US 3328223 A
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June 27, 1967 M. H. FINK METHOD OF CONSTRUCTING A DIELECTRIC LADDER 2 Sheets-Sheet 1 Original Filed Aug. 19 1963 INVENTO'R. 7W4J PM United States Patent 3,328,223 METHOD or CONSTRUCTING A DIELECTRIC LADDER Miller H. Fink, Centralia, Mo., assignor to A. B. Chance.
This is a division of application Ser. No. 303,728, filed Aug. 19, 1963, now U. S. Patent No. 3,225,862, which in turn is a continuation of application Ser. No. 132,665, filed Aug. 21, 1961, now abandoned.
This invention relates to a dielectric ladder and the method of making same, and more particularly, it relates to a joint between the rail member and respective rung members of said ladder.
Ladders are often used around high tension lines to position a workman to reach certain components. Often the work is performed at a considerable elevation off the ground, thus making a relatively long ladder mandatory. Metallic ladders are strong, but they are subject to the disadvantage that they conduct electricity and thus increase the hazard to a lineman working from the ladder.
v Referring to FIG. 1, 11 is a ladder embodying the present invention. It has elongated rails 13 and 15 interconnected by a multiplicity of tubular rungs 21.
Referring now to FIGS. 24, it will be seen that rail 13 has an outer casing or sleeve 17 and a central core 19. The casing is preferably made of a reinforced plastic, mg, a glass fiber reinforced epoxy, and the core is preferably a unicellular plastic foam that is light and substantially impermeable to moisture, e.g., polyurethane. For a superior dielectric pole, the casing and the core should be bonded together to form a hermetic seal there: between. My Patent 2,997,529 discloses the preferred materials and method of manufacture for an elongated electrical insulating rod, which I prefer to utilize for rails 13 and 15.
Rungs 21 are mad-e of a strong material, preferably a I dielectric. Various materials can be used, e.g., fiber glass a tendency to soak ,up moisture; moreover, their mechanical strength for a given weight is rather low.
It is therefore the object of the present invention to provide a ladder which will overcome the disadvantages of metal and wood in the prior art; more specifically, it is an object to provide such a ladder made of reinforced plastic with a moisture-resistant unicellular core and having rungs of a suitable dielectric material; it is a further object to provide simple and eflicient joints between the rails and rungs on. said ladder; and yet another object is to provide a method for making such a ladder and particularly for forming the joints for same.
In summary, the present invention concerns a ladder composed of spaced parallel rail members made of annular reinforced plastic casing or sleeve filled with a foam center and amultiplicity of dielectric rungs interconnecting said rail members. The joints between the railsand rungs depend on a bonded plastic collar which extends from the end of each rung intoa socket-like interior in the rail. The collar is bonded to internal. rail surfacesand it is ofa larger diameter than the mug receiving opening which extends outwardly through the sides of the casing to receive and engage the rung end. The method of forming the joint contemplates that a hole will be drilled into the side of the rail and that an enlarged cavity will be dug out in the foam interior in communication with the exterior of the casing through the hole therein. The collar is formed in situ by filling the cavity with a curable liquid plastic material and curing said material to harden it while a rung is supported in place.
For a better understanding of the invention, reference may be made to the accompanying drawings wherein:
FIGURE 1 illustrates in elevation a ladder made in accordance with the present invention;
FIG. 2 is a transverse section, taken along line 22 through FIG. 1;
FIG. 3 is a section taken along line 33 of FIG. 2;
FIG. 4 is a section taken along line 4-4 of FIG. 2; and
FIG. 5 is a transverse section illustrating the assembly of the rung to the rail.
rein-forced polyester rod, but I prefer that rungs 21 be made of the same materials as rails 13 and 15. Thus each rung 21 has a tubular portion 23 of glass reinforced plastic and an internal core 25 of unicellular plastic foam. The end portion 27 of rung 21 extends into the interior of rail 13 through the generally circular opening 29 in sleeve 17. The extremity of end 27 abuts the internal surfaces of casing 17 that lie opposite thereto, -as indicated at 31 and 33 in FIG. 2. The foam core 25 of rung 21 terminates short of the outer extremity of rung end 27, thus providing exposed internal surfaces 35 of the reinforced plastic tubular portion 23.
