How do load cell rollers work?

A load cell rollers, also called tension rollers or transducer rollers, work on the same principles of any load measuring device. If you can weigh yourself on a bathroom scale, then you can understand a transducer roller.

How does a bathroom scale work? You stand on the scale and the needle moves to indicate your weight. Inside a bathroom scale is a spring. When you step on the scale, you compress the spring. The needle is mechanically linked to the spring. The heavier you are, the more you compress the spring, the more the needle moves, and your higher weight is revealed.

How do you know a bathroom scale is reading correctly? You need to calibrate it. There are two parts to calibrating a scale: zero offset and gain. The zero offset is used to counter the weight of the scale’s components, such as the plate you stand on. The gain sets the relationship of the spring deflection to the applied force.  Since all bathroom scales are designed to work under Earth’s gravity and expect you to use them on a flat surface, the gain is preset at the factory, leaving you to only set the zero for a reasonably calibrated scale.

A load cell roller is a roller mounted on two transducers, one supporting each end of the roller, that convert the applied force into an analog signal. Typically, the transducers are installed between the shaft and the equipment side frames or support structure. One company, Dover Flexo, has a unique and patented tension measuring roller design where the load cell is inside the roller between the rotating shell and the roller shaft.

Like bathroom scale, the load cell or force-to-current transducer measures load by monitoring the deflection of a spring (in this case, the spring is a bending beam). Load cell rollers measure three forces: the weight of the roller, the tension of the incoming web, and the tension of the outgoing web. However, the load cell roller does not measure these three forces independently; instead, it measures a component in the sensing direction of the vector sum of the three forces.

How do you know a load cell roller is reading correctly? You need to calibrate it. Like a bathroom scale, there are two parts to calibrating a load cell roller: zero offset and gain. The zero offset is used to counter the weight of the roller. The gain adjusts for the relationship of the output signal to the load cell geometry. The gain will change with different wrap angles and the orientation of the wrap angle and gravity relative to the load cell sensing direction.

I’m a big fan of load cell rollers. I like load cell rollers because they measure tension. Dancer rollers have their place and unique function, but they don’t tell you what the tension is. Since understanding tension (and strain from tension) is so important to resolving web handling problems, I always recommend load cell roller for all but the simplest web handling processes.

I’m also a big fan of having a fast responding analog or graphical display of the measured tension. Monitoring your tension and tension variations is like measuring the heartbeat of a patient. You can learn a lot from watching the pulses of tension, when they happen, and how big they are. If you only have a digital readout, especially one that is filtered and updates at a slow rate, you are missing out on valuable process information.

How much wrap angle should a load cell roller have?

Many equipment suppliers will by default tell you to use 45 or even 90-degree wraps on load cell rollers. More wrap creates a stronger force on the roller, making is easier to measure the web tension. However, in many cases, it can be difficult to create even a 45-degree wrap. Does this mean you can use a load cell roller? No, I like to think about a minimum acceptable wrap in terms of signal to noise (or maybe I should call it load-to-sensitivity). By signal to noise, I’m referring to the force exerted by the web’s tension relative to the roller’s weight. If you have a high tension web and a light roller, then you can use a small wrap angle. However, if you are running ultra-light tension, you will need more wrap angle. I’d like to see the vector sum of the web tensions be greater than 40-50 percent of the roller weight.

Regarding load to sensitivity, many load cells are accurate to less than one percent of the full range, so don’t buy an over-sized load cell if you want to run with small wrap angles. You can use a smaller load cell and smaller wrap angles if you set up the sensing direction perpendicular to gravity.

What is a dancer roller? How does it work?

A dancer roller is a roller that pushes into the web with a controllable force. The dance of a dancer roller, in either a pivoting or sliding motion, is how it moves to accumulate or dispense web in response to any speed variations between the nearest upstream and downstream driven elements. If the incoming web is faster than the outgoing web, the dancer will accumulate. If the incoming web is slower than the outgoing web, the dancer will dispense.

When used as a tension feedback device, the position of the dancer is monitored and used to adjust the speed of one driven element relative to another, maintaining a relatively constant tension within the range of accumulation and preventing the tension extremes of web breaks or slackness. The dancer is usually wrapped with a 180-degree wrap, so the tension created by the roller load is relatively consistent through the accumulate and dispensing range. A dancer has a null position, usually defined as the center of its travel range.

The feedback of the dancer is the distance it moves from the null position, usually monitored with a linear or rotational transducer. If the input web is at speed V1 and the output web is at V2, the amount of web that needs to be accumulated or dispensed is (V1-V2) x time. The greater the speed differential and longer the differential exists, the more dancer range is needed. In a closed-loop tension control system, the dancer position is fed to a controller that quickly adjusts the input our output speed to drive the dancer back to its null position. In operation, a dancer roller will constantly dither between above and below the null point.

