GB2305955A

GB2305955A - An extension ladder with telescopic legs

Info

Publication number: GB2305955A
Application number: GB9520075A
Authority: GB
Inventors: Gary Kuo
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-10-02
Filing date: 1995-10-02
Publication date: 1997-04-23
Anticipated expiration: 2015-10-02
Also published as: GB2305955B; GB9520075D0; GB2305956B; GB9620518D0; GB2305956A

Abstract

An extension ladder includes a pair of telescopic legs 1A, 1B consisting of a plurality pairs of straight round tube members, a plurality of rungs in the same numbers as that of the tube member pairs and each having two lateral ends for the two legs to pass therethrough, a plurality sets of locking mechanisms 80 and lock ensuring mechanisms, and a general switch assembly 90 disposed beneath the lowest rung. The tube members each have a sleeve member 60 and a plug assembly 30A disposed around a top end and in a bottom end, respectively. Each plug assembly includes a set of cover members 33A, 33B and a set of inserts 31A, 31B having an upper and a lower movable hook members 32A, 32B connected thereto. The upper hook member in a lower tube member separately links with the lower hook member in a higher tube member, limiting the tube members of the telescopic legs to be sequentially extended stair by stair. The upper hook member each has an air cock provided at a bottom end thereof to close or open an air port formed on a base of the inserts. The air cocks and the air ports, together with two one-way valves separately disposed at a lower portion of the lowest tube member pairs, enable the ladder to be quickly extended from a collapsed state and be collapsed from an extended state at moderate speed. The locking mechanisms and lock ensuring mechansims are disposed beneath each rung to effectively prevent the ladder from unexpectedly collapsed due to any accidental touch of the locking mechanisms. The general switch assembly serves to actuate and control the sequential and moderate collapse of the rungs.

Description

An Extension Ladder with TelescoPic Legs The Present invention relates to an extension ladder with telescopic legs, and more particularly to an extension ladder which can be quickly extended for use or be telescoped in a slow and sequential manner for safe storage when it is not in use. Moreover, the extension ladder of the present invention can be easily assembled and is Provided with automatic locking and lock ensuring means to further ensure the safety in use.

Fig. I shows a conventional extension ladder with multiple rungs. The ladder includes a Pair of legs 800, 900 each consisting of multiple tapered telescopic sections (in the drawing, four sections of each leg are shown, separately indicated by reference numerals 100A 200A 300A, 400A and 100B 200B, 300B 4008, from bottom to top. Please see Fig.

1A for the structure of these legs), aultiple rungs (in the drawing, three rungs are shown separately indicated by reference numerals 500, 600. 700, from bottom to toP) each having a reverse U-shaped cross section, and multiple sets of locking mechanism for firmly locking the telescopic sections of the legs in Place after they are fully extended.

The numbers of the rungs are equal to that of the telescopic sections of the legs.

The telescopic sections are provided at their upper and middle portions with locating holes (in the drawing, locating hole 111 on the upper portion of the telescopic section 100A, locating holes 211. 221 respectively on the upper and the middle portions of the telescopic section 200A and locating hole 321 on the middle section of the telescopic section 300A are shown). The rungs 500. 600, 700 each has two ring-like ends 500A and 5008. 600A and 600B, 700A and 7008, respectively, for tightly engaging with a pair of reduced parts 112. 212, or 312 at the upper portions of their corresponding telescopic sections 100A, 100B; 200A, 2008; or 300A, 3008.In addition, the ring-like ends 500A, 5008; 600A, 6008; 700A, 700B of the rungs 500. 600. 700.

resepctively, are provided with a hole 501A, 601A, 701 A, respectively, corresponding to the locating holes 111. 211, etc. on the upper portion of the telescopic sections.

Retaining rods 512, 612, etc. respectively having a Push ari 514. 614. etc. and a return spring 513. 613. etc. are disposed near the ring-like ends 500A, 500B; 600A, 600B; 700A, 700B of the rungs 500. 600. 700. respectively, to form the locking mechanisms.Moreover. wear-proof plug members 222. 322. 422 with vents 222A 322A, 422A are seParately plugged into a bottom end of the telescopic sections 100A, 200A, 300A, 400A, 100B, 200B, 300B, 400B Following disadvantages are found in the above-described conventional extension ladder: 1. To assemble the extension ladder, each tapered telescopic section thereof must be engaged into a lower section by inserting its top end into a lower end of the lower section one by one, and the rungs must be riveted to each top of the telescopic sections.Such assembling procedures can not be easily performed and the sections are not easily aligned with one another when they are assembled according to the above procedures.

2. When the ladder is extend for use, the higher rungs are always separated from each other and locked in place before the lower rungs are completely Pulled out to their true positions. Therefore. the telescopic sections of the conventional extension ladder can not be sequentially extended and located in Place one by one from bottom to top.

Also, the ladder might unexpectedly and dangerously collaPse due to operational mistake by an operator.

3. To collapse the extension ladder. the Push arms of the locking mechanisms of the lowest rung is Pulled to disengage the retaining rod thereof from the corresponding locating hole, allowing the next higher telescopic section to descend until the locking mechanisms of the next higher rung touches the lower rung below it. The contact of the higher rung with the lower rung shall release the higher telescopic sections from the locking mechanisms of the higher rung.

With the same ooveaent the remaining telescopic sections and rungs collaPse and descend one by one. However, there is not any buffer provided between every two of the telescopic sections, the rungs descend at a speed high enough to unexpectedly and dangerously injure a user's fingers. A buffer member disposed at the bottom end of each telescopic sections might effectively slow the descending of the rungs, the extension of the telescopic sections will, however, become slow, too.

4. The locking mechanisms can be easily unlocked by pulling the Push arm. In the case the push arm is unexPectedlY pulled by someone. especially a child, accident might very possibly occur.

5. The telescopic sections of the extension ladder fri ct i ona y contact with one another when they are extended or collaPsed and are therefore worn out and become looselY connected after being used for a long period of tine.

Therefore, it is necessary to provide an improved extension ladder to eliminate the above shortcomings.

It is a primary object of the Present invention to provide an extension ladder having a safer and more effective structure. The extension ladder of the present invention includes two legs which each consists of a plurality of sequentially telescoped sections. These telescopic sections each is a straight round tube having uniform diameter which facilitates the assembly of the telescopic sections and the rungs.

Another object of the present invention is to Provide an extension ladder in which a plug assembly is mounted to a bottom end of each telescopic section thereof. The plug assembly each consists of an upper and a lower movable hook members. When the extension ladder is in a fully extended state, the upper hook members may seParatelY extend their pointed end into retaining holes formed on the telescopic sections. The upper hook member each has a bevelled projection formed at an upper corner thereof on which a lower hook member of the plug assembly in the next higher telescopic section hooks when the extension ladder is in a fully collaPsed state. When the ladder is extended and the upper hook members separately extends into the retaining holes on the telescopic sections, the lower hook members of the upper plug assemblies disengage from the bevelled projection of the upper hook member, allowing the telescopic sections of the ladder to be Pulled upward one br one from bottom to top.

