Many web converters 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. While addressing all the various processes that can affect line speed is impractical here, we can address how to optimize line speed by enhancing or adjusting other key line components, in particular, 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 if you work 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 startup 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.

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. Each roll is assigned a balance tolerance.

Balance tolerance is the amount of imbalance a roll can withstand as a function of its mass and speed (rpm). The faster the operating speed, the tighter the balance tolerance. The heavier a roll, the greater imbalance it can tolerate (See Figure 1).

Allowable imbalances are specified for various speeds; for example, "dynamic balance within 1 oz-in. at 1,500 fpm." This means that at 1,500 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-in. roll running at 1,500 rpm, there can be no more than 0.5 oz of imbalance at the roll's outer diameter when rotating at 1,500 rpm.

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.

The critical speed of a rotating mass is the speed (RPM) at which harmonic vibration or resonance occurs. This happens when centrifugal forces move the center of gravity away from the true geometric center (See Figure 2). 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 either the deflection or the RPMs. Increasing the roll diameter is a simple way to accomplish both at the same time.

Consider the unwinding of a 100-in.-wide paper at 6,000 fpm with a 90-deg 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 percent of critical speed, the deflection must be controlled to approximately 0.003 in. Options for roll material and corresponding specifications are as follows:

Steel rolls: 10-in.diameter with a 1-in.wall weighing approximately 1,000 lbs. This option is the most economical but also the heaviest, with the highest inertia values.

Aluminum rolls: 12-in. diameter with a 0.5-in. wall thickness weighing approximately 250 lbs. This is more expensive than a steel roll, however this solution provides a good balance between roll cost, weight and inertia.

Carbon-fiber rolls: 10-in. diameter with a 0.375-in. wall thickness, weighing about 100 lbs. The 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.

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 can often result in a wrinkle.

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 that is not squeezed out by web tension becomes entrapped, which can cause the web to lose traction, resulting in tracking, wrinkling or "floating."

Grooving the roll surface is one of the best methods for controlling airflow and eliminating entrapment. The most common 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, a microgroove roll using 10–30 microgrooves/in., can generate uniform airflow over the roll (See Figure 3).

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. It's 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 compared to steel, and does so at a fraction of the price. Carbon-fiber rolls, with a mass 20 percent less than steel and near equal strength, offer another excellent choice for high-speed applications.

The downside of a lightweight roll design is the possibility that it cannot handle web load and resulting deflection. Here, special design considerations must be worked out to assure correct engineering of the roll.

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, dwell time of web and roll contact decreases, reducing the efficiency of the heat-transfer process.

Entrapped air that occurs with faster moving webs acts as insulation that can also reduce the transfer of heat. 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, use several smaller heat-transfer rolls, instead of one large roll (See Figure 4).

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. Larger custom-manufactured rolls typically cost more than smaller standard rolls.

When properly adjusted or engineered, rolls can be instrumental in helping converters satisfy their need for speed. Use this article as a guide to prepare for increased line speeds before contacting your roll supplier. Gather all pertinent data on web material, processes, speed, deflection forces, and any issues that have surfaced during previous attempts to increase line speed. With the help of a good roll engineer, you should be able to quicken the pace of your line and increase output.