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PAPER INDUSTRY WEB (PIW) CAMERON GAP TEST
INTRODUCTION
When shipping roll structure quality becomes an issue, the internals
of the shipping roll must be examined to understand what is happening
in the winding process. There are many roll structure problems
that are reasonably simple to diagnosis. Prior experience, basic
knowledge and intuition may lead to an easy solution. There are
other problems however that are not easily cured and must be investigated
more thoroughly to understand the condition and apply a solution.
There are many tools available that can be used to measure roll
condition and roll structure. They range from the simple to complex,
their cost generally proportionate to the complexity. There are
also destructive and non-destructive means to measure roll structure.
The accuracy of testing is dependent on the tools to be used and
the expertise and care used by the person performing the test.
To name the more common measuring devices, The WIT-WOT, Density
Analyzer, Beloit Rhometer, Smith Needle, Schmidt Hammer, Core
Torque Measuring, J-line, Gap and the Back Tender Stick. These
all have their specific uses and when applied properly will give
acceptable results. All but the Back Tender Stick are or can be
configured to be quantitative tools. THE CAMERON STRAIN TEST
The Cameron Strain Test is an interesting and useful tool to measure
roll structure. This test was originated by The Cameron Machine
Company and although Cameron referred to it as the Cameron Strain
Test it is more commonly known as the "gap test."
The gap test in summary:
With the roll resting on the floor,
loose outer wraps are removed to expose a uniform, clean outer
wrap. The outer wrap is slit across the face. The gap that results
is measured and applied to a formula to establish wound in tension
or wound in stress. The accuracy of the test is dependent on the
care taken by the person doing the test, the number of consecutive
wraps tested, the number of tests across the roll face and axially
through the roll. Cameron described the test procedure in the early 60s as follows:
The procedure hasn't changed since the above description appeared in TAPPI Standards. Experience indicates that using multiple layer testing and increased number of incidences of gap measuring across the face of the roll as roll width increases, result in improved statistical results. Many labs and paper mills use the gap test frequently for roll structure analysis. The only tools needed are a tape measure, a magnifying glass with graduations to .0005", a calculator and marker pen.
Step 1. Clean clean roll so all loose paper is removed. Using the tape measure, carefully measure the outside diameter to a close tolerance-preferably down to 1/64". Mark the edge of the roll with increments for testing. This can be a broad range-say ½" increments for a "quickie" or down to thousandths of an inch depending on the accuracy required. On some occasions, a combination of broad testing in general areas and very close stepped testing in known problem areas is a suitable procedure to expedite testing and still get accuracy. Step 2. Cleanly slit across the face of the roll. Be careful only to cut the outer wrap. Using a hand on each side of the roll, pull the loose wrap up towards the slit without inducing undue tension in the wrap. Step 3. Using the magnifying glass or loupe, measure the gap and record it. Again, for a "quickie" you may want to only measure a single wrap at each point-for accuracy, measure two or three successive wraps.
Assuming the chart above results from values taken at a single point at the center of the roll, the structure can be analyzed using the chart above. The gap test is more useful two or more three points across the face are used. Front, center and back is not uncommon on wider rolls, say over 40". The analysis then consider the cross roll profile as well as the radial profile giving a more definitive picture of the shipping roll structure. The number of points radically, across the face and number of subsequent wraps for each reading is a judgment based on the character and seriousness of the defect being studied. GAP TEST LIMITATIONS Most people using the gap test on a regular basis recognize that gap test measurements may not be as reliable near the core as the outer wraps. This is principally due to the weight factor (or nip) at the floor line as the paper weight is removed during testing. Calculating the roll weight of a 40" diameter shipping roll vs. a 15" diameter shipping roll of almost any grade of paper will quickly make an awareness of the variability of roll weight and floor nip as the roll size is reduced. A couple of examples are listed in the table below.
ROLL NIP TO FLOOR
It is recognized that these conditions corrupt the gap testing
results at smaller test roll diameters. The practical question
is, to what extent? Unfortunately there does not appear to be
documentation that answers this important question.
WHAT'S IN THE FUTURE FOR GAP TESTING?
The limitation outlined above is a "red
flag" in gap testing on grades that
have a very low paper to paper coefficient of friction and roll
structuring is a critical issue at the winder. A future paper
will present Gap Test II that extends the practical use
of the gap test. Luigi Bagnato-Paper
Industry Web
CREDITS: Although the information of this article is a culmination of experiences of the writer, in the course of preparing this article, several papers were reviewed in preparation of this article. Pekka Komulainen, Wartsila, Roll Quality Measurement and Control, Tappi Finishing & Converting Proceedings, 1982. David R. Roisum, Winder Consultant, Roll Quality Measurement, Tappi Finishing & Converting Proceedings, 1988 Duane Smith, The Black Clawson Company, The Art of Winding Quality Rolls, Tappi Finishing & Converting Proceedings, 1991 J. David Pfeiffer, McGillUniversity, Internal Pressures in a Wound Roll of Paper M. Hussain & W. R. Farrell, Abitibi Paper Company, Roll Winding-Causes, Effects and Cures of Loose Cores in Newsprint Rolls Alexis Olshansky, Beloit Lenox, Roll Structure Analysis Rudi Deeg, Beloit Lenox, Supplemental information and art work ideas.
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