It is a further object of the present invention to Provide the above-aentioned extension ladder in which the plug assembly each is provided a-t a bottom portion of the upper hook member with an air cock, and at a base thereof with an air port corresponding to the air cock, and the lowest telescopic sections and the lowest rung are provided with one-way valves and cooperating air tube, respectively, whereby when the extension ladder is extended and the upper hook members pivot to extend through the retaining holes on the telescopic sections with their bottom portions lifted, the air ports are oPened to together with the one-way valves and the cooperating air tube allow the air in the telescopic sections to quickly flow through the legs and thereby permits the ladder to be quickly extended.Or, when the ladder is collaPsed and the upper hook members pivot to disengage from the retaining holes on the telescopic sections with their bottom portions and accordingly the air cocks descended, the air ports are closed to slow the air flow in the telescopic sections and the collapse sPeed thereof.

It is still a further object of the Present invention to Provide an extension ladder in which locking mechanisms are separataely mounted below the rungs centered on each end thereof and a lock ensuring mechanism is disposed behind each locking mechanism, whereby when the lock ensuring mechanisms are in a locked state, the corresponding locking mechanisms would not be unexpectedly and dangerouslY unlocked by pulling an Pushing arm thereof.When the top surface of a lower rung touches the lock ensuring mechanisms of the next higher rung, the lock ensuring mechanisms of the next higher rung are unlocked to free their associated locking mechanisms, permitting the rung and the telescopic sections & the next higher stair of the ladder to collaPse.

Therefore, the rungs and the telescopic sections of other higher stairs of the ladder can be sequentially and safely collapsed from bottom to top in the same manner as described above.

It is still a further object of the present invention to provide an extension ladder in which a wear-proof ring member is fitted around a bottom outer periPhery of each telescopic section to Prevent the same from directly frictionally contacting with each other at their wall surfaces lest the telescoPic sect ions should become wearing and loosely contacting with each other which is dangerous to any user.

The detailed structure and functions of the present invention can be best understood through the following detailed description of the Preferred embodiments and the accompanying drawings, wherein: Fig. 1 is a fragmentary perspective view of a conventional extension ladder, a part of which is cut away to show the manner in which the rungs are connected to the legs of the ladder Fig. 1A is a fragmentary, exploded perspective view of Fig.

1 showing the structure of the leg of the ladder; Fig. 2 is a fragmentary, exploded perspective view of an extension ladder according to the Present invention a part of which is cut away to show the assembling of a rung to two vertically telescoPed sections of the legs; Fig. 2A is a fragmentary, sectional view taken on line A-A of Fig. 2; Fig. 3 is an exPloded perspective view of a plug assembly disposed at a bottom end of each telescopic section of the present invention; Fig. 3A is a fragmentary, asseabled, sectional view showing the positions of an upper and a lower hook members of the plug assembly in a telescopic section of the Present invention;; Fig. 4A is a fragmentary, sectional view showing the manner in which the telescopic sections of the legs of the Present invention is sequentially extended from a collaPsed state; Fig. 4B is a fragmentary, sectional view showing the manner in which the telescopic sections of the legs of the present invention is gradually collaPsed from an extended state; Fig. 5A is a fragmentary, exploded PersPective view of a locking mechanism and an associated lock ensuring mechanism disposed in a rung other than the first or the lowest rung of the Present invention; Fig. 5B is a fragientrau, exPloded Perspective view of a general switch assembly and the associated locking mechanism disposed in the first or the lowest rung of the present invention;; Figs. 6A to 6E are fragmentary, sectional views showing the operation of the mechanisms at different stairs of the ladder of the present invention to collaPse the same; and Fig. 7 schematically shows another embodiment of the Present invention Please refer to Fig. 2.The extension ladder of the present invention mainly includes a pair of legs 1A, 1B each consisting of a PluralitY of. telescopic section pairs, a plurality of rungs each having a peripheral wall which gives the rung a substantially reverse U-shaped cross section, a Plurality of locking mechanisms 80 for locking the telescopic section pairs of the legs 1A, 1B in Place, and a plurality of lock ensuring mechanisms 82 cooperating with the locking mechanisms 80. The numbers of the rungs are equal to that of the telescopic section Pairs. In the first embodiment of the present invention, reference numerals 50A and 50 are designated to the first (the lowest) and the other higher rungs, respectively.The telescopic sections of the legs 1A, 1B each is formed from a straight round tube member having uniform diameter through its length. A lower telescopic section or tube member always has an inner diameter larger than an outer diameter of a telescopic section or tube member immediately above it, such t-hat every higher telescopic section can allays be fitly received in a telescopic section immediately below it. In this embodiment, the telescopic sections of the legs 1A, 1B are sequentially named the first tube member 10, the second tube member 20, the third tube member 30 from bottom to top.

A sleeve member 60 and a plug assembly 30A are disposed on. a top end and a bottom end of each telescopic section, respectively. Figs. 2 and 2A illustrate the manner in which the next lower or the second rung 50 (that is, the rung immediately above the first rung 50A) and the next lower telescopic section pairs (that is, the second tube members 20) are assembled and locked in place. Since every other stairs of the ladder of the present invention and n d either lateral side thereof are assembled in the same manner, they are not rePeatedlY explained herein. Of course, in the actual assembling, it must begin with the lowest (that is, the first) Pair of telescopic sections and the lowest rung from either side thereof.

Please refer to Figs. 2 and 2A. To form the second stair of the extension ladder, first insert the third tube members 30 downward to pass through the sleeve members 60 fitted over the top end of the second tube members 20. The third tube members 30 each is Prenounted at its lower end with a plug assembly 30A. The manner in which the plug assembly 30A is assembled and functions will be described in more details later in this specification. Then, the second rung 50 is pUt over and engaged with a top end of the third tube members 30. Allow the third tube members 30 and the plug assemblies 30A at their bottom ends to extend into the second tube members 20.The sleeve member 60 each is formed at a top end with an outward extended upper flange on which an upper and a lower locating bosses 61A, 61B are provided to respectively engage with a first locating recess 55 formed on the second rung 50 and a second locating recess 21 formed near an upper edge of the second tube member 20, causing a first locating hole 62 to aligne with a locating hole 54 formed at one end of the second rung 50. The second tube members 20 are firmly engaged into two ends of the second rung 50 with the sleeve members 60 between them without the risk of turning relative to the the second rung 50.An elongated slot 51 is formed at either end of the second rung 50 and a pair of threaded holes 52 are oppositely formed on the peripheral wall of the second rung 50 at two sides of each slot 51, whereby fastening screws 53 can be passed through the threaded holes 52 to narrow the slots 51 and therebY tightly bind the second rung 50 with the seconds tube members 20.

The locking mechanisms 80 are disposed near the ends of the second rung 50 on a central axis thereof and the lock ensuring mechanisms 82 are seParately disposed just behind the locking mechanisms 80. For the first (lowest) stair of the ladder, a general switch assembly 90 can be additionally provided beneath the first rung 50A to cooperate with the lock ensuring mechanism to control the extension or collapse of the entire ladder.

Fig. 3 illustrates in details the manner in which the plug assembly 30A and the third tube member 30 are assembled.

