Calculation of Excess Fiber Length in Loose tube

Optical fibers loaded at the Fiber pay-offs need some tension in order to maintain a constant paying off process. This tension applied on the fiber is taken by the glass part of the fiber mainly as the strain bearing capacity of silica is higher than the acrylic coating. The fibers then enter into the plastic tube simultaneously filled with jelly. The loose tube enters a hot water trough. Gradient cooling of the tube is necessary in order to keep the plastic properties. Passing through the wheel capstan, the tube goes through the Cold-water trough. 

The transition from Hot water to Coldwater results in the contraction of the tube. After drying off, the tube passes through the diameter monitoring unit and then pulling the caterpillar. The pulling force between the wheel capstan and pulling caterpillar creates elongation of the tube. The tube is then taken to the bobbin at the take-up unit through pulleys. 

The winding of the tube is to be done by applying tension in order to keep the layers of winding intact. The winding process results in the elongation of the tube at the take-up unit.

The tube experiences contraction at some points and elongation at some points.

excess fiber length in tubes

So the above process of Tube making can be expressed by the following mathematical formula:

Excess Fiber Length = ContractionStrain


Contraction of the tube can be calculated by the following formula:

Contraction in % = ΔT


= Co-efficient of Thermal Expansion of the tube material,

ΔT = Difference in temperature between Hot water and Coldwater troughs.

ΔT = T1 – T2  (T1 = Temperature of Hot water trough, T2 = Temperature of cold water trough)


Elongation or strain can be calculated by the following formula:

Elongation/Strain (%) =       T /YxA  


T = Tension (Pay-off Tension, Traction force, Winding Tension, etc.)

Y = Young’s modulus of the Tension bearing material.

A = Area of the cross-section of the Tension bearing material.

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