pin 27 inserted in the end of the rung 23 and the shoulder 29 flush with the outside surface 34 of the inner strap 21, only the surface sections 30, 31 protrude from the strap 21 at the end of the respective rectangular, rigid member 19. The rung 23 and the inside end portion of the pin 27 are fixedly secured to the strap 21 by a weld passing through a hole 32 and through a suitably located cut 33 machined in the underside of the cylindric section 28 of the in 27 as is shown in Fig. 4. The rung 23 can be secured to the strap 21 in a great number of different ways only some of which are described as the particular way of securement in itself does not form part of the invention. The free end of the first and second surface sections 30, 31 is within the plane of a peripheral groove 35 machined in the cylindric segment 30 and complementary with a C-clip 36. The C-clip 36 cooperates with the face 34 of the strap 21 which face is flush with shoulder 29 as mentioned above, to form therewith which is generally referred to as axial displacement limiting means which allows only very slight axial displacement of the associated outer strap 37 along a pivot axis A as will be described later.

The described rung securement means for fixedly attaching each rung to the strap 21 and the shape of the part corresponding to the pin 27 can be modified in many ways and to a very substantial degree. As a matter of fact, the pin 27 could even be made independent from the rung 23 and integral with the strap 21. The aforesaid modifications could encompass a flat strap with serrated horizontal ribs, etc.

Fig. 5 shows one such possible modification of the structure of the support pin. Here the pin is designated with number 38. It is of a generally rectangular cross-section of its part passing into the rung 39 and is provided with a pin hole 40 for a fixing pin 41. The cross-section of this arrangement of the pin corresponds to the shape of a slot 42 in the inner strap 43. The free end portion of the support pin 38 is formed by first and second surface sections of the same type as those in Fig. 4, with an identical arrangement of the axial displacement limiting C-clip. The last three parts are therefore not referred to with a separate reference number, even though they are visible in Fig. 5. A ladder having the structure as shown in Fig. 5 would be the ideal solution if it were fixed in a suspended position. In other words, from the top the rungs could be pulled up and would automatically fold into a retaining housing (not shown) when not in use.

Thus, each of the rectangular, rigid members 19 presents a generally rigid, rectangular structure whose ends are formed by a pair of rungs fixedly secured to a pair of side straps such as straps 21, 22.

Except for the end members of the ladder, wherein the continuity of the ladder structure ends either by way of the pair of hooks or by simply not attaching any further second members or elements, a typical rectangular, rigid member is secured at each end to a second member. The term "second member" is used for convenience and does not designate an integral structure such as the rectangular, rigid member. Instead, the "second member" is comprised of a pair of outside straps of aluminum rectangular pieces having cross-sectional configuration similar to that of any one of the inside straps 21, 22.

One of the outside straps has already been introduced above with reference number 37. The other strap 37 is designated with number 37a. It is apparent from the representation of Figs. 2 and 3, that, typically, there are two "second" members associated with each rectangular, rigid member. They are so disposed that one end of each of straps 37, 44 coincides with one end of the rectangular, rigid member 19, while another pair of the outside straps coincides with the other end of the rectangular, rigid member 19. It will be explained that the connection between the first and second members is pivotal. Accordingly, there are two pivot axes A and B (Fig. 2) to each rectangular, rigid member.

Each of the outside straps 37, 44 forms, at each end thereof a part of pivot securement means which, in the embodiment shown, is bearing means of the type of a machined bore 45 having the shape of a distorted keyhole. The cross-section of the bore 45 is best seen in from Figures 4 and 5. The keyhole 45 is elongated in the downward direction of Figure 4 or Figure 5, which is the direction of elongation of the strap 37 in which the bore is machined. It includes a narrow end portion which is directed toward the adjacent end of the strap 37 and is designated with reference number 45. The narrow end portion 46 defines a straight or flat side surface 47 which is complementary with the second outer surface section 31 of the pin 27. With the remaining parts of the narrow end portion 46 being rounded in general compliance with the radius of the cylindric segment 30 of the pin 27, it can be seen that the first and second outer surface sections 30, 31 of the pin can lockingly engage the narrow end portion 46 of the keyhole shaped bore 45. The wide end portion of 48 which is more remote from the adjacent end of the strap 37 than the narrow portion 46, is of a cross-section sufficient to rotatably receive the first outer surface section 30 of the support ping 27 so as to allow pivotal movement of the strap 37 about the axis A when the ladder is being either extended from a folded position or brought back to the folded position from extended state. It can be appreciated on review of Fig. 2 that the keyhole shaped bore of each strap 37, 44 at the opposite end thereof is of identical configuration but is oriented such that its narrow end portion points out in an opposite direction, to its adjacent end of the strap 37, 44. This is indicated by the arrangement of the keyhole shaped bore 45a in the lower strap 37 of Fig. 2. It can thus be seen that the mutual position between the strap 22 and 37 is determined by the instant state of the assembly which includes the pin 27. When the pin 27 is arranged as shown in Figure 4 and shifted with its second outer surface 31 to engage side surface 47 (as shown in Fig. 2), the pin is generally coaxial with axis A and holds the ladder in an aligned state by virtue of the engagement between surfaces 31 and 47. This state can be achieved by suspending the ladder which results that the individual sections of the ladder bring the pin 27 into the narrow end portion 46 of the keyhole shaped bore 45 by gravity.

