Web Handling & Converting Blog

Nip Roller Workshop

Nip Roller Workshop by Timothy J. Walker and Dr. Kevin A. Cole on February 16 – 18 in Rochester, NY.

Nipped rollers are the most common method to create high pressure many value-adding web processing, including coating, printing, laminating, winding, and embossing. Nipped rollers are also commonly used to prevent air lubrication in heating, cooling, corona treating, and tension control driven rollers.

In most cases, the goal of a nipped roller system is to provide uniform pressure across the web's width, but diameter variations, hardness variations, misalignment, deflection, web thickness variations, and uneven loading can all lead to undesired pressure distributions and a variety of defects. Understanding the root causes of these defects and their common remedies can quickly improve your profitability.

The goal of this workshop is to show how to improve your roll and web quality through a better understanding of the winding process, winder design, and roll measurements.

Unlike any other option in web handling education, the AIMCAL series of web handling "workshops" are one-of-a-kind training that combines classroom seminars with on-machine demonstrations at Optimation's Media Conveyance Facility.

Barring and Compression Set and Grinding

A covered roll that is barred must obviously be ground down to the lowest point on the cylindrical surface. However, that is not good enough. The material at the low spot has a compression set. This means that the low spot may have a different modulus and/or a different hardness than the neighboring high spot. This means that the just serviced roll will vibrate again in the nip because of these cyclical (original pattern) variations in elastic compression. This means the brand new surface, no matter how perfect of geometry, will start to vibrate again in the same pattern as before it was ground. Moral: grind much deeper than seemingly needed for barred rollers to get below the invisibly compression-set damaged areas of the cover.

Name That Defect 12 - Slip Pimple

This is the second in a series of user submitted defect descriptions. Send me your pic and problem description and I will do my best to figure it out or tell you where to go (for more help). I promise to oh so sanitize the description so thoroughly to protect the oh so guilty.

This defect should be pimples, but we use anti-block JC-30 (2500 ppm), slip (2000 ppm) in outer layers, and our film of CPP is co-extrusion with 3 layers. Can you tell me what the defect is and possible remedies?

Don’t second guess yourself. Just because you use anti-block, does not mean you are anti-blocking. A salesman’s words for something does not make that something happen. It could be the wrong chemistry; it could be the right chemistry but simply not enough.

The slip pimple defect was described in my 2/21/06 post and much better on Name That Defect 04 dated 8/3/09 so I won’t repeat those discussions here. I will just parallel the ideas presented in Name That Defect 11 where we call on science’s gift of prophecy.

Note that defects don’t necessarily have to appear identical to textbook cases to have the same primary mechanism. True, the classic slip pimple is shaped very much like a very weathered mountain. However, it can elongate into something like a gage band if there is a collection of troubles in the same CD position. Also, similar mechanisms can cause a really bad-assed bell on the slit edges rolls of of tacky film. The mechanism here, though not known nearly so well as most other defects, involves two testable predictions. 1) If you wind tighter (or look at a known gage band where tightness is locally tighter), the defect count would be expected to be higher. This can be tested, for example, by running two trials; one with minimum layon load (max speed) and the other with max layon nip load (and min speed if possible) and compare the incidence. 2) If you change the amount of or chemistry of the anti-block, the incidence also changes. We expect that more anti-block is the same (either both bad or both good) or better but cannot predict the direction with regard to chemistry, only that a strong response to chemistry is compatible to the slip pimple working theory.

Name That Defect 11 - Crepe Wrinkle

This begins the first in a series of user submitted defect descriptions. Send me your pic and problem description and I will do my best to figure it out or tell you where to go (for more help). I promise to sanitize the description so thoroughly to protect the guilty.

We have many crepe wrinkles as well as defects such as shown in the picture above. The material is slippery uncreped tissue. We are winding this on an old two-drum winder that had been used for linerboard that has a rider roller with an 8 PLI minimum load. Could these defects be associated with baggy webs/lanes? - Anonymous

Possibly, bagginess already causes enough troubles on just about any material, especially when wound with a nip. However, lets start with what we can know for sure and then proceed to best guesses. This defect is certainly a web quality defect in that the locations of the defects are places where the web is different than its neighbors or the mean/average. In other words, the web rather than the winder mostly owns the locations of the defect. That is not to say the winder does not also have a role to play. It is most certainly also a winding defect because these defects show up after winding.

