Cable ducting and method for installing cables through ducts

Abstract: A duct for transmission cable comprising polymeric tubing having spiral ribs (58) located on the inner wall (50). The valleys (56) are adapted to receive lubrication and the peaks (54) form a surface over which cable may travel at reduced friction. The spiral makes one revolution from about one to six linear feet, at rib heights from about 0.127 mm to 3.175 mm (0.005 to 0.125 inch), and the spacing between the ribs is about 0.025 to 0.500 inch. The duct may be located in an outer duct which is normally buried underground, or directly buried or ploughed in. There may be a plurality of ducts placed in the ground with an outer duct. Longitudinal ribbing (59,60) on the external walls prevents rotation. A cable is laid by pulling the ducts through the outer ducts over the longitudinal ribs (59,60) and subsequently pulling the cable through the ducts along the spiral ribs (58), thus reducing the friction and increasing the length of pull that may be done without exceeding the tension strength of the cable.

Description: The present invention relates to cable ducting and to cable installation. When it was discovered that fibre-optic cable could be used more efficiently than electric wires for the transmission of telephone conversations, a need for an enormous replacement undertaking became apparent. The copper cable that was previously used underground had to be replaced by fibre-optic cable in a cost-efficient manner which would maintain the integrity of the transmission cable. When installing the fibre-optic transmission cable, which is usually sheathed in a thermoplastic insulator, there are a number of problems that must be avoided. The plastic sheathing itself must not encounter any sharp surfaces that damage or shave it away to any substantial degree. If this were to happen, exposure of the light guide would occur, along with the pertinent problems. Moreover, tension must not exceed a certain safety margin of the transmission cable or it might break. Because of the forces necessary to pull the transmission cable through a duct, it is highly advantageous to lower the friction between the transmission cable and its duct. When the coefficient of friction of the duct is lowered, the resulting lower forces to pull the cable through the duct allow longer lengths to be strung without a relay. Previously, transmission cable was pulled through a duct which either had no interior ribs or had longitudinal interior ribs. An example of the ribbed duct is shown in US-A-4,565,351. Alternatively, transmission cable was strung through corrugated ducting, i.e. ducting having transverse ribs. Each of these ducts caused problems. Smooth-walled ducting had the highest coefficient of friction, and therefore required high forces to be used when pulling a transmission cable there-through. As a result, relatively short lengths of transmission cable could be placed underground before the forces built up to the point where the cable itself became endangered. Corrugated and longitudinally ribbed ducting reduced surface friction with the cable. Both the longitudinally ribbed and smooth-walled ducts, however, had a tendency to damage the cable sheath. Such damage to the sheath would not normally be discovered until a later date, and replacement of the cable would be necessary. The replacement process was highly undesirable and needlessly expensive. Corrugated tubing had a relatively low coefficient of friction relative to the transmission cable, and did not have a tendency to damage the sheathing. However, corrugated ducting is relatively thin and has uneven walls because of the necessity of continuously blow-moulding it during its manufacturing process. Moreover, the thin-walled corrugations had a tendency to stretch or break during field installation because of their own relatively low tensile strength. The flexible nature of corrugated tubing also allowed it to rotate and shear. The thin wall thickness also wore quickly and perforated when transmission cable was pulled through it. The depth of the corrugations is generally two to four times as deep as longitudinal ribs, thus causing the lubricant to well up between successive corrugations and impede normal cable lubrication efforts. DE-A-3529541 discloses a duct of the kind for transmission and other cable, the duct comprising polymeric tubing having spiral ribs with peaks and valleys along the inner wall, the peaks forming a surface over which the cable may travel to reduce the friction of travel of the cable through the duct. DE-U-8901210 discloses a cable which has a plastics sheath and which is to be drawn through a cable duct. Friction between the cable sheath and the cable duct is reduced by forming projections on the outside of the plastics sheath. Such projections can be in the form of spiral ribs which have alternating directions of slope. A lubricant may be placed in groove like depressions in the tips of the ribs if these tips are of sufficient width. According to the present invention, a duct of the above kind is characterised in that the spiral ribs along the inner wall of the duct turn in one direction and then in the other direction. Preferably, lubricant is received in the valleys. The invention includes a duct system comprising:- an outer duct under the surface of the ground and extending between two points; a