A rebar straightener in a tunnel job feeds a rebar bender downstream and ultimately a rebar cutter at the cage build-up station, so its diameter window, throughput, and residual curvature budget dictate the whole cage line [S2].
Tunnel cages typically consume 12-32 mm deformed bars, with TBM segment cages leaning to 12-20 mm and cavern / launch-shaft walls pushing to 32 mm; 8 mm wire for stirrups and mesh is usually handled by a separate wire straightener, not a heavy bar line.
What the tunnel job actually demands from a straightener
Tunnel reinforcement yard volume is tied to the boring method: a slurry or EPB TBM advancing 8-12 m/day in a metro contract will burn through 25-60 t of rebar per ring cage, while cut-and-cover and NATM drives use shorter, denser bar runs in the 16-25 mm range [S2]. The straightener on the front of that line must hold a residual yield-strength loss under 5%, otherwise the bar has been cold-worked past code limits before the rebar bender ever touches it.
Three numbers decide a tunnel-yard straightener: bar diameter window, line speed in m/min, and mandrel / roller count. A unit rated 8-32 mm at 30 m/min with five-roller geometry is the minimum credible specification for 2026 tenders; cheaper three-roller units over-curve 25 mm+ bars and the residual camber shows up in the finished cage [S1].
Diameter windows, line speed, and the coupler upstream
Bar grade selection has moved with coupler approvals: B500B and B500C per EN 10080 are now the default European tunnel grades, while ASTM A706 Grade 60 (weldable) is the U.S. equivalent when rebar couplers are specced rather than lapsed splices [S1]. A straightener that quotes a 500 MPa yield retention certificate on 32 mm B500B is the bar to compare against; do not accept a generic "low-carbon bar" claim on the data plate.
Line speed benchmarks: 8-16 mm at 30-50 m/min, 16-25 mm at 18-30 m/min, and 25-32 mm at 8-18 m/min. The pinch-roll pressure and flywheel diameter scale with bar size; a 32 mm pass needs a 60-80 mm solid flywheel, smaller flywheels on heavy bars mark the surface and trip ultrasonic inspection at the rebar cutter station.
Straightener, bender, cutter: how they cascade on a tunnel line
A tunnel cage line is three machines in series: straightener, bender, cutter. A straightener output of 25 m/min feeding a two-head bender at 12-15 bends/min and a 32 mm-rated cut-off is the matching trio for a metro segment yard; mismatched throughputs leave the bender idle or the cutter starved, both expensive in a night-shift operation. [S2]
Mandrel design and bend radius matter: a mandrel radius of 3-5d (bar diameters) on the straightener sets the residual camber under 1 mm/m, which is the threshold where the bar will sit flat in a rebar jig without spring-back during cage assembly. Units that skip the mandrel and rely on crossed rolls alone are fine for stirrups, not for primary tunnel bars.
Mechanical vs servo-electric vs hydraulic: which fits a tunnel site
Mechanical flywheel straighteners are the workhorses: 4-7.5 kW main motor, shear-pin or torque-limited overload, 5-roller geometry, throughput dominated by bar diameter. Servo-electric straighteners add closed-loop pinch-roll control, typically 3-5 kW per axis, and win on 12-20 mm accuracy and noise (a tunnel site at night often has a 75 dB(A) cap). Hydraulic units are reserved for 32-50 mm heavy bar; they trade energy cost for a 15-25 kW draw but hold bar tension constant under fluctuating mill scale. [S1]
Comparison of the three types on tunnel-yard criteria: mechanical (cost low, maintenance high, diameter 6-32 mm, throughput best, accuracy 1-2 mm/m residual cammer); servo-electric (cost medium, maintenance low, diameter 6-25 mm, throughput good, accuracy under 1 mm/m); hydraulic (cost high, maintenance medium, diameter 16-50 mm, throughput medium, accuracy 1-2 mm/m). For metro segment cages in 12-20 mm, servo-electric is the 2026 default. For cavern and shaft walls at 25-32 mm, mechanical still wins on capex.
Standards, sourcing, and what to demand on the data plate
Demand EN 10080 mill cert traceability on the input bar plus a CE / UKCA mark on the straightener itself, and require the supplier to state residual yield-strength test results on the heaviest bar in the window (e.g. "32 mm B500B, 97% yield retention"). Machines bundled with construction tools packages (rebar tier, spacer, and bender) simplify spare-parts holding on a remote tunnel site [S1].
Insist on a documented bend test: the straightened bar must wrap a mandrel of 3d (bar diameter) for 180 degrees without surface cracking visible at 10x magnification. Also confirm the IP rating (IP54 minimum for tunnel-yard dust) and that the flywheel guard is interlocked, not just a removable cover. A straightener without an interlocked guard is a tender-disqualification item on European metro jobs.
Failure modes the spec sheet will not tell you
Three failure modes dominate tunnel straightener returns: (1) mandrel wear on 16-20 mm high-yield bar, which lifts residual camber past 2 mm/m and shows up as "drunken" stirrups; (2) flywheel bearing failure on 32 mm bar driven past rated speed, audible as a 4-6 kHz whine before seizure; (3) pinch-roll slip when surface rust or mill scale is present, leaving longitudinal scoring and tripping the rebar straightener inspection protocol. [S1]
Mitigation: keep a spare mandrel set on site, run incoming bar through a descaling pass for rusty deliveries, and load the straightener motor to 70-80% of nameplate, not 100%, to keep the flywheel in its sweet spot. The most common operator error is overspeeding the line on small bars to "make rate" while a 32 mm run is queued, which is the path to the second failure mode above.
For plumbing jobs that share bar yards with tunnel sites, straightener picks for plumbing installation overlap on 8-16 mm selection but diverge on mandrel geometry and footprint.