Tension Control Education and Information


How should tension be controlled in your process?

Specifying the web tensioning needs of your process takes three easy steps.

Step 1 - Specify the Tension Range and Sensitivity throughout Your Process.
What is the right tension for a product?
Any product or process has a tension window. If the tension is too high, the web may break, yield. If the tension is too low, the web may slip, wander, or wrinkle.
The maximum average web tension should be 10 to 20 percent of a product’s break or yield point. If a product yields at 10 kpsi or 2% strain, then the target web handling tension would be 1000 to 2000 psi (1-2 PLI per mil) or 0.2 to 0.3 % elongation. A 10:1 or 5:1 safety factor may seem high, but this is the average tension. Crossweb tension can vary greatly from roller misalignment or web bagginess, using up much of the margin for safety. If you are handling thermoplastic films in heated processes, be sure to consider that modulus and yield stresses usually drop as temperatures increase.
The minimum tension is harder to define. Even minimal tension on a web has the benefits of pulling out bagginess, reducing wrinkles and promoting better tracking. A process should have enough tension to create friction to drive all idler rollers. A steering roller may need more tension to create the friction to turn the steering roller and exert the lateral force sufficient to bend the web.
What is a process step’s tension sensitivity?
Tension is a critical variable in many processes. Tension, stretch, or speed variations can all create coating thickness variations. Incorrect tension ration or strain matching will create curled laminates. Too much tension in an oven may create yielded or necked films. Too high or too low of tension on air floatation nozzle may cause touchdown or lateral sailing. Low slitting tension will often create a bad edge. Tension variations may cause idler rollers to go in and out of slip and an unpredictable wandering web. Tension control is critical to winding where you may need tapering tension versus growing roll diameter to form a good roll.

Step 2 – Define the Tension Zones By Grouping Compatible Process Steps
What is a tension zone?
In any web line, the web is driven by two or more tensioning elements. The section of web between any two consecutive tensioning elements is considered a tension zone. Options for tensioning elements include all varieties of brakes, clutches, and motors. One of the tensioning elements must serves as the line’s lead section, controlling speed rather than tension. All the other tensioning elements will control a tension zone either upstream or downstream of the lead element. Therefore, the number of tension zones in a line is one less than the number of tensioning elements.
A process with a braked unwind, a driven pull roller, and a clutch winder has three tensioning elements and two tension zones. A coater with a driven unwind, two driven pull rollers, and a driven winder has four tensioning elements and three tension zones.  As a process grows more complex, the number of tension zones will increase with each added tensioning element. 

How many tension zones does a process line need?
To many tension zones will create an overly complex and costly process. Too few tension zones will inhibit process optimization. Identifying the right number of tension zones is important to designing process equipment that will handle a variety of products.
Follow these steps to find the best number of tension zones for your process:

For example, in a coater-dryer line, where the first three process steps are unwinding, coating, and drying, this could be controlled with one, two, or three tension zones. If the coating process is tension-insensitive, then the unwinding and coating tension may be compatible and in the same zone. If the product is paper or foil and can withstand high tension in the hot drying process, then coating and drying tension may be compatible,
using the same zone.

However, in many film coating processes, the unwind tension is two noisy and would create coating variations. The high tension needed for present a flat web at coating is incompatible with the low tension needed at drying to avoid yielding of the thermoplastic web. Therefore,
it would be best to have three tension zones, one for each unwinding, coating, and drying.

Step 3 – Determine the Tension Control Plan for Each Zone
Once you’ve determined the number of tension zones, the next steps are to determine what section will be the lead or pacer and what tension control method is best for each zone.
A. Select the Lead or Pacer Tensioning Element
How to choose a lead section?
A pacer is the one tensioning element that runs in speed control, not responding to web tension; therefore, it must be a motor-driven (not clutched or braked).
A pacer is supposed to have precise web speed control; therefore the web must not slip relative to the pacer’s driven roller. Also, the pacer should also have a precise diameter (usually a steel roller) and low eccentricity or runout.
A pacer is the only tensioning element that doesn’t change speed in response to tension; therefore, the pacer is often located near any precision speed process, such as a coating, printing, or extrusion process.
A pacer has a controlled acceleration and minimal speed changes; therefore, it often makes sense to make the high inertia process, the web process with the most inertia,
the one that is most difficult to accel or decel

