Vacuum Rolls & Vacuum Belts
Table of Contents
Vacuum rolls (sometimes called vacuum drums or suction rolls) and vacuum belts have been growing in popularity in converting applications. The essential functions of these devices are tension control and/or web metering. But why use a vacuum roll instead of “S” rolls or nip rolls? At what points in a web processing line should vacuum devices be considered?
To answer these questions, this overview begins with a description of vacuum rolls and belts. Then useful characteristics of these vacuum devices are listed, and typical applications explained. Finally, a summary of design considerations is included.
Description & Operation
As shown in Figures 1 and 2, a vacuum roll is typically a driven hollow steel roll with a large number of holes. A vacuum is applied inside the roll to pull the web against the roll surface through the required wrap angle. Deckles may be used inside the roll to adjust the vacuum zone to match web width.
Porous drums or porous inserts in a steel roll have also been used. Special concerns for this approach are that typically a higher vacuum is needed and there is a tendency for the porous material to plug up with dirt over time.
Figure 3 shows a vacuum belt, which is quite similar. In this case, a belt with a large number of holes is used. It runs in a loop over two rolls, one of which is driven. A vacuum belt allows tension control even when very little or no wrap angle is available. Internal deckling may be included to adjust for web width.
Vacuum tension rolls and belts provide web tension control by generating a web force against the roll or belt that is independent of the web tension difference (Tension In and Tension Out) shown in Figures 2 and 3. The discussion that follows is based on vacuum rolls, but generally applies to vacuum belts as well.
Two things happen when the vacuum is applied to the roll. First, the vacuum under the wrap angle creates a pressure difference between the vacuum level and atmospheric conditions. This pressure difference forces the web against the roll and is independent of web tension differential.
Second, boundary or captured air carried along by the revolving roll and moving web is removed by the vacuum. This allows the web to be in intimate contact with the roll through the wrap angle.
Air removal becomes especially important as roll and web speed increase. The vacuum removes much more boundary air than can be squeezed out by the web as it begins to wrap over the roll.
To prevent the web from slipping on the roll, the frictional force must be greater than the desired tension differential. Frictional force for a vacuum roll is the product of the normal force to the surface and the coefficient of friction.
The normal force pushing the web against the roll has two components. One is the resultant force produced by web tension. The other is the force produced by the differential pressure. This second component is by far the larger force for most vacuum rolls, and accounts for the ability of vacuum rolls to achieve much higher tension differences than other tension control devices.
Three key variables affect the amount of frictional force resulting from differential pressure.
- Coefficient of friction is important. Rougher roll surfaces increase this coefficient, as does better boundary air removal. The less air that remains between web and roll, the higher the coefficient of friction will be.
- Vacuum level is also important, and should be maximized to the extent practical. But this must be carried further, since there is also the question of how much vacuum can be achieved at the interface between web and roll. That vacuum level, like coefficient of friction, depends on how well boundary air is removed.
- Surface area of contact between web and roll is another obvious factor. More contact area means more vacuum roll holding power for a given web width. Therefore, wrap angle and roll diameter are also important in determining the frictional force from the pressure difference.
Suppliers of vacuum rolls can recommend the right roll surface or covering, roll diameter, vacuum level, and hole geometry to achieve a required tension difference for a given web width and wrap angle. Hole sizes and locations help determine how well boundary air is removed so that better contact is made. At times, grooving may also be used to give boundary air an escape path.
Vacuum rolls and belts have a number of characteristics that are potentially useful to converters. What benefits are derived depends on the application, which will be addressed later. A summary list of characteristics follows.
- Single Side Contact – A very popular feature of vacuum rolls is the ability to control tension in a web while touching only one side.
- No nip is used, so a potential source of injuries is eliminated.
- Vacuum rolls can transport a wet web, coated but not yet dried or cured, without touching the coated side.
- Once the coating is dried or cured, it can be kept free from damage or contamination because it is not touched by the vacuum roll.
- Large Tension Differentials – Vacuum rolls and belts can sustain larger differences between Tension In and Tension Out than other technologies.
- A vacuum roll between two sequential process steps in a converting line can enable each step to be carried out at its optimal tension level, even when there is a big difference between the two levels.
- Extremely Low Tensions – Tensions can be minimal since the holding force from the pressure difference does not depend on tensions.
- Low line tensions can be achieved.
- High precision web metering can be implemented.
- Less slippage means more accurate speed control can be provided.
- Web accumulation can be done.
- The web can be held in place when a web break occurs.
- Small Wrap Angle – Compared to “S” wraps, the wrap angle required for a vacuum roll is fairly small. For belts, wrap angle can be very small for a given tension differential. Line layouts can be improved, for better use of space.