What makes a good dancer roller system?

Dancers need to dance (accumulate and dispense), but this motion will be opposed by Newton’s first law (sometimes called the Law of Inertia).

An object at rest will remain at rest unless acted upon by an external and unbalanced force.
An object in motion will remain in motion unless acted upon by an external and unbalanced force
.

So for a dancer to dance, the forces applied to it must overcome inertia. Inertia is related to mass, so reducing mass improved a dancer roller’s performance. Besides inertia, dancers also need to avoid high friction, and for dancers using pneumatic cylinders to increase or decrease the load on the web, the pneumatic system should avoid significant restrictions to the dancer’s dance.

In general:

  • Minimize friction and inertia (dancers are the most logical application of low inertia idlers)
  • Avoid using weights to counter-balance a dancer roller. Instead, use either a horizontal dancer motion or a pneumatic counterbalance.
  • To avoid hysteresis, design any pneumatic system with sufficient piping diameter, high volume flow controllers, and air accumulators.
  • Reduce geometry-related tension variations with 180-degree wrap and sufficient entry and exit span lengths.
  • Magnify the measurement of the dancer roller motion by gearing it with a mechanical advantage over the rotational encoder.
  • Pivoting dancer assemblies will almost always have fewer stick-slip and misalignment problems than linear dancers.
  • As with any pivoting element, make sure your equipment is design for ease of measuring and aligning the dancer roller’s pivot shaft. A dancer roller’s parallelism starts with a well-aligned pivot shaft.
  • Sufficient range to accumulate and dispense the length of web required based on DV x time. Usually, a dancer range of 200-250 mm (8-10 inches), storing 0.4-0.5 m (16-20 inches) of web, is enough.

What is better for tension feedback, a load cell or dancer roller?

This is one of the oldest debates in web handling. Early in my web handling career, I asked many people this question. Based on the input from many people and seeing many applications, here are my key thoughts on this debate and how I answer the question:

Advantages – Load Cell Roller

  • Actually measures tension
  • Often small wrap angles and one-sided contact
  • Fast and accurate Low maintenance

Disadvantages – Load Cell Roller

  • Small accumulation, web path changes create large tension changes
  • Poor response to high frequency upsets

Advantages – Dancer Roller

  • Accumulation
  • Minimal tension change with web path changes.
  • Compensates for poor motor control.

Disadvantages – Dancer Roller

  • Doesn’t measure tension
  • Large wrap angle + difficult to align = wrinkle sensitive
  • Creates tension variation from friction and inertia
  • Poor response at high or resonant frequencies
  • Higher maintenance

I think dancer rollers are over-used. Dancer rollers are a mechanical system used to compensate for mediocre controls. In most cases, today’s motors and control systems don’t need dancers, but many suppliers and converters stay with dancers based on poor experiences with load cells many years ago.

My advice:

  1. If you can afford it, install load cell rollers in every tension zone, if not for closed-loop control, to monitor your process.
  2. Ensure your load-cell rollers are properly calibrated.
  3. If your process will have section to section speed differential of magnitude and time to require accumulation and dispensing of web, use a dancer roller.
  4. I support using dancers on winders and unwinders to absorb speed variations from out-of-round rolls and speed variations of automatic splicing processes.

What is an accumulator?

In many cases, an accumulator is simply a series of rollers used in the same function as a dancer roller, but with the ability to accumulate and dispense much longer lengths of web.

Accumulators are most commonly used with unwinders or winders to allow a zero-speed splice, without interrupting the speed of other processes on the web line, such as coating or drying.

What is the sequence using an accumulator for zero-speed splicing?

  1. The main process continuously runs at constant speed.
  2. The winder or unwinder decelerates to zero speed.
  3. The accumulator’s festoon moves, accumulating (ahead of winders) or dispensing (downstream of unwinders) the amount of web equal to the speed differential x time.
  4. The manual or automatic splice is made at the winder or unwinder.
  5. The winder or unwinder is accelerated to 50-100 percent over the main line speed to either refill the unwinder accumulator or dispense the web in a winder’s accumulator.
  6. As the accumulator returns to its starting position (fully dispensed for winder or fully accumulated for unwinders), the winder or unwinder ramps down to equal the main line speed.
  7. The accumulator is ready for the next splice.

Most accumulators are force loaded and float to a position determined by the input and output speed differential. Some accumulators are speed controlled, driving the web to a calculated position, again based on the input and output speed differential and usually coupled with a standard dancer roller to make up for small speed variations.