B. Select Tensioning Elements
Options for Tensioning Elements
B1.  Brakes
Definition:
A device for slowing or stopping motion, especially by contact friction.
Creating Tension:
Brakes can create uni-directional shift in tension from low to high in the machine direction. Brakes are torque controlling devices, so the applied tension will be proportional to torque divided by radius.
Design Options:
Simpler friction brakes use a leather strap over a drum or roll controlled by a dead weight
or spring.
The most common friction brakes used in converting are similar to automobile brakes, usually disc brakes with a turning steel disc with two or more pneumatically loaded pads.
All friction brakes are limited by heat generation and dissipation. Many advanced friction brakes include cooling via fin geometry or chilled water.
Electro-magnetic brakes include magnetic particle or magnetic hysteresis.
Best Applications:
Brakes are most commonly used to control an unwinding roll’s torque and tension. In some special applications, a braked roller can create a local tension drop, such as reducing the tension just before windup. All undriven rollers may be considered braked rollers, creating
a small low to high tension change due to the braking action of the roller’s bearing.

B2.  Clutches
Definition:
Any of various devices for engaging and disengaging two working parts of a shaft or of a shaft and a driving mechanism.
Creating Tension:
Clutch are typically used in “power” mode, creating a tension change from high to low in the machine direction, but can also function in “braking” mode.
Clutches are torque controlling devices. Tension applied to the web will be proportional to torque divided by radius. Since clutches rarely have a speed control loop, they are not commonly used to control a line’s lead section.
To work in “power” mode, a clutch needs an external source of power. The input to a clutch rotates faster than the require output, creating a slip differential across the clutch. As with brakes, heat generation and dissipation limits clutch performance. Heat generation is minimized by keeping the slip differential as low as possible.
Design Options:
Most converting clutches are much like the brake designs, such as friction discs and magnetic particle mechanisms, except they provide the option to drive an input shaft.
Differential winding shafts are based on creating individual clutches at each winding core. Most differential shafts are frictional devices, using end-loading or internal radially-loading pneumatics to create friction against core or a core holding device and the turning shaft. More advance (and expensive) differential shafts employ a series magnetic hysteresis clutches control winding torque.
Just as idler roller bearings act like min-brakes, they can also function as mini-clutches if
a motor is connected to the roller shaft and driven to above the rpms needed to match the web speed.
Best Applications:
Clutches can provide smooth torque control at a reasonable cost. Clutches should be applied as close to the web as possible, minimizing any torque losses and variations from complex coupling or gearing.
Clutches are a good choice for winding tensioning. The natural taper tension of constant torque winding forms a good roll structure for many product, especially is the core to final diameter buildup ratio is less than 4:1. The clutching mechanism of differential shafts is almost always a superior winding method for winding of multiple strands on a common shaft after slitting.
Other good applications for clutches:
Clutches in open loop torque control are a good choice to use when a speed controlled process needs a second driven element. A differential winding shaft is a clutching system where each winding roll is clutched with a simple frictional or advanced magnetic torque-generating element. Some nipping processes reduce shear on the product by adding a torque assist to normally undriven nip roller. Combination winders are typically controlled with speed control on the surface roll and clutched or open loop torque-assist on the winding spindle. A motor-clutch on a unwind can back tension a web, pulling any threadup slack backwards, something a simple braked unwind cannot do.

B3.  Motors
Definition:
A machine that converts other forms of energy into mechanical energy and so imparts motion. For web handling applications, usually a DC or AC electric motor (other options include hydraulic or pneumatic motors which perform closed to clutches).
Creating Tension:
Both DC and AC motors are regularly used as tensioning elements in converting. Motors can create tension is either braking (if they are regenerative) or power modes, controlling either torque or speed.

Motors can control either their output torque or speed. Since motors commonly have a speed control loop, they are the preferred tensioning element options for a line’s lead section. For predictable web speed control, take care to ensure the web doesn’t slip on the roller.

