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Unwind guiding with an edge sensor

Unwind Guiding

Feed Perfect Alignment from Imperfect Rolls. Precision Shifting Stands.

In roll-to-roll processing, the material entering the machine must be perfectly aligned, even if the source roll is telescoped or loaded off-center. An Unwind Guide (or Shifting Stand) physically moves the entire heavy unwind roll left or right to ensure the web enters the process at the exact correct cross-machine location. Unlike intermediate guides that bend the web, this system shifts the bulk mass of the roll, requiring high-thrust actuation and specific mechanical geometry to maintain stability. 

The Challenge: Heavy Loads and Telescoped Rolls

Unwind guiding faces unique challenges compared to standard web guiding because it deals with the highest mass in the system:

  • Variable Roll Quality: Incoming rolls are rarely perfect. They may be telescoped (uneven edges) or wound with camber. If these errors aren't corrected immediately at the unwind, they propagate through the entire machine, causing wrinkles and registration errors downstream.
  • Massive Inertia: The system must move the entire weight of the roll (often thousands of pounds) plus the stand structure. Legacy systems relied on hydraulic cylinders to generate this force. These systems are messy, prone to leaks that contaminate products, and require costly maintenance of filters and seals. 
  • Operator Loading Errors: If an operator loads a new roll slightly off-center, the web guide must immediately correct for this steady-state error before the web enters the machine.

The Cost of Inaction: What Legacy Systems Are Costing You

Every month you continue operating with legacy hydraulic unwind systems, you're accumulating hidden costs:

  • Direct Costs: Hydraulic fluid replacement, filter maintenance, seal repairs, and the downtime required for each service event. 
  • Hidden Costs: Operators repositioning narrow sensors for every width change, extended changeovers, and the labor time that never shows up in downtime reports.
  • Risk Costs: A single hydraulic leak can contaminate an entire roll of product. In food, pharmaceutical, or medical applications, this means scrapped material and potential compliance violations.
  • Opportunity Costs: Limited to edge-only guiding? Many slitter rewinder applications require line guiding on printed materials, but traditional systems cannot switch between modes without hardware changes.

Plants using legacy hydraulic systems typically spend 10-20 hours annually on unwind guide maintenance alone—time that could be running production.

The Solution: Roll-2-Roll® Actuators

We replace leak-prone hydraulics with clean, high-precision electromechanical actuators that provide the thrust needed to move heavy stands without the mess.

The Roll-2-Roll Advantage:

  • High Thrust, Zero Fluids: Our actuators utilize stepper or servo motors coupled with ball screws or planetary roller screws to move loads up to 30,000 lbs (with low-friction linear bearings). This eliminates the risk of oil spills ruining your production.
  • Stiff Control Loop: Large masses are prone to mechanical resonance. Roll-2-Roll® Actuators are designed with high structural stiffness to ensure the natural frequency of the stand is well above the control frequency (typically 25-50 Hz), preventing oscillation during rapid corrections.
  • Wide-Area Sensing: Our wide-band sensors allow for significant web wander without the sensor losing the edge, accommodating poorly wound rolls that would cause traditional narrow sensors to fault out.

Engineering Guide: The "Fixed Sensor" Rule

The physics of Unwind Guiding are the exact opposite of Rewind Chasing. To ensure stability, you must follow these specific installation rules:

  1. The Sensor MUST Be Fixed: Unlike a rewind system where the sensor moves, in an unwind application, the sensor must be fixed to the machine frame (floor).
    • The Logic: You are trying to guide the web to a specific target in the machine. Therefore, the sensor acts as that target. The controller moves the shifting stand until the web aligns with the fixed sensor. 
    • Common Mistake: If you mount the sensor on the moving stand, the sensor moves with the error, and the system cannot detect the misalignment relative to the machine.
  2. The "Shifting Idler" Requirement: You must install at least one idler roller that moves with the unwind stand (a shifting idler) before the web reaches the fixed sensor.
    • Why? As the roll diameter decreases from full to core, the web plane changes. If the web went directly from the changing roll to a fixed sensor, the pass line angle would constantly change, affecting sensor accuracy.
    • The Geometry: The shifting idler creates a consistent web plane relative to the roll. The sensor is placed immediately downstream of this shifting idler.
  3. Actuator Sizing: Sizing an unwind actuator requires calculating more than just the load mass; it is critically dependent on friction.
    • Breakaway Force: The actuator must provide enough force to overcome the static friction of the linear bearings.
    • Recommendation: Use low-friction linear rail bearings (coefficient ~0.01) rather than sliding shafts (coefficient ~0.25). High-friction bearings require significantly larger actuators to achieve the same response time.
    • Alignment: Precise alignment between multiple linear rails is critical to prevent binding. Misalignment artificially increases friction, requiring additional thrust to move the stand.

