[Home] [APPW 2004] [Journal papers]

 

UN-NIPPED ROLL COVER TECHNOLOGY

Author

Thomas Cody Merrion

Company

Industrial Polymer Solutions, Inc.

Keywords

roll covers, un-nipped, rubber, polyurethane, slippage, wear, traction, case study

 

 

 

 

PREMISE

Un-nipped roll covers have often been considered "non technical" roll covers in the paper machine and for more than thirty years little has been done to improve the performance or attributes of roll covers operating in a un-nipped position. This lack of advancement is due largely in part to the misconception that un-nipped roll covers do not have a direct affect on the art of papermaking or the quality of paper, yet they typically represent 80% or more of the covered rolls in a paper machine.  They seemingly perform a modest task, to carry the web, wire, pulp, felt and sheet from the head box to the reel. While the task may appear modest, it is also fundamentally important.

Why has the advancement in technology of un-nipped roll covers averted the attention of major OEM paper machine manufacturers for so many years? Why has the papermaker been reduced to "accepting" the flaws and limitations of rubber in the un-nipped roll covering market, while every other performance roll cover in the paper machine has been upgraded from rubber to polyurethane or some other performance polymer? The answer is simple….

WHY CHANGE IS NEEDED

If you ask a papermaker what their highest cost non-chemical consumable item is, most of them will tell you that the machine clothing is by far the most expensive, regularly replaced consumable.  Why is this?  One reason is because the "non technical", un-nipped roll covers are ill designed for the purpose of efficiently carrying a costly wire and felt through a paper machine. By definition, these wire rolls, felt rolls, guide rolls, breast rolls, forming rolls and table rolls are meant to transport the wire and felt; and whether the roll is driving the wire/felt, or the wire/felt is driving the roll, traction is paramount and the relationship between the roll cover and the wire/felt must be akin to a sprocket and a chain.

Fabrics wear out because the knuckles on the fabric become worn and effectively reduce the open area in the fabric, which is a precursor to poor drainage.  Poor drainage from a fabric in the wet end and press sections will increase the cost to remove water in the drying section. In addition, an improperly functioning piece of clothing will affect sheet formation and paper quality. So the papermaker is forced to regularly change out his clothing to maintain a consistent paper quality and to keep his operational costs in the drying section to a reasonable level.

So, why is the clothing wearing so quickly and how can steps be made to correct or extend clothing life?

THE PAST AND THE PROBLEM

Rubber is renowned for its traction properties, just look at what your automobile is running on.  So why isn't rubber effective in providing traction in a paper machine? The answer lies in the chemistry of rubber.  Most un-nipped, rubber roll covers in the paper industry operate at a hardness of 0-1P&J "bone hard" in order to extend the life of the roll cover. Yet, putting a hard cover on a roller whose primary function is traction is counterproductive.  The hard cover will invariably slip against the wire/felt and this slippage is one of the main causes of friction, heat and wear in wires/felts.

The paper machine already has several strikes against it when it comes to traction.  It is operating in a very wet environment and it is operating at ever-increasing speeds. So why put a bone hard roll cover in an area where traction is paramount? Because the materials used to produce roll covers in the wet end of a paper machine have traditionally been rubber, a very hydrophobic material that does not adsorb moisture. And the morphology of these materials is such that a harder material is much, much more durable than a softer one. This is where the contradiction occurs. In order to make the roll covers last longer, they have continually made the roll covers harder. They have been limited by the chemistry of the rubber and have been forced to make the covers harder to prevent frequent roll change outs and grindings. The industry has adopted a technology that promotes longer cover life but sacrifices traction to the wire and actually causes premature wear in the most expensive consumable item in a paper machine. 

To make my point, we will revisit a topic I mentioned at the top of this paragraph.  Remember my analogy to the tires of your car providing excellent traction? Well, check the hardness of a tire on your car and you will quickly see that it is relatively soft.  If we assumed that a roll cover was made of the same rubber used on cars, in order to provide exceptional traction against the clothing, this is approximately how long the roll cover would last in a paper machine:

EXAMPLE 1
Roll Cover Outside Diameter (OD): 500 mm
Speed Of the Paper Machine:1000 m/min
Rating Of the Tire:70,000 km

[ 70,000 km ] / [(1000 m/min) X (1 km / 1000 m)] X [1 hour / 60 min] X [1 day / 24 hours] = 48.61 days

This means that if your roll covers were made of the same rubber that your tires are made of, they would last approximately 49 days. Soft rubber provides excellent traction, but sacrifices on durability.  For further proof, ask a paper maker what hardness his drive roll covers are and typically he will tell you they are between 10 and 15 P&J, whereas the rest of his un-nipped roll covers are between 0-3 P&J.  He will also tell you that he replaces his drive roll covers approximately 2-3 times as often as his "bone hard", non-driven rolls. This again, points to the correlation between hardness and durability in rubber.

