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

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Taming the Wild Web

This article offers a clear, concise overview of web handling problems and solutions, focusing on the most common converting roller — the idler.

All web converting operations have at least one interest in common: They want to increase production. Yet, no matter what material you are running, or which type of machinery you are using, there are only three ways to do this: You can add more people and run more hours each day, run your machinery faster, or run more material by using a wider web.

In some cases, converters try to go both faster and wider. The critical mistake they make is trying to do it using rolls that are the same size and diameter. The line will eventually reach the first critical speed, and the machine will shimmy and shudder. Then the line must be slowed down–and little or no additional production is ever realized.

Vibration, however, is just one of the limitations operators face when trying to increase production. There are a host of other web handling “abnormalities” that can plague a process line even at the slowest speeds, such as stretching, slipping, bagging, wrinkling, marking, and sticking. These problems can occur at different places along the line, at different times, on different materials. They can slow down production and result in an inferior product.

One thing is clear: There is never one absolute solution to every web handling problem. There are simply too many variables that can affect the performance of a web. In presenting an overview of “wild web” problems and solutions, we’ve focused specifically on the use of idler rolls. Because the idler is the most common converting roller, one that carries, turns, deflects, or supports the web, many of the problems we will address can be solved by the proper design and application of idlers.

The following information touches on some of the more common web handling problems–including–wrinkles, stretching, bagging, and creasing–and recommended solutions. Remember that what works for another converter may not work for you. However, through proper exploration, attention to detail, and the counsel of a qualified roller manufacturer, you are more likely to find the right solution for your operation.

Poor Machine Alignment
Machine alignment is the first thing you should check when experiencing web performance problems. If machine stands are not properly aligned, purchasing new rollers may be a waste of money. Though most machines are aligned when first installed, misalignment can develop later due to vibration, improper machine or roller maintenance, tension over time, or even, in colder climates, from freeze-thaw conditions that can result in shifting floors.

If machines tilt and pitch in different directions between one another, the web will surely be on a winding road, no matter how subtle the turns. Poor alignment between rollers can ultimately cause a wide variety of web problems, including drift, flutter, and wrinkling. Straightening out even the slightest of curved paths is critical to achieving higher speeds and wider web widths.

Though alignment services can be costly, they may save you time and money over other remedies that simply do not work. Checking machine alignment is an essential starting point and a good investment.

Poor Roll Geometry
Idler rolls that are not cylindrical in shape are sometimes cone-shaped. Imagine a race car whose tires are larger in diameter on one side of the car than the other, so the car turns naturally inward on the oval track. Likewise, the web will naturally turn if the roll is bigger on one side than the other. Some operators attempt to compensate for this by trying to turn the web back the other way at the next roller, which generally results in a wrinkle.

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

Even poorly wound parent rolls represent a type of cylindrical geometry on the line that can cause web handling problems. Excessive parent roll runout can translate into vibration and web flutter. In other words, the condition of the material itself, as a poorly wound parent roll, will end up creating wrinkles through the whole process. No one's rollers can totally cure that.

Most unwind operators can tell when they have a bad parent roll on the stand. Don't accept inferior parent rolls from your suppliers. If this can't be avoided, a good tension dancer system can help adjust for much of the web tension variance coming off the parent roll.

Converters running different web materials on the same line have to find a balance among various solutions that will work best for all runs.

Excessive Roll Deflection
Deflection in an idler is caused by the force of the web as it changes direction. When the web starts to apply force, or load, to the roll surface, the roll bows, or deflects. The greater the load, the greater the force that can result in roll deflection. If you carry that deflection too far, you can create a tension upset, resulting in wrinkling or bagging.

Roll deflection is often identified when the web bunches near the midsection of the web directly around the roller. However, the presence of deflection may be much more subtle. Consider a situation in which you have twenty thousandths of an inch deflection on an idler, and 50 idlers in the system. By the time the web gets from one end of the machine to the other, the web on the outside has to travel one inch more than the web on the inside. This is an example of accumulated effect due to deflection. It’s not the effect of a single roll that causes the problem–it’s the effect of the total system.

Like so many other situations, the extent of the problem will depend greatly upon the web characteristics. Some webs recover quickly edge to edge when stretched slightly, others do not.

Small-diameter idlers with long face lengths should raise a red flag for any qualified roll supplier. Deflection is, perhaps, the single most important factor that determines roll construction. Roll material (steel, aluminum, or carbon composite), roll diameter, wall thickness, face length, web width, shaft sizing, and bearing location all have to take load, or deflection, into consideration.

Excessive deflection can also lower critical speed–the point at which harmonic vibrations occur in a roll. A lower critical speed will force you to run the line slower. Be aware that two identical rollers could have different critical speeds based on different degrees of web wrap load, deflection, and/or mounting.

