INTRODUCTION
Winding Technologies, Part 5, titled "The Cameron Gap Test"
describes the background and procedures of the gap test. To review
that article click here.
Gap II is a procedure developed in the field by Rudi Deeg, an
engineer for Beloit Lenox. I had the good fortune to be at the
mill during this period working on the same project as Rudi, participating
as a consultant.. The Gap II story is my observations combined
with valuable technical input from Rudi. Gap II is not a replacement
for the Cameron Gap Test but an extension to enhance the test
when superior roll structure control is required or unique conditions
prevail.
THE CAMERON STRAIN TEST
At the risk of being repetitive, the following summary is for
those that may not have access to Part 5, "The Cameron Gap
Test." If you have reviewed the information in Part 5 you
can skip ahead to the Gap II section of this paper.
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.
The 3 basic steps to perform the gap test are:
Step 1. Clean up the 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.
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.
40" diameter, 40" wide Shipping Roll
| Grade | 40" dia roll wgt
| 15" dia roll wgt |
40" dia floor nip | 15" dia floor nip
|
| 30# Newsprint | 1200 #
| 160 # | 40 PLI
| 4 PLI |
| 30# Roto | 2000 #
| 283 # | 50 PLI
| 7 PLI |
PAPER NIP TO FLOOR
An
additional factor that effects the accuracy of the gap test at
smaller test roll diameters is introduced during the slabbing
of paper from the roll for gap testing. As the paper is slit and
falls away, the roll being tested rests on an increasingly higher
pile of paper. If 12" of paper is slabbed from the radius,
the roll being tested rests on a 12" cushion of paper. This
cushion, lightly nipped, can cause instability and slipping during
the gap test procedure, particularly if the paper has a low coefficient
of friction.
It is recognized that the conditions outlined above corrupt the
gap testing results at smaller test roll diameters. The practical
question is, at what point and to what extent? Unfortunately there
does not appear to be documentation that answers this important
question. It would appear that increased floor nip would reduce
the tendency to corrupt to findings.
To compensate for this change in roll weight and nip force at
the floor level, it is recommended that a rod of as substantial
weight as possible be inserted in the core during testing to prevent
layer to layer slippage during the last few inches of the roll.
GAP II - A MODIFIED GAP TEST
In
the course of my experience in finishing I was fortunate to be
involved in a major duplex winder project and startup by Beloit.
The purpose of this article is not to sell Beloit or their machinery
but to relate the findings concerning the use of the Cameron Gap
Test as it effected this startup. The experience just happened
to occur on a Beloit machine.
During the start-up of the winder, the gap test was extensively
used to tune the winder and eventually confirm anticipated and
desired roll structures. The winder was a duplex winder using
center torque at the core. The system, through which the center
torque was applied had two integrated AC motors, one each end
of each individual rewind station, driving via a right angle gear
drive into the core with torque expanding chucks. Very direct,
no losses. Whatever power was transmitted in the form of torque
had to be in the roll unless there was slippage at the winder
drum which did not occur.
After
the drive was tuned and electrically confirmed that the set points
followed, the roll structure should at least reflect those same
trends. Instead the winder was getting regressive values instead
of progressive values as it neared the core. The curve shown is
typical.
A couple of things subtly happened. The paper being highly coated
and supercalendered had a very low paper to paper coefficient
of friction (slippage occurred earlier than anticipated from testing
other grades), consequently slippage in the roll during the gap
test occurred relatively early, sometimes as early as 22"
to 24" diameter.
This was helped by a softer nip between the cut paper on the floor
and the remaining roll as well as higher tensions run with this
LWC compared with earlier tests with other grades. Another phenomena
was that while cutting and handling the roll during testing, the
roll moved minutely, i,e. rocking it back and forth. This gave
the paper an opportunity to slip through the nip at floor level
long before this would occur had everything remained static.
Once the inefficiencies of the measuring practice was recognized,
corrective measures were taken. to be sure any slippage in the
nip was excluded due to instability of the nip.
For positive control of the roll during gap testing, a small "sawhorse"
was made with a 5" angle bar on top (shown in black), The
sawhorse was 60" in length to accommodate the widest roll
to be tested. A pair of special removable C-clamps with V-blocks
on the clamping plates were also made (shown in red). The clamps
are fitted up on each end of the roll and the down force adjusted
to the roll via a reasonably stiff aluminum tube through the core.
The OD of the aluminum should be consistent with the ID of the
core. All this equipment could be handled easily by one person.
This arrangement insures that the roll being gap tested would
be under a controlled nip with the sawhorse throughout the testing
procedure. Each time paper is slabbed from the roll, the clamp
is adjusted to stabilize the system.
The
modified procedure was to gap the test roll in the usual manner
on the floor down to 30" diameter. At 30" diameter,
the outer wraps of paper were fastened as securely and tight as
possible. The 30" roll was lifted from the floor and it's
"cushion" of paper and placed on the sawhorse. The clamps
were fixed in place and adjusted and testing continued.
From the very first roll after continuing testing, the residual
strain followed the tension curve of the winder to the "T".
Because of the design of the winder, the direct center drive and
100% controllable nip which was constant in the 6 to 8 PLI area
throughout the roll buildup, it could be reasonably assumed that
the programmed operating parameters came to the roll 100% without
any impairing geometry, similar to the two drum winder's increasing
nip with the weight of the roll.
Once the merits of clamping the roll down were recognized, in
retrospect, the entire roll could have been placed on such a stand
with an adequately designed support as long as an available crane
is in the area. This will assure the same stable conditions throughout
the entire testing process.
There are a number of curves in the data base from this project,
which, if looking at them in view of the above, might not reflect,
so much the actual residual strain, as they show slippage during
the gap test.
One never knows, but to make sure that this does not happen, the
above procedure is mandatory.
The Gap II procedure as outlined above, in no way invalidates
or minimizes the importance and use of the Cameron Strain Test
as originally developed by Cameron Machine Company. Gap II is
an important contribution in extending the use of the gap test
into the core area, which, in today's world of jumbo rolls, is
of fundamental importance. Using the described procedure, Gap
Test II provides a tool that can be used with a high level of
confidence on those installations that have unique requirements,
and particularly useful in setting up a new winder installation.
Note: Minor corrections (inconsequential typos) and feedback note
added August 16, 1997.
Luigi Bagnato-Paper
Industry Web
Note: Rudi Deeg is retired from
Beloit Corporation, Lenox Division