Webex Inc. Idler rolls and heat transfer rolls for web converting

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Satisfying the Need for Speed

With proper roll engineering and application, more converting lines can be modified
to achieve optimum line speed.

By Pete Eggen, Roll Products Manager

Optimum speed — it’s a common goal of web converters who want to push their converting machinery to the speed limit while maintaining process integrity and material quality. Assuming product quality remains acceptable, higher speeds mean more output, lower production costs, and more profit. So, where does the limit to a machine line speed come from?

The first limitation is often the converting process itself. It is not practical to address all the various processes that can affect line speed. It is, however, much more feasible to address how line speed can be optimized by enhancing or adjusting the line’s rollers — the backbones of the converting process that carry, pull, support and convert the web as it travels from one end of the line to the other. While there are many limitations to speed in a given process, rollers (or rolls) should not be one of them.

When properly engineered and manufactured, any roll should be capable of handling high speeds. Several factors must be considered first, however, before increasing line speed. Even though you may end up working with an experienced roll engineer, it will help if you know something about roll design and construction.

For starters, an engineer will want to know about intended line speeds and existing processes that may be affected by web speed. For example, chill rolls will have less time to cool the web as the web travels faster. Higher speeds can cause the machine to vibrate violently (known as critical speed) when the balance tolerance of a roll is exceeded. Air entrapment may increase with web speed, resulting in the web “skipping” or even free-floating. Or the web might be dragged over rolls and marred, or possibly stretched or broken when roll weight and inertia specifications are not properly configured to start-up speeds, web tension and web strength.

To better plan for an increase in line speed, one must understand the basic parameters that affect roll performance. With the right rolls, converters can quicken the pace of their web lines and enjoy greater output. And while there is no substitute for an experienced roll engineer, it will be helpful to have the following insights regarding roll design and construction.

Balance
Rolls must be balanced at a speed and tolerance appropriate to the actual running conditions. Improperly balanced rolls can cause tension upsets or vibration problems. To achieve balance, each roll is assigned a balance tolerance.

Balance tolerance is the allowable amount of imbalance a roll can withstand as a function of the roll’s mass and speed (rpm). The faster the operating speed, the tighter the balance tolerance must be. Additionally, the heavier a roll, the greater imbalance it can tolerate.

At slower speeds, a static balance may be specified. Here, the roll has “zero rotation” and will not rotate to a heavy side when placed on low-friction bearings.

As speeds increase, uneven mass will cause a roll to vibrate. Now a dynamic balance is required. Dynamic balance today is done on a sophisticated roll balancing machine that utilizes computer accuracy to identify the locations and weights of needed counter balances — usually metal pieces welded or glued to the inside of the roll’s shell. In some cases, weight is also removed by machining precision holes on the roll’s journal.

Allowable imbalances are specified for various speeds; for example, “dynamic balance within 1 oz-in at 1500 FPM.” This means that at 1500 FPM, there must be less than 1 oz-in of imbalance. The inches represent the radius of the rotor and the amount of imbalance is multiplied by the radius at which it occurs. Therefore, if a 1 oz-in tolerance is applied to a 4-inch roll running at 1500 RPM, there can be no more than 0.5 oz of imbalance at the roll’s outer diameter when rotating at 1500 RPM.

It is important to note that all rolls will have some level of inherent imbalance. If a roll is set in low-friction bearings, gravity will rotate the roll to its heavy side down. This occurs when the breakaway torque (the torque required to start the roll in motion) of a roll mounted in low-friction bearings is less than the imbalance tolerance. It does not signify that the roll is out of balance.

Critical Speed
The critical speed of a rotating mass is the speed (RPM) at which a harmonic vibration or resonance occurs. This happens when centrifugal forces move the center of gravity away from the true geometric center. Critical speed calculations are theoretical and assume rigid mounting. Therefore, it is not recommended to rotate a roll at more than 60 to 70 percent of its calculated critical speed.

Critical speed must be analyzed as web and roll width and speed increase. Critical speed is determined by the directional forces on a roll coming from web tension and degree of web wrap, and the ability of the roll to withstand the load. If a roll is running too close to critical speed, you must decrease the deflection or decrease the RPMs. Increasing the roll diameter is a simple way to accomplish both at the same time.

Consider the unwinding of a 100-inch wide paper at 6000 FPM with a 90-degree web wrap on rolls and 3 PLI web tension: What should roll size and deflection be to operate below 70 percent of critical speed?

To keep the roll operating at less than 70% of critical speed, the deflection must be controlled to approximately .003″. Options for roll material and corresponding specifications are as follows:

Steel rolls: 10″ diameter with a 1″ wall weighing approximately 1000 lbs. (This option is the most economical but it is also the heaviest, with the highest inertia values.)

Aluminum rolls: 12″ diameter with a 0.5″ wall thickness weighing approximately 250 lbs. (More expensive than a steel roll, however, this solution provides a good balance between roll cost, weight and inertia.)

Carbon fiber rolls: 10″ diameter with a .375″ wall thickness weighing about 100 lbs. (Cost of this roll would be the highest, but roll performance will be superior. If roll speed changes frequently with acceleration or deceleration, this would be the best solution.)

Roll Geometry
As line speeds increase, roll geometry becomes increasingly important. Out-of-round rolls, or rolls with poor concentricity, can create vibration problems or other issues impacting web quality. Idler rolls that are not perfectly cylindrical in shape will cause the web to naturally turn when the roll is bigger on one side than the other. A roll with poor geometry will skew the web and then the next roll will attempt to correct its own entry angle by trying to turn the web back the other way. This usually results in a wrinkle.

Poor roll geometry can come from poor craftsmanship in the manufacturing of the roll, from excessive surface wear, or from inadequate design specifications leading to roll failure or diminished performance.

