A strength member is used to increase the tensile force which the cable will experience starting from manufacturing stage to the installation through ducts or directly buried or pulled through. To design a fiber optic cable, it is a must to know what are the elements that contribute to increase the tensile force. This depends on
The practical rules are:
If the fiber optic cable contains only a central member, only this is included in the strength calculations. The tensile force is directly transferred to the central strength member. All other elements contribute negligibly to the counteract the cable tensile force.
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.
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:
Excess fiber length in loose tube after stranding is 0%
Permissible axial fiber elongation is 0%
Minimum permissible fiber bending radius is 40mm
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/mm2
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. Please note that these are purely from the experience of fiberkids and no authority is claimed on these data.
2 to 4F per tube - 1.8/1.2 mm (Tube OD/ID)
6F – 2.0/1.4 mm
12F – 2.2/1.5 mm
How to calculate the number of elements over CSM ?
There are many complicated formula available in the books. Fiberkids do not want to confuse you with complicated mathematical formula. The most friendly and easy formula is:
Where R = Stranding radius, sum of half diameter of (CSM + Loose tube) i.e.: CSM dia/2 + Loose tube dia /2
LT dia = Outside diameter of loose tube
If you put this formula in excel sheet, this is best way to calculate the central strength member diameter too.
How to calculate the fiber bundle diameter?
With the introduction of micro duct cables with small loose tubes the knowledge and importance of fiber bundle diameter has become a must. In older times of fiber cable history, there was enough margin between the inner diameter of loose tube and the fiber bundles. So cable design engineer never worried about the fiber bundle diameter as the big inner diameter will ensure safe positioning of fibers. But when the micro duct cables demanded neck to neck placement of fiber bundles inside very thin tubes, the fiber bundle diameter became apparently important. The formula to calculate the fiber bundle diameter is given below:
Where Fbd = Fiber bundle diameter
Fd = Fiber diameter (generally 0.255 mm is taken)
N = Number of fibers in bundle or tube