To further complicate the issue, it has been recommended that these hard roll covering materials have a very smooth surface, sometimes approaching sub 0.5mM Ra surface finishes.  Why would this recommendation be made?  It is because the wire manufacturers and the roll cover manufacturers have reconciled themselves to the fact "bone hard" rubber covers are going to slip. In order to minimize the abrasive effects of the rubber against the wire, they recommend a slick, smooth surface.  So now you have a roll cover that is hard and slick and it is supposed to provide or maintain traction to a wet, nylon wire moving at speeds up to 2200 meters per minute.  This is not a formula for success.  This is why papermakers are replacing more wires than they should.

Another issue to consider is power consumption.  With the ever rising cost of global energy, conserving power is a major factor when looking at paper machine efficiency and profitability .  If rolls are not properly tracking with the wire, then they are effectively acting as a brake as they slip against the wire. Traction considerations are not just for the driven rolls, but for all rolls in the wire and felt sections. Remember that most wires are driven by only one or two rolls and that the transfer of energy is extremely important. Whether the transfer of energy is from the driven roll to the clothing or from the clothing to a roll, power and energy are being transferred via traction and friction between the clothing and the roll cover. By putting roll covers in the paper machine that are designed for optimum traction, you can reduce the power consumption required by the drive rolls, breast rolls, wire return rolls, couch rolls, etc.

THE LACK OF PROGRESS

So why haven't companies introduced roll covering materials that would provide better traction and durability than rubber? They have the technology, but it would force the paper maker to replace roll covers more often. What about materials other than rubber, can't they employ a technology that would allow them to put a softer cover in the same position currently occupied by "bone hard" rubber covers, while not sacrificing cover life? They have, but they have limited those materials to press rolls and soft nip calendar rolls.  The roll covering material is polyurethane and almost everyone in the paper industry is educated on the cost and operational benefits of polyurethane versus rubber. Many papermakers have switched to polyurethane in the press section because of its superior properties as compared to rubber.  The one problem with polyurethane has been its tendency to adsorb moisture, or to hydrolyze over a period of time. Even polyurethane press rolls typically need to be taken out of the machine on a routine schedule and "dried out".  So companies did not want to put polyurethane roll covers in un-nipped positions because those rolls typically ran continuously for years and there were not sufficient spare rolls to allow a roll to be taken out of the machine to be "dried out" every year. Also, the traditional method required to manufacture a polyurethane roll cover (vertical casting) was cost prohibitive and the papermaker could not be expected to pay the same price for a polyurethane breast roll as he was currently paying for a polyurethane press roll.  So the cycle has perpetuated itself and the dilemma of traction and durability with un-nipped roll covers has continued to go unchecked.
 
THE SOLUTION AND THE FUTURE

IPS, Inc., in conjunction with National Urethane Industries (PTY) Ltd. of South Africa (NUI), is introducing new products on a global scale and Aklin Carbide (PTY) Ltd. is our exclusive distributor in South Africa. For the last 6 years, Aklin Carbide has been providing new technology for the South African pulp and paper market that finally brings a solution to the often overlooked problem of un-nipped roll cover performance. 