- Boundary Air Removal – At higher speeds, boundary air dragged into the area of a roll wrapped by the web can significantly lower coefficients of friction and thereby compromise tension control.
- Vacuum rolls help remove boundary air allowing higher speed operation.
The characteristics listed above lead to a wide variety of applications for vacuum rolls and belts. A vacuum roll may replace a standard driven roll to do the same job better, or its unique characteristics may be used to alter and improve a process.
For an idea of how some converters have applied vacuum rolls to advantage, consider the following.
- Transporting Wet/Coated Webs – This takes advantage of single side contact, and is a traditional application for vacuum rolls. In this case, a nip roll or “S” wrap simply cannot be used. Figure 4 shows a vacuum roll located between coater and dryer.
- Replacing Nip Rolls – While initially a bit more expensive, vacuum rolls and belts have proven popular nonetheless as replacements for nip rolls. Vacuum devices may be considered more desirable for one or more of the following reasons.
- Safety improvement by getting rid of nips, a potential source of injuries
- Scrap reduction by eliminating scratches, imprints, wrinkles, and slippage due to nip rolls
- Time savings by eliminating nip adjustment for web thickness, providing better access for web threading, and avoiding the need to manually close nips before starting a line
- Improved line layout from less wrap angle
- Handling Light Weight Film – Moving light weight film through a coating line is very challenging with traditional driven rolls. The positive grip and control of a vacuum roll, even at very low tensions, helps minimize stretching and web breaks.
- Guiding Webs – Gaining popularity is the use of vacuum rolls as part of a web guide. This takes advantage of the vacuum roll’s ability to keep a good grip on the web even as path adjustments are being made by the guide. At the same time, the vacuum roll can replace a nip roll or “S” wrap.
- Dealing with Web Breaks – When a web break occurs, vacuum rolls remain functional. At a minimum, this can keep most of the web in place so less threading is required to restart the line. In more sophisticated applications, a vacuum roll can be used to initiate an alternate path when a web break is detected.
- Implementing a Short-Term Loop Accumulator – Figure 5 shows two vacuum rolls being used to accumulate a length of web. Some process lines, like bag production, may use a recurring start-stop operation. If the web can only be touched on one side, vacuum rolls still allow a start-stop process section to be isolated from the balance of the line. The inlet vacuum roll continuously feeds into the loop, while the exit vacuum roll meters material into the process step intermittently. The reverse is done on the other side of the process step.
- Establishing a Constant Loop – Process lines in which an abrasive or bulky material like sand or reflective particles is added to a web may use a constant loop. This allows excess material that did not adhere to be removed from or fall off the loop in a position below important machine components, reducing repairs.
- Metering and Drawing a Web – Certain processes require both precision metering and a constant pull. Figure 6 shows a vacuum device metering the web, then holding it while another device pulls it with constant tension.
- Releasing Web from a Tenter Frame – The ability of a vacuum roll to provide a very low tension on the inlet side makes it ideal for controlling web tension at a tenter chain release point. See Figure 7.
- Controlling Tension in a Floatation Dryer – Achieving low tensions through a floatation dryer is a classic application for vacuum rolls, shown in Figure 8. A typical layout has vacuum rolls before and after the dryer to isolate tension, and perhaps between zones to help stabilize and guide the web.
One simple barrier to using vacuum rolls and belts is that they are more complicated than alternatives. A shell material must be selected and the diameter and pattern of holes in the shell specified. A variety of components are needed to form the vacuum zone within the shell, which may need to be adjustable. A shell coating or covering, chosen from many options, may be required to spread the vacuum, protect the web or to improve the coefficient of friction.
Besides the roll itself, a vacuum system is required. This would comprise of an exhaust connection from the roll, a vacuum gauge, an airflow control valve, the necessary ducting, and a blower.
Fortunately, most vacuum roll manufacturers take on the burden of doing the needed calculations and applying their experience to arrive at the correct vacuum roll specification. In order to do so, the converter or machinery builder will need to provide the following parameters.
- Maximum and minimum web width
- Wrap angle
- Substrate types and thicknesses
- Line speed
- Tension and tension differential ranges
- Orientation of the center of the vacuum zone
- Operating environment, including web temperature, air temperature, explosion-proof requirements, elevation, and electrical power available
In addition, any special considerations with regard to the substrate, how the roll is to be driven, where it is to be mounted, or locations of adjustments or connections should be discussed.
With that information in hand, the vacuum roll manufacturer can respond with a quotation including specifications for the vacuum roll or belt.
Vacuum rolls have traditionally served as the only way to accomplish certain functions within web processing lines. But as experience with vacuum rolls and belts has grown, so has the number of ways in which they are applied. Given the unique characteristics of these vacuum devices, the likelihood is that even more applications will be found as converters and machinery builders collaborate on product development and improvement.