Air Entrapment
As a web moves over a roll, air is pushed into the tangent point where the web and roll meet. This increases air pressure under the web. Air pressure that is not squeezed out by web tension becomes entrapped air.

Entrapped air can cause the web to lose traction with the roll resulting in web tracking issues, and/or web wrinkling. Entrapped air can also cause damage to the web as it floats or “skips” on the roll surface.

Grooving the roll surface is one of the best methods for controlling airflow and eliminating air entrapment. The most common groove used to treat entrapped air is a spiral “V” groove. Here, one or more grooves traverse the roll in both right and left directions to create a crisscross pattern on the roll surface. If web marking is still an issue, then a microgroove roll using 10 to 30 micro-grooves per inch, can generate uniform airflow control over the roll.

While air entrapment is difficult to predict, there are published formulas to help address air entrapment problems. One such reference is “The Mechanics of Rollers” by Dr. David Roisum. This guide contains formulas for calculating boundary layers as a function of web speed, roll diameter and tension. Likewise, a quality roll supplier with an engineering staff can make recommendations on airflow management through a thorough analysis of the web application.

Roll Weight and Inertia
As a web changes speed, the roll’s mass moment of inertia is a critical limiting factor to acceleration and deceleration. The mass moment of inertia of a solid object is a measurement of that object’s ability to resist changes in rotational speed about a specific axis. The moment of inertia of a roll is not only related to its mass, but also the distribution of the mass throughout the roll. Therefore, two rolls of the same mass may possess different moments of inertia.

A low-inertia roll will be considerably more responsive to changes in web speed and less likely to damage the web. Low-inertia rolls are directly related to roll weight. Therefore light materials, such as carbon fiber, or thin-walled aluminum tubes, are commonly used in high-performance rolls for high-speed, low-tension applications. Aluminum provides a very good strength-to-weight ratio when compared to steel, and does so at a fraction of the price, creating a good, balanced approach to increasing speed. Carbon fiber rolls, with a mass twenty percent less than steel and near equal strength, offer another excellent choice for high-speed applications. The following case study provides an example for roll selection based on roll weight and inertia.

In addition to coating paper, a coating line will begin coating polyester film at low tension. The tension range will be widened from 2.5 PLI to 0.5 PLI, while increasing the speed from 1000 FPM to 1500 FPM.

With the decrease in tension, the idler rolls will need to be replaced with lower inertia, free turning idlers balanced for the higher line speed. At the same time, the idler rolls must be able to withstand the higher range tension of 2.5 PLI. A 6″ diameter, light-weight aluminum roll will provide a good combination of low inertia and low deflection. The 6″ roll will also keep the roll running at a safe distance from the first critical speed. Carbon fiber rolls would also work well in this application.

Idler rolls must be designed to remove entrapped air at higher speeds and lower tensions. At 0.5 PLI the calculated air boundary layer is 0.0015″. Spiral grooves 0.060″ wide x 0.050″ deep would be added to the roll face to relieve the entrapped air.

A tendency drive system could be added to the oven rolls to accommodate the lower tension range and higher speed.

The downside of a lightweight roll design is the possibility of creating a low-strength component that cannot adequately handle web load and resulting forces of deflection. Special design considerations must be worked out to assure correct engineering of the roll for these applications.

Heat Transfer
For optimal heat transfer to occur, a web must have contact with the heat transfer roll for a set period of time. As line speeds increase, the “dwell time” of web and roll contact decreases, thereby reducing the efficiency of the heat transfer process. Additionally, entrapped air that occurs with faster moving webs acts as insulation that can also reduce the transfer of heat energy. Because a larger roll diameter results in a longer “dwell time,” increasing heat transfer roll diameter is one of the more common adjustments when increasing line speed.

However, a limit will be reached when the boundary layer of air will diminish any gains from increased roll diameter. In these cases, using several smaller heat transfer rolls, instead of one large roll, will better accomplish heat transfer.

The smaller diameter rolls will have smaller boundary layers while providing the same dwell time of a single, larger roll. When using several smaller rolls, heat transfer is also applied to both sides of the web — all the more important with thicker webs. Worth noting, larger rolls are usually custom-manufactured, and typically cost more than smaller rolls that are “standard” sized and more affordable.

The heat transfer rolls must also be designed to cope with the proportional increase in heat load that will be generated with an increase in line speed. An increase in heat load will also require a proportional increase in fluid flow rate to maintain the same temperature differential across the web width. Following is a case example on meeting these additional requirements.

An extruder is extruding 0.015″ thick vinyl at 100 FPM and wants to increase to 150 FPM.

Provided the web thickness and width remain unchanged, the mass flow rate of the material, and therefore the heat load, will increase 50%.

The flow rate of the cooling medium must increase 50% to maintain the same delta-T.

The cooling system heat exchanger must provide additional capacity to remove 50% more energy from the cooling fluid.

Where a single 24″ diameter roll will work at 100 FPM, a 52″ diameter roll will be required at 150 FPM. Alternatively, two 16″ diameter rolls will also cool the web as well as one 52″ roll at 150 FPM.

Conclusion
There are many obstacles to achieving the balance between web quality and optimum line speed. Rolls do not have to be one of those barriers. When properly adjusted or engineered, rolls can be highly instrumental in helping converters satisfy their need for speed.

The information within this article can be used to guide your preparation for increasing line speed. Before contacting your roll supplier, gather all the pertinent information and parameters related to web material, processes, speed, deflection forces, as well as any issues that have surfaced during previous attempts to increase line speed. With the help of good roll technician or engineer, you should be able to effectively quicken the pace of your line and increase production output.