They now provide a new range of polyurethane roll covers for un-nipped roll cover applications in the paper machine. These covers do not adsorb water and do not show any indication of hydrolytic degradation, regardless of the term of service. Aklin can offer the papermaker the benefits of polyurethane (durability and traction) without the drawbacks of hydrolysis and prohibitive cost. Six years ago, Aklin started producing polyurethane roll covers with a fixed ratio system that applied the polyurethane roll cover in multiple passes (layered). At that time,  they were the only ones able to produce one type of roll cover in one P&J hardness.  Today they can manufacture a variety of homogeneous polyurethane roll covers in one continuous pass down the face of a roll at thicknesses up to 25mm per side.  Aklin can offer the papermaker a 4-25 P&J polyurethane roll cover where they are currently using a bone hard rubber cover.  But the roll covers they are currently manufacturing are not just softer sitting on your shop floor in ambient conditions, where the P&J rating of roll covers is established. IPS, Inc. and NUI have chemically engineered their covers to perform at optimum hardness in the paper machine at paper machine operating temperatures.  The covers provided will operate at about 10-15 P&J in your paper machine, softening slightly from the rated P&J reading taken at ambient temperatures.  Our softer covers provide better traction without sacrificing cover durability. In many instances, we find our softer cover is more durable than the harder, traditional bone-hard rubber covers. This is because of the chemical prowess that polyurethane has over rubber in its chemical morphology and structure and also because once we eliminate the slippage between the wire and the roll cover, there is nothing to cause wear against the roll cover.  This provides a significant advantage to a papermaker, especially those running faster machines where traction is at a premium.

Aklin Carbide is now reintroducing its line of hydrophobic polyurethane roll covers for the pulp & paper industry in South Africa.  Major improvements to their processing equipment and to the raw materials over the last 6 years have culminated in their current product offering. Today's products outperform the products of the past by leaps and bounds.  The new line of products is much more thermally stable and will maintain its visceolastic properties throughout its operational temperature range.  The new products are more hydrophobic and have better elastomeric properties.  Aklin has four new products to offer:

WireTrax™ roll cover is a polymer composite material comprised of a polyether / polyester polyurethane matrix blended with micro particles.  It has proven itself to be a superior roll covering material for the forming and press sections of a paper machine. It has been engineered to offer the papermaker a roll cover with optimum wear resistance, durability, hydrophobic properties and doctorability.  By focusing strictly on un-nipped roll positions, in a way currently unavailable to press roll covers.  We have maximized the properties that matter most in un-nipped roll cover applications.

Aklin's PolySlick™ roll cover is a polymer composite material comprised of a polyurethane matrix blended with Teflon® micro particles. The polyol backbone  is part of the same chemical family as our polyurethane roll covers. It has the same wear resistance, durability, hydrophobic properties and doctorability as WireTrax and is grindable to maintain optimum surface finishes.  The uniform teflon content throughout the thickness of the cover allows for several regrinds without losing any of the release characteristics.  Scratches and marks can develop on the roll's surface over time due to doctoring, repetitive cleaning and scraping .  These marks can easily be ground out and the cover profile (surface finish) restored. These roll covers can be easily doctored with a UHMW doctor blade. This is the only Teflon containing elastomeric roll cover in the industry that can be doctored.

Many of the same properties we found to be beneficial in un-nipped roll cover positions were also applicable to reel spool applications.  Our SprayCast™ process also has a unique attribute that it imparts into the resulting polyurethane polymer. The roll cover is naturally stress relieved during the manufacturing process and because no post cure is needed, the low internal stress in our covers mean that a knife cut will not propagate and spread like a similar cut would in a rubber cover.

Aklin's ReelTrax™ roll cover is a polymer composite material comprised of a polyurethane matrix blended with micro particles. This has proven itself to be a superior roll covering material for the reel section of a paper machine.  It has been engineered to offer the papermaker a roll cover with optimum wear resistance, cut resistance, durability and traction properties.  The micro particles we incorporate into the polyurethane impart unparalleled cut resistance and traction. We believe it is quite simply the best reel spool cover on the market today.

FeltTrax™ is the one of the latest technological advances from Aklin, NUI and IPS. This brings the benefits of polyurethane into applications where higher temperatures have traditionally excluded polyurethane roll covers. At 1 P&J, these covers are harder than traditional WireTrax roll covering materials, but as a composite still provide exceptional traction.  In addition, the chemical engineering and formulation work ensures that the cover will run at 10 P&J when placed in operating environments with elevated temperatures. These covers are designed to perform in hotter environments approaching 150°C, such as around dryers, size presses, breaker stacks and coaters. The remarkable thermal stability means that it can also operate inside the dryer hood as a dryer felt roll cover. Finally, the papermaker will be able to solve the corrosion problems that are inherent with dryer felt roll covers running bare steel shells.

Unlike all other major suppliers of roll covers, we design and manufacture our own polyurethane materials, giving us unequaled ability to custom design our formulations to the very specific needs of the paper machine.  Advances from other industries in nano-particle technology combined with our own developments in blended polyurethane polyols will keep new and improved products coming out for the foreseeable future.

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