To avoid problems of the kind, be sure your rolls are properly specified. When ordering, you should know the amount of web wrap (usually stated in terms such as 10 o’clock to 3 o’clock), web tension (PLI), any nip load (PLI), web width, and maximum expected line speed. If you haven’t thought of them in advance, a good supplier will walk you through these questions and can even help you troubleshoot much of this over the phone.

You can’t change the laws of physics. You can have the truest running roll in the world, but if the roll material or design can’t stand up to the force of the web, you are going to have problems.

Shaft Deflection
Shaft deflection is just as critical as roll deflection. In fact, in a dead shaft roll, shaft deflection is the problem more often than roll deflection. The problems surface in several forms. The shaft may start vibrating and causing wrinkles in the web. The changing angle of the shaft can also cause premature bearing failure.

Sometimes changing the mounts form a quickie mount to a rigid mount can help. If you have a short-faced roll with an extra-long shaft, like a 60-in. roll with a 120-in. shaft, you may be better off using an 80- or 90-in. roll. The resulting angle is going to be less, and you’ll have a stiffer shaft/roll configuration. The bearings will also last longer because they experience less side thrust. In other cases, using a larger tubular shaft within the roller may reduce shaft deflection significantly.

Getting the right shaft is as important as the right roll. Count on a qualified roller supplier to help you determine the specifications based on the same criteria: wrap, load, speed, and deflection.

The Unsupported Web
You must have enough idlers to avoid long areas of unsupported web. However, there is no special formula or rule of thumb to tell you how many. Too little support can result in bagging and wrinkling. Too much support can result in unwanted drag from unnecessary idlers.

Unsupported webs are also susceptible to other environmental factors, such as the drafts of a nearby motor or normal plant ventilation. We’ve seen examples in which an open door caused enough draft to result in wrinkling. As so often is the case, the strength and weight of the web will have a lot to do with the number and location of idlers needed.

We encourage customers to send an elevation side view indicating roller locations and where the wrinkling occurs. A qualified roller supplier can use this to help troubleshoot your problem. If there’s a short span between rolls where it’s wrinkling, we investigate support solutions or additional idlers. When evaluating whether your span is short or long, consider the width of the web. A tentative rule of thumb is: short = less than the width of your web; long = 3x the width of your web.

Improper Web Tension
Excessive web tension can contribute to web marking by pulling on the web and intensifying any flaw that may be in the roller’s surface. It can wrinkle, stretch, or break the web and cause unwanted roll deflection. Excessive tension can come from too much drag on idler rolls (bearing friction, roll weight, etc.) or from too many idler rolls between drive sections. Excessive tension is often identified by stretched edges and slack centers on the web.

Lack of tension causes “bagging,” meaning web edges are loose while the center is slack or just the opposite. Baggy edges will result in concave-looking finished rolls–thicker on the edges than in the middle. Loose webs can result in folds, creases, and wrinkles. Lack of sufficient tension can come from loosely wound parent rolls, unsupported spans of web travel, drive sections out of sync, and/or lack of nip or web pull through the line.

If you have a tension problem from rolls that are too heavy (drag), be cautious about simply replacing them with smaller rolls. For example, if you replace a single idler with two idlers weighing half as much, you will still have the same problem.

Lighter weight rolls may be the solution for tension problems experience during startup or speed changes. They can supply support without excessive drag. If your problem occurs once you are up to speed, an idler designed with special free-running bearings may be a good solution.

If you suspect your tension problems come from too many rolls or rolls that are too heavy, try some experiments. Spin individual idlers by hand. Are they hard to turn? Can you selectively remove or replace some rolls and see problems lessen?

If you are running a light web at light tension, and all your rolls are 6-in. steel idlers with sealed bearings, you could try 3-in. idlers with free-running bearings to reduce roll weight and drag significantly. Rolls of the same size but made of lighter material, such as aluminum, with free-running bearings could help. A good roll supplier should be able to provide rolls with free-running bearings with optimal lubrication.

Converters running different web materials on the same line have to find a balance among various solutions that will work best for all runs. Often, you need to determine which material will be run most and focus on maximizing production for that material. Your roll supplier needs to know this so that your rolls can be designed and balanced to meet the most demanding specifications of any web run.

Air Entrapment
Air entrapment is often found when rolls do not spin up to line speed. It becomes more of a problem as the line goes faster. For example, as plastic garbage bag material has become stronger, it has been produced in thinner gauges. These lines can now be run at higher speeds, meaning air entrapment can become a problem.

