In a twin or quad bundle, all subconductors should reach the tower at equal tension and equal sag. In practice they rarely do, and the difference is not cosmetic. Unequal tension means one subconductor carries more than its share, spacers sit under permanent stress, and the bundle twists and slaps in wind. The causes are mechanical, measurable, and correctable during the pull if the crew is watching the right instruments.
The typical report is that the bundle "looked fine" until the sag was checked. One conductor sits low, the spacers will not line up, and re-sagging means re-clamping four conductors instead of one. Worse, imbalance discovered after the pull cannot be corrected by tensioning: the geometry of the bundle is already fixed by clamps and spacers, so the remedy becomes a re-string of the section.
Each subconductor sees a slightly different path, and each difference adds or subtracts tension. Drum brake settings change payout resistance. Sheave bearings with different friction change the tension required to move the conductor. Nearside and farside routes are rarely the same length. Tensioner capstan grooves wear unevenly, so one groove grips differently from another. And on quad work, four lines multiply these small differences rather than averaging them out.
| Source | Mechanism | Signature |
|---|---|---|
| Drum brake setting | One drum pays out with more resistance | Imbalance present from the first metre |
| Block friction | Worn or dry bearings on one stringing block | Imbalance grows with distance; one path runs warm |
| Route length | Subconductors follow slightly different geometry | Imbalance appears in specific spans only |
| Capstan groove wear | Uneven grip between tensioner grooves | Constant offset independent of section |
| Pull synchronisation | Multiple lines not coordinated | Oscillating imbalance during the pull |
| Conductor length mismatch | Drum lengths and splice positions not planned | Imbalance jumps at a splice |
Tensioners designed for bundle work provide independent tension control rather than one common brake. The 8 ton twin-bundle tensioner is published at 2×45 / 1×90 kN maximum intermittent tension and 2×40 / 1×80 kN maximum continuous tension, with 2×20 / 1×40 kN available at maximum speed, 2×35 / 1×70 kN maximum reverse pulling force, 1300 / 1500 mm bull-wheel and a 77 kW (103 hp) water-cooled diesel. The GL2×70 is specified for twin-bundle 630 mm² and below in mountainous and hilly terrain, 720 mm² and below on flat land, and single conductor 720 mm² in various terrains. On the pull side, the GL140TP is published for one to four ropes or bundled conductors at 140 kN maximum intermittent tension, with hydraulic dynamometers featuring set-point and automatic maximum pull control.
These are catalogue ratings. When judging an actual pull, the operative numbers are the dynamometer readings logged during that pull plus the machine's own force record, not the datasheet.
One further mechanism matters on quad work: a bundle is only as balanced as its slowest line. If one subconductor's pilot rope passes over a block with a failing bearing, that line runs at higher tension from that point onward, and the tensioner holds the offset because its brake receives no signal that the other three lines are lighter. Continuous logging, rather than a single end-of-pull reading, is what makes this visible while it can still be corrected.
Hardware relevant to bundle balance: the 8 ton twin-bundle tensioner, the 16 ton four-conductor tensioner, the GL2×70 77 kW twin-bundle tensioner and the GS280 quad conductor puller.
No. It means one subconductor carries measurably more load and the bundle sags unevenly. Target within 5 percent and investigate anything above it.
Only by re-sagging, because clamps and spacers have already fixed the bundle geometry. Balance during the pull.
Yes. Four lines accumulate four sets of friction differences, so check blocks more often and keep the tolerance tighter.
コンタクトパーソン: Mr. Marble Wu
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