Web Tension Measurement and Control: A Design Guide

A design guide for measuring tension in a moving web — what the measurement solves, how the transducers work and mount, how wrap angle and roll weight set the force you size for, and how to choose instrumentation and protect it in hazardous areas.

What Web Tension Measurement Solves

Running a web at its optimum tension is what produces high-quality material across paper, film, foil, rubber, non-wovens, metal, and plastic, and across converting steps like coating, laminating, printing, and slitting. A measurement system turns tension from a guess into a controlled variable.

• Quality. Accurate tension eliminates over- and under-stretching, improves registration, removes wrinkles, and tightens dimensional control. Dancer rolls that can damage fragile webs can often be eliminated.
• Faster setup. A direct tension readout lets an operator reproduce the right tension every time, so different materials and web widths can run on a single machine.
• Higher productivity. Higher running speeds, breakage alarms that catch a web before it parts, and a feedback signal for manual or automatic drive, brake, and clutch control reduce scrap and downtime.

How the Measurement Works

A tension system is one or two transducers (a module adds the mounting hardware), their junction boxes, and instrumentation. BLH Nobel uses SR-4 foil strain gages in a full Wheatstone bridge that is temperature-compensated and deadweight-calibrated — giving accuracy and sealing that hold up in hot, corrosive, high-tension environments, roughly ten times the rangeability of semiconductor designs in low-tension work, and no need for recalibration.

• Low-tension transducers. Cylindrical in design. They install at idler-roll ends — one per end for left, right, and total tension — replace a cantilevered idler on single-sided machines, or attach to in-line pulleys for single strands. Range: milligrams to 500 lb.
• High-tension modules (HTU). Mount beneath a standard pillow-block bearing on a load plate, with the transducer at the bearing centre line so bearing friction cancels during start-up and shutdown. Mount under both roll ends for left, right, total, and differential tension. Range: 500 to 100,000 lb.

Getting an Exact Measurement

The goal is to capture the true tension force, Fr, while rejecting vibration, roll weight, and other noise. Because that force acts along the bisector of the wrap angle, a transducer that reads only horizontally or vertically loses part of an asymmetrical signal. BLH Nobel low-tension transducers are cylindrical: at start-up you rotate the transducer while watching the readout, then lock the bolt at the angle that captures the full Fr. High-tension modules read both axes at once, and an Expert Series instrument computes the exact resultant-force angle continuously.

Entry and exit angles are usually fixed, but where a roll is adjustable the wrap angle can vary, and the resultant force stops being proportional to tension. Deflector rolls pin down the geometry around the measured roll:

• One-roll The wrap angle is defined by other fixed rolls in the line, so no deflector roll is needed.
• Two-roll The geometry is defined on one side of the measured roll but not the other; a single deflector roll fixes the open side.
• Three-roll The geometry is undefined on both sides; two deflector rolls fully define it. An HTU system computes the resultant angle automatically.

Choosing Where to Install

Where several locations could carry the measurement, these factors — in order of importance — optimize the result. Thanks to module accuracy, there are no limits on wrap angle or roll weight.

Imbalance and speed
Balance the roll dynamically at maximum speed. Some imbalance is acceptable, but if the imbalance force plus the carried force exceeds the transducer's linear range, the signal distorts. Keep running speed away from — or moving quickly through — the roll's critical speed.
Fimb = e · m · (2πf)²
Imbalance force, from eccentricity e (ft), imbalance mass m (lb), and rotational frequency f (Hz).
nres = 187.5 √(k / w)
Critical speed (rpm), from spring constant k (lb/in) and w = half the total weight of roll and bearings (lb).

Sizing the Transducer

Sizing is straightforward vector mechanics. The force a transducer must support combines the resultant web tension with the roll weight; size for the larger end load and pick the next capacity up.

θ = α + β
Wrap angle: the sum of the entry and exit angles.
Fr = 2 · T · sin(θ / 2)
Resultant web tension force — the vector sum of the entry and exit tensions.
γ = (−α + β) / 2
Angle to rotate the transducer from vertical for maximum sensitivity along Fr.
Ft = √(Fr² + W² + 2 · Fr · W · cos γ)
Total force the transducers carry — tension and roll weight combined by the law of cosines.

Picking the capacity. The force at each roll end is Fmax = Ft/2; select the next-larger transducer. A T-MAC system's capacity is twice a single transducer's, and sizing assumes the load splits evenly between ends. Short bursts above capacity but below the safe load do no harm, but continuous operation above 120% of rated capacity can cause fatigue failure.

Selecting Instrumentation

Because tension modules are strain-gage transducers, the instrument supplies excitation, amplifies and sums the signals, and provides the digital interface most plants need.

• Environment Mount instruments as close to the transducers as possible. Choose NEMA 4/4X enclosures for humid or wash-down areas and EMI/RFI shielding, and look for FM and CSA approvals.
• Per-transducer evaluation Simple systems just sum the signals into a total. To track trends, find web centre, or watch left and right tension, use an instrument with an A/D converter per transducer and a high-resolution (16-bit) analog output.
• Response speed High-tension drives respond in hundreds of milliseconds, so a 20-per-second (50 ms) update rate is more than adequate. Fast low-tension webs — 2000 ft/min and up — may need up to 120 updates per second.
• System size and control Rule of thumb: one instrument per roll, though per-transducer A/D lets one instrument cover several rolls. Tie multiple instruments to a supervisory PLC or DCS over Allen-Bradley Remote I/O, Modbus Plus, or Profibus.

Intrinsic Safety in Brief

Where the web runs through flammable gas, dust, or fibers, the same hazardous-location rules that govern electronic weighing apply. Areas are classified by hazard type, likelihood, and substance.

Instrumentation in a Division 1 area must be purged or explosion-proof. In Class I, Division 2 areas, non-incendive equipment can be used without special enclosures, and nearly all BLH Nobel instruments carry FM and CSA non-incendive approvals. If the transducers themselves sit in a Division 1 area, intrinsic safety barriers are required — even with the instrument in a safe area, barriers limit transducer voltage levels.
Classifications, protection methods, and barrier wiring are covered in depth in the companion handbook, A Guide to Safe Electronic Weighing in Hazardous Locations.

Key Terms

Fill in the Web Tension Worksheet and BLH Nobel Applications Engineering will recommend the transducer capacity and model. The full TC0012 handbook includes every formula, the sign conventions, diagrams, and the complete glossary.