You say you also have many crepe wrinkles. I am also going to guess that the pictured defects are in fact just another way of a crepe-like behavior to develop, call it an incipient or baby crepe if you want. Certainly the picture resembles a partial core burst such as seen just above the unwinds on printing presses supporting large roll weights (if you cut the defect down its centerline to make an 'edge' much like a core has an 'edge’). The picture also resembles even more closely a 'caliper shear burst', a rare and debated separately listed defect in Roll and Web Defect Terminology. What connects all of these supposedly different defects together is mechanism, interlayer slippage under a heavy rotating nip.

Friction Limited Thrusting (in Nips)

Ever wondered why wound roll edges are rough? Ever wondered why bearings on nip rollers don’t seem to last as long as others do? The answer to both may be friction limited thrusting. Few people know that a mutual thrust load is developed when two cylinders are in nip. This could be in the roller-roller nip of a calender, laminator or printer or the roll-roller of a winder. This thrust load is caused because the world is not perfect. Specifically, the diametral profile of the rollers (alignment etc) is not perfect. This causes the two rollers to act like a cone drive which mechanical designers know causes thrust.

The thrust developed is not very dependent on the size of the error; the tiniest of errors can cause similar thrusting as the largest. The thrust is in fact friction limited to the normal (nip) load times the coefficient of friction between the two rolls. Thus, if you had a modest 1 PLI on a 50” wide film winder lay-on roller with a COF of 0.5, you would develop 25# of side load. On a wide paper winder, thrust loads have been measured at #10,000 lbs. Thrust loads on calenders can even be higher because the nip loads are higher on calenders than on almost any other nip in the web industry.

The problem is first with the bearings and frameworks. Every designer knows that almost all bearings (except thrust bearings) used on rollers do not take side loads very well at all. One (new) calender builder did not realize this and sized loads only for the obvious radial loads. The result was a 24 hour bearing life with a bearings that cost several thousand dollars each. Also this side load tends to spread framework that can cause a wide variety of problems.

On winders we have a different issue. Here the side loads can cause the roll edges to dish. This looks something like an offset or telescope. It happens right near the core when side loads first develop and then push the (wound) roll one way and the (nip) roller the other way. Curiously, the more level you get the web profile and machine alignment the worse the problem becomes. If things are way out of whack, the winding set will immediately thrust to one side and stop against the bending frame. If, however, things are nearly balanced, the set may toggle from one side of the play/flexibility band to another causing offsets throughout the wind.

The moral: keep the frame stiff and ball (roller) bearings strong when engaging nips.

Calendering Machine Knobs versus Process Knobs

There are many more ‘knobs’ or things that can be adjusted on a machine than there are independent parameters for that same process. Take a calender for example. You may increase the calendering effect (however you measure or define it) by increasing load, increasing cover hardness, decreasing calender roller diameter, decreasing speed, increasing temperature/moisture (for most products) and using multiple nips such as in tandem or stack calenders.

This number of adjustments is, however, beguiling. It makes you think you have much more control than you really do. The reality is that calendering as a process is only a one knob process even though there are many knobs on the machine. All of the knobs listed above essentially do the same thing; increase calendering.

If you know this you can trade one knob for another. For example, if cover life is short you can increase cover hardness and correspondingly decrease nip loading. In many cases the product can end up identical with the only difference is improved cover life. If you know this as product designers and process troubleshooters you can avoid trying to chase mutually opposing goals and think you can achieve them. You would then know better than to, for example, try to increase laminator bond and still maintain bulk (for malleable materials).

Calibrating Nips

Nips are just like any other control; they must be calibrated. Unfortunately, few do. This means that we don’t know where we are at. It also means that we can’t necessarily repeat a successful run on a different machine or even this machine a year from now.

First, we must use proper units as described in the next post. Second, we must define/measure zero as described a couple of posts ago. Third, we must calculate gain. Fourth, we must check gain (more than once due to hysteresis described a couple of posts ago. How to do this depends on the machine. However, it is usually done by pulling against a tension load cell (simple shipping or crane scale may work) strapped to a fixed surface such as a beam or by a compression load cell between the moving and fixed rollers. Finally, this process should be repeated on a regular basis, perhaps annually, for critical processes.

