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Author Topic: Rubber comparative test.  (Read 8437 times)
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AndresF1G
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« on: January 29, 2012, 08:59:16 PM »

Engine rubber comparative test.
By Andres Pietrocola.

As we know, we have a hard time getting consistent and accurate information on how to handle our balls of rubber, which measure is best, how much torque, and so on.
At present there are conflicting opinions, different ways of thinking about what you believe is best, either alone because he always used the same glue, same method of lubrication, the same lubricant, equally, and there is the first error, we also hear and see it, therefore we are wrong too. Solution? Make your own test, talk to people who really know and always try to reach the sources of real information.
As I said, you hear or read different opinions modelers, qualified or not. In my case, I have resorted to the study of the subject, to speak with people of great experience as Tony Mathews, Roger Morell, Dave Sacks, Remo Bovio, Alejandro Marchese, among others. In all cases, we have reached conclusions very consistent, which rescued all my tests have been fully accurate, and we will address them to learn a little more so when choosing or buying rubber.
To begin with, in all cases using TAN SUPER SPORT rubber provided by FAI Model Supply USA. The measures used were the 1/8 and 1/16, the first of these traditional and more contemporary second. The batch that I received is in production in July 2011. The elements used for the tests were: a dynamometer, a torque wrench, a stopwatch and a model F1G. Note that all my tests are based on the category F1G engines, rubber 10 grams.
I began by preparing five engines 1/8. First we must remove some dust that coats the gum but not washed, just shaking fingers Hank, remember that it is not powder, but a component of the rubber. Short a length of 9.6 grams, join both ends with the classic rubber seal knot. Then I put 0.4 grams of lubricant for rubber USA SIL-SLICK in a plastic bag, where I introduce the skein for lubrication. This method may vary depending on each one. The final weight is around 9.7 to 9.9 grams.
Then I prepared another five engines but 1/16, with the same method as described.
Important: Use a lubricant such as liquid silicone inappropriate untreated, makes our engines lose performance over time of storage. I recommend using specialized lubricant like SIL-SLICK LUBE.

-Test: dynamic force.
Rubber 1/8, 12 strands, seting dynamometer to a maximum of 2 kg to 50 cm., We obtained 1.61 kg and a dynamic torque of 32.2 grams per linear centimeter.
Rubber 1/8, 14 strands, dynamometer seting to a maximum of 2 kg to 50 cm., We obtained 1.71 kg and a dynamic torque of 34.2 grams per linear centimeter.
Rubber 1/16, 24 strands, dynamometer seting to a maximum of 2 kg to 50 cm., We obtained 1.89 kg and a dynamic torque of 37.8 grams per linear centimeter.
Rubber 1/16, 28 strands, dynamometer seting to a maximum of 2 kg to 50 cm., We obtained 1.97 kg and a dynamic torque of 39.4 grams per linear centimeter.

-Test: Maximum usable (in turns) and discharge time in flight.
Rubber 1/8, 12 threads: 417 laps, 41 seconds.
Rubber 1/8, 14 threads: 378 laps and 38 seconds.
Rubber 1/16, 24 threads: 440 laps and 48 seconds.
Rubber 1/16, 28 threads: 402 laps and 43 seconds.

-Conclusion.
As you have seen yourself, rubber 1/16 is far superior to that of 1/8. Besides providing better download times and torque, it is much easier to make our engines as accurate, and that they are all equal in length. Finish the engine is a bit more tedious than gum 1 / 8, but the end results are worth the effort.
There are new engine assembly techniques, as well as a new node to join and seal both ends, but that would be another topic to develop in another report.
Any questions please write to me: [email protected]

I thank the people I named above for helping me and led to do all these tests, and so have learned more about our exciting hobby.

Andres Pietrocola
ARGENTINA


PD: Sorry for my poor english.
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« Reply #1 on: January 30, 2012, 11:20:01 AM »

Andres
Thank you for taking the trouble to send us your findings on Super Sport rubber.  Your English was quite adequate to get your information across but unfortuneately I do not understand your torque figures.  The figures you give such as 32.2 and 37.8, are they the maximum torque when the rubber is fully wound and are the units gram.cm?

John
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AndresF1G
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« Reply #2 on: January 30, 2012, 12:08:20 PM »

John,
the torque meter grams per centimeter, not for the rubber loaded but expressed the dynamic force on the dynamometer. The test is based on taking the engine, grabbed one end of the dynamometer and the other end is stretched 50 cm. With this, I get the static torque and dynamic torque. For full wounded torque, I use only a winder with torquemeter.
Thanks for your comment.

Andrés Pietrocola
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« Reply #3 on: January 30, 2012, 03:49:08 PM »

Andres
Thank you.  The dynamometer test do seem to indicate that the 1/16 rubber is stronger but in the tests with the Coupe propeller the rotational speed is considerably higher with the 1/8 rubber which would mean that the 1/8 rubber is giving more torque.

John
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AndresF1G
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« Reply #4 on: January 30, 2012, 09:35:20 PM »

John,
no, in my test the 1/16 rubber is better in all, more torque and more discharge time.
Remember, it's only my opinion and my test.
Regards,

Andrés
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Olbill
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« Reply #5 on: January 31, 2012, 12:58:03 PM »

I spent some time recently reading about rubber for slingshots. There was a statement made somewhere that the force generated by a given weight of rubber was in some way proportional to the surface area of the rubber. The conclusion was that using more strands was better because the surface area was higher.

I haven't done any testing specifically designed to prove the worth of multi-strand motors for my indoor models but I do use multi-strand motors and I haven't found them to be inferior to normal 2 strand motors. Some are better and some aren't which is much like my test results in general. I have had some very good flights with 3 or 4 strand motors.

For my flying the one unquestionable value of multi-strand motors is that I can frequently find uses for thin remnants of rubber left over after cutting regular motors. They're not making any more 5/99 or 3/02 rubber so being able to put these scraps to use is a big bonus.
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Dave Andreski
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« Reply #6 on: January 31, 2012, 01:05:49 PM »

Is this info helpful?
"There has not been a bad batch of Tan SS rubber since the January “B” batch of 2009. Just buy it!
The Tan SS available today is overall the best rubber that has EVER BEEN AVAILABLE. It is much more consistent from batch to batch than Tan II, is far more durable, and has none of the mysteries of Tan II (such as heat sensitivity).
-Don D.
Owner of about 50 lbs of 2009-2011 rubber. All my 2003-2008 is now test rubber!
FFML, 2/7/11 message# 26163
Rick you are chasing rabbit trail. There has not been any bad rubber since the January B batch of 2009. This is when the new additive was included that has made Super Sport very consistent and DURABLE. Much more durable than any batch of Tan II ever was."
Don DeLoach
Editor, NFFS Free Flight Digest
FFML, 11/22/11 message # 33196

Dave Andreski
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AndresF1G
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« Reply #7 on: January 31, 2012, 01:11:14 PM »

Dave,
very good and helpful info, thanks for shared.

Andrés
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« Reply #8 on: January 31, 2012, 01:27:05 PM »

I guess that means that Don will sell me his 5/99