In the embodiment shown in Figs. 1 through 5, each of the outside straps 37, 44 is provided at its outer surface and at each end with a flat spring 49 whose one end is fixedly secured, by a rivets 49a, only one shown in Fig. 4, adapted to pass through openings 50 in the respective end of spring 49, to anchor the end to a respective strap 37, 44. The opposite end of the spring 49 carries a locking bolt 51 which, in the embodiment shown in Figs. 1 through 5, is a flat disc of a generally D-shaped cross-sectional configuration complementary with the wide end 48 of the associated keyhole shaped opening 45. The D-shaped configuration is provided by cutting off a part of the side of the originally circular disc to provide a flat surface 52 which, in a fully extended state of the ladder, abuts against the first outer surface section 30 of the associated pin 27. The mechanism of the spring 49 is also provided, at an intermediate portion of the spring 49, with a push-out tab 53 which is slidably received in a suitable opening 52a in the respective strap 37 or 44. The push-out tab 53 serves the purpose of disengaging the locking bolt 51 from the associated pin 27 by displacing the bolt 51 outwardly and away from the opening 45 to allow sliding of the pin 27 back into the wide end 48 of the keyhole opening upon subjecting the elements to the suitably oriented force.

Figs. 1 and 2 also show another important feature of the invention, which makes it virtually impossible to assemble the ladder of the present invention in a wrong way.

As mentioned above, each rigid member 19 includes a total of four support pins 27,27a, 27b, 27c, one at each corner of the rigid member 19. The pins 27 - 27c are arranged in pairs of adjacent support pins 27-27a, 27a-27b, 27b-27c and 27c-27. The pairs are disposed either longitudinally - 27-27a and 27b-27c or transversely 27a27b, 27c-27, of the rigid member 19. There is a particular relationship between the pins of each of said pairs which is visible in the drawings referred to. It can be observed, with reference to the location of the surfaces 31. 31a. 31b and 31c as an example, that the pins of any of said pairs are symmetrical with each other with respect to a point S1, S2, S3 and S4 of symmetry, each of which points is on the respective one of four reference axes r1, r2, r3 and r4 extending each between the respective two support pins. For instance, axis r1 extends between the pins 27 and 27a, while axis r4 extends between pins 27c and 27. Each of the points of symmetry is equidistantly spaced from the respective two support pins. See as one of the four examples the spacing between point S1 and its associated pins 27, 27a.

Similarly, the outside straps 37, 44a are each provided with a pair of said bearing means, in Figs. 1- 4 keyhole shaped openings 45, 45a, one at each end of the respective outside strap 37, 37a. The shape of the upper opening 45 of each strap 37 is symmetrical with the shape of the other opening of the same outside strap 37 with respect to another point of symmetry T1 which is on a reference axis t1 extending between the openings 45 of the respective outside strap 37 and is equidistantly spaced from the respective two openings 37, 37. The orientation of the openings 45a in the opposite outside strap 37a is similarly symmetrical with respect to its associated point t2 of symmetry.

It will be readily appreciated that with the outside surface of the straps 37 readily recognizable by the location of the springs 49, it is impossible to wrongly place any strap with respect to any support pin. This also facilitates the manufacture as only one type of the rigid member and one kind of the outside strap is required. The arrangement utilizing the general shape of the keyhole in the connecting straps 37, 44 is not necessarily the only arrangement according to the present invention which provides the advantages as set forth above.