The plug assembly 30A each consists of a front cover 33A a rear cover 338. a front insert 31A, and a rear insert 31B Both the front insert 31A and the rear insert 318 are formed at their lower outer wall with a lower flange. A first pin 32I is used to pass an inner side of the front insert 31A while threading through a first return spring 32H and PivotallY connecting an upward extended and movable upper hook member 32A to the front insert 31A. The upper hook member 32A is provided on its one side surface at an upper corner thereof with a bevelled projection 32G.A roller 32F is disposed in a hollow space formed at a front portion of the upper hook member 32A with a somewhat flexible cover plate 32J fittied in front of the roller 32F, such that when the upper hook member 32A extends into a locating hole 33C formed on the front cover 33A, the locating hole 33C is airtightly sealed. An air cock 32E is provided at a bottom end of the upper hook member 32A. An air port 31D is formed on a base of the front insert 31A corresponding to the air cock 32E, wherebY the air port 31D is oPened or closed when the upper hook member 32A is pivotally turned about the first pin 32I to lift or lower the bottom end, respectively.

A ring ear 31E is connected to an underside of the front insert 31A. A downward extended and movable lower hook member 32B is pivotally connected to the ring ear 31E bu means of a second pin 32D which is threaded through a scond reuturn spring 32C and an upper portion of the lower hook member 32B and the ring ear 31E. The lower hook member 32B is eccentrically Positioned below the front insert 31A and is vertically aligned with the bevelled projection 32G of the upper hook member 32A.A projected block 31C is formed near a front portion of the front insert 31A such that when the front insert 31A is placed into the third tube member 30 and fitted into a locating hole 30B thereof, the front cover 33A is engaged at its bottom flange 33H with the bottom end of the third tube member 30 with a locating projection 33E thereof extending into the locating hole 30B of the third tube member 30 and vertically abutting against the projected block 31C of the front - insert 31A just beneath it, Permitting a front half of the plug assembly 30A to be firmly fixed to the third tube member 30.

Then, the rear insert 318 having an air outlet 31F is inserted into the third tube member 30, permitting an opening 33F on the rear insert 318 to communicate with an opening 30C on the third tube member 30. Then, the rear cover 33B is fitted around the third tube member 30 with a bottom flange 33G of the rear cover 33B engaging into a clearance between the bottom end of the third tube member 30 and the rear insert 31B At this point, a locating projection 33D on an inner wall of the rear cover 33B extends through the opening 33F of the rear insert 318 and engages into the opening 30C, permitting the rear half of the plug assembly 30A to be firmly fixed to the third tube member 30 As shown in Fig. 4A, when the assembled ladder is to be extended from a collapsed state, the lower hook members 32B of the plug assemblies 30A in the next higher tube members 20 that are connecting with the upper hook members 32A of the plug assemblies 30A in the lowest tube members 10 by means of the bevelled projections 32G shall become disengaged from the upper hook members 32A after one-war valves 70 disposed near bottom ends of the first tube members 10 are oPened due to a Pressure differential resulted from the extending operation. The open of the valves 70 permits air to be suPPlied to the first tube menbers 10.Due to the air flowing in the tube members forming the legs of the ladder, the telescopic sections of the legs can be quickly extended. When the upper hook members 32A of the plug assemblies 30A in the second tube members 20 are moved to a point in alignment with locating holes 10A formed on the first tube members 10, the upper hook members 32A are sprung toward the locating holes 10A under the spring force of the first return springs 32H and thereby open the air ports 31D.At this point the air further flows upward. meanwhile, the lower hook members 32B of the immediately higher telescopic sections 30 are released from the bevelled projections 32G of the upper hook members of the second tube members 20 because they move upward along with their upper hook members 32A. At this point, the next higher rung, that is, the second rung 50 can be Pulled upward. After the upper hook members of the next higher tube members 30 are moved uPward and engaged into the corresponding locating holes formed on the lower tube members 20, the next higher rung, that is. the third rung 50. can be further pulled upward. In the same manner, other higher telescopic sections and rungs can be sequentially extended.

Reversely. when the ladder is downward Pushed to collapse it, the rollers 32F in front of the upper hook members 32A in the second tube members 20 rotate when theu are pressed against an inner wall of the tube members 20, allowing the upper hook members 32A to disengage from the locating holes 10A of the tube members 10. A compression effect similar to the function of a piston will be created on each of the Plug assemblies at the bottom of the tube members, forcing the one-wau valves 70 to close.Air in the tube members is compressed and can onlu escape from the relatively small air outlet 31F on the rear insert, permitting the ladder to be collapsed slowlY When the tube members 20 are collapsed to such an extent that the lower hook members 32B thereof engage with the bevelled projections 32G of the upper hook members 32A of the plug assemblies 30A in the first tube members 10 due to the spring force of the return springs 32H and the bevel surfaces of the bevelled projections 326, the higher tube members 20 are therefore connected to the lower tube members 10.In the same manner, the other higher tube members are sequentially collaPsed and connected to their respective immediate lower tube members, permitting the completely collaPsed ladder to be carried by just holding its highest rung without the risk of unexpectedly extending again. Moreover, with the locking mechanisms, the lock ensuring mechan i sms, and the general switch assembly which will be described in more details later, the ladder can be automatically and sequentially collapsed stair by stair from bottom to top.

Fig. 5A shows the locking mechanism and the lock ensuring mechanism of the present invention, wherein the locking mechanism-80 each consists of a sleeve member 80A, a locking Pin 81, a touch arm 81A, a push block 818, a first pin member 81C, and a spring 81D. The sleeve member 80A is fitted into the locating hole 54 formed at each end of the second rung 50. One end of the locking pin 81 is inserted into the sleeve member 80A while the other end of the locking Pin 81 is threaded through the spring 81D to extend into a substantially reverse U-shaped stopper 57 disposed in the second rung 50.Below the locking pin 81, the first Pin member 81C is used to connect the touch arm 81A to the Push block 81B and fixedly locate them in the rung 50. such that when the touch arm 81A is touched, the locking pin 81 can be moved backward so as to unlock the locking mechanism 80.

The lock ensuring mechanism 82 each is comPosed of a locking block 83, an unlocking touch ari 83D, a first stop screw 56, and a third return spring 83. The lock ensuring block 83 is formed at each side with a straight slide channel 83B to receive a slide block 58 in the second rung 50, whereby the lock ensuring block 83 can be vertically slided relative to the second rung 50. The lock ensuring block 83 has an indicating block 83C Projecting from a top thereof, whereby when the lock ensuring block 83 is moved upward to a top Position of the second rung 50, the indicating block 83C protrudes beyond a warning opening 59 formed on a toP surface of the second rung 50 to indicate the actual operating state of the lock ensuring mechanism 82.