Retainer collar 37 is bonded securely to the rung end 27 and extends radially outwardly therefrom to such a point that the over-all diameter of end 27 and collar 37 is significantly greater than the diameter of opening 29. This collar extends continuously from the inner wall surfaces 39 of casing 17 lying adjacent opening 29 to the inner wall surfaces 41 of casing 17 lying opposite opening 29, the casing being bonded tightly to these surfaces. The outer lateral surfaces. 43 of collar 37 are configured generally as a right circular cylinder.
Foam core.19 encircles and jackets surfaces 43. Collar 37' extends into the recessed interior in the extremity of rung end 27 and in bonded engagement with the exposed internal rung surfaces 35 as well as with the'exposed end surface of foam core 25.
It will be evident from the foregoing that rung 21 is securely retained from axial motion outwardly from rail 13 since collar 37 is larger than opening 29 and cannot pass therethrough and because of the variously described bonded connections between the rung end and the internal surfaces of rail casing 17. Moreover, the joint between rung 21 and rail 13 is so constituted that transverse loads applied to rung 21, acting in cantilever thereon, are resisted since the collar 37 integrally bonds the rung to the internal wall surfaces 39 adjacent opening 29' as well as to the opposite wall surfaces 41. Thus,viewing the rung as a beam loaded transverse to its axis, it effectively has two points of support in its joint with rail 21: one at its end-most extremity, and the other displaced therefrom along the axis of rung 21 approximately the internal diameter of casing 17. 7 1
Although the collar .37 is bonded. securely with the jacketing foam su'rfacesin rail 13, and with the circular end surfaces of foam core 25 of rung 21, these joints cannot be depended on to ,add substantial strength to the over-all joint because of the structurally weaknature of the unicellular foam, it being poor to resista load pin either tension or compression. So, while the foam-plastic collar bonds are excellent for sealing out moisture, they are of little assistance in adding joint strength.
The above described joint is formed in a relatively simple manner. Referring to FIG. 5, hole 29 is drilled through casing 17 and on through foam center 19, terminating against the internal wall surfaces 41 of the casing lying generally opposite hole 29. Then a suitable tool, for example, a drill with an expandable reamer means, is manipulated to remove additional, foam and enlarge the passageway through the foam to define enlarged cylindrical foam passage 51, which is of considerably greater diameter than hole 29. A knife or other instrument is used to scrape the foam from against the internal wall 41 opposite opening 29 and from the internal wall areas 39 just adjacent opening 29. The loosened particles of foam are removed from the internal rail cavity by inverting the rail and tapping it lightly or by blowing air into the opening with an air hose.
A recess in the end of rung 21 is prepared by removing foam to expose inner wall surfaces 35.
A measured quantity of liquid collar-forming material 53 is poured into cavity 51 while :the rail is positioned with the opening 29 upright so that the collar forming liquid 53 will not run out. The collar-forming liquid is a polymerizable plastic resin that will bond quite tightly with the exposed internal wall surfaces 39 and 41 of rail 13 and of the outer exposed surfaces of rung end 27 and with internal rung surfaces 35. Likewise, it should form a good moistureseal bond with the foam cavity defining surfaces 51 and the circular foam end of rung core 25. The material should be quite strong yet it should not be brittle. I find that amine-cured epoxy resin which has been modified by an additive to make it somewhat more flexible, and thus more resistant to crumbling, is quite suitable. One acceptable resin may be obtained by mixing two components commercially available from Shell Chemical Company under the designations, Shell Epon 828 and Shell X7l additive. Approximately 1:4 ratio of additive to resin, by weight, is preferable. Another commercially available resin is Cibas Araldite No. 502. This resin requires no further additive. Both of the above resins cure at room temperature, and, depending on temperature and other ambient conditions, curing takes twenty minutes to an hour after addition of 6% to 8% by weight of an amine-curing agent, for example, diethylenetriamine.
