BACKGROUND OF THE INVENTION With respect to fiber optic cables, which are being used increasingly in the telecommunications industry, an important concern is the protection of the cables against damage during the installation process, as well as any damage that might occur after the cable has been installed. For these reasons, flexible and durable innerducts made of thermoplastic polymers, such as various grades of polyethylene (e. g., high molecular weight, high density polyethylene) or their blends, are being used with increasing frequency. In practice, the innerducts are typically placed into existing conduits, such as subterranean pipes, with the cable then being installed or introduced into the innerduct.

Alternatively, the innerduct may be buried directly and the cable then installed in the innerduct.

The mechanical properties of the innerduct which are essential for protection of the cable after it has been buried in the ground are relatively easy to obtain by making a suitable selection of the base polymeric material in combination with the geometric dimensions of the innerduct.

The situation is more complicated when it is required to protect the cable from damage during installation into the innerduct. Here, the basic prerequisite is an absence of any sharp or irregular surfaces at the physical contact point between the cable sheath and the innerduct surface which could potentially damage the cable sheath or disrupt the transmission function of the optic fibers. A smooth inner surface of the innerduct can be achieved by carefully selecting suitable polymers, technological parameters, and geometry of the extrusion tool, thus permitting the avoidance of surface defects in the

flow during processing.

The requirement to reduce the coefficient of friction between the cable and the innerduct surface is particularly important given that there is a limitation on the force to be used when pulling a transmission cable through the innerduct, as well as the desire to achieve longer cable pulls without the need for a relay (e. g., in order to achieve an improvement in cable installation economy). Similarly, friction will limit the lengths through which such cable may be installed by blowing.

A lower coefficient of friction can be attained by using one or more lubricants introduced into the innerduct interior during the cable installation (i. e., external lubrication) or by using a lubricant-containing polymer in the manufacture of the interior innerduct surface in contact with the cable, i. e., a material having a reduced coefficient of friction (i. e., permanent lubrication).

In either manner, the result is a highly lubricous surface or layer being formed on the interior innerduct wall.

Another method of lowering the coefficient of friction consists in reducing the contact area between the cable sheath and interior innerduct surface, i. e., increasing specific load. The reduction in the contact area between the cable sheath and the interior innerduct surface is achieved by providing the interior wall of the innerduct with longitudinal ribs or ribs forming a spiral-like structure in the interior wall of the innerduct, the spiral ribs being used particularly when external lubrication is applied. In this manner, cross- sections of the ribs forming grooves in the interior wall of the innerduct have a shape of a rectangle or a triangle with its top ridge facing towards the innerduct interior.

Although both methods of lowering the coefficient of friction between the cable and the interior innerduct surface described above have their unquestionable advantages, the use of one or the other did not have the desired result of protecting the cable from damage during installation into the innerduct.

Therefore, a combination of the above methods, i. e., combination of the

ribbed innerduct surface with external or permanent lubrication, is increasingly being used in practice. Such a solution is, for instance, an innerduct manufactured by co-extrusion of two polymeric layers with an inner layer of lower thickness having good sliding properties (e. g., a low coefficient of friction) and longitudinal ribs. The sliding properties of the inner layer are obtained by incorporating a specified amount of lubricants or slide-increasing additives (such as graphite, silicone, MOS2, PTFE (e. g., TEFLON@), etc.) directly into the polymer. An example of this technology can be found in U. S.

Patent No. 4,892,442 issued to Shoffner on January 9,1990, the entire specification of which is incorporated herein be reference.

A relative disadvantage of this method, which otherwise appears to be the most advantageous with respect to lowering of the coefficient of friction, is that the relatively expensive material comprising the inner lubricated or slide layer is contained even in those portions of the interior innerduct wall which are not in direct physical contact with the cable, thus unnecessarily increasing the price of the whole innerduct.

Therefore, there is a need for an innerduct having a lubricated, ribbed inner layer or surface, wherein only those portions of the ribs that actually physically contact the cable are lubricated.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a new and improved innerduct.

It is another object of the present invention to provide a new and improved innerduct having a plurality of radially protruding, longitudinally extending members disposed on an interior surface thereof.

It is another object of the present invention to provide a new and improved innerduct having a plurality of radially protruding, longitudinally extending members disposed on an interior surface thereof, wherein a lubricous material is disposed on at least a portion of the members.

In accordance with another embodiment of the present invention, an

innerduct having an outer circumference and an inner circumference is provided, the innerduct having a plurality of alternating grooves and radially projecting, longitudinally extending members disposed on at least a portion of a surface of the inner circumference, wherein a lubricous material is disposed on at least a portion of the surface of the members, wherein the grooves are substantially free of the lubricous material.

In accordance with another embodiment of the present invention, an innerduct having an outer circumference and an inner circumference is provided, the innerduct having a lubricous material disposed on at least a portion of the surface of the inner circumference in a discontinuous pattern.

In accordance with still another embodiment of the present invention, a method for making a lubricated innerduct is provided, comprising: providing an innerduct having an outer and inner circumference; and applying a lubricous material to at least a portion of the surface of the inner circumference in a discontinuous pattern.

A more complete appreciation of the various embodiments and aspects of the present invention and the scope thereof can be obtained from a study of the accompanying drawings, which are briefly summarized below, the following detailed description of the invention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a cross-sectional view of a duct, in accordance with the general teachings of the present invention; and Figure 2 illustrates an enlarged fragmentary cross-sectional view of a detail of the ribs and grooves of the duct depicted in Figure 1, in accordance with the general teachings of the present invention.

The same reference numerals refer to the same parts throughout the various Figures.

