Where compressed air does the work in web handling
Converting, printing, laminating, coating, slitting and rewinding lines, along with paper, film, foil, nonwoven and textile processes, use compressed air in several tension-related ways:
- Pneumatic brakes and clutches on unwind and rewind shafts, where air pressure sets braking torque.
- Dancer rollers loaded by low-friction cylinders or rolling-diaphragm actuators, where air pressure sets web tension.
- Nip and lay-on rollers, where cylinder force sets contact pressure.
- Expanding air shafts and chucks that grip the core.
- Splice, cut and transfer mechanisms that fire during roll changes.
In every case, the force at the web is proportional to the pressure acting on the actuator, minus friction and other losses. If the pressure at the actuator wanders, tension wanders with it, and the result shows up as wrinkles, register errors, telescoped rolls, web breaks or uneven coating.
From web force to actuator pressure
The starting point is the tension the process needs, normally given by the material supplier or the machine builder as force per unit width. Multiply by web width to get total tension. For a dancer, the actuator must balance that tension through the dancer geometry; for a brake, the torque needed is tension multiplied by roll radius.
Actuator pressure then follows from force divided by effective area, allowing for friction:
Required pressure (gauge) ≈ (required force + friction allowance) ÷ effective piston or diaphragm area
Worked example (illustrative only): a dancer needs 600 N from a single low-friction cylinder with a 63 mm bore. Area is about 31.2 cm², or 0.00312 m². Pressure ≈ 600 ÷ 0.00312 ≈ 192,000 Pa, about 1.9 bar(g) or 28 psig, before friction. If friction and seal breakaway add a further 10% of force at low speed, the working setting is nearer 2.1 bar(g). The same line may run at 1.2 bar(g) for a light film and 3.5 bar(g) for a heavy board, so the air supply and regulator must give fine resolution at low pressures as well as enough headroom at the top of the range.
Two points often overlooked:
- Seal friction and stick-slip matter far more at low pressures. Low-friction cylinders, rolling diaphragms or bellows actuators are chosen for this reason.
- For brakes, the required torque falls as the unwind roll diameter falls, so the brake pressure must reduce continuously through the roll.
Why steady running is not the real test
At constant speed with a full roll, almost any reasonable air supply gives acceptable tension. Problems appear when something changes:
| Event | What the air system must do | Typical symptom if it cannot |
|---|---|---|
| Acceleration to line speed | Change brake or dancer pressure quickly as inertia loads change | Slack or tension spike; dancer hits end stop |
| Deceleration and emergency stop | Increase braking smoothly without overshoot | Web break or loose wraps on the rewind |
| Roll diameter decreasing | Reduce brake pressure in small, smooth steps | Tension rises through the roll |
| Flying splice or roll change | Fire splice cylinders while tension actuators hold steady | Tension disturbance at every splice |
| Other machines drawing from the header | Keep regulator inlet pressure above its working minimum | Random tension variation unrelated to line events |
A specification should therefore state the speed profile, acceleration and deceleration rates, roll diameter range and splice frequency, so that response can be assessed, not just steady pressure.
Regulators, proportional valves and tubing
The component between the plant air and the actuator decides how well tension is controlled.
- Manual precision regulators suit fixed-tension lines with gentle speed changes. Choose one with good low-pressure resolution, low hysteresis and adequate relief flow so pressure can fall as fast as it rises.
- Electro-pneumatic proportional regulators allow closed-loop tension control from a load cell or dancer position sensor. Their response time, resolution and flow capacity must match the actuator volume.
- Volume boosters can increase flow capacity where a proportional regulator’s pilot output is too small for a large brake or cylinder.
- Tubing length and bore set the volume the regulator must fill and vent. Long, large tubing slows response; very small tubing restricts flow. Mount the regulator close to the actuator wherever possible.
Many regulators need their inlet to stay a margin above the set pressure to regulate accurately. If the header falls close to the set point during peak plant demand, regulation degrades even though the regulator is working correctly.
Separating supply problems from control problems
Tension faults from a weak air supply and from poor control tuning can look similar on the product. Log the right signals on one time base to tell them apart:
- Header pressure at the machine connection.
- Regulator inlet pressure.
- Actuator pressure, measured at or near the actuator port.
- Regulator command signal, if it is electro-pneumatic.
- Tension feedback from the load cell or dancer position.
- Line speed and roll diameter.
Then read the record:
- If actuator pressure follows the command closely but tension still oscillates, the problem lies in tuning, mechanics or sensor filtering, not the air supply.
- If actuator pressure lags or droops while the command is correct, look at regulator capacity, tubing and inlet pressure.
- If regulator inlet pressure dips at the same moments as tension errors, and those dips match other plant loads or compressor load/unload switching, the supply is the cause.
- If tension errors appear only at splices, check whether splice cylinders share the tension actuators’ supply.
Air quality and condition at the line
Proportional regulators and low-friction actuators are sensitive to contamination. Water causes sticking and corrosion; oil carry-over can swell seals or contaminate the web; particles damage pilot orifices. A practical baseline is dry air from a refrigerated dryer (typical pressure dew point around +3 °C), coalescing filtration close to the machine and a particle filter immediately upstream of precision regulators. Where the web is food packaging, pharmaceutical foil or a clean film, the product owner may require oil-free compressed air or a stated ISO 8573-1 class at the point of use. Paper and board mills often have humid, dusty areas, and in Telangana and Andhra Pradesh summer ambient heat can push compressor-room temperatures high, which raises the moisture load on dryers. Check the dryer is sized for those conditions.