plurality of ducts in the outer duct, each of the ducts comprising polymeric tubing with spiral ribs along the inner walls of the tubing, the spiral ribs having peaks and valleys, the peaks forming a surface over which cable may pass at reduced friction without contacting the valleys and cables in the ducts; characterised in that the spiral ribs along the inner wall of each duct turn in one direction and then in the other direction in alternating fashion. Preferably, lubricant is received in the valleys. The invention also includes a method for installing a cable in a duct and a method for installing a cable in a duct and installing the duct in an outer duct. The use of spiral ribs has been found to give the best combination of results when used with a transmission cable for reducing the friction between the cable and the duct, providing structural integrity of the tubing, and not damaging the sheath on the cable. The spiral ribs act much like corrugations in that they do not shave off the sheathing because the cable tends to pass over them in a transverse manner. The internally spiralled duct is much stronger than corrugated tubing and does not have a tendency to rip or tear during its installation. It is also flexible enough to go around corners. The valleys between the spiral ribs also act as a resrvoir for lubricant which is commonly used to reduce friction between the cable and the duct. The invention is further described, by way of example, with reference to the accompanying drawings, in which:- Fig.1 is a schematic representation of the installation of multiple ducts which are surrounded by an outer duct; Fig.2 is a longitudinal section of a duct according to this invention; Fig.3 is a transverse cross section of the duct of Fig.2; Fig.4 is a developed view of internal spiral ribs of the duct of Figs. 2 and 3; Fig.5 is a perspective view which illustrates one embodiment of exterior ribs of a duct according to the invention; Fig.6 is a similar view which illustrates an alternative embodiment of exterior ribs on the duct; Fig.7 is a cross-section which illustrates a plurality of ducts and transmission cables; Fig.8 is a schematic representation of the installation of ducts in an open trench; and Fig.9 is a schematic representation of the direct installation of ducts by means of a plough. As particularly illustrated in Fig.1, a plurality of takeoff reels 10, 12 and 14 on stands 16, 18 and 20, respectively, are located so that they feed a plurality of ducts 22, 24 and 26 into a feeder tube 30. The feeder tube 30, as well as the apparatus for pulling the ducts through an outer duct 31, is described in US-A- 4,296,157; US-A-4,028,473; and US-A- 4,326,605. As described therein, lubricant is also often used and is fed into the feeder tube with the transmission cable. The ducts 22, 24 and 26 are pulled by means of a draw line 32 towards a take-up reel 36 mounted on a stand 38. The draw line 32 is attached to the duct, as described in the above-noted US-A- 4,028,473. The draw line 32 is wound on the reel 36 and is followed by the duct. As a result of this invention, the distance from the work area 40 to the work area 42 has been substantially increased for the length of the transmission cable that may be pulled through the duct. At the present time, this distance may be 914 to 1828m (3000 to 6000 feet) with the use of this invention. Normally, the duct transmission or other cable enters a work area, such as 40, by means of an outer duct 44 and may be passed back into another outer duct, such as 46 or 48, to relay them to another location. As particularly illustrated in Figs, 2, 3 and 4, the inner wall 50 of the duct 22 has peaks 54 and valleys 56 forming spiral ribs 58 along the interior of the tubing. Although not shown, the direction of spiral of the ribs 58 alternates. The material which comprises the duct is normally a thermoplastic, such as polyethylene, although other materials are well known in the art. The lubricant used with the transmission cable is that such as described in US-A- 4,111,820. The use of spiral ribs has been found to reduce dramatically the coefficient of friction without noticeably damaging the cable sheaths or jackets when the cable is pulled through. The coefficient of friction (f) of the transmission cable as it passes over the spiral ribs can be measured over various load and velocity conditions by pulling cable through a duct coiled about a fixed drum. The following mathematical relationship can be used to calculate values of f from cable tension measurments leading into and out of a duct loop: f = (1/2πn).