Friction in Conduit Systems

Friction is a critical limiting factor in determining the type and length of cable
installation. Although very little information on cable installation is provided in this
guide, this section has been made available as a background reference on frictional
properties.
Definitions
Friction:the nature of interaction occurring between two surfaces. The basis of
friction has its roots in the mechanical and physical-chemical makeup of the
interface created by bringing together two surfaces.
Coefficient of friction, COF:the ratio of the force required to move a body relative to
the normal, or clamping force, acting to keep the bodies together.
Static COF:the ratio of forces required to bring about the onset of motion between
two bodies at rest with each other.
Kinetic COF:the ratio of forces acting on a body already in motion. It is essentially a
measure of the effort required to keep the body in motion.
Friction Reduction
Friction reduction can be promoted by reducing mechanical interactions, grounding
electrostatic charges, reducing polar interactions, selecting dissimilar polymers, and
employing methods and mechanisms which act to dissipate heat. Although many
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times little can be done to control the composition of cable jacket materials, choices
can be made to select friction-reducing conduit designs and lubricating mechanisms.
The use of lubricants is strongly recommended during the placement of the conduit
or cable, or may be included in the manufacturing process of the conduit. Typical
lubrication methods would include:
Water-soluble lubricantsare available in many different forms including low
viscosity free-flowing petuitous liquids, creamy consistencies, and stiff gels. Low
viscosity liquids are best suited for placement of long lengths of lightweight cables,
such as fiber cables. Heavier, cream-like consistencies are useful on lightweight
power conductors. Stiff gels are used in vertical applications in buildings, or where
high sidewall loads are expected in placement of heavy power cables or innerducts.
Polymeric water-soluble lubricantsare commonly used in the field to lubricate the
placement of cable, or of the conduits themselves. In this case the lubricant is applied
either ahead of, or in conjunction with, the advancing cable. Water-soluble polymer
chemistries include a number of different enhancements including surface wetting
and cling, modification via fatty acids or their derivatives, or by inclusion of various
friction-reducing oils, including silicones.
Conduits may be pre-lubricatedduring the manufacturing process by incorporation
of lubricants directly onto the conduit inner wall, or via a lubricant-modified
coextruded layer. The most common type of lubricant used for this type of
application is silicone polymer, although other agents such as mineral oils, fatty acid
derivatives and glycols have also found use.
Prelubrication finds particular value with fiber cable push-blow systems. Because
the sidewall loads with these techniques are quite low compared with pulling,
and the distances so great, the viscous drag contributed by water-soluble lubes
can be detrimental. The ultra-light amount of lubricants employed by factory prelubrication methods can be a real advantage.
Geometry of the inner surface of the conduit can also play a role in friction
reduction. As the normal load increases, the COF is found to decrease, unless the
surface is damaged in such a way so as to increase the contact area, or heat is allowed
to build up at a rate faster than it can be conducted away. Ribs formed on the inner
conduit wall are a common design feature to reduce friction.
Longitudinal ribbingresults in a reduction of the contact surface between the cable
and the conduit wall from an area to a line of contact. Decreasing the area of contact
under the same sidewall load results in a higher localized normal force. Within a
limited range of sidewall loads, the COF is found to go down – at least until the
loading causes localized damage to the jacket sheath.
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Spiral ribbingfurther reduces the contact area from a line to a series of points. In
addition, because the advancing cable is alternately on and off the ribbing, there is
an opportunity for cooling and re-lubrication. Constantly changing the direction of
the spiral eliminates the tendency to accumulate spiral-induced torque in the cable.
Transverse ribbing,or corrugated profiles, results in similar friction reducing
geometries. However, there is a tendency for field-added lubrication to be scraped
off the cable by the corrugations. In addition, the high degree of flexibility requires
careful placement of the duct to reduce the buildup of friction due to path curvature.