A practical guide to grooving suction rolls.

Editor's Note: The information in this article first appeared in "Ahead of the Curve," a free weekly electronic newsletter. See "Additional Resources" at left for more information on subscribing to the newsletter.

Grooving suction rolls has been done for some time. Here are some of the most frequently asked questions.

WHY ARE SUCTION ROLLS GROOVED?

There are a number of reasons why suction rolls are sometimes grooved. Grooving provides knuckles of high pressure, thought to aid in the transfer of water from the sheet to the felt under the action of the pressure in the nip. Just as the suction roll provides a shorter path for the water to go before it reaches an area of no or low pressure, grooving of that suction roll will shorten those water transfer paths even more.

Grooving a suction roll cover also provides a more compliant surface to the cover, which compensates for irregularities in the web, the felt, and the mating roll. It is for this reason that grooving is sometimes used on suction pressure rolls for Yankee dryers.

Also, grooving will compensate somewhat for the inability of harder covers to conform properly to the mating roll. This is the reason why many polyurethane covers are grooved. Presently, polyurethane covers can be manufactured to around 55 P & J and under. (P & J refers to the Pusey & Jones index, a measure of hardness; the lower the value, the harder the cover.) However, one must be aware of potential groove closure, i.e. the lands will buckle under load and "mushroom." Thus, the top of the groove will be narrower than its bottom, which will restrict the intended water flow considerably. Cover manufacturers have calculation programs available to determine how much load can be applied before that is expected to happen; a rule of thumb is, though, to choose a cover hardness of up to 12 P & J for such cases.

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In some cases, grooving a suction roll cover has eliminated or minimized shadow marking. This is generally considered to be the result of flow patterns generated in the sheet. To eliminate shadow-marking, a softer cover can be applied. This lowers the pressure in the nip, and it increases the time to go from zero pressure outside of the nip to maximum pressure near its centerline. It further reduces the velocity of water flow in the nip; thus it does not reorient the fibers. In short, grooving the drilled cover breaks up the flow pattern so that orientation of the fibers does not occur.

Often, shadow marking becomes unacceptable as the cover thickness decreases due to grinding and normal wear. It is sometimes possible to get one or two more grinds out of an existing cover before recovering when grooves are added during a scheduled regrind.

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WHAT IS THE EFFECT OF GROOVING ON COVER P & J?

Let's start by considering a plain roll, which has not been drilled, and consider PSI in the nip as the major factor. Drilling the plain roll with the usual 20% open area will increase the nip width at a given load and consequently decrease the pressure calculated on the total nip width by about 20%.

If one considers only the bearing surface in the nip, the average pressure is actually increased by about 5% by drilling. If that drilled roll has grooving added, (usually 8 grooves per inch, .020" wide, depending on the grade and basis weight produced), there will be little effect on the nip width since the bulk modulus of the cover will not change much due to the surface grooving.

All of this has occurred on the same cover--no changes were made to its type or hardness. In the rubber industry, hardness is considered to be resistance to indentation. The P & J plastometer is a device that measures the indentation of the 1/8" diameter ball into the surface of the rubber under a load of one kilogram. Trying to position the ball of the P & J instrument on a drilled roll so that it is equidistant from any of the holes is a difficult enough task. If we are dealing with a drilled and grooved cover, it becomes almost impossible to get reproducible results because of the difficulties in accurately positioning the ball.

This means the answer to the question concerning the effect on P & J is that higher P & J readings will be obtained. A drilled and/or grooved cover will "act" softer than a plain cover.

P & J hardness should be specified based on an undrllled cover; this is the basis for determining the modulus of the cover. The bulk modulus mentioned above is the modulus of the cover as a system. Not only the P & J, but the drill pattern and open area are used as input for the computer programs used to calculate pressures, nip residence times and other nip related quantities.

WHAT ELSE HAPPENS WHEN A SUCTION ROLL IS GROOVED?

Grooving a suction roll decreases the surface area to the nip load. It is easy to see that this will increase the amount of abrasion inflicted on that smaller area. A drilled and grooved cover surface also becomes discontinuous to the extent that the little pillars that support the load are free to move in the nip; this also increases the wear rate. Refer also to the remark made earlier about possible groove closure.

Chemical attack on covers is dependent on a number of things, only one of which is compatibility of the cover type and material with the prevalent chemical environment. Another factor is the surface to volume ratio. Suction rolls have a larger surface area to absorb chemicals because of the drilling, but at the same time, they usually run cooler than plain rolls. This affects the rate of any chemical attack; it is less at lower temperatures than at higher ones. When the drilled roll is also grooved, it further increases the surface to volume ratio and can tilt the balance so that chemical resistance can become a very real problem.

WHAT YOU WILL LEARN

* Why suction rolls are sometimes grooved.

* What happens when a suction roll is grooved.

* A comparison of suction roll materials.

ADDITIONAL RESOURCES

* The information in this article first appeared in "Ahead of the Curve," a free weekly electronic newsletter published by TAPPI that presents articles from a variety of authors on trends and ideas the pulp and paper industry, as well as practical papermaking articles authored by Metso Paper. To subscribe, go to www.tappi.org, click on Publications, click on Online Newsletters, and choose Ahead of the Curve.

RELATED ARTICLE: SUCTION ROLL MATERIAL COMPARISON

Suction rolls are expensive yet necessary components of modern papermaking machines. These components are used to aid in water removal and to control the wet paper web during the paper making process. A modern suction roll shell can be up to 1800 millimeters (mm), (70 inches) outside diameter, with a 101 mm (3.9 inch) thick wall and 11 meters (433 inches) long. To function, this large shell can have a million holes drilled through it. Their geometrical factors, along with operating stresses and corrosive operating environments require a deep understanding of the material requirements for long-lived suction roll shells.

There are several material features that define the most suitable material for a suction roll shell, including:

1. Microstructure: Hot rolling leads to a more homogeneous and finer microstructure, improved strength and corrosion fatigue strength.

2. Chemical composition: Certain elements protect better against corrosion. For example, Metso Paper's MetShell Pro suction roll contains molybdenum. This alloying element provides improved corrosion resistance, especially against crevice and chloride pitting attacks.

3. Corrosion fatigue strength: Higher corrosion fatigue strength directly results in a longer shell life.

4. Threshold fatigue crack growth: The best threshold fatigue crack growth rate proves the material performance.

5. Thermal fatigue strength: Wrought microstructure produces the best thermal fatigue resistance.