If the cable is made with strength members in its outer layer such as aramid yarns or steel wire armoring, these outer elements are also included to the strength calculations. The consideration of central strength member for tensile calculation depends on the installation method of the fiber optic cable.
When the fiber optic cable is installed by pulling it from the sheath with a pulling eye, the central strength member can not be considered for strength calculations.
When the cable is installed by pulling it with a pulling eye which connects central as well as peripheral strength members, then you must include both strength members for calculation.
Cable components such as plastic tapes, binder yarns, sheathes are not generally not considered for tensile force calculation. Only wires and strength yarns can be considered.
The tensile strength of cable depends on the Y, the young’s modulus of strength member and cross sectional area of the material and the permissible cable elongation.
Ts = Sum of ( Y A Ep )
Y = Young’s modulus of cable materials
A = Area of cross section of cable components
Ep = Permissible elongation of cable
To meet high tensile strength, the strength members must possess several requirements including high Young’s modulus, high elastic limit, low weight per unit length and flexibility. As we can see the young’s modulus and elastic limit are contradicting parameters, an optimum value must be sought. That is the work of a fiber optic cable design engineer; to find a compromising solution between the characteristics of strength members. Stability over a wide range of temperatures and friction coefficient in relation to other cable components are also important parameters in deciding the strength members in fiber optic cable design.
The Minimum Young’s modulus in kN/sq.mm of some strength members are given below:
GI wire – 180, Stranded steel wire – 160, Stainless steel – 170, FRP – 50, Aramid Yarns – 120.
Pre-assumptions and design principles:
Maximum permissible elongation of strength member vary as per the type of material. For steel strength member the maximum elastic range is 0.5%, the Young’s modulus is 40kN/mm2. For FRP the maximum elastic range is 1 % and the Young’s modulus is 120-200 kN/sq.mm.
The standard tube diameters, which were in practice till around 2000 had drastically reduced with the market observing severe competition and the OSP contractors demanding smaller cables easy to handle and install. This has forced manufacturers to develop micro duct cables with small diameter loose tubes. Even though for our academic purpose, let us see some of the conventional loose tube diameters which are in practice.
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