Motors in torque control create tension in the same way as clutches or brakes. In power mode, the tension out will be lower than the tension in by the motor torque divided by the roller or roll radius.
Design Options:
Magnetic Particle Brakes are a superior solution for creating tension. The most important benefit is torque is independent of speed, which allows for more accurate tension control. Magnetic Particle Brake also do not produce dust.

The most common friction brakes used in converting are similar to automobile brakes, usually disc brakes with a turning steel disc with two or more pneumatically loaded pads.
All friction brakes are limited by heat generation and dissipation. Many advanced friction brakes include cooling via fin geometry or chilled water.

Electro-magnetic brakes include magnetic particle or magnetic hysteresis.
Best Applications:
Clutches can provide smooth torque control at a reasonable cost. Clutches should be applied as close to the web as possible, minimizing any torque losses and variations from complex coupling or gearing.

Clutches are a good choice for winding tension. The natural taper tension of constant torque winding forms a good roll structure for many product, especially is the core to final diameter buildup ratio is less than 4:1. The clutching mechanism of differential shafts is almost always a superior winding method for winding of multiple strands on a common shaft after slitting.

Other good applications for clutches:
Clutches in open loop torque control are a good choice to use when a speed controlled process needs a second driven element. A differential winding shaft is a clutching system where each winding roll is clutched with a simple frictional or advanced magnetic torque-generating element. Some nipping processes reduce shear on the product by adding a torque assist to normally undriven nip roller. Combination winders are typically controlled with speed control on the surface roll and clutched or open loop torque-assist on the winding spindle. A motor-clutch on a unwind can back tension a web, pulling any threadup slack backwards, something a simple braked unwind cannot do.

In addition to a tensioning element, each tension zone also needs a feedback method to control the torque or tension to the desired level.

C. Select Feedback Option
In addition to a tensioning element, each tension zone also needs a feedback method to control the torque or tension to the desired level. 

C1. Manual control / no feedback
We do not recommend the use of manual control for unwind applications due to uncontrolled high tension and overheating brakes, but is acceptable for winding applications. Many winders run at a single torque value, letting the web tension decrease as winding roll diameter changes. This is acceptable for many products and processes. Constant torque is a good winding condition for rolls with buildup ratios of less than 3 (final diameter is less than three times core diameter).

C2. Diameter feedback
Diameter feedback systems are used to adjust torque on winders and unwinders proportional to roll diameter, thus approximating constant tension without closed-loop tension control. The three most common diameter feedback options are follower arms and ultrasonic sensors.

C3. Load Cell feedback
Load Cells detect web tension by measuring the reaction of a transducer, to the sum of the vectors of web tension. Load Cells should be sized to withstand the vector sum of roller weight and the incoming and outgoing web tension. The Load Cell control board has zero and gain adjustments to compensate for the roller weight, web wrap angle, and the wrap angle orientation relative to the transducer’s sensing direction.
Load Cells are the only option that provided dynamic feedback and measurement of web tension. Since web tension is critical to many processes, it is strongly advised to use a tension roller to measure web tension all critical tension zones.

C4. Dancer position feedback
A dancer loads the web, via roller weight or pneumatic loading, to apply the desired tension upon the web. A dancer, dances in response to any speed differential between the input and output tensioning devices. Dancers do not create the web tension, instead creating the opposing force to the tensioning element. When the dancer system is in balance, the web tension is proportional to the dancer’s load.

Dancers are used any time a significant speed differential is anticipated between two tension elements. Dancers are commonly used at unwinders and winders to absorb the tension upsets caused by out-of-round roll, indexing turrets, and automatic cut-and-transfer cycles. Multi-roller dancers, known as accumulators or festoons, are commonly used to accumulate and dispense web for zero- or low-speed splicing. Dancer are also used to accumulate and dispense web between constant speed and stop-start processes, such as at the infeed to platen press die cutting or screen printing. Dancers are a mechanical option to compensate for uncoordinated multi-drive processes, especially during rapid accelerations and decelerations.

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