In the embodiment of Figure 6, a rung 54 is of the type of a tube in which is welded a support pin 55 whose outside end portion 56 is provided with a grove 57 complementary with a locking clip 58. The inside strap of this embodiment is designated with reference number 59 and the corresponding outside strap with reference numeral 60. In a fully assembled state, the rung 54, the inside strap 59 and the pin 55 form a rigid, integral unit. The outside strap 60 is provided with bearing means of the type of a bore 61 which is of the size allowing pivotal movement of the outside strap 60 relative to the inside strap 59, about the axis A. The inside strap 59 is provided with a pair of bores 62, 63 which are aligned with the corresponding bores 64, 65 in the outside strap 60 when the two straps 59, 60 are aligned in a coextensive arrangement as indicated by the exploded view of Figure 6. This embodiment is particularly suitable in applications wherein the ladder is assembled only at the beginning of its use and then remains in assembled state, much as a regular, right ladder.

As in the first described embodiment, a pair of springs 66, 67 is riveted to the outer surface 68 of the outside strap 60 with their locking bolts 69, 70 being of a size sufficiently smaller than that of the corresponding bores 64 - 62 and 65 - 63 to pass through same in order to interlock the inside strap 59 and the outside strap 60 in an aligned position. The bolts 69,

70 are preferably tapered at front to assist alignment with the corresponding bores. The clip 58 does not allow any axial displacement of the outer strap 60 along the axis A, when it is mounted on the outside end portion 56. The tolerance of the axial spacing is only sufficient for allowing pivotal movement of the outside strap 60 relative to the inside strap 59. For the sake of simplicity, Fig. 6 does not show the counterpart of the push-out tab 53 shown in Fig. 4 even though, obviously, it would be included in the structure of Fig. 6 if required. Thus, in operation, when the ladder is in a fully extended state in which the locking bolts 69, 70 are in engagement with the bores 62 - 64 and 63 - 65, there is no possibility of inadvertent disengagement of the aligned position by forces that can be reasonably encountered during the use of the ladder. The displacement which would disengage the locking bolt 69 70 from the aligned bores would either have to overcome the locking clip 58 or would somehow have to deform the structure of the generally rectangular rigid member 19. Thus, the release of the locking bolts 66, 67 can be only achieved by dexterous manipulation of the springs 66 and 67.

The feature which is common to both embodiments of the locking of the straps in aligned position is in that it is achieved by resiliently urging locking bolt means in a direction parallel with the axis A and preferably inwardly of the ladder while at the same time limiting axial displacement between the two straps to a minimum, just enough for allowing pivotal movement between the connected inside and outside straps. In normal use, including emergency fire escape application, there is never a force exerted on the joint which would release the locked engagement between two adjacent straps. Such disengagement could happen, for instance, in case of the aforesaid British Patent 1,518,958, where a sudden impact directed outwardly from the centre of the ladder, for instance by a person trying to quickly descend the ladder, could disengage the lock by displacing one end of the connecting strap axially outwardly.

Both embodiments allow the folding of the ladder as indicated in Figs. 3 and 7 such that the locking bolts 69, 70, 51 of the springs 66, 67, 49, respectively, rest, on misalignment of the folded state, against the surface of the strap 68 (in the case of Fig. 6) or against the outer face of the pin 27 in case of the first embodiment described. In both disengaged positions, the locking bolt means is urged inwardly by the resiliency of the associated springs 49, 66 or 67. As soon as the respective straps become aligned on extension of the ladder, the bolts snap into the locked position to maintain the ladder in an extended state.

Those skilled in the art will readily appreciate that many modifications may be carried out of the embodiments described, without departing from the gist of the present invention. Strictly as an example, it may be feasible to provide the locking means only at one side of the ladder. Another readily conceivable modification resides in providing only one locking spring 66 and leaving out the other locking spring 67. The support pins can be molded integral with the inside straps and thus do not have to be exactly aligned with the respective axes of the rungs. A great number of other modifications exists which, however, are still within the scope of the invention. Another such modification would be the provision of stops, which are not shown, but which would prevent the pivoting of the strap relative to the associated rigid member such that the strap cannot pass the position of automatic locking in the extended position. Another modification could be made by extending the length of the strap 22 and providing a hole therein into which the bolt 51, which would, of course, be increased in length, could enter to provide an additional interlock of the embodiment of Fig. 4 to reduce wear in applications in which the ladder is subjected to heavy transverse loads, e.g. in use as a horizontal support.

Accordingly, I wish to protect by letters patent which may issue on this application all such embodiments as properly fall within the scope of my contribution to the art.