Key Advantage: Dual-Mode Sensing for Slitter Rewinders

Traditional unwind systems face a limitation: they can only guide off the web edge. But many converting operations, especially slitter rewinders, need to guide off a printed line or registration mark for certain products.

With Roll-2-Roll Technologies wide-aperture sensors (up to 960mm sensing range), you gain capabilities that legacy systems simply cannot match:

  • Edge AND Line Guiding: A single ODC 960 sensor can switch between edge guiding and line/contrast guiding without hardware changes. Process opaque materials using edge detection, then switch to printed film using line detection—all from the same sensor.
  • No Sensor Repositioning: Wide sensing range accommodates multiple web widths and multiple web paths without moving the sensor. Reduce changeover time and eliminate operator errors.
  • Multiple Web Paths: Some operations run different materials through different web paths on the same machine. Wide sensors handle both configurations without reconfiguration.

For Slitter Rewinder Applications: This dual-mode capability is particularly valuable. You can guide off the edge for standard materials, then seamlessly switch to guiding off a printed line for laminated or pre-printed substrates—all without changing sensors or stopping the line.

System Configurations (Select Your Kit)

Unwind Guiding Kit

Best for: Loads less than 3000 lb and for replacing leaking hydraulic cylinders on existing shifting stands.

Heavy-Duty Unwind System

Best for: Metals, heavy paper, and large diameter film rolls.

  • Actuator: High-Thrust Ball Screw or Planetary Roller Screw option.
  • Sensor: Wide Roll-2-Roll® Sensor (Fixed to machine frame) allow for web width change without sensor repositioning.
  • Controller: Integrated drive with regenerative braking compensation to handle the inertia of large shifting masses.

Technical Specifications

SpecificationValue
Load CapacityUp to 30,000 lbs (13,600 kg) with low-friction bearings
Thrust Range50 lbf (222 N) to 1,500 lbf (6,670 N)
Stroke Length1 in (25 mm) to 12 in (300 mm)
Maximum SpeedUp to 2 in/sec (51 mm/sec)
Sensor Range48 mm to 960 mm (ODC Family)
Control Frequency25-50 Hz typical
Industrial ProtocolsEtherNet/IP, PROFINET, EtherCAT, Modbus/TCP
Voltage Options24 VDC or 48 VDC

Frequently Asked Questions

Sizing an unwind actuator requires more than knowing the total load weight. The key factors are:

  1. Breakaway Force: The actuator must overcome static friction to start movement. This depends on the bearing type:
    • Low-friction linear rail bearings (coefficient ~0.01): A 10,000 lb load requires only ~100 lbf to start moving
    • Sliding shaft bearings (coefficient ~0.25): The same 10,000 lb load requires ~2,500 lbf
  2. Acceleration Requirements: Higher acceleration for faster response requires proportionally more thrust
  3. Safety Factor: Include 20-30% margin for binding, misalignment, and wear

Recommendation: Always use low-friction linear rail bearings. The cost savings on a smaller actuator typically exceeds the bearing cost difference, plus you get better dynamic response.

Roll-2-Roll Technologies RLA and BLA series actuators provide thrust from 50 lbf to 2,000 lbf, handling loads up to 30,000 lbs when paired with proper bearing systems.

Actuator sizing starts with one question: how much force does the actuator need to produce to move your guide or shifting stand reliably? The answer depends on more than just the weight on the bearings.

Roll-2-Roll Technologies uses a 6-term force model that accounts for every significant resistance the actuator must overcome:

  • Bearing friction — the dominant term for heavy loads on linear bearings
  • Inertia — force to accelerate the mass at your target correction rate
  • Web tension lateral component — the sideways pull from web wrap angle on guide rollers
  • Umbilical drag — hoses, cables, and air lines that resist carriage motion
  • Floor grade — gravity component if the travel axis is not perfectly level
  • Misalignment friction — additional drag from rail or bearing misalignment

Our actuator sizing calculator offers two approaches:

  • Simple mode — enter load weight and bearing type for a quick estimate. Uses a factor of safety of 2.0 or higher to compensate for forces not explicitly modeled.
  • Detailed mode — model all six force terms individually for a precise result, typically with a factor of safety of 1.5 to 1.75.