Promptly after the liquid resin and curing agent, which are mixed just before pouring, are introduced into the cavity, rung 21 is placed. into position by inserting rung end 27 into the cavity 51 and pushing its end-most portion into contact with the opposing inner wall surfaces 41 of rail casing 17. The positioned rung and rail are supported by a suitable ji-g until curing is substantially completed.
If an excess of liquid collar-forming material is used, it will be forced out through the slight clearance between hole 29 and rung 21 when the rung is inserted. This can be easily cleaned off by rubbing with a methyl ethyl ketone soaked rag.
Considering the assembly of a plurality of rungs, it will of course be appreciated that all the joints for at least one rail will have to be inserted substantially simultaneously. Since the resin takes several minutes before substantial curing occurs, there is ample time to fill each of the cavities with the liquid resin and position all of the rungs.
If desired, the ladder assembly may be accomplished by assembling the rungs to the rails, with all rung ends in place, and, while supporting the assembly by clamps or the like, introducing the collar-forming fluid into each of the joint cavities. This may be accomplished by drilling v a small hole through the tubing of the rails to communicate with each of the internal cavities and introducing the liquid therethrough. This has the advantage of per- Initting that a seal be made between rung 2-1 and rail 13 beforehand in order that resin may not run out therebetween to require clean-up effort.
By way of example, a ladder of the instant invention may have side rails of approximately 2 /2 inches diameter the rungs being 1% inches in diameter. The diameter of the foam cavity in the rail at each joint would run say A to of an inch greater than the diameter of the rung receiving opening in the outer tubular portion of the rail, which opening is just a bit larger than the diameter of a rung. For a 1% inches rung, the hole would be about 1.26 inches. The quantity of resin and curing agent required is approximately 50 grams when the wall thickness of the outer tubing of the 2 /2 inch rail is 0.155
1 inch, the Wall thickness of the 1%, inch rung 0.090, and
the depth of the end cavity in the 1 /4 inch rung /2 inch.
It will be appreciated that while my ladder is expected to find its greatest use as a safety ladder for work around high tension electricity, it will find some degree of application as a general-purpose ladder because of its light, strong construction.
Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:
1. The method of joining an elongated tubular plastic sleeve filled with a foam core to a tubular rod-like member of smaller diameter than said sleeve and also filled with a foam core, comprising:
(1) forming a hole through the side of said sleeve;
(2) forming a cavity communicating with said hole by removing foam material from the interior of said sleeve, said cavity depth being defined by rough internal wall surfaces of said sleeve adjacent as well as generally opposite the hole in said sleeve, said cavity diameter being made greater than the diameter of said hole;
(3) forming a depression of substantial depth in an extremity of the member by removing foam material therefrom to produce a depression in the member of substantial depth and exposing a portion of the rough internal surface of the member of major area;
(4) introducing a sufiicient quantity of curable liquid plastic into said cavity and the depression to completely fill the same; and
(5) inserting said one end of said member through said hole and into said cavity and permitting it to remain so positioned while the plastic liquid material cures to form a solid mass bonded to said wall surfaces of the second member and to the internal surfaces of said sleeve. 2. The method of claim 1 wherein said hole is round and said cavity is formed coaxially inwardly and generallyround in cross-section. Y
3. The method of claim 1 wherein is included the step of scraping the foam material out of the sleeve and said one end of the member to assure that the exposed interior surfaces of the sleeve and member are of generally rough characteristics prior to step (4).
References Cited UNITED STATES PATENTS 1,558,498 10/1925 Peck et al 264-2 62 X 1,742,625 1/ 1930 We-ckerle 264-262 X 2,040,126 5/ 1936 Grieve 144-309 2,862,650 1 2/ 1958 Scott et a1. 182-46 3,009,532 11/1961 Richard et a1. l82-4 6 3,042,140 7/ 1962 Basile et al 18246 3,272,894 9/ 1966 Roach 264-262 X EARL M. BERGERT, Primary Examiner.
H. ANSHER, Assistant Examiner.