Supply stability: what the compressor room can do
Even with good local regulation, a stable header makes tension control easier. Useful steps include:
- A local receiver near the line, with a check valve, to isolate the machine from header dips.
- A dedicated, regulated branch for tension and precision devices, separate from blow-off and cleaning air.
- Correct compressor control: a narrow pressure band, sequencing across machines, or a variable-speed trim compressor so switching does not cause steps in header pressure.
- Avoiding an excessively high header set point to mask droop, since each extra bar costs about 7% more compressor power.
Checklist for a web-handling air brief
- Web materials, widths and tension range.
- Actuator type, bore or effective area and maximum pressure.
- Required actuator pressure range, in bar and psig.
- Regulator type, response and flow capacity.
- Line speed range, acceleration and stop times.
- Roll diameter range and splice method.
- Other air users sharing the supply.
- Air-quality requirement at the line.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad, has served industrial compressed-air users since 2002. We are an independent multi-brand specialist, not an authorised dealer, and sell, rent and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other compressors. We have tie-ups with all major and reputed brands. For printing, packaging, paper and film lines we can:
- Log header, branch and point-of-use pressure alongside line events to show whether the air supply is the cause of tension faults.
- Recommend receivers, dedicated branches, dryers and filters to stabilise the supply.
- Review compressor control and sequencing to reduce pressure swings.
- Supply new, used/refurbished or rental compressors, including oil-free machines where the product requires them.
- Provide service, AMC and spares through more than 20 service engineers and technicians covering Telangana and Andhra Pradesh, with supply across India.
Tension control tuning and machine safety functions remain with your machine builder and controls engineer; we focus on getting reliable, clean air to the machine.
What to send us
- Line type, maker and the materials it runs.
- Actuator details and the pressure settings used for each product.
- Description of the tension fault and when it occurs.
- Compressor, dryer and receiver details for the supply.
- Any pressure or tension trend data you already have.
Frequently asked questions
Why does web tension vary when air pressure looks steady?
A gauge shows a slow average and hides short dips. Tension can also vary because of regulator hysteresis, actuator friction at low pressure, roll diameter changes or control tuning. Log actuator pressure and tension feedback on the same fast time base. If pressure follows its command but tension still moves, the cause is mechanical or in the control loop rather than the air.
What air pressure is needed for a pneumatic dancer roller?
It depends on the tension required, the dancer geometry and the actuator area. Divide the required force by the effective piston or diaphragm area and allow for friction. Many dancers run at low pressures, often a few bar, which is why low-friction actuators and precision regulators with fine low-pressure resolution are preferred. Your machine builder’s data sets the actual range.
Can compressor load and unload cycling affect web tension?
Yes. A compressor switching between load and unload causes the header pressure to swing. If a tension regulator’s inlet falls close to its set point, its output may droop or respond slowly. A local receiver, a dedicated regulated branch, or better compressor control with a narrower pressure band usually removes the disturbance at the line.
Should tension actuators share air with splice cylinders?
It is better to keep them on separate regulated branches. Splice and cut cylinders draw a sudden burst of air exactly when tension control is most critical. If they share a regulator or small supply line, actuator pressure can dip during the splice. Separate branches, with local storage for the splice mechanism, help keep tension steady through roll changes.
Do web handling lines need oil-free compressed air?
Not always. Oil-injected compressors with good coalescing filtration serve many lines. Where the web is food packaging, pharmaceutical material or a sensitive film, the product owner may require oil-free compressed air or a specified ISO 8573-1 class. Precision regulators also last longer on dry, clean air, so dryers and filters are recommended in either case.
Related pages
Calculators and worked examples
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Actuator pressure for a web tension or force
Turn a web tension (per unit width) or a known actuator force into the gauge pressure a dancer or loading cylinder needs, add a friction allowance, check regulator headroom, and give the brake torque for a roll radius.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Tension per unit width (N/m; 0 to enter the force directly): 0
- Web width (m): 1
- Dancer geometry factor: actuator force per N of tension: 1
- Required actuator force (N; used when tension per width is 0): 600
- Number of actuators sharing the force: 1
- Actuator bore or effective diaphragm diameter (mm): 63
- Friction and breakaway allowance (% of force): 10
- Regulator inlet pressure at the worst moment (bar g): 6
- Margin the regulator needs above its set point (bar): 1
- Roll radius for brake torque (m; 0 to skip): 0
Result
- Force per actuator: 600 N
- Effective area: 31.17 cm² = 0.00312 m²
- Pressure before friction: 1.92 bar g = 1,92,478 Pa = 27.9 psi
- Working pressure with 10% allowance: 2.12 bar g = 30.7 psi
- Regulator headroom: 3.88 bar above the working pressure: OK
Method. Web tension = tension per unit width × width (or enter the actuator force directly). Actuator force = tension × dancer geometry factor ÷ number of actuators sharing it (a dancer roll with a 180° wrap carries about twice the web tension; use the machine builder’s geometry). Effective area = π × D² ÷ 4. Pressure (Pa) = force ÷ area; 1 bar = 100,000 Pa. Working pressure = pressure × (1 + friction allowance ÷ 100). Brake torque = web tension × roll radius; it falls as the roll diameter falls. The regulator needs its inlet a margin above the set pressure.
What to take from it. Low set pressures make seal friction and regulator resolution matter most: use low-friction actuators, a precision regulator close to the actuator, and a supply that never sags near the set point.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.