The lock ensuring block 83 has a pin hole 83A Provided at a front end thereof to receive one end of the locking pin 81 extending through the reverse U-shaped stopper 57. The third return spring 83E and the unlocking touch arm 83D are disposed on a side of the locking block 83 near a rear end thereof. The unlocking touch arm 83D is formed at an outer side facing the peripheral wall of the second rung 50 with a recess.A notch 83F is Provided on a Peripheral" wall surrounding the recess of the unlocking touch arm 83D, such that when the lock ensuring block 83 is placed into the second rung 50 and the first stop screw 56 is screwed through the PeriPheral wall of the second rung 50, a lower side of the screw 56 abuts against the peripheral wall of the recess of the unlocking touch arm 83D to limit the lock ensuring block 83 to travel only within a distance defined by the unlocking touch arm 83D.

Fig. 5B shows the general switch assembly 90 and the locking mechanism 80 mounted in the first (the lowest) rung 50A of the present invention Since these components are somewhat different from those mounted in other rungs 50, they are separately described herein. The general switch assembly 90 includes a shifting wheel 90K having a shifting arm 90B, two lock ensuring Push plates 90J seParately disposed at two ends of the first rung 50A, two resilient engaging pieces 90C, and two pull rods 90A each being connected at one end to the shifting wheel 90K and at the other end to a locking pin 81 of the locking mechanism 80 by extending through a central pin hole 90I formed on each lock ensuring Push plate 90J.

To assemble the general switch assembly 90 and the locking mechanisms 80 to the first rung 50A, the shifting wheel 90K is fixed to a bottom central Portion of the first rung 50A by means of extending a second Pin member 909 through the peripheral wall of the first rung 50A and through the shifting wheel 90K. The lock ensuring push Plates 90J each is disposed in the first rung 50A with two lateral edges thereof seParatelY received in slide channels 90N formed on the PeriPheral wall of the first rung 50A near each end thereof, allowing the lock ensuring Push plates 90J to vertically move in the first rung 50A up and down.Second stop screws 900 are separately screwed into the peripheral wall of the first rung 50A to extend into dents 90P separately formed on a side edge of the two ensuring Push plates 90J, so as to limit the lock ensuring Push Plates 90J to vertically move within a distance defined by the dents 90P. The lock ensuring push plates 90J each has a top indicating block 83C which protrudes beyond the warning opening 59 formed on the first rung 50A. The lock ensuring push plate 90J each is formed with a central pin hole 90I and a projected block 90D.The central pin hole 90I each has a vertical length larger than a diameter of the locking pin 81, wherebY the locking pin 81 can be moved into or out of the central pin hole 901. The resilient engaging Pieces 90C each is engaged with the Projected block 90D while its two lower ends are received in slide channels 90Q formed in the first rung 50A.

With the above arrangements, the extension ladder of the Present invention can be- automatically and sequentially collapsed in a manner as shown in Figs. 6A, 68, 6C, 6D and 6E where in the plug assemblies 30A are only schematically illustrated.

Fig. 6A illustrates the extension ladder in an extended and locked state. At this point, the lock ensuring blocks 83 of the rungs 50 and the lock ensuring push plates 90J of the rung 50A serve to disengage the locking pins 81 from the pin holes 83A and the central pin holes 901, resPectivelY allowing the rungs 50 and 50A to be in a firmly locked position relative to the higher tube members and the first tube members, respectively. Meanwhile, the resilient engaging pieces 90C separately engage with the projected blocks 90D of the lock ensuring Push plates 90J.

Fig. 6B ilAustrataes the first steP to collars the extension ladder. First, the lock ensuring Push plates 90J are pushed uPward to disengage the resilient engaging pieces 90C from the projected blocks 90D of the lock ensuring Push plates 90J. At this point, the indicating blocks 83C on the top of the lock ensuring push Plates 90J moves upward to Protrude beyond the warning openings 59, indicating the rungs and the telescopic tube members of legs are released from the safelY locked position. The indicating blocks 83C can be painted in red color or with any other warning mark. Before using the extension ladder, first check for any indicating blocks 83C protruded beyond the rungs. Any Protruded indicating block 83C means the ladder is not in a locked position for safe use.At this point, the locking Pins 81 each abuts against a lower edge of the central pin holes 901, leaving a gap between the locking pin 81 and an upper edge of the central pin hole 901. The locking pins 81 at this position are allowed to extend into the central pin holes 90I to interact with the Pull rods 90A and the shifting wheel 90K.

That is, as shown in Fig. 6C, when the shifting arm 90B of the shifting wheel 90K is shifted to Pull the pull rods 90A, causing the locking pins 81 to move toward the center of the rung 50A until the locking Pins 81 abut against the resilient engaging Pieces 90C. At this point, the resilient engaging Pieces 90C are Pushed by the locking pins 81 to disengage from the projected blocks 90D of the lock ensuring push plates 90J Meanwhile, the gaps between the locking pins 81 and the upper edge of the central Pin holes 91I cause the lock ensuring Push Plates 90J to move downward which further causes the resilient engaging Pieces 90C to seParate from the projected blocks 90D.Therefore, when the locking pins 81 are further inserted into the lock ensuring Push'plates 90J and be locked thereto, the lock ensuring push Plates 90J will not be stopped by the resilient engaging ~Pieces 90C and are therefore, permitted to slide downward to a firmly and safelY locked Position. When the shifting wheel 90K is shifted to an upper Position, a retaining leaf spring 90H on the shifting wheel 90K is stoPPed by a retaining projection 90U formed on a bottom center of the first rung 50A and the shifting wheel 90K is Prevented from further rotating and is restricted to a fully unlocked position. This avoids undesired Partial collaPse of the extension ladder due to instantaneous extension by manual operation.

When the immediate higher rung 50 descends and the unlocking touch arms 83D touch the first (lowest) rung 50A, as shown in Fig. 6D, the lock ensuring blocks 83 are forced to move upward. At the same time, the indicating blocks 83C Protrude beYond the warning openings 59. When the lock ensuring blocks 83 move to an upmost position, the pin holes 83A are aligned with the locking pins 81, allowing the locking pins 81 to move into the Pin holes 83A. Meanwhile, the notches 83F of the unlocking touch arms 83D are aligned with the first stoP screws 56. At this point, a downward projected Pin 90F disposed on a central Position of the upper rung 50 aPProaches to and is aligned with a hole 90E formed on the first (lowest) rung 50A.

As shown in Fig. 6, before the upper rung 50 is collaPsed to its lowest Position, the unlocking touch arms 83D and the touch arms 81A connected to the locking pins 81 are forced to pivotally turn sideward away from the lock ensuring blocks 83 and the locking pins 81 are further moved into the Pin holes 83A, leaving the telescopic tube members in an unlocked state and permitting the upper rung 50 to descend.

At this Point, the projected pin 90F fully engages into the hole 90E to press the retaining leaf spring 90H of the shifting wheel 90K and disengage the same from the retaining projection 90U. The shifting wheel 90K shall restore to its home position due to the springs 81D at two ends of the rung. At this point, the locking pins 81 are in a position with their outer ends closing to the tube members for readily springing into the tube members and lock the same and the rung together.

Fig. 7 illustrates another embodiment of the present invention wherein a hollow fixing beam 95 is connected between the two one-way valves 70 oppositely disposed near the bottom of the first tubes 10 below the first rung 50.