When you get an air barrier buildup, the web loses contact with the roll. The roll then does not spin at the same speed. When it does make contact, it can drag and mark the web. This happens mostly with light webs at high speeds. You get web flutter, side-to-side drifting, and lack of web control. If this happens on web guide rolls, your web guide will quickly become ineffective.

In extreme cases of air entrapment, the web doesn’t make any contact with the roll, and the roll stops spinning. If all your rolls are wrapped or configured the same, and one isn’t rotating, or it is rotating slowly, then suspect bearing drag. If the web is floating over numerous rolls, then expect air entrapment.

Some roll manufacturers offer grooved or machined surface options that can help eliminate air entrapment, such as a spiral V-groove surface, a micro-groove surface, and a diamond-knurled surface. A spiral V-groove design is specified most often for air entrapment problems. Other patterns are determined by specific application and type of web.

When considering solutions, think about what grooved configuration could be used without marking your web. If a standard V-groove won’t work, you may need to use a micro-groove pattern, which is a much smaller groove but has a lot more of them. A diamond-knurl and a micro-groove surface perform much the same. Your roll supplier should be able to guide you through the various details of specifying the right surface configurations.

Through proper exploration, experimentation, and consultation, you can eliminate or reduce most of these problems. Do your homework.

Improper Web Release
Web release problems are most often associated with coating processes, such as adhesive coating operations. When the web adheres, or sticks to the roll surface, it can pick off coating from the web as well as cause tension fluctuations and web marking.

The most common solution to this problem is to increase the release characteristics of the roller. This can be achieved by decreasing the surface contact area of the roller and/or using special coverings, such as plasma or ceramic coatings or silicone rubber coverings, on the roller surface.

A good release surface will eliminate or reduce coating buildup on the edge of the roll and also facilitate roll cleanup. For example, a plasma-coated roll lets operators easily peel off accumulated coating on the roll’s edge. Rolls cleaned right on the line don’t have to be removed for soaking and cleaning. Be aware that any coating buildup can also affect the balance of the roll.

Should you want to move or turn a coated or sensitive web without any roller contact whatsoever, an air turn may be the ideal solution. Here the web travels on a controlled blanket of air emitted from the air turn. Air turn designs can provide simple through paths or wraps of 250 deg+. Designs vary among manufacturers. Talk to your roll supplier to find out what is available.

The Roll: Defects Can be a Drag
Web marking is readily caused by flaws in a roll, drag on the line, web slippage, or drag in a roller when a roll is not turning up to speed. A repetitive mark is likely caused by a spot defect in a roll. A streak normally comes from roll drag or a circumferential defect in the roll. A repetitive streak or blemish may also come from the roll scuffing the web as air entrapment or web tension fluctuates.

A lot of web marking is caused by rolls not spinning up to speed due to air entrapment or poor bearing performance. If a roll is not spinning up to speed, check for air entrapment or poor bearing performance. A lighter weight roll may also be a simple solution, or consider a rubber-covered roll for increased traction.

Excessive Vibration
The causes of vibration can vary greatly, from inadequate machinery mounting to poor roll balance to external influences. Before spending money on rolls or machinery, be sure you investigate all the possibilities.

Check if your line is properly isolated from the vibration of adjacent machinery. Improper roll mounting can also cause vibration. Check to see if your roller supplier can help identify your first critical speed based on information shared by phone. Knowing this, you can better plan your line’s performance.

Roll balance is also very important. When ordering new rolls, you should know that ISO’s standard (G6.3) is the recognized industry specification for spindle balancing.

Rolls can become unbalanced due to a number of causes, including “tossing” or losing counterweights; excessive wear; O.D. (outside diameter) buildup (such as from coatings); or careless handling during maintenance or changeovers. You’re less likely to encounter problems associated with tossed weights by specifying rolls with mechanically attached balance weights.

You can often locate the vibration-causing roll simply by feel. Vibration testing is also available through firms specializing in preventive maintenance. Has that roll always vibrated at that speed? Has the mounting changed? The wrap? Sometimes, a little historical investigation can turn up the variable that started the problem.

Summary
Regardless of the many precautions and cures you might apply to your web converting operation, sometimes problems, such as wrinkling, are just going to happen. In some instances, a special roll will help. In others, it won’t. The problem may be in how the parent roll was wound. It might be in the makeup of the material itself. The cause may be a combination of things, some of which may be out of your control.

This simply means there is no one magic cure for taming wild webs. Through proper exploration, experimentation, and consultation, you can eliminate or reduce most of these problems. Do your homework. Then look to the experience and advice of a qualified roller manufacturer to help you work through the options. A qualified supplier will work hard to ensure that no roller is overdesigned or underdesigned. With the former you pay for something you don’t need; with the latter you end up with a roller that just won’t live up to your process demands.