Split duct with integral hinge

Ducting particularly suited for placement over an existing cable run comprises a tubular duct, for example glass filament wound epoxy duct, the wall of which is slit along the length thereof. Opposed to the slit the wall is weakened e.g. by perforating to form integral hinges upon which the duct may be hinged open for the introduction of the cable therein.

This invention is concerned with service installations of power and signal 
distribution cable or the like; it is particularly concerned with ducting 
suitable for use therewith. 
In service installations of the above type it is upon occasion necessary to 
replace the ducting protecting an existing cable run. Hitherto it is 
believed that such replacement has been effected by the use of ducting 
which is longitudinally split into two separate halves. To install the 
duct, the two halves must be separately placed about the cable and 
manually retained until they are at least temporarily secured together. 
This method of installation is both tedious and slow, additionally due to 
the exigencies both of the manufacturing processes and of the method of 
installation, it is found to be difficult to maintain the register of 
circumferential mating portions and of axial end mating portions of the 
duct halves, hence the task of sealing the duct subsequent to its 
installation is time concuming and the results are not wholly 
satisfactory. 
My invention contemplates a duct which has been found to be more suitable 
for use in the above installations whereby the difficulties earlier 
experienced may be largely overcome. Briefly expressed, a duct constructed 
in accordance with my invention comprises a tube made from a resilient 
plastic material, the wall of which is axially slit along the length 
thereof. The wall of the tube duct generally radially opposed to the slit 
is weakened sufficiently so as to form an integral hinge whereby the tube 
duct may be resiliently hinged open for the introduction of the cable 
therein, and the tube duct then snapped in position to surround the cable. 
The materials from which the resilient tube duct of my invention may be 
manufactured are as a class plastic resins. Suitable resins are presently 
employed for making integrally walled tube duct and also the separately 
halved duct referred to above. Amongst these resins may be particularly 
mentioned thermoplastic resins such as polyvinyl chloride, polyethylene 
and polypropylene, which resins may be reinforced, for example with glass 
or other fibers. Certain fiber reinforced thermosetting resins may also be 
employed such as polyester resins or epoxy resins. 
The precise manner in which the wall of the tube duct is weakened to form 
the integral hinge will depend largely upon the material from which the 
duct is constructed. Where certain of the thermoplastic materials are 
employed, their modulus of elasticity may be relatively low, and the wall 
of a duct constructed from these materials may be continuously grooved to 
weaken it along the length thereof. Tube duct made from these materials is 
commonly extruded, hence the duct of my invention may be formed with both 
an axial slit and a continuous groove opposed thereto in a single 
extruding operation. As an alternative, integrally walled tube duct may be 
sawn or otherwise slit lengthwise and the integral hinge formed by 
grooving the wall by hot pressing or rolling for example. Where materials 
such as the glass filament reinforced thermosetting plastics are employed 
their elastic modulus may be relatively high generally, and the force 
necessary to hinge open a duct sufficiently for the cable to be introduced 
is generally much greater than can be generated manually even when the 
wall of the duct is deeply grooved. A preferred manner of weakening the 
wall under these circumstances to form the integral hinge is to perforate 
the wall. In a pipe duct of the helically wound glass filament reinforced 
epoxy resin type constructed, for example, in accordance with the method 
disclosed in Canadian Pat. No. 1,025,904 issued Feb. 7, 1978 to Buehler et 
al, commonly assigned herewith, I find that a plurality of collinear 
axially aligned slots can be cut into the wall of the duct to occupy from 
about 90 to 99% of the length thereof, leaving some 10 to 1% to function 
as the hinge. Typically in a duct having a length of 20 ft., the total 
axial length of the hinge sections may be only some 3 to 6 inches. 
Desirably on long duct lengths the total hinge length is divided into two 
or more parts whereby the duct halves may be retained more readily in 
parallel relationship. However where duct couplings and fittings are 
integrally hinged, such parts are normally of limited axial extent and a 
single hinge usually suffices.