An air tube 95A is disposed in the fixing beam 95 to communicate with the two one-wau valves 70. In addition, a quick breather adapter 95B is provided at a center of the fixing beam 95 to communicate the air tube 95A with the atmosphere. As mentioned previously, the upper hook member 32A may extend into the locating hole 33C on the front cover 33A and airtightly seals the locating hole 33C, whereby a compressed air source can be connected to the quick breather adapter 95B for the air pressure in the ladder to automatically and quickly increase.

In conclusion, the extension ladder of the present invention can be sequentially extended for safe use. In addition, the extension of the ladder can be Performed quickly while the collaPse thereof can be achieved in a slower manner.

Moreover, the extension ladder of the present invention can be easily manufactured. Therefore, the shortcomings and possible dangers in using the conventional extension ladders as mentioned in the beginning of the specification can be effectively eliminated.

What to be noted is the above embodiments are onlu used for illustrati~ng the present invention not intended to limit the scoPe thereof. ManY modifications of the embodiments can be made without departing from the spirit of the Present invention

Claims

CLANS 1. An extension ladder comprising a pair of legs, a plurality of rungs each having two lateral ends for said legs to pass therethrough, and a Plurality of locking mechanisms disposed at a bottom side of said rungs, being characterized in that each of said legs consists of a plurality of telescoped straight circular tube members and a sleeve member is disposed in an upper end of each said tube member near where said tube member connecting with one of said rungs, and that said sleeve member each has a top flange which just rests on said upper end of each said tube member when said sleeve member is mounted in said tube member to support and fitly engage with one of said rungs.
2. An extension ladder as claimed in claim 1, wherein said sleeve member each is formed at said top flange with an upper locating boss and said rungs each is formed at a Position corresponding to said upper locating boss with a locating recess, permitting said sleeve member to fully connect said rung by engagement of said locating boss with said locating recess.
3. An extension ladder as claimed in claim 1 or 2, wherein said sleeve member each is further formed with a lower locating boss and said tube member each is formed at a position corresponding to said lower locating boss with a I oce---recess, Permitting said sleeve member to firmly attach to said tube member without turning relative to said tube member.
4. An extension ladder as claimed in claim 1, wherein said two lateral ends of each said rung are in the form of a ring and are respectively Provided with an elongated slot at a predetermined position, and threaded holes are formed on a peripheral wall of each said rung at two opposite sides of each said elongated slot, Permitting fastening screws to seParatelY extend through said threaded holes to narrow the elongated slots such that said tube member each is tightly received within said ring-shaped end of said rung.
5. An extension ladder as claimed in claim 1, wherein a plug member is mounted to a bottom end of each said tube member, said plug member each having a lower flange at a bottom end thereof to extend beyond said bottom end of said tube member to serve as a buffer when said telescopic tube members are collapsed.
6. An extension ladder as claimed in claim 5, wherein said plug member each is formed with a locating Projection at an outer portion thereof to correspond to a hole formed on each said tube member for said plug member to firmly locate in said tube member.
7. An extension ladder as claimed in claim 1. wherein cover members are fitted around each said tube member to prevent said telescopic tube members from directly frictionally contacting with each other.
8. An extension ladder as claimed in claim 7, wherein said cover members each is formed at an inner wall with a locating projection to correspond to a locating hole formed ow each said tube member for said cover member to firmly attach to said tube member.
9. An extension ladder as claimed in claim 1 wherein a plug assembly is mounted to a bottom end of each said tube member, said plug assembly each consisting of an insert member and a cover member, said insert member being disposed in each said tube member while said cover member being disposed around each said tube member, said insert member each having a lower flange at a bottom end thereof to extend beYond said bottom end of said tube member and contact with a lower end of said cover member, serving as a buffer when said telescopic tube members are collaPsed and preventing said telescopic tube members from directly frictionally contacting with each other.
10. An extension ladder as claimed in claim 9, wherein said cover member each is formed at a bottom portion with an inward flange and said insert member each is formed with a locating projection at an outer portion thereof to correspond to and extend into holes seParatelY formed on each said tube member and on each said cover member for said insert member to firmly locate in said tube member with said inward flange of said cover member abutting against said lower end of said tube member and said lower flange of said insert member extending beyond said inward flange of said cover member, such that said plug assembly is firmly attached to said tube member.
11. An extension ladder as claimed in claim 9, wherein said insert member each consists of a front insert and a rear insert and said cover members consists of a front cover and a rear c o re r.
12. An extension ladder as claimed in claim 11, wherein said front and said rear covers are respectively formed at their bottom portion with an inward flange, said front insert is formed with a locating projection at an outer portion thereof to correspond to and extend into holes separately formed on each said tube member and on each said front cover, and said rear cover being formed at an inner wall with a locating projection to correspond to and to extend into holes separately formed on each said tube member and on each said rear insert, such that said inward flanges of each said front and said rear cover abut against said bottom end of each said tube member and said lower flanges of each said front and said rear insert extend beyond said inward flanges of said cover member, permitting said plug assembly to firmly attach to said tube member.
13. An extension ladder as claimed in claim 5 or 9, wherein said plug member or said plug assembly each is provided with a resilient movable upper hook member having a hook Portion which extends into a corresponding locating hole formed on each said tube member to retain said tube member in a fixed position without the risk of unexpectedly sliding down again when said ladder is pulled to extend from a bottom end toward an upper end thereof.
14. An extension ladder as claimed in claim 13, wherein a roller is disposed in a front Portion of each said uPPer hook member, whereby said upper hook member can move along an inner wall of each said tube member in a rolling manner without abrading said inner wall of said tube member.
15. An extension ladder as claimed in claim 13, wherein said uPPer hook member each has a bevelled projection disposed on one side at an upper rear corner thereof, and wherein said plug member or said Plug assembly each is further eccentrically Provided at a bottom side with a resilient movable lower hook member such that said lower hook member is vertically aligned with said bevelled projection of said upper hook member, whereby when said ladder is in a collapsed state, each said upper hook member, by means of its bevelled projection, links with a lower hook member in a next higher tube member of said ladder, and when said ladder is to be extended from the collaPsed state and a first pair of said upper hook members are caused to extend into said corresponding locating holes formed on a first pair of said tube members, said lower hook members of said next higher tube members originally linking with said upper hook members disengage from said upper hook member now, permitting said first pair of telescopic tube members to be extended, and other higher pairs of telescopic tube members can extended sequentially from bottom to top in the same manner.
16. An extension ladder as claimed in claim 13 wherein said upper hook member each is provided at a lower bottom with an air cock and said plug member or said plug assemblY each is provided at its base portion with an air port corresponding to said air cock on said upper hook member, such that when each said upper hook member extends into said corresponding locating hole formed on said tube member and causes its lower bottom and accordingly said air cock to lift, said air port is opened, and when each said upper hook member moves out of said corresponding locating hole formed on said tube member and causes its lower bottom and accordingly said air cock to lower, said air port is closed, thereby, air flow in and through said legs of said extension ladder can be controlled to facilitate the quick extension or the moderate collaPse of said extension ladder.
17. An extension ladder as claimed in claim 16, wherein a one-way valve is provided near a bottom end of the first or the lowest pair of said tube members to supply additional air into said legs to facilitate a quick extension and a moderate collaPse of said extension ladder.
18. An extension ladder as claimed in claim 17, wherein a cover Plate having dimensions slightly larser than those of said locating hole formed on each tube member corresponding to each said upper hook member is provided in front of said upper hook member for the latter to extend through a middle oPening on said cover Plate before it extends into said locating hole, such that said locating hole is airtightly sealed when said upper hook member extends into it.
19. An extension ladder as claimed in claim 18, wherein an air tube is connected to each said one-way valve at said first Pair of tube members for supplying pressurizing air into said first pair of tube members which have been airtightly sealed at said locating holes to speed the extension of said extension ladder.
20. An extension ladder comprising a Pair of legs each consisting of a Plurality of telescopic tube members, a Plurality of rungs having a substantially reverse U-shaped cross section and two ring-like ends for separately receiving said legs therein, a plurality of locking mechanisms disPosed beneath said rungs near said two ends thereof, and a plurality of lock ensuring mechanisms separately' disposed behind said locking mechanisms; said locking mechanisms each comprising a locking pin caPable of resilientlr extending and engaging into a corresponding locating hole formed on each said tube member and a touch arm pivotally connected to said locking pin for moving said locking pin out of said corresponding locating hole by Pulling said touch arm; said lock ensuring mechanisms each comprising a lock ensuring block having the form of a block stop and an unlocking touch arm pivotally connected to said lock ensuring block; and said locking pin of said locking mechanism after extending into said corresponding locating hole on said tube member, being stoPPed by said lock ensuring block without being easily moved out of said corresponding locating hole by pulling said touch arm unless said unlocking touch arm is Pushed to displace said lock ensuring block.
21. An extension ladder as claimed in claim 20, wherein a Push block is fixedly disPosed in each said touch arm of said locking mechanism with an arcuated surface thereof abutting against said locking Pin, whereby when said touch arm is pivotally moved, said arcuated surface of said Push block is pivotally turned to translate said locking pin toward or away from said corresponding locating hole on said tube member.
22. An extension ladder as claimed in claim 20, wherein each said lock ensuring block is formed at each lateral surface with a straight slide channel and wherein each said rung is formed on each side wall with a slide block corresponding to said slide channel on said lock ensuring block, such that the engagement of said slide block with said corresponding slide channel allowing said lock ensuring block to vertically move up and down below said rung.
23. An extension ladder as claimed in claim 22, wherein said unlocking touch arm has a recess formed on a surface thereof facing an inner surface of said side wall of said rung, forming a peripheral wall surrounding said recess; wherein a stoP screw is threaded through said side wall of said rung facing said unlocking touch arm and presses against said peripheral wall of said recess, and therebY limits said lock ensuring block to vertically move in said rung only within a limited distance.
24. An extension ladder as claimed in claim 23, wherein said unlocking touch arm is formed with a dent on said peripheral wall surrounding said recess thereof and said peripheral wall has a curved lower portion, such that when said unlocking touch arm is subjected to an uPward force from a lower end thereof, said lock ensuring block is moved to slide upward along said slide blocks inside said rung until it abuts against a toP wall of said rung, and when said unlocking touch arm is further forced to move upward, said stop screw moves along said curved lower portion and into said dent on said peripheral wall, permitting said unlocking touch arm to idle without damaging the entire lock ensuring mechanism.
25. An extension ladder as claimed in claim 24, wherein a return spring is connected to said unlocking touch arm at where said unlocking touch arm is pivotally connected to said lock ensuring block, such that whenever said unlocking touch arm is released from said uPward force applied to it, said return spring immediately causes said unlocking touch arm to return to its home position. alloning said lock ensuring block to move down again.
26. An extension ladder as claimed in claim 22, wherein said lock ensuring block has a Pin hole formed on one end facing said locking pin such that when said lock ensuring block is moved upward to a certain position, said Pin hole becomes aligned with said locking Pin, allow said locking pin to moved into said pin hole and at the same time to disengage from said corresponding locating hole formed on said tube member.
27. An extension ladder as claimed in claim 22, wherein each said lock ensuring block is provided with an indicating Panel Projected from a top Portion thereof and each said rung is provided at its top surface with a warning hole corresponding to said indicating Panel for the latter to Project therefrom when said lock ensuring block is moved upward to contact said top Portion of said rung, wherebY a user is warned that said rung is in a collapsible state.
28. An extension ladder substantially as hereinbefore described with reference to any of Figures 2-7 of the accompanying drawings.