ℓn (tension out/tension in) where n is the number of complete duct wraps about the fixed drum and ℓn is the natural logarithm. It has also been found that velocity has an effect on the measured values, such that either load or velocity must remain constant to compare test data. In testing, it was found that the relationship between f and velocity varied in a predictable way which could be mathematically expressed as K = f/log v , where K is known as the velocity-compensated frictional load factor and v is the average velocity. By comparing values of K a more accurate picture is expressed relating friction, load and pulling velocity. For any constant incoming load, both f and K can be calculated. In actual tests, the following load factors K were found for unlubricated duct: Thus, even at relatively low degrees of spiralling, such as one turn per 1.828m (six feet), the velocity-compensated load factor (coefficient of friction) is substantially reduced to 0.0764 at the 11.34kg (25 pound) incoming load, whereas its non-spiralled counterpart fused the cable to the duct wall. In addition, the coefficient of friction is minimized, at between one third and one turn per 0.305m (1 ft). The testing showed other important ramifications. Regardless of how sharp the spiral ribs were pointed, no jacket cutting or abrasion to the cable was observed. This is a significant improvement over both smooth-walled and longitudinally ribbed ducts. In those cases, resulting damage from shearing, cutting, and fusion to the cable commonly occurred. The spiral ribbed duct did not itself experience any damage such as that commonly found in corrugated tubing. Moreover, although the ribbing is spiral, there was no tendency to twist the transmission cable. Because lubricants are commonly used during transmission cable placement, it is desirable to keep the frequency of spirals as low as possible in order to have the lubricant pass efficiently along the cable. Lack of passage of the lubricant is another drawback of corrugated duct, since the lubricant tends to be wiped off with the initial corrugations. With a low frequency spiral, however, lubricant is free to move along the duct, again gaining the benefit of low friction corrugations with ease of lubrication. With lubrication manufactured by Arnco Corp., Model No. SF150DF, load factors as low as 0.0050 have been measured for cable travelling at 30.48 m/min (100 ft/min) with an incoming load of 90.72 kg (200 pounds). The duct diameter is not critical, although typically ducts range from 25.4 mm to 152.4 mm (one inch to six inches) inside diameter. Wall thicknesses may also vary, but useful thicknesses are sizes such as Standard Thermoplastic Pipe Dimension Ratio or standard inside dimension ratio (SIDR) 5 to 21. The SIDR is equal to the average internal diameter divided by the minimum wall thickness. Rib height preferably is from about 0.127 mm to 3.81 mm (0.005 inch to 0.150 inch), with the preferred being about 0.381 mm (0.015 inch). The rib spacing typically varies between about 0.635 mm and 12.5 mm (0.025 inch and 0.500 inch), with the preferred spacing being 3.175 mm (0.125 inch). The frequency of spirals can range between 32.8 turns per m and 0.16 turns per m (10 revolutions per foot and 0.05 revolution per foot), the preferred being about 1.08 turns per m (0.33 revolution per foot), or one revolution every 0.914 m (three feet). The direction of spiral rotation is altered in a periodic fashion to create a sinusoidal wave. The outer wall 52 of the duct may be smooth, as shown in Figs. 2 to 4, or, alternatively, as illustrated in Figs. 5 and 6, may be ribbed along the length thereof. A spiral rib 59 may be used along the outer wall 52. The spiral nature of the rib will add additional strength, and also will cause a type of interlocking with other ducts if they are placed together as illustrated in Fig.7. Alternatively, longitudinal ribs 60, as shown in Fig.6, may be used along the outer wall 52 to aid in inserting the duct through an outer duct. The use of the ribbing reduces friction in this regard, as does the internal spiral ribbing 58. The benefit of alternating the direction of the spiral ribs 58 is that a cable passed through the duct will not have a tendency to rotate with the spiral. By reducing the area of the duct that touches the outer duct, the friction, and therefore the force needed to pull the duct through the outer duct, is substantially reduced. As illustrated in Fig.7, a series of ducts 62, 64, and 66 having inner walls with spiral ribs 68, 70, and 72, respectively, are used to hold transmission cables 74, 76 and 78. Exterior ribs 82, 84 and 86 interlock with each other, whether spiral or longitudinal, in order to prevent rotation of one duct relative to the other. Typically, there are four to five ribs per linear 25.4 mm (one inch) around the duct of this invention. The three conduits shown in Fig.7 may be connected along their length in the moulding process or, alternatively, may be moulded separately and placed in the ground separately. Fig.8 illustrates an open trench 90 having side walls 92, 94 and a bottom 96. Ducts 100, 102 and 104 are laid in the trench 90 in a generally triangular fashion and are bound together by a band 106. The ducts have spiral ribs 108 on their outer wall 110. Similarly, there are spiral ribs on the inner wall of the duct. The ribs 108 may also be longitudinal so that the ducts interlock and do not rotate relative to one another. After the ducts have been laid, they are covered with soil. Another method of laying the ducts is known as "direct plough". By this method, a plough 112 is attached to a large tractor (not shown). As the plough moves forward, or from left to right as shown in Fig.9, the point 114 disrupts the earth and loosens it so that the ducts 116 and 118 having spiral ribs 120, 122 can pass through the hollow inner section, out of the back of the plough and into the ground. In this embodiment, it is not necessary to cover the ducts in a subsequent step. Accordingly, this process is relatively fast. As previously discussed, the optical cables are then passed through the ducts after they have been laid in place.