The Gold Standard for Nip Control

Much operator confusion can result when the nip control cylinder is most conveniently piped on one side. Here, depending on the geometry, a certain amount of pressure may be required to lift the nip roller assembly against gravity. In this case, zero (cylinder) pressure does not equal zero nip. Worse yet, with this configuration, an increase in cylinder pressure results in a decrease of nip pressure. How confusing, zero is not equal to zero and up (on the control panel) is down in the real world.

This and many other problems can be quite simply solved by piping. Here, we pipe both sides of the cylinder. One side is called ‘fixed’ and the other side is called ‘control.’ The fixed side is set to just counterbalance assembly weight. This is a service engineering function and the operator never needs to see or adjust it. The other side is the control side where the operator sets the nip load (or dancer web tension) on the product. Zero equals zero and up is up.

So how do you make the assembly open and close? Simple, we use valves. Typically the fixed side is either vented or brought to full pressure depending on the geometry and the opposite is done on the control side. The exhaust, in this condition, is routed through an adjustable needle valve to control speed. So how can we call one side ‘fixed’ when it is in fact varied? What we mean by ‘fixed’ is that it is constant only during run. Other conditions, such as raise or lower, will have other pressures.

Friction is Bad

Nip friction is bad. It does two bad things. First, it increases the hysteresis or uncertainty of the load that is actually applied to the web. Thus, for example, you might dial in 10.0 on your computer but the value of the applied load on the product might be 9-11 if the friction is 10%. Second, it will not allow you to run low nip loads. You must always be above the friction band with the pressure applied to the cylinder. Thus, with a 1 PLI friction you can never apply a nip load under ½ PLI above the middle of the hysteresis band. If you go lower, the nip will often bounce up or off the product and stay off. Extra pressure is required to keep the nip closed.

Thus, excessive friction does not allow fine or repeatable control nor does it allow for delicate loading. There is little that control fanciness can do to help you here. You could, for example, put a load cell in the pin of the cylinder and close the loop. However, you are closing the loop inside the friction band and the control system will hunt trying to nail the load. Also, there are seldom ever maintenance solutions. True, you occasionally get the totally starved lubrication or bent or binding assembly. However, most friction was built in when the mechanical design choices were made even before the machine was even built.

How bad can it be? Well, it is hard to make general conclusions because the world is a really big place with lots of variation. However, most dancers are troubled or very troubled because friction exceeds 10% of the web’s tension. In contrast, most nips tend to not as bad or not as critical. Even here, we must be careful. I once ran into a tissue winder where a desired nip load might be something like 3 PLI yet the friction was near 30 PLI. A NOISE to signal ratio of 10:1. Caveat emptor. Buyer beware. Measure before you buy as oiling the assembly may do little to effectively reduce the friction after the (design) fact.

Nip Friction

Friction destroys the accuracy and especially the repeatability of instruments and controls. This is especially true of nip control because the friction might be high. Friction comes from valve/piping losses, pivots, chains/gear-racks used to time the two sides and, most especially, the seals on the piston and rod.

System friction can be easily measured on almost any load controlled nip as well as other devices such as dancers. Merely set up a configuration (with web off) where you can just counterbalance the weight of the nip assembly. Then slowly decrease the pressure until the assembly just begins to drop/close. Write down that close value of pressure. Then slowly increase the pressure until the assembly just begins to raise/open. Write down the open value of the pressure. An engineer can then easily calculate the system’s total friction from the difference between the two pressures times the acting area of the cylinder and considering the appropriate geometries such as the swing of pivoting systems.

What do you do with this value of friction? Compare it to the applied load on the web. It is something like a signal to noise ratio. You usually want the friction to be less than or far less than 10% of the load to the web product. How fussy process/products are varies greatly. For example, a laminating (bonding) calender is not so fussy; as long as you have a minimal pressure you do the job and you might be able to take 10X that without damage to machine or web. On the other hand, a calender that is used to form the web would be very fussy. Thickness and other properties would vary if the actual load applied to the web (as opposed to what was merely set on the machine’s controls) varied inadvertently due to friction/hysteresis/uncertainty.