28. An extension ladder comprising a pair of legs each consisting of a Plurality of telescopic tube members, "a plurality of rungs having a substantially reverse U-shaped cross section and two ring-like ends for sePartately receiving said legs therein, a Plurality of locking mechanisms disPosed beneath said rungs near said two ends thereof, and a general switch assembly provided beneath'one of said rungs forming the lowest or the first stair of said ladder; said locking mechanisms each comprising a locking pin caPable of resilientlu extending and engaging into a corresponding locating hole formed on each said tube member and means capable of Pulling said locking pin out of said corresponding locating hole at a moment when said means is Pushed upward by one of said rungs immediately below said means, allowing said telescopic tube members of that stair to be collaPsed; and said general switch assembly comprising a general switch and Pull rods for Pulling said locking pin of said locking mechanism out of said corresponding locating hole, whereby an operation of said general switch shall actuate said lowest rung said tube members forming the second stair of said ladder to collapse, and, the collaPse of said tube members of said second stair shall, with said locking mechanisms separately mounted beneath each said rung, automatically actuate said rungs and said telescopic tubemembers of higher stairs of said ladder to collaPse stair by stair from bottom to top.

29. An extension ladder as claimed in claim 28. wherein said means for Pulling said locking Pin out of said corresponding locating hole each is a touch arm pivotally connected at a position immediately below said locking pin, said touch arm having an upper end contacting with said locking Pin and a lower end pointing downward, and, said touch arm being pivotally movable when an upward force is aPPlied to said lower end.

30. An extension ladder as claimed in claim 29, wherein said touch arm, has a curved bottom periphery extending from a substantially vertical upper portion of said touch arm to a substantially horizontal lower Portion thereof, pointing toward said rung immediately below said touch arm and Providing an increased distance within which said rung immediately below said touch arm keePing in contact with said touch arm.

31. An extension ladder as claimed in claim 29 or 30, wherein a push block is fixedly disposed in each said touch arm of said locking mechanism with an arcuated surface thereof abutting against said locking pin, whereby when said touch arm is pivotally moved, said arcuated surface of said push block is pivotally turned to translate said locking pin toward or awar from said corresponding locating hole on said tube member.

32. An extension ladder as claimed in claim 28, wherein said locking pins beneath said rung of said lowest stair of said ladder is formed with a hole at one end and said means for pulling said locking pins out of said corresponding locating holes formed on said tube members of said lowest stair are said pull rods of said general switch assembly; said pull rods pulling said locking pins by seParatelY extending their one end to hook in each said hole on said locking pins.

