会社のニュース Why Continuous Pulling Force Matters More Than Maximum Pulling Force in Long-Distance Stringing

A pull that stops halfway across a river does not stop because the machine ran out of strength. It stops because the oil reached 90 °C, the relief valve lifted, and the operator lost the ability to hold tension while the conductor hung in the span. The number that predicts this is not the maximum pull on the quotation sheet; it is the continuous pulling force, and the duty cycle behind it.

The construction problem: the pull that cannot be paused

On short sections, a contractor can tolerate overheating: stop, let the oil cool, restart. On a 900 m crossing or a long straight run between two tension towers, stopping is expensive and sometimes unsafe. The conductor is suspended, the traffic or navigation window is ticking, and restarting a partially sagged conductor risks snatching and slip. So the real question on site is not "how hard can this machine pull" but "how long can this machine hold this pull without derating".

Technical cause: heat load grows with continuous duty

Pulling power is force times speed. At 2.5 km/h, which is 0.69 m/s, a 100 kN pull requires about 69 kW at the rope; 120 kN requires 83 kW; 250 kN requires 174 kW. Every kilowatt lost in pump, motor and relief valve becomes heat in the hydraulic oil, and the cooler must reject that heat continuously.

Once oil passes about 80 °C, viscosity falls, internal leakage rises and the machine derates itself: the same pressure setting produces less rope pull and more heat. Above 90 °C seals harden and pump life falls sharply. That is the mechanism behind a pull that "just gets slower" and then stops.

Parameter examples: how the two ratings differ

Published specifications show the gap clearly. The SA-QY60 lists 60 kN maximum intermittent pull against 50 kN maximum continuous, with 2.5 km/h at max pull, 5 km/h maximum speed and 25 kN available at maximum speed. The SA-QY120 lists 120 kN intermittent against 100 kN continuous, 50 kN at maximum speed, on a 129 kW (173 hp) engine. The SA-QY150 moves to 150 kN intermittent, 120 kN continuous and 60 kN at maximum speed. The GS280 states 280 kN maximum tractive force against 250 kN maximum sustained traction at 2.5 km/h, with corresponding tension 120 kN.

The pattern is consistent: continuous capability sits at roughly 80 to 85 percent of the peak, and the pull available at maximum speed is lower again, typically 40 to 50 percent of peak. A specification that quotes only a peak figure is telling you what the relief valve is set to, not what the machine can do for an hour.

Field handling: planning a long pull correctly

• Calculate the worst-case section pull, add 20 to 25 percent margin, and compare against the continuous rating, never the peak.
• Ask for the continuous rating at your site ambient temperature, not at 20 °C, and confirm the cooling package: hydraulic oil cooling system, radiator type and thermostatic bypass.
• Log oil temperature every 15 minutes on long pulls and set three thresholds: investigate at 75 °C, reduce load at 80 °C, stop at 85 to 90 °C.
• Verify the machine can hold set pull at near-zero speed, which is what a stalled crossing requires.
• Where possible, split a long section into two pulls rather than one continuous overload; the cheapest cooling upgrade is a shorter duty cycle.
• Keep the cooler core clean at every shift change. A fouled radiator derates a healthy machine faster than age does.

Compare the published sustained ratings of the SA-QY60 60 kN puller, the SA-QY120 120 kN puller, the SA-QY150 150 kN puller and the GS280 280 kN machine before specifying for a long crossing.

FAQ

Does a higher maximum pull mean a stronger machine?

It means a higher relief setting. Structure, pump displacement and cooling capacity decide what the machine can sustain, and that is the number that finishes a long section.

What oil temperature should trigger a stop?

Investigate at 75 °C, reduce load at 80 °C and stop at 85 to 90 °C. Continuing past that turns a routine pull into a pump repair.

Can oil grade raise sustained pull?

Only marginally. A higher viscosity index oil delays the derate but does not add cooler capacity, so the steady state limit stays where the radiator puts it.