Detailed Description
Why Fiber on a Reeling Drum?
A stacker/reclaimer in a coal terminal has a boom length of 40–60 meters, a travel distance of 500–1 000 meters along the stockyard rail, and a motorized cable reeling drum that pays out and takes up cable as the machine travels. The reeling drum stores 500–1 000 meters of cable in multiple layers. Every meter of that cable must carry:
- CCTV video from cameras on the boom and machine house to the operator cab and remote control room
- PLC-to-PLC communication between the machine's onboard control system and the terminal's central control system
- Ethernet data for the terminal operating system (TOS) and machine monitoring
- Safety interlocks — emergency stop, fire detection, belt misalignment — that must function without interruption
Fiber optic reeling cable solves all three problems: no EMI, no ground loops, and lighter weight per meter than an equivalent copper data cable.
Stainless Steel vs PBT Loose Tubes — Why Stainless for Reeling
The loose tubes that house the optical fibers must withstand the compressive force of multiple layers of cable wound onto the drum. A reeling drum stores cable in layers — 3, 5, 8 layers or more, depending on drum diameter and cable length. The bottom layer experiences the compressive force of all layers above it — potentially 500–1 000 N per 100 mm of cable length.
| Property | PBT Loose Tube | Stainless Steel Loose Tube |
|---|---|---|
| Crush resistance | 1 000 N/100 mm (typical) | 4 000 N/100 mm (typical) |
| Thermal expansion | High (α ≈ 1.3 × 10⁻⁴/K) | Low (α ≈ 1.7 × 10⁻⁵/K) — matched to fiber |
| Temperature range | –40°C to +70°C | –50°C to +85°C |
| Moisture barrier | Depends on gel fill | Metal tube = absolute moisture barrier |
| Weight | Lighter | Heavier |
| Cost | Lower | Higher (+30–50% premium) |
| Rodent resistance | Vulnerable | Resistant |
For indoor or light-duty reeling (short drum, few layers, controlled environment), PBT loose tubes are acceptable. For outdoor, heavy-duty reeling — stacker/reclaimers, STS cranes, mining equipment — stainless steel loose tubes are the standard specification. The cost premium is justified by the elimination of tube crush failures that would otherwise require a complete cable replacement on a machine that costs $5 000–20 000 per hour of unplanned downtime.
Water Blocking — Three Layers of Defense
A reeling cable on an outdoor machine is exposed to rain, snow, condensation, and occasional immersion. Water entering the cable follows the interstitial spaces between tubes and migrates along the cable length. When it reaches the fiber, two things happen:
- Hydrogen darkening: Water reacts with the silica glass fiber surface, creating hydroxyl (OH⁻) absorption peaks at 1 383 nm that increase attenuation
- Freeze-thaw damage: Water that freezes inside the cable expands by ~9%, creating mechanical stress on the fibers and tubes
- Layer 1 — Gel in tubes: Thixotropic gel fills the space between the fiber and the inner wall of each stainless steel tube. The gel is hydrophobic — it repels water and prevents it from contacting the fiber. It also lubricates fiber movement as the cable bends
- Layer 2 — Swellable tape: A water-swellable tape is helically wrapped around the assembly of stainless steel tubes. If the outer jacket and armor are breached, water contacts the tape, which swells and forms a gel plug that blocks longitudinal water migration
- Layer 3 — Swellable yarns: Water-swellable aramid yarns fill the interstices between the tubes and under the armor layer. These yarns provide additional longitudinal water blocking and also serve as tensile strength elements
Yichi Fiber Optic Reeling Cable Manufacturing
- Stainless steel tube welding: Tubes are formed from AISI 304 stainless steel strip, longitudinally welded, and drawn down to final diameter. The weld seam is continuously tested by eddy current — any weld defect triggers automatic rejection of the affected tube length
- Excess fiber length (EFL) controlled: EFL is measured by OTDR on every tube before cabling. Target is 0.2–0.5% — slightly higher than festoon fiber (0.1–0.3%) to accommodate the greater elongation of the cable structure under reeling tension
- Armor coverage verified: Steel wire armor coverage is optically measured — minimum 90% coverage over the inner sheath. Gaps in armor coverage create weak points where the inner sheath is exposed to mechanical damage
- Attenuation tested pre- and post-cabling: Fibers are tested for attenuation before cabling (bare fiber), after tubing (fibers in tubes), and after final jacketing (finished cable). Any increase greater than 0.05 dB/km at any stage is investigated — it indicates a manufacturing problem (excessive tension, tight bend, crushed tube) that must be corrected