33. An extension ladder as claimed in claim 28, wherein said general switch of said general switch assembly is a shifting wheel pivotally rotatably mounted to a central position beneath said lowest rung, said shifting wheel being formed at two lateral side surfaces with two holes for receiving the other end of said pull rods opposite to those ends hooking in said locking pins and further having a shifting arm extending from an outer periphery thereof, such that when said shifting arm is pushed to move said shifting wheel, said locking pins are pulled by said Pull rods to disengage from their corresponding locating holes on said tube members, allowing said ladder to be collaPsed sequentially.

34. An extension ladder as claimed in claim 33, wherein said shifting wheel has a retaining leaf spring disposed opposite to said shifting arm, said retaining leaf spring being stoPPed br a retaining projection formed beneath said lowest rung when said shifting wheel is moved to its upmost Position below said lowest rung by pushing said shifting arm, preventing said shifting wheel from further rotating while said shifting arm being prevented from any undesired quick release before said first stair of said extension ladder is completely collaPsed.

35. An extension ladder as claimed in claim 34, wherein said lowest rung is formed at a top surface with a hole corresPonding to said retaining leaf sPring and a second rung immediately above said lowest rung is formed beneath a top surface with a downward Projected pin corresponding to said hole of said lowest rung, whereby when said second rung is collapsed to touch said lowest rung, said Projected pin extends through said hole to touch said retaining leaf spring and disengage the same from said retaining projection of said lowest rung, releasing said shifting wheel and permitting the same to return to an operative state.

36. An extension ladder as claimed in claim 28. wherein a lock ensuring push Plate is disposed between each said locking Pin and each said pull I rod, said lock ensuring push plate each being formed with a central pin hole having a vertical length larger than a diameter of said locking pin for said locking pin to extend therethrough while allowing said lock ensuring push Plate to vertically move below and within said lowest rung with two lateral ends of said push plate received in two slide channels formed on two inner side walls of said lowest rung at a Position corresponding to said lock ensuring Push plate, each said locking pin becoming aligned with said Pin hole of said lock ensuring push Plate when the latter is Pushed upward, allowing each said Pull rod of said shifting wheel to pull said locking Pin out of said corresponding locating hole on said telescopic tube members, and each said locking Pin becoming out of alignment with said pin hole when said lock ensuring push plate is moved downward. causing said general switch assembly to be in a securely locked state.

37. An extension ladder as claimed in claim 36, wherein said lock ensuring Push plate each has a toP indicating block and said lowest rung is formed at said top surface with a warning opening corresponding to said indicating block for the latter to protrude therefrom when said lock ensuring push Plate is Pushed upward to abut against said toP surface of said lowest rung, serving as a warning to a user that said ladder is in a collapsible state.

38. An extension ladder as claimed in claim 36 or 37.

wherein said lock ensuring push plate each is formed at one of said lateral ends with a dent to just receive a stop screw threaded through said lowest rung to limit said lock ensuring Push Plate to vertically move onlY within a distance defined by said dent.

39. An extension ladder as claimed in claim 38. wherein said lock ensuring Push plate each is provided below said Pin hole with a transverse projected block and below said projected block with a resilient engaging Piece, each said engaging Piece having two lower ends which are separately slidably engaged with said inner side walls of said lowest rung, and a resilient bent portion which is engaged with said projected block to retain said lock ensuring Push plate in place when the latter is pushed upward and thereby facilitates the operation of said general switch assembly, and each said engaging piece being Pushed to disengage from said lock ensuring Push Plate by each said locking Pin when said locking pin is pulled out of said corresponding locating hole and backward extends through said pin hole of said lock ensuring Push plate, allowing said lock ensuring push plate to slide down and return to a lock ensuring state.

40. An extension ladder substantially as hereinbefore described with reference to any of Figures 2-7 of the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS 1. An extension ladder comprising a pair of legs, a plurality of rungs each having two lateral ends for said legs to pass therethrough, and a plurality of locking mechanisms disposed at a bottom side of said rungs, being characterized in that each of said less consists of a plurality of telescoPed straight circular tube members and a sleeve member is disposed in an upper end of each said tube member near where said tube member connecting with one of said rungs, and that said sleeve member each has a top flange which just rests on said upper end of each said tube member when said sleeve member is mounted in said tube member to support and fitly engage with one of said rungs.

2. An extension ladder as claimed in claim 1. wherein said sleeve member each is formed at said toP flange with an upper locating boss and said rungs each is formed at a position corresponding to said upper locating boss with a locating recess, permitting said sleeve member to fitly connect said rung by engagement of said locating boss with said locating recess.

3. An extension ladder as claimed in claim 1 or 2, wherein said sleeve member each is further formed with a lower locating boss and said tube member each is formed at a Position corresponding to said lower locating boss with a locating ~recess. Permitting said sleeve member to firmly attach to said tube member without turning relative to said tube member.

4. An extension ladder as claimed in claim 1, wherein said two lateral ends of each said rung are in the form of a ring and are respectively Provided with an elongated slot at a predetermined Position, and threaded holes are formed on a peripheral wall of each said rung at two opposite sides of each said elongated slot, permitting fastening screws to seParatelY extend through said threaded holes to narrow the elongated slots, such that said tube member each is tightly received within said ring-shaped end of said rung.

5. An extension ladder as claimed in claim 1, wherein a plug member is mounted to a bottom end of each said tube member, said plug member each having a lower flange at a bottom end thereof to extend beyond said bottom end of said tube member to serve as a buffer when said telescopic tube members are collapsed.

6. An extension ladder as claimed in claim 5, wherein said plug member each is formed with a locating projection at an outer portion thereof to correspond to a hole formed on each said tube member for said plug member to firmly locate in said tube member.

7. An extension ladder as claimed in claim 1, wherein cover members are fitted around each said tube member to prevent said telescopic tube members from directly frictionally contacting with each other.

8. An extension ladder as claimed in claim 7, wherein said cover members each is formed at an inner wall with a locating projection to correspond to a locating hole formed on each said tube member for said cover member to firmly attach to said tube member.

9. An extension ladder as claimed in claim 1, wherein a plug assembly is mounted to a bottom end of each said tube member, said plug assembly each consisting of an insert member and a cover member, said insert member being disposed in each said tube member while said cover member being disposed around each said tube member, said insert member each having a lower flange at a bottom end thereof to extend beYond said bottom end of said tube member and contact with a lower end of said cover member, serving as a buffer when said telescopic tube members are collapsed and preventing said telescopic tube members from directly frictionally contacting with each other.

10. An extension ladder as claimed in claim 9. wherein said cover member each is formed at a bottom portion with an inward flange and said insert member each is formed with a locating Projection at an outer portion thereof to corresPond to and extend into holes separately formed on each said tube member and on each said cover member for said insert member to firmly locate in said tube member with said inward flange of said cover member abutting against said lower end of said tube member and said lower flange of said insert member extending beyond said inward flange of said cover member, such that said plug assembly is firmly attached to said tube member.

11. An extension ladder as claimed in claim 9, wherein said insert member each consists of a front insert and a rear insert and said cover members consists of a front cover and a rear cores.

