single fiber loose buffer
In the hollow loader, the secondary protection consists of a tube in which one or more fibers are located. This results in optimum protection of the fiber against transverse forces. The transverse forces are absorbed by the tube and cannot affect the fiber. The outer diameter of the hollow loaders is in the range of 1.2 to 4.5 mm, depending on the design. Tensile forces and high as well as low temperatures have a direct effect on the fiber via the secondary coating of the hollow loaders.
At low temperatures, the tube becomes shorter than the fiber according to the thermal expansion coefficient. The fiber now takes the path of least resistance and spirals against the inside of the tube. As long as the pitch of the spiral is not below a critical value, this process remains damping neutral.
At high temperatures, the tube becomes longer. Initially, the existing excess length is used up. With a further increase in temperature, tensile stress is applied to the glass fiber.
When tensile forces occur, the excess length is also used up first. Tensile loads exceeding this lead to tensile stress on the fiber.
Hollow wires are manufactured with up to 12 fibers in the tube. From 2 fibers upwards, the term " loose tube" is used. In the case of hollow conductors with a longitudinally watertight design, the cavity in the tube is filled with a core filler. This compound must not be too highly viscous so that fiber mobility is not restricted even at low temperatures. On the other hand, the core filler must not drip out at higher temperatures.
The advantage of a hollow core is its insensitivity to transverse forces that can arise from stranding and temperature fluctuations. The mechanical strength of the hollow core is much higher than that of the solid or compact core. In the cable abbreviation, the second letter is an "H" for an unfilled Hollow core, and a "W" for a filled one.