Powerband
Static Testing for Spearfishing Enthusiasts
PKeep
in mind in our testing we are using an uncalibrated scale and statistically
limited samples of rubber material. Many of the tests are performed on
a single band of rubber. The results that we get are unofficial information,
casually obtained by enthusiasts, not engineers. The goal of this website
is to educate other spearfishing enthusiasts in methodology whereby they
can make some decisions about their own equipment. The goal of this website
is not to endorse any particular manufacturers product but to educate the
individual interested in building his own equipment. John
Warren, 3/19/2001
In
order to adequately power your speargun, or figure out why your existing
equipment seems to be somewhat lacking you can do some very simple STATIC
TESTING of your speargun bands. With these test results
it is easy to choose between available rubber material types on the market.
With this data you can also determine HOW LONG (?)
you should cut the bands for your speargun. Please
be aware that by increasing the strength or number of your speargun bands,
you may be subjecting the sear mechanism, which holds the spearshaft,or
gunbody to forces which exceed it's designed strength. By increasing the
strength or number of bands you may cause the mechanism or gunbody to FAIL
CATASTROPHICALLY discharging the spearshaft
unexpectedly. A situation where this occurs can be lethal to anyone around.
Please be aware that there is no intention expressed or implied for the
reader of this website to endanger himself or anyone else by overloading
his speargun with rubber. Never point a loaded
gun at anybody under any circumstances on land, boat or underwater.
The
equipment you will need consists of a TEST BENCH, PULLING DEVICE and SCALE.
Of course you need a sample of rubber to test. It is also important to
systematically write down your recordings and keep track of the data over
time. Please notice the static testing setup below. The test bench is a
stout 2 X 10 board. The pulling device is a ratchet "Come-along" puller.
The spring scale has capacity to 220 lb. on one side and 100 kg. on the
other.
Notice
that the green band is being tested. This band is cut to a reasonable length
for a blue water tuna gun. By doing some initial testing on this band we
were able to determine the actual length needed for the modified Jack Prodanovich
blue water speargun. DON WERT, master spearfisherman
and gun builder is holding the testing board.
On
the right frame notice how the board is layed out. The unstretched band
is at the zero elongation mark. It's tension is practically zero. When
this band is doubled in length, it is said to be at 100% elongation. This
means that it's static length has been increased 100% by stretching. Since
the (double) length of the band is a measured dimension, we have placed
data points on the board at 25% increasing intervals. The scale of percentage
elongation goes from 0% (unstretched) to 300%. Where the "knuckles" of
the band cross the front of the tape mark is considered to be the point
where the tension on the scale is measured. At each data point a reading
on the scale is taken and written down for later charting.
Here
you see the very first stretch of this rubber band. This band was actually
stretched to 300% elongation intitially. For later testing, we decided
that we were elongating this band excessively. In real use, this band would
never be stretched to this elongation, although some other material could
be effectively used at 300% elongation. It was with great caution we tested
this band to this much tension. Please be
cautious if you test your bands. If a band breaks the scale or puller could
go flying and hurt you or a bystander. Don't stand directly in front or
back of the testing bench while the band is under tension.
The
goal of static testing rubber is to learn something about it's properties
prior to cutting your bands and putting them on a speargun. There are several
important types of data that can be gained by testing, initial tension
and tension over a time period.
The
initial tension is usually the rated power of a band at a certain percentage
elongation. This is just the beginning. What we do is a "SOAK DOWN" test.
In this test, the band is taken up to a certain percentage elongation,
in this initial case, 300%, and then readings are taken on the scale at
time intervals. In some of our testing we took a reading every 5 minutes
for up to 45 minutes.
Please
see the next image. It is the FIRST PULL of
this RUBBER BAND. It is very significant to
notice that the tension on the scale reads 190 lb. I was unable to capture
the highest reading of 215 lb.
After
a period of time this tension continued to drop. In the second frame you
can see that the tension has dropped significantly.
In
the third frame you can see that the tension is continuing to drop. A band
which started at 215 lb. tension is now starting to approach 160 lb.
The
next frame is a picture of this same band immediately upon removing the
load placed upon it by the ratchet pulling device. You can see that the
tension on the scale is almost zero. Please
note that the length of this band has changed. What
started out as a band which had a total rubber length of 32" has become
somewhere around 35" or more in length. What happened? It is pretty obvious
that some of the crosslinking in the material has been broken. Now the
band has a non-recoverable increased length of almost 10%.
This
is not the same band as we started with. It has changed. After the first
initial pull to 300% this band never contracted to it's 32" starting length.
Please keep in mind that these are static tests and do not necessarily
predict how "snappy" this rubber actually is.
Charted
Tension Data
The
following chart represents the plotted tension in units of 25% elongation.
The range is between 0% to 300%. There are three series of data plotted.
The black line represents the very first test where the band was taken
up to 300%. The terminal datapoint was 215 lb. (but it rapidly soaked down
in tension). The second data series is represented by the red line. Notice
carefully that this line is well below the first (black line). Our methodology
was a little flawed in that we used the same band and did not adjust the
percentage elongation scale to accomodate the new longer length. With the
third data set, the yellow line represents a tension test where the percentage
elongation scale was re-calibrated to accomodate the new length of the
rubber.
The
yellow line more closely resembles the black (first) test. Also note that
we did not stretch the band all the way to 300%. We stopped at 250%, which
is a more reasonable elongation.
Soak
Down data
In
actual diving, most of the time the diver spends in the water swimming
about looking for fish. In blue water hunting, the diver may swim for several
hours without pulling the trigger of his speargun. It is important for
the spearfishing enthusiast to know what the performance of his band material
is after a period of time. This data is plotted for a fixed percentage
of elongation in 5 minute time intervals.
Note
that there are 4 tests in this chart. The first three correspond to the
first three data series tests in the previous tension tests. At the end
of the tension test, the band is held at it's maximum elongation and data
readings are taken from the spring scale. A steep curve means that the
rubber is rapidly losing tension. The third series of data points represented
by the yellow line is the most accurate representation of the rubber in
actual spearfishing conditions. The band is only taken to 250% elongation.
Note that the starting tension is 167 lb. and after 45 minutes this band
has a tension of 142.5 lb.
Conclusions
Our
intitial two tests of this rubber material was confusing. We had not realized
that the percentage elongation scale needed to be readjusted for the new
band length. After adjustment of the scale, we feel that we got repeatable
results.
In
advising my personal spearfishing associates, who intend on using this
tested material, I would advise them to PRE-STRETCH
the complete length of rubber material they intend to use for an hour at
300%. After the tension of the band is taken off, the band can be reliably
cut to desired percentage elongation. If you are after a band that has
less initial tension make it a little longer. In my opinion 200% elongation
is just over a 100 lb. band. At 250% you will get much higher tension.
Remember,
EVERY BATCH of every BRAND and DIAMETER of RUBBER has different tension
and soak down properties.
The
rubber material that was used to illustrate this methodology of testing
was MASAHIRO MORI'S MEAN GREEN SPEARGUN RUBBER. I am not commercially affiliated
with Masahiro Mori, but I feel that this is a very fine quality speargun
band material. JOHN WARREN, 3/19/2001
Contact
information for Masahiro Mori
Write
to Mori
Special Mori Wishbones page
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