PEER REVIEWED PRESSING

Techniques for nip ABSTRACT

New technology has led to faster run-

impressions ning paper machines with ever

increasing operating pressures in the
press and other sections of the paper
JOSEPH A. ZWINAK machine. It is important that the
operating pressure between two or
more rolls have even distribution
across the face of the nipped rolls to
ensure product quality and roll cover

T
HERE ARE MANY VARIABLES AT loads. The running characteristics of integrity. Nip impressions can ascer-
work during the operation covers are highly pressure depen- tain if there is a uniform pressure dis-
of a paper machine. They dent. A noninclusive list of cover tribution.This paper reviews the need
make it difficult to judge the properties dependent on pressure for nip impressions, the various media
condition of an individual nip from includes heat buildup, speed differ- available to perform the work, gen-
the entering or exiting sheet charac- ential with the mating roll, wear, and eral instructions for taking the
teristics. This means that the mois- surface deterioration. All these fac- impressions, information to analyze
ture content of the sheet or a felt in tors would be uniform if the nip the results, and discussion of a crown
a press section does not necessarily were equal across the roll face. correction equation based on nip
indicate that the roll crowns are cor- Uneven heat buildup during impression measurements.
rect or that the nip pressure is uni- operation can result in thermal Application:
form across the roll face. crowning that will distort the speci-
The latest information on techniques
Why should the papermaker care fied crown. In severe cases, it may
to use nip impressions can provide
about the nip if the product is within result in cover separation from the
proper roll operating pressure.
specification? If all the components metal roll core. A meltdown starting
of a system are operating at maxi- below the surface and above the
mum efficiency, it would seem intu- metal may be the result with some
itive that the system as a whole is cover materials. Internal water cool-
also operating at maximum effi- ing may alleviate this condition. tion by ensuring that a uniform nip
ciency. For a paper machine, this Even loading of nipped rolls load exists across the entire roll face.
should translate into factors such as results in a speed variation between
higher speed, less steam consump- rolls that have different cover hard- NIP IMPRESSION
tion, and longer roll cover life. The ness.Varying the pressure across the A nip impression is the footprint of
roll covers can have many composi- roll face also results in varying speed two mating rolls obtained under con-
tions including polymeric, compos- differentials. The results of these vari- trolled conditions. For maximum
ite, and filled. ations in speed differential are accel- use, it should be continuous across
Many things happen to roll cov- erated cover wear and more frequent the face of the rolls forming the nip.
ers when they run under nonuni- grinding to return the surface to its It also requires materials that can
form cross direction (CD) condi- desired operating condition. show a reasonably sharp transition
tions. Across the roll, the cover Rubber can undergo stretching between the loaded and unloaded
would be under nonuniform pres- and return to its original length on a areas on the paper, film, or foil used
sure. A nip forms when two rolls load recurring basis without deteriora- to obtain the impression. There are
against one another. The expression tion. This remains true as long as the three major commercially available
for this load pressure is usually kilo- amount of stretch is below its critical products to obtain nip impressions:
newtons per meter (kN/m) or elongation. After exceeding that crit-
pounds per linear inch (pli). Rolls ical elongation number, cracking will • The most common technique
normally have a positive or negative occur quickly. Since rubber is incom- uses paper and carbon paper
crown to compensate for core bend- pressible, it has to stretch when together so the carbon paper
ing and distortion under specified going through the nip. The amount produces a copy of the pressur-
loads. If the crown is incorrect, the of stretch depends on the line load. ized area on the plain white
load will vary across the roll face. In the case of the harder cover mate- paper. Since it requires dry condi-
Some areas will have higher loads rials that are used today, this means tions, a waterproof envelope is
while adjacent areas may have lower one can only achieve crack preven- necessary in a wet nip.