Referring now to FIG. 1, a tube duct which is indicated generally by the 
numeral 10 comprises an axially elongated, generally cylindrical tube 
which may typically have a length of up to 20 feet. The embodiment 
illustrated is constructed of glass filament reinforced epoxy resin; for 
general details of the construction reference may be had to the 
aforementioned Buchler et al patent. Duct 10 is axially slit along the 
length thereof at 12, which operation may be carried out by sawing, for 
example, or by abrading. Radially opposed to slit 12 the wall of duct 10 
is weakened by slots of perforations 14, leaving sections 16 and 17 which 
form resilient integral hinges whereby the two halves of pipe duct 10 are 
interconnected. Slots 14 may be formed in the wall of duct 10 in the same 
manner as the duct is slit at 12, e.g. by sawing. The width of slots 14 is 
not critical, and in general the saw kerf formed by a heavy duty carbide 
tipped blade, approximately 0.25 inch, may be adequate. When the halves of 
duct 10 are hinged open for the introduction of a cable therein, a 
considerable stress is imposed locally in the hinge areas of the 
reinforced duct, and as a consequence the areas may be strained 
considerably beyond their elastic limit. It is desirable to retain a 
sufficient integrity of the hinge areas such that duct 10 will snap shut 
subsequent to the introduction of a cable therein, thus ensuring a proper 
register of the opposed edges of slit 12. The strain experienced by the 
hinge areas 16 and 17 may be reduced by further weakening the wall of duct 
10, for example by increasing the width of slots 14. An alternative method 
contemplated by my invention and illustrated in FIG. 2, is by forming 
transverse slots 20 which intersect slots 14 at the closed ends thereof. A 
still further alternative, as illustrated in FIG. 3 is to form two or more 
radially spaced apart axial slots 14, 14'. Other methods of weakening the 
wall of duct 10 and delocalizing the imposed stress forces in the region 
of the integral hinges will be apparent to those skilled in the art. 
The axial extent and location of integral hinges 16 and 17 along duct 10 is 
not critical. Assuming duct 10 to have an extended length, i.e. from about 
4 to 20 feet, such as may be employed in commercial duct systems, it is 
generally preferred to locate each hinge with about 10 to 25 percent of 
the length of the duct from the ends thereof. A third or more hinge or 
hinges intermediate hinges 16 and 17 may be found to be desirable 
particularly where duct 10 has a length some 20 feet. The axial extent of 
the hinges will depend upon the materials and manner of construction of 
the duct 10 and the method of delocalizing stress forces in the vicinity 
of the hinge. For the simple hinge structure shown in FIG. 1, the axial 
extent of each hinge may be in the range of about 1 to 4 inches depending 
upon the wall thickness and diameter of duct 10. In general the combined 
axial extent of the hinge sections is approximately in the range 1 to 10 
percent of the axial length of the duct, or as a corollary, the weakening 
slots 14 occupy approximately 90 to 99 percent of the axial length of the 
duct. When the duct is of smaller axial extent e.g. in the range of about 
1 to 4 feet, a single hinge will normally suffice and provide adequate 
register of the mating edges of slit 12. 
Where the duct is linear and of constant radial cross section, the number 
and placement of the hinge areas will be guided by the above 
considerations. Where the duct is not linear, and or where the radial 
cross section changes, the hinge placement will be guided by the 
desirability of the hinges being recticollinear. Thus, in the duct 10 of 
FIG. 1 which is shown as having a bell mouth end 19, hinge 17 provided at 
that end of the duct will not normally locate on the bell mouth but will 
be adjacent thereto, and one slot 14 will normally extend continuously 
through the bell mouth end. It will be apparent that provision must be 
made to permit areas of the bell mouth 19 located along the hinge line to 
move towards each other as the duct is hinged open. The wall thickness of 
duct 10 constructed accordingly to the aforesaid Buehler patent is 
normally not greater than about 0.1 inch, hence the difference in external 
diameter of the bell mouth 19 and the spigot end 21 of the duct is usually 
not greater than about 0.25 inches. Whilst the width of slot 14 may be 
specifically adjusted where it passes through bell mouth 19, the normal 
method of formation of slot 14 by sawing or abrading as earlier described 
usually provides an adequate clearance between the opposed surfaces of 
slot 14 in this area for the purpose described. 
Whilst my invention has been particularly described with respect to a 
preferred embodiment thereof, such embodiment is not to be taken as being 
limiting but merely as being illustrative of my invention. A wide range of 
resilient plastic materials may be utilized within the scope of my 
invention and and the precise means by which the wall of the duct is 
weakened to form the integral hinge structure will depend upon the known 
physical properties of the materials employed. The scope of the invention 
is to be taken according to the spirit of the appended claims.