Claims

The embodiments of the invention in which an exclusive right or privilege is claimed are defined as follows: 1. A collapsible ladder of the type comprising a plurality of subsections pivotally linked to each other, each subsection comprising:

a) a generally rectangular, rigid member which includes a pair of generally parallel, opposed inside straps, and a pair of rungs generally perpendicular to and integral with the inside straps and extending therebetween, one rung near each end of the rigid member;

b) a pair of generally parallel, opposed outside straps each having a first end portion partially overlapping an outside surface portion of the adjacent inside strap, both outside straps being pivotally secured to said rigid member near one end and at opposed sides thereof;

c) pivot means providing pivotal securement of said outside straps to said frame member, said pivot means comprising two pivot assemblies coaxial with each other about a transverse pivot axis and disposed at said one end of the rigid member;

d) each said pivot assembly comprising a support pin and a complementary bearing means, said support pin and said bearing means being each formed in one of the inside and outside straps, at the respective side of the rigid member, said pivot assembly further including axial displacement limiting means preventing displacement of the respective outside strap relative to the adjacent inside strap in both directions of said axis only to a minute axial spacing required for allowing pivotal movement between the straps about said axis;

e) lock means for generally rigidly but releasably connecting the outside straps to the rigid member to maintain the outside straps generally coextensive with the respective inside straps when the ladder is in an extended state, by preventing the outside straps to pivot relative to the respective inside straps about the pivot axis;

f) locking bolt means disposed at one end of spring means whose other end is fixedly secured to the surface of one of said straps, said spring means being adapted to resiliently urge the bolt means in a direction generally parallel with the direction of the pivot axis, to assume a locked position, wherein the locking bolt means interengages a first engagement portion which forms a part of said spring and a second engagement portion which forms a part of one of said outside straps such that the frame member is maintained in a generally rigid, coextensive position relative to the associated outside strap, said spring means allowing displacement of said bolt means to a released position, wherein the bolt is removed from at least one of said engagement portions to render said outside strap free to pivot relative to the frame member about the pivot axis.

2. A collapsible ladder as claimed in claim 1, wherein each support pin protrudes outwardly of the inner strap and wherein the respective bearing means is disposed in the respective outside strap.

3. A collapsible ladder as claimed in claim 2, wherein the support pin is fixedly secured to the rung at said one end of the rigid member.

4. A collapsible ladder as claimed in claim 2, wherein the spring means is arranged to urge the associated bolt means in an inward direction. 5. A collapsible ladder as claimed in claim 4, wherein said other end of the spring means is fixedly secured to an outer surface of the outside strap.

6. A collapsible ladder as claimed in claim 3, wherein each support pin has an outer surface portion complementary with the associated bearing means and being of a generally "D"-shaped contour in cross-section, said cross-section being formed by an outer surface sections and a second outer surface section, said first outer surface section having the shape of a segment of a cylindric surface, said second surface section being flat, generally parallel with the plane defined by the pivot axis and by the axes of elongation of the respective first member and extending peripherally between ends of the first outer surface section. 7. A collapsible ladder as claimed in claim 6, wherein each bearing means has the general shape of a distorted keyhole whose cross-section is elongated in the direction of elongation of the respective outside strap and includes a narrow end portion which comprises a generally flat side surface complementary with said second surface section for engagement therewith to maintain the respective inside and outside strap in a coextensive position, said bearing means further including a wide end portion communicating with said narrow end portion and generally complementary with said first outer surface section to allow relative pivotal movement between the support pin and the respective outside strap when the respective inside strap and the respective outside strap are at an angle relative to each other, and also to allow the sliding of the support pin relative to the bearing means to bring the second surface section into facing relationship with the flat side surface when the respective inside strap and outside strap are aligned.

8. A collapsible ladder as claimed in claim 7, wherein said bolt means is of a generally D-shaped cross-section complementary with the space defined by said wide portion of the keyhole shaped bearing and by said support pin when the latter engages said narrow portion, whereby the bolt, when in a locked position, engages the support pin and the bearing to prevent the return of the support pin from the narrow portion of the bearing means back into the said end portion.

9. A collapsible ladder as claimed in claim 8, wherein the spring means is arranged to urge the bolt means in an inward direction.

10. A collapsible ladder as claimed in claim 9, wherein the other end of the spring is secured to the outer face of the outside strap.

11. A collapsible ladder as claimed in claim 7, wherein the narrow part of the keyhole is directed along the elongation of the outside strap and toward the adjacent end of the respective outside strap.