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• Embossed metallic foil (usually For the loading pressure, a gauge 2. Tape the paper to one of the
aluminum) is another commonly pressure reading will require the rolls, being sure that the nip
used medium. It has the advan- equivalent in kN/m or pli. The con- paper is straight along the roll
tage of use under wet and very version of gauge kPa, psi, or bar is width. For a two part system,
hot conditions. It is more difficult complex and requires press loading ensure that the two layers are
to measure than the carbon system data. Such information is put together correctly.
paper and does not store as well. probably available from the original 3. Rotate the roll, if necessary, so
It has a tendency to wrinkle, mak- equipment supplier. It was probably the nip paper will be in the cen-
ing the results difficult. a conversion chart presented at ter of the nip when closed.
• The third material consists of press installation. 4. Close the nip using the minimum
two plastic films. One has a coat- amount of pressure to bring the
ing of micro capsules with gradu- TAKING NIP IMPRESSIONS rolls together. Load both sides at
ated resistance to rupture under The following instructions have a the same time. Remember that all
pressure. The other film has a general nature. There are a variety of nip impression media measure
coating that reacts with the con- press configurations and roll types. It the maximum pressure they have
tents of the ruptured micro cap- would be impossible to provide seen. If a nonuniform rate of
sules to generate a color of vary- comprehensive information for all loading (one end loading before
ing intensity. One can use this cases. For those instances where the other) occurs, the nip
material statically like carbon there are multiple nips or controlled impression will reflect this. It will
paper and foil or dynamically. crown rolls, one should contact the not give a realistic representation
Optimum use of this method press or calender stack supplier to of the pressure distribution in
requires an instrument to mea- obtain detailed instructions on tak- the nip.
sure color intensity differences. A ing nip impressions on their equip- 5. Gradually increase the pressure
distinct advantage of the method ment. on both ends at the same time
is its use in dynamic and static Rolls require new grinding until reaching the desired
nip impressions with the felt(s) before taking nip impressions that loading.
in place. will be used to establish crown 6. Hold for one minute.
changes. A roll’s crown compensates 7. Gradually decrease the pressure
RECORDING NIP IMPRESSIONS for deflection due to roll weight and uniformly from both ends until
Every nip impression should have normal running nip load. A nip the rolls disengage.
the following information: impression on worn rolls will not 8. Remove the nip impression
produce an accurate indication of paper and examine the impres-
1. Centerline diameter to the the deflection. It will show the sion obtained to ensure it
closest 4.0 mm (1/64 in.) on loading characteristics under the appears representative of the
both rolls current conditions of operation. entire nip.
2. Machine number and nip Rolls require rotation for at least one 9. If taking another nip impression,
identification such as first hour before taking the impression to rotate both rolls so the new nip
press, size press, etc. eliminate any effects of core sag. impression will not occur in the
3. Mill roll identification number Observe all safety precautions, since same place as the previous
4. Identification of tending and the nip points are areas of high risk. one(s).
drive sides of the impression “Nip paper” will refer to the nip
5. Date and time impression medium, although it may SUCTION ROLLS
6. Loading pressure used to be any of the three media referred to Suction rolls present a problem. Part
include any difference on the above.While the general instructions of the core bending or distortion
tending and drive sides that follow are guidelines for taking load is the result of the application
7. An indication if the rolls were nip impressions, the suppliers of the of vacuum. One can address this by
newly ground or old rolls various nip impression media do increasing the pressure to compen-
showing the installation date provide detailed information for use sate for the vacuum. Another
8. Design crown for both rolls of their products. method is by sealing off the suction
9. Other items of interest to box area with plastic and applying
include information about the 1. Pull out a length of nip paper an amount of vacuum equal to that
roll temperature and direc- long enough to overhang both normally run in the roll. If using the
tion of rotation. ends of the roll. increased pressure method, contact

