A crossing is not a long span with a bigger machine bolted on. It is a separate construction problem in which span length, ground clearance, live traffic, navigation and wind all constrain a pull that cannot be paused halfway. Most crossing failures are not equipment failures; they are planning failures made visible at the worst possible moment, with the conductor already off the ground.
A 900 m river span at 2.5 km/h takes over 20 minutes of steady load before any slowdown, and if the conductor stalls over a navigation channel the hold can extend much further. During that period the puller must maintain tension without interruption while the tensioner holds back-tension and the blocks take the full conductor weight in the span. Add a traffic window with a fixed closing time and the margin for error disappears.
Three numbers govern the design: crossing span, required minimum clearance and the sag the conductor reaches at final tension and maximum design temperature. For a given conductor weight and tension, sag rises roughly with the square of the span, so doubling the span at unchanged tension multiplies sag by four. That is why crossings are pulled at higher tension, and why the puller's sustained rating rather than its peak rating sets the limit.
| Parameter | What it controls | Planning consequence |
|---|---|---|
| Crossing span | Sag, pull duration, pilot rope length | Longer span means longer continuous pull; heat load matters |
| Required clearance | Minimum tension at maximum temperature | Higher tension raises contact pressure at every block |
| Water level or traffic height | Safety zone and fall protection | Defines the abort point and the stop signal |
| Wind exposure | Blow-off and galloping risk | May require temporary ties or hold-downs |
| Station space | Puller, tensioner, drum stand layout | Usually the tightest real constraint on site |
Crossing work usually sits in the 120 to 280 kN class on the puller side. The SA-QY120 is published at 120 kN intermittent and 100 kN continuous with 50 kN available at maximum speed on a 129 kW engine. The SA-QY150 raises that to 150 kN intermittent and 120 kN continuous with 60 kN at maximum speed, and is specified for four-bundle 630 mm² conductor in mountainous terrain and 720 mm² on flat land. The GS280 states 280 kN maximum tractive force and 250 kN sustained at 2.5 km/h for four or six conductors, with 120 kN corresponding tension. On the tensioner side, the GL140TP is published at 140 kN maximum intermittent tension, configured for one to four ropes or bundled conductors, in a 4700 × 2300 × 2700 mm envelope with hydraulic oil cooling and negative self-acting hydraulic brakes.
All of these figures come from published specifications. For a specific crossing, confirm the sustained duty at the actual ambient temperature and confirm brake holding behaviour at the calculated tension, because that is the condition the machine will actually face.
Machines and tooling typically deployed: the 280 kN hydraulic brake cable puller winch for 800 to 1000 kV work, the GL140TP 140 kN puller-tensioner, the 16 ton four-conductor tensioner and matched seven-sheave running-out blocks.
It can be held, but not repeatedly relaxed and restarted. Holding requires maintaining set tension at zero speed, which is exactly the capability to verify before mobilisation.
The tensioner sets sag and protects the conductor; the puller sets pace and heat load. On a crossing both are fully loaded at once, so both should be sized on sustained ratings.
Enough to keep clearance at maximum design temperature after creep. Calculate it, because the same extra tension also raises contact pressure at every block in the run.
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