12. A collapsible ladder as claimed in claim 2, wherein the locking bolt means is a pin operatively associated to become engaged in a pair of complementary coaxial bores, one of said bores being located in each of the inside and outside straps disposed in a face-to-face arrangement, the bores being so arranged in the inside straps and in the outside straps that they become aligned when the respective inside strap and the respective outside strap assume a coextensive position corresponding to the extended state of the ladder, and non-aligned when the respective inside strap and the respective outside strap assume a non-coextensive position corresponding to a state in which the ladder is at least partly collapsed.

13. A collapsible ladder as claimed in claim 12, wherein the spring means is arranged to urge the bolt means in an inward direction.

14. A collapsible ladder as claimed in claim 13, wherein the other end of the spring is secured to the outer face of the outside strap. 15. A collapsible ladder as claimed in claim 1, wherein the spring means is arranged to urge the associated bolt means in an inward direction.

16. A collapsible ladder as claimed in claim 15, wherein said outer end of the spring means is fixedly secured to an outer surface of the outside strap.

17. A collapsible ladder as claimed in claim 16, wherein each support pin protrudes outwardly of the inner strap and wherein the respective bearing means is disposed in the respective outside strap.

18. A collapsible ladder as claimed in claim 15, wherein each support pin has an outer surface portion complementary with the associated bearing means and being of a generally "D"-shaped contour in cross-section, said cross section being formed by an outer surface sections and a second outer surface section, said first outer surface section having the shape of a segment of a cylindric surface, with second surface section being flat, generally parallel with the plane defined by the pivot axis and by the axes of elongation of the respective first member and extending peripherally between ends of the first outer surface section.

20. A collapsible ladder as claimed in claim 19, wherein each bearing means has the general shape of a distorted keyhole whose cross-section is elongated in the direction of elongation of the respective outside strap and includes a narrow end portion which comprises a generally flat side surface complementary with said second surface for engagement therewith to maintain the respective inside and outside strap in a coextensive position, said bearing means further including a wide end portion communicating with said narrow end portion and generally complementary with said first outer surface section to allow relative pivotal movement between the support pin and the respective outside strap when the respective inside strap and the respective outside strap are at an angle relative to each other, and also to allow the sliding of the support pin relative to the bearing means to bring the second surface section into facing relationship with the flat side surface when the respective inside strap and outside strap are aligned. 21. A collapsible ladder as claimed in claim 19, wherein said bolt means is of a cross-section complementary with the space defined by said wide portion of the keyhole shaped bearing means and by said support pin when the latter engages said narrow portion, whereby the bolt, when in a locked position, engages the support pin and the bearing means to prevent the return of the support pin from the narrow portion of the bearing means back into the wide end portion. 22. A collapsible ladder as claimed in claim 19, wherein the narrow part of the keyhole is directed along the elongation of the outside strap and toward the adjacent end of the respective outside strap. 23. A collapsible ladder as claimed in claim 19, wherein each support pin protrudes outwardly of the inner strap and wherein the respective bearing means is disposed in the respective outside strap. 24. A collapsible ladder as claimed in claim 15, wherein the spring means is arranged to urge the bolt means in an inward direction.

25. A collapsible ladder as claimed in claim 24, wherein each support pin protrudes outwardly of the inner strap and wherein the respective bearing means is disposed in the respective outside strap.

26. A collapsible ladder as claimed in claim 24, wherein the spring means is arranged to urge the bolt means in an inward direction. 27. A collapsible ladder as claimed in claim 26, wherein the outer end of the spring is secured to the outer face of the outside strap.

28. A collapsible ladder as claimed in claim 1, wherein each support pin has an outer surface portion complementary with the associated bearing means and being of a generally "D"-shaped contour in cross-section, said cross-section being formed by an outer surface sections and a second outer surface section, said first outer surface section having the shape of a segment of a cylindric surface, said second surface section being flat, generally parallel with the plane defined by the pivot axis and by the axes of elongation of the respective first member and extending peripherally between ends of the first outer surface section.

29. A collapsible ladder as claimed in claim 28, wherein each bearing means had the general shape of a distorted keyhole whose cross-section is elongated in the direction of elongation of the respective outside strap and includes a narrow end portion which comprises a generally flat side surface complementary with said section surface section for engagement therewith to maintain the respective inside and outside strap in a coextensive position, said bearing means further including a wide end portion communicating with said narrow end portion and generally complementary with said first outer surface section to allow relative pivotal movement between the support pin and the respective outside strap when the respective inside strap and the respective outside strap are at an angle relative to each other, and also to allow the sliding of the support pin relative to the bearing means to bring the second surface section into facing relationship with the flat side surface when the respective inside strap and outside strap are aligned.