DETAILED DESCRIPTION OF THE INVENTION The present invention overcomes, or at least eliminates to a

considerable extent, the shortcomings of conventional methods relating to the installation and mechanical protection of fiber optic cables within innerducts.

Although primary reference will be made to the use of the present invention in conjunction with fiber optic cables, it should be appreciated than many other types of cables, lines, and/or conduits may also be used to practice the present invention, such as power transmission lines, fluid transmission lines, and so forth.

The innerduct of the present invention is preferably comprised of a polymeric material, such as, but not limited to polyethylene (e. g., high density polyethylene) wherein the inner or interior surface of the innerduct is provided with a system (e. g., a plurality) of radially protruding, longitudinally extending members (e. g., a series of ribs and corresponding grooves) which are preferably evenly spaced around the circumference thereof. The shape of each of the grooves is given by the base cylindrical surface of the duct inner surface in its bottom area and by surfaces of the adjacent ribs in the area of its sides, the ribs having a cross-section of a substantially drop-like shape narrowing towards the base cylindrical surface of the duct inner surface.

The rib height preferably constitutes about 20 to about 60% of the innerduct wall thickness and the rib peak width is preferably about two to about four times the rib foot width. The present preferred minimum groove width between adjacent ribs is about 1 to about 2 times the rib peak width.

Further, it is advantageous if the end of the rib top has a shape of a cylindrical surface and if at least a portion of the surface of the rib in this top area (i. e., in the place of the possible physical contact with the cable sheath) are covered with a layer of a lubricant or slide material attached firmly to the rib body.

The main benefit of the present invention is the fact that the grooves in the duct inner wall have, in comparison with the prior art, a more closed shape given by ribs of a drop-like cross-section. This will manifest itself favorably (with respect to aerodynamics) when blowing is used as the main or auxiliary process in the cable installation.

Practical blowing conditions inside conventional innerducts lead to a flow which is turbulent in nature. On the contrary, the present invention provides a closed shape of the groove which creates favorable conditions for occurrence of a streamline (i. e., laminar) or predominantly streamline flow due to the existence of a laminar sublayer (produced during the turbulent flow along the rib wall) filling a large part of the groove cross-section. It is generally known that pressure losses during turbulent flow are typically higher than those in a streamline (i. e., laminar) flow due to an exchange of particle momentum in the course of particle mixing motions between adjacent layers of flowing gas when the particles, besides the basic motion in the flow direction, perform their own motions outside of the flow direction. The different pressure losses result in a flow from the groove volume into the innerduct internal space caused by a pressure difference in the given innerduct cross- section. This direction of the flow assists in shifting the cable towards the innerduct center thus reducing the friction drag on the cable when it is being transported (e. g., by pulling, pushing, blowing, and the like) through the innerduct.

The drop-like shape of the rib with a cylindrical end at the rib top reduces considerably the risk of formation of sharp projections (in the innerduct manufacture) that might damage the cable sheath during its installation.

Because the cable sheath is in contact with the rib tops, both of which have oppositely curved surfaces contacting each other virtually in a straight line, the advantage of ribbed innerducts, i. e., the minimum contact area between the cable and the duct inner surface, is maintained.

As the lubricous material is applied or disposed only in the rib top area where physical contact with the cable being installed is expected, a savings (up to 50 percent) of relatively expensive lubricous material is achieved. This will permit the increase in the thickness of the lubricous material layer and thus reduce the risk of wearing it through, or, alternatively, the use of even more expensive lubricous materials permitting the reduction of the coefficient of friction in a more pronounced way without increasing material costs of the

innerduct production.

Referring to the Figures, a non-limiting example of an innerduct structure in accordance with one embodiment of the present invention is shown. Innerduct 1, which is intended primarily for installation and mechanical protection of fiber optic cables is preferably made of polyethylene and has an outside diameter D of about 40 mm, a wall thickness h1 of about 2.9 mm and is preferably provided with a plurality or system of about 50 radially projecting, longitudinally extending ribs 3 and a corresponding number of grooves 4, which are preferably evenly spaced around its internal circumference.

The shape of each of the grooves 4 is given by the base cylindrical surface 5 of the innerduct 1 inner surface in the groove bottom area and surfaces 9 of adjacent ribs 3 in the area of the groove sides. The separate ribs 3 have a drop-like cross-section narrowing towards the base cylindrical surface 5 of the innerduct 1 inner surface. The height h2 of ribs 3 is preferably about 1.2 mm and, consequently, constitutes about 40% of the thickness h1 of the innerduct wall 2. The top width s1 of ribs 3 is preferably about 0.8 mm which constitutes about twice the rib foot width s2 (i. e., about 0.4 mm). The minimum width s3 of grooves 5 between the adjacent ribs 3 is about 1.2 mm, being consequently 1.5 times the rib 3 top width s1.

The ends of the ribs 3 in the rib top area 7 have a shape of a cylindrical surface, which is provided with a layer of a lubricous or slide material 8 having a thickness h3 of about 0.5 mm at maximum. The lubricous material 8 is preferably connected firmly to the body of the rib 3, by any number of suitable methods such as co-extrusion, applying, spraying, pouring, painting, brushing, rolling, gluing, laminating, and so forth.

The foregoing detailed description shows that the preferred embodiments of the present invention are well suited to fulfill the objects above-stated. It is recognized that those skilled in the art may make various modifications or additions to the preferred embodiments chosen to illustrate the present invention without departing from the spirit and proper scope of the

invention. For example, while the innerduct of the present invention has been disclosed for use with fiber optic cables, it is also well suited for use with other types of cables or the like. Accordingly, it is to be understood that the protection sought and to be afforded hereby should be deemed to extend to the subject matter defined by the appended claims, including all fair equivalents thereof.