Claims: A duct for transmission and other cable, the duct comprising polymeric tubing having spiral ribs (58) with peaks (54) and valleys (56) along the inner wall (50), the peaks forming a surface over which cable may travel, to reduce the friction of travel of the cable through the duct, characterised in that the spiral ribs (58) along the inner wall of the duct turn in one direction and then in the other direction. A duct as claimed in claim 1, in which lubricant is received in the valleys (56). A duct as claimed in claim 1 or 2, wherein each rib makes about one revolution for about 0.305m to 1.829 m (one to six linear feet) in one direction and then in the other direction in an alternating fashion. A duct as claimed in claim 1, 2 or 3, wherein the tubing has an inside diameter from 12.5 mm to 152.4 mm (1/2 inch to 6 inches) and a wall thickness from SIDR 5 to 21. A duct as claimed in any of claims 1 to 4, wherein the rib height is from about 0.127 mm to 3.175 mm (0.005 to 0.125 inch). A duct as claimed in claim 5, wherein the rib height is about 0.381 mm (0.015 inch). A duct as claimed in any of claims 1 to 6, wherein the rib spacing is from about 0.635 mm to 12.5 mm (0.025 to 0.500 inch). A duct as claimed in claim 7, wherein the rib spacing is about 3.175 mm (0.125 inch). A duct as claimed in any of claims 1 to 8, wherein the outer wall (52) has spiral or longitudinal ribs (59 or 60). A duct as claimed in claim 9, wherein there are about one to six longitudinal outer ribs (60) per linear 25.4 mm (linear inch) measured about the circumference of the tube. A duct as claimed in any of claims 1 to 10, wherein a velocity-compensated coefficient of friction K, where K = f/ln v, f being the coefficient of friction and ln v being the natural logarithm of the velocity, measured in feet per minute, is about 0.04 to 0.0936. A duct system comprising: an outer duct (31) under the surface of the ground and extending between two points; a plurality of ducts (22,24,26) in the outer duct (31), each of the ducts (22,24,26) comprising polymeric tubing with spiral ribs (58) along the inner walls of the tubing, the spiral ribs having peaks (54) and valleys (56), the peaks (54) forming a surface over which cable may pass at reduced friction without contacting the valleys and cables in the ducts (22,24,26); characterised in that the spiral ribs (58) along the inner wall of each duct turn in one direction and then in the other direction in alternating fashion. A duct system as claimed in claim 12, in which a lubricant is received in the valleys (56). A method for installing a cable in a duct and installing the duct in an outer duct, wherein one end of a draw line (32) is attached to a duct (22,24,26) according to any of claims 1 to 11, the other end of the draw line being sent through the outer duct (31), and the duct (22,24,26) is pulled through the outer duct (31; and wherein a cable is pulled over the peaks (54) of the spiral ribs (58) in the duct (22,24,26), thereby reducing the force necessary to install the cable and increasing the length of cable that can be pulled through the duct without breaking it. A method for installing a cable as claimed in claim 14, wherein a second polymeric tubing is pulled through the outer duct (31) with the polymeric tubing, the outer walls of the first and second tubing having ribs (82,84,86) along their length which cause an interlocking when the first polymeric tubing and the second polymeric tubing are held together. A method for installing cables in ducts, wherein: a trench (90) is opened in the soil and ducts (22,24,26) in accordance with any of claims 1 to 11 are laid in the trench (90), ducts (22,24,26) being bound to each other longitudinally to stop them from rotating relative to one another; and wherein a cable is passed over the peaks (54) of the spiral ribs (58) in each duct (22,24,26), thereby reducing the force necessary to install the cables and increasing the length of cables that can be pulled through the ducts without breaking them. A method of installing a cable in a duct, wherein: a plough (112) is pulled through the soil, thereby disrupting the soil and loosening it and at least one duct (22,24,26) in accordance with any of claims 1 to 11 is passed through an opening from the top to the bottom rear area of the plough; and wherein the duct or ducts (22,24,26) is or are placed in the soil; and a cable is pulled through the or each duct (22,24,26) over the peaks (54) of spiral ribs (58) along the inner wall of the duct, thereby reducing the force necessary to install the cable and increasing the length of cable that can be pulled through the duct without breaking the cable.

Inventors: WASHBURN ROBERT B, WASHBURN, ROBERT B.
Applicants: ARNCO CORP, ARNCO CORPORATION
Application year: 1990
Main classification: H02G1
IPC codes: G02B6, H02G9, H02G1, H02G3