30. A collapsible ladder as claimed in claim 28, wherein said bolt means is of a generally D-shaped cross-section complementary with the space defined by said wide portion of the keyhole shaped bearing and by said support pin when the latter engages said narrow portion, whereby the bolt, when in a locked position, engages the support pin and the bearing to prevent the return of the support pin from the narrow portion of the bearing means back into the wide end portion.

31. A collapsible ladder as claimed in claim 28, wherein the spring means is arranged to urge the bolt means in an inward direction. 32. A collapsible ladder as claimed in claim 31, wherein the other end of the spring is secured to the outer face of the outside strap.

33. A collapsible ladder as claimed in claim 28, wherein the narrow part of the keyhole is directed along the elongation of the outside strap and toward the adjacent end of the respective outside strap.

34. A collapsible ladder as claimed in claim 28, wherein each support pin protrudes outwardly of the inner strap and wherein the respective bearing means is disposed in the respective outside strap.

35. A collapsible ladder as claimed in claim 28, wherein the support pin is a support pin fixedly secured to the rung at said one end of the rigid member. 36. A collapsible ladder as claimed in claim 1, wherein the locking bolt means is a pin operatively associated to become engaged in a pair of complementary coaxial bores, one said bores in each of the inside and outside straps disposed in a face-to-face arrangement, the bores being so disposed in the inside straps and in the outside straps that they become aligned when the respective inside strap and the respective outside trap assume a coextensive position corresponding to the extended state of the ladder, and non-aligned when the respective insider strap and respective outside strap assume a non-coextensive position corresponding to a state in which the ladder is at least partly collapsed.

37. A collapsible ladder as claimed in claim 36, wherein each support pin protrudes outwardly of the inner strap and wherein the respective bearing means is disposed in the respective outside strap.

38. A collapsible ladder s claimed in claim 37, wherein the spring means is arranged to urge the associated bolt means in an inward direction.

39. A collapsible ladder as claimed in claim 38, wherein said other end of the spring means is fixedly secured to an outer surface of the outside strap.

40. A collapsible ladder as claimed in claim 39, wherein two pairs of said coaxial bores are provided near each support pin, each pair of coaxial bores being associated with one locking bolt means.

41. A collapsible ladder as claimed in claim 1, wherein: a) each pair of adjacent support pins located next to each other either longitudinally or transversely of the respective rigid member are symmetrical with each other with respect to a point os symmetry which is on a reference axis extending between the respective two support pins and is equidistantly spaced from the respective two support pins; and

b) the outside straps are each provided with a pair of said bearing means, one at each end of the respective outside strap, the shape of one bearing means of each pair of said bearing means being symmetrical with the shape of the other bearing means of the same outside strap with respect to a point of symmetry which is on a reference axis extending between the bearing means of the respective outside strap and is equidistantly spaced from the respective two bearing means .

42. A collapsible ladder as claimed in claim 41, wherein each support pin has an outer surface portion complementary with the associated bearing means and being of a generally "D"-shaped contour in cross-section, said cross-section being formed by an outer surface sections and a second outer surface section, said first outer surface section having the shape of a segment of a cylindric surface, said second surface section being flat, generally parallel with the plane defined by the pivot axis and by the axes of elongation of the respective first member and extending peripherally between ends of the first outer surface section.

43. A collapsible ladder as claimed in claim 42, wherein each bearing means has the general shape of a distorted keyhole whose cross-section is elongated in the direction of elongation of the respective outside strap and includes a narrow end portion which comprises a generally flat side surface complementary with said second surface section for engagement therewith to maintain the respective inside and outside strap in a co-extensive position, said bearing means further including a wide end portion communicating with said narrow end portion and generally complementary with said first outer surface section to allow relative pivotal movement between the support pin and the respective outside strap when the respective inside strap and the respective outside strap are at an angle relative to each other, and also to allow the sliding of the support pin relative to the bearing means to bring the second surface section into facing relationship with the flat side surface when the respective insider strap and outside strap are aligned.

44. A collapsible ladder as claimed in claim 43, wherein said bolt means is of a generally D-shaped cross-section complementary with the space defined by said wide portion of the keyhole shaped bearing and by said support pin when the latter engages said narrow portion, whereby the bolt, when in a locked position, engages the support pin and the bearing to prevent the return of the support pin from the narrow portion of the bearing means back into the said end portion.