If you want a reality check before running the calculator, try a spring-scale pull test: attach a calibrated spring scale to the carriage and pull horizontally at a steady, slow rate. The peak reading gives you the actual installed friction force — often 2 to 5 times the catalog bearing friction value.

For a deeper walkthrough of the force model and worked examples, see the full actuator sizing technical article.

This question highlights an important distinction that causes confusion: load weight is not the same as thrust force.

The RLA Series supports loads up to 30,000 lb on precision linear bearings. That is the weight sitting on the bearings — the roll, the chuck, the shifting stand structure. The actuator does not lift this weight. It pushes the carriage sideways along the bearings, overcoming friction and other resistance forces.

The actual thrust force required to move a 30,000 lb load is typically 120 to 600 lbf, depending on bearing type, rail condition, and secondary forces like web tension and umbilical drag. The RLA Series provides thrust across three models:

  • RLA-050 — 500 lbf thrust
  • RLA-100 — 1,000 lbf thrust
  • RLA-200 — 2,000 lbf thrust

To illustrate with rough numbers: a 20,000 lb load on profiled linear rails with a friction coefficient of 0.005 (installed, not catalog) produces approximately 100 lbf of friction force. Add inertia, web tension lateral component, umbilical drag, and a factor of safety — and the required thrust might be 250 to 400 lbf. An RLA-050 handles that with margin.

The key takeaway: do not select an actuator based on load weight alone. Run the force calculation to determine actual thrust demand. Our actuator sizing calculator does exactly this.

Roll-2-Roll Technologies RLA actuators are deployed in hundreds of installations across converting, printing, and packaging lines, reliably guiding loads from a few thousand pounds up to the rated 30,000 lb capacity — many running continuously for years.

The "fixed sensor rule" is fundamental to unwind guiding physics and is the opposite of rewind chasing:

The Logic: In unwind guiding, you are positioning the web to enter the machine at a specific target location. The sensor acts as that target. The controller moves the shifting stand until the web edge aligns with the fixed sensor position.

The Common Mistake: If you mount the sensor on the moving stand (as you would in a rewind chasing application), the sensor moves with the error. When the stand shifts left, the sensor shifts left too—so from the sensor's perspective, nothing has changed. The system cannot detect or correct the misalignment relative to the machine frame.

Proper Configuration:

  • Sensor: Fixed to the machine frame (floor or fixed structure)
  • At least one idler roller: Mounted on the shifting stand (moves with it)
  • Sensor position: Immediately downstream of the shifting idler

This configuration ensures the sensor sees true web position relative to the machine, while the shifting idler maintains a consistent web plane as roll diameter changes.

Let us be direct: servo motors are excellent. They offer smooth torque delivery, high bandwidth, and precise closed-loop control. For many motion control applications, they are the right choice. The question is whether web guiding is one of those applications — and for OEM-shipped actuators, the answer favors steppers for specific, defensible reasons.

The OEM shipping problem: Roll-2-Roll Technologies builds actuators that ship to converting, printing, and packaging facilities worldwide. Servo drives require load-dependent tuning — reflected inertia ratio, friction characterization, and mechanical resonance all depend on the installed machine, not the actuator alone. A stepper motor sized conservatively at design time requires no field tuning. It works out of the box regardless of what it is bolted to.

The web sensor outer loop: This is the key enabler. In web guiding, position feedback comes from a Roll-2-Roll® Sensor observing the actual web edge — not from an encoder on the motor shaft. Every source of actuator imprecision (lost steps, backlash, drivetrain compliance) is corrected by the sensor on the next correction cycle. The actuator does not need to be a precision positioning device. It needs to move reliably in the commanded direction.

What we give up — honestly:

  • Efficiency — steppers draw current at rest to hold position; servos draw only what the load demands
  • Speed — stepper torque drops off faster at higher speeds than servo torque
  • Noise — steppers are audibly louder during motion, especially at certain speeds
  • High duty cycle — continuous rapid cycling favors servo thermal characteristics

For web guiding — where corrections are slow (0.5 to 2 Hz), the motor is at rest most of the time, and field simplicity matters — these tradeoffs are acceptable. The result is a reliable, zero-tuning actuator that has performed across hundreds of installations.

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