12. An extension ladder as claimed in claim 11, wherein said front and said rear covers are respectively formed at their bottom portion with an inward flange. said front insert is formed with a locating projection at an outer portion thereof to correspond to and extend into holes separately formed on each said tube member and on each said front cover, and said rear cover being formed at an inner wall with a locating projection to correspond to and to extend into holes separately formed on each said tube member and on each said rear insert, such that said inward flanges of each said front and said rear cover abut against said bottom end of each said tube member and said lower flanges of each said front and said rear insert extend beyond said inward flanges of said cover member, permitting said plug assembly to firmly attach to said tube member.

13. An extension ladder as claimed in claim 5 or 9, wherein said plug member or said plug assembly each is provided with a resilient movable upper hook member having a hook portion which extends into a corresponding locating hole formed on each said tube member to retain said tube member in a fixed position without the risk of unexpectedly sliding down again when said ladder is Pulled to extend from a bottom end toward an upPer end thereof.

14. An extension ladder as claimed in claim 13, wherein a roller is disposed in a front Portion of each said upper hook member, whereby said upper hook member can move along an inner wall of each said tube member in a rolling manner without abrading said inner wall of said tube member.

15. An extension ladder as claimed in claim 13, wherein said upper hook member each has a bevelled projection disposed on one side at an upper rear corner thereof, and wherein said plug member or said plug assembly each is further eccentrically provided at a bottom side with a resilient movable lower hook member such that said lower hook member is vertically aligned with said bevelled projection of said upper hook member, whereby when said ladder is in a collaPsed state, each said upper hook member, by means of its bevelled projection, links with a lower hook member in a next higher tube member of said ladder, and when said ladder is to be extended from the collaPsed state and a first pair of said upper hook members are caused to extend into said corresponding locating holes formed on a first pair of said tube members, said lower hook members of said next higher tube members originally linking with said upper hook members disengage from said upper hook member now, permitting said first pair of telescopic tube members to be extended, and other higher pairs of telescopic tube members.

can extended sequentially from bottom to top in the same manner.

16. An extension ladder as claimed in claim 13, wherein said upper hook member each is provided at a lower bottom with an air cock and said plug member or said plug assemblY each is provided at its base portion with an air port corresponding to said air cock on said upper hook member, such that when each said upper hook member extends into said corresponding locating hole formed on said tube member and causes its lower bottom and accordingly said air cock to lift, said air port is opened, and when each said upper hook member moves out of said corresponding locating hole formed on said tube member and causes its lower bottom and accordingly said air cock to lower, said air port is closed, thereby, air flow in and through said legs of said extension ladder can be controlled to facilitate the quick extension or the moderate collapse of said extension ladder.

17. An extension ladder as claimed in claim 16, wherein a one-waY valve is provided near a bottom end of the first or the lowest pair of said tube members to supply additional air into said legs to facilitate a quick extension and a moderate collaPse of said extension ladder.

18. An extension ladder as claimed in claim 17, wherein a cover plate having dimensions slightly larger than those of said locating hole formed on each tube member corresponding to each said uPPer hook member is provided in front of said upper hook member for the latter to extend through a middle opening on said cover Plate before it extends into said locating hole, such that said locating hole is airtightly sealed when said upPer hook member extends into it.

19. An extension ladder as claimed in claim 18, wherein an air tube is connected to each said one-wau valve at said first pair of tube members for supplying pressurizing air into said first pair of tube members which have been airtightly sealed at said locating holes to sPeed the extension of said extension ladder.

20. An extension ladder comprising a pair of legs each consisting of a Plurality of telescopic tube members, a plurality of rungs having a substantially reverse U-shaped cross section and two ring-like ends for separately receiving said legs therein, a plurality of locking mechanisms disposed beneath said rungs near said two ends thereof, and a Plurality of lock ensuring mechanisms separate 1 T' disposed behind said locking mechanisms said locking mechanisms each comprising a locking Pin capable of resilientlu extending and engaging into a corresponding locating hole formed on each said tube member and a touch arm pivotally connected to said locking pin for moving said locking Pin out of said corresponding locating hole by pulling said touch arm; said lock ensuring mechanisms each comPrising a lock ensuring block having the form of a block stoP and an unlocking touch arm pivotally connected to said lock ensuring block; and said locking pin of said locking mechanism, after extending into said corresponding locating hole on said tube member, being stoPPed by said lock ensuring block without being easily moved out of said corresponding locating hole by pulling said touch arm unless said unlocking touch arm is pushed to displace said lock ensuring block.

21. An extension ladder as claimed in claim 20, wherein a push block is fixedly disposed in each said touch arm of said locking mechanism with an arcuated surface thereof abutting against said locking Pin, whereby when said touch arm is pivotally moved, said arcuated surface of said push block is pivotally turned to translate said locking pin toward or away from said corresponding locating hole on said tube member.

22. An extension ladder as claimed in claim 20, wherein each said lock ensuring block is formed at each lateral surface with a straight slide channel and wherein each said rung is formed on each side wall with a slide block corresponding to said slide channel on said lock ensuring block, such that the engagement of said slide block with said corresponding slide channel allowing said lock ensuring block to vertically move up and down below said rung.

23. An extension ladder as claimed in claim 22, wherein said unlocking touch arm has a recess formed on a surface thereof facing an inner surface of said side wall of said rung, forming a PeriPheral wall surrounding said recess; wherein a stop screw is threaded through said side wall of said rung facing said unlocking touch arm and presses against said peripheral wall of said recess, and thereby limits said lock ensuring block to vertically move in said rung only within a limited distance.

24. An extension ladder as claimed in claim 23, wherein said unlocking touch arm is formed with a dent on said peripheral wall surrounding said recess thereof and said peripheral wall has a curved lower portion, such that when said unlocking touch arm is subjected to an upward force from a lower end thereof, said lock ensuring block is moved to slide upward along said slide blocks inside said rung until it abuts against a top wall of said rung, and when said unlocking touch arm is further forced to move upward, said stop screw moves along said curved lower portion and into said dent on said peripheral wall, permitting said unlocking touch arm to idle without damaging the entire lock ensuring mechanism.

25. An extension ladder as claimed in claim 24, wherein a return spring is connected to said unlocking touch arm at where said unlocking touch arm is pivotally connected to said lock'insuring block, such that whenever said unlocking touch arm is released from said upward force applied to it, said return spring immediately causes said unlocking touch arm to return to its home Position. allowing said lock ensuring block to move down again.

26. An extension ladder as claimed in claim 22, wherein said lock ensuring block has a Pin hole formed on one end facing said locking pin such that when said lock ensuring block is moved upward to a certain position, said Pin hole becomes aligned with said locking pin, allow said locking pin to moved into said pin hole and at the same time to disengage from said corresponding locating hole formed on said tube member.

27. An extension ladder as claimed in claim 22, wherein each said lock ensuring block is provided with an indicating panel projected from a top portion thereof and each said rung is pro v i d e d at its toP surface w it h a warning hole corresponding to said indicating Panel for the latter to project therefrom when said lock ensuring block is moved upward to contact said top Portion of said rung, whereby a user is warned that said rung is in a collapsible state.