122 TAPPI JOURNAL JULY 1997

the original equipment supplier to 4. If possible, the pressure roll N1 = Nip width at the center of the roll
obtain the correct amount to use. If should have run approximately N2 = Nip width 50 mm (2 in.) in
this information is not readily avail- 24 h before the test. from the ends of the roll
able, one can approximate the incre- 5. The pressure roll under examina- D1 = Diameter of the top roll
mental pressure addition by using tion should be loaded to the D2 = Diameter of the bottom roll
the one of the following formulas: value for which it is crowned.
6. Immediately after ceasing to If the rolls have equal diameters,
Vacuum, kN/m = 1.332x10–4WVF (1) make paper, reduce the machine then Eq. 3 becomes the following:
speed to crawl and pass the film
Vacuum, pli = 0.4912 WVF(2) through the nip. C = (N22 – N12)/D (4)
7. Interpretation of the results is
where done by using instrumentation If C is negative, then the nip is
to measure the color intensity, overcrowned.
W = width of the vacuum box, mm which relates to the nip kPa. As an example, let us assume that
V = vacuum level in mm (in.) of we have two 762-mm (30-in.) diame-
mercury INTERPRETING THE NIP ter rolls, and we find that the nip
F = box seal efficiency factor IMPRESSION widths are 22.9 mm (0.9 in.) on the
(0.9 for most suction rolls) Usually the first interpretation of the ends and 17.8 mm (0.7 in.) at the
nip width occurs immediately follow- center under the loading at which
Only 70–75% of the vacuum is an ing the removal of the nip paper from we desire to run the rolls. Then by
addition to the applied loading. the roll surface. Take the paper and our formula:
put the two ends together so that the
YANKEE PRESSURE ROLLS impression of each end can be exam- N1 = 17.8 mm (0.7 in.)
Paper machines with Yankee cylin- ined to see if there is a variation. This N2 = 22.9 mm (0.9 in.)
ders present special problems when will also establish the centerline of D = 762 mm (30 in.)
taking nip impressions. The tempera- the nip impression. Visually compare C = [(22.92 – 17.82) / 762] = 0.27 mm
ture is too high for using carbon the widths of the impression from (0.011 in.)
paper. A rubber-covered roll held the ends to the centerline. This test is
against a hot Yankee cylinder during to see if the nip width is symmetrical. There are many techniques for
the taking of a nip impression would If it is, then proceed to the next step. measuring the width of the nip. Even
damage the cover. The best method If it is not, try to determine why it is though carbon paper has been care-
to use is the micro-encapsulated plas- not and make whatever changes are fully chosen to give as sharp an edge
tic film. An envelope of some type necessary to make it approach sym- as possible, it is still slightly fuzzy.
must be used to protect the film metry. Take nip impressions until The technique that has given the
from moisture and heat. Heavy paper symmetry is attained. best and most reproducible results is
or plastic has been successfully used If the only object of taking the to first mark off the nip impression
as the envelope. In this case, a impression is to establish if crown in 21 crown station intervals. Then
dynamic nip impression probably correction is required, the only mea- using a straight edge and a pencil
would be more accurate than a static surements needed are the nip width with a very sharp point, draw a line
impression. The following conditions at the centerline and at each end of about 100–150 mm (4–6 in.) long at
should exist to obtain a meaningful the roll. The nip widths at each end the impression edge at each side of
nip impression: are measured at crown station 0. the impression and centered at the
Once these measurements have crown station. Using a scale gradu-
1. The pressure roll should be been made, the amount of total ated in 0.5-mm (0.02-in.) increments,
newly ground and the felt should crown correction can be calculated measure the width of the nip as
be no more than ten days old. from the equation: accurately as possible. These mea-
2. The Yankee cylinder should be surements are dependent to some
hot and contain the normal run- C = (N22 – N12) (D1 +D2)/2D1 D2 (3) extent on the person who takes
ning steam pressure. them. They are not completely sub-
3. If a second pressure roll is nor- where jective. It therefore is best for the
mally used, it should be loaded to same person to draw the lines and
its operating pressure. C = Diametric crown deficiency for take the measurements.
the nip in mm (in.)

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KEYWORDS equation using nip widths and roll Thermal crowning due to operat-
Carbon papers, composites, foil diameters, it is recommended that ing conditions can be a problem. It is
papers, laminates, linear load, loads, you seek assistance from your roll true that water cooling minimizes
machine components, nip, plastic films, cover supplier who can suggest a thermal crowning, but it can occur
pressure sensitive papers, roll crown, solution based on the nip impres- even when water cooling is used.
rolls, specialty papers, transfer papers. sion that has been taken. One way to establish that thermal
GENERAL NOTES crowning is taking place is to take a
Most of the above information has nip width impression not only on
stipulated that the nip width impres- newly ground rolls when they are
sion be taken from newly ground reinstalled, but also after the rolls
Nip width impressions can be used roll covers. The nip width impres- have reached running equilibrium
to determine not only the amount of sion also can be used for trouble- temperature. This technique has
crown necessary for a given nip load shooting problems with worn rolls. proven especially valuable in size
but also the shape of the crown. The Many mills routinely take nip impres- presses and calender stacks.
usual crown shape is a 70° cosine sions on rolls every time they have Although both static and
curve but the use of 90° cosine an opportunity. This gives a picture dynamic techniques are used in
curves is not uncommon. Higher of what is going on under actual obtaining nip impressions in super-
degree cosine curves are used to cor- operating conditions and when cou- calenders, it should be noted that
rect problems which appear at the pled without crown measurements fully loading the calender under sta-
quarter points. Size presses can be can often lead to changes in crowns tic conditions may possibly damage
particularly prone to quarter point or other operating conditions. For a filled rolls or composite covers.
problems. Compound crowns are visual display of the nip or crown Dynamic nip impressions for verifi-
used where relief from overloading profile, plot the 21 crown station val- cation of CD nip uniformity are
of the roll ends is required. The cal- ues on a graph. This helps create a made with pressure sensitive type
culations resulting in the accurate more comprehensive way of seeing papers. TJ
determination of these crown what is occurring in the nip. Ongo-
shapes are complex and require the ing records of these plots may iden- Zwinak is director of technical services, Beloit
use of sophisticated computer pro- tify a trend that would not be easily Manhattan, Inc., P.O. Box 155, Clarks Summit,
grams which are regarded as propri- found with just a list of numbers. PA 18411.
etary by machinery builders and roll Keeping accurate and complete
cover suppliers. If problems are not records for troublesome nips can Received for review June 28, 1996.
solved by the simple changing of the assist greatly in determining correc- Accepted Aug. 9, 1996.
amount of crown indicated by the tive actions. Presented at the TAPPI 1996 Engineering
Conference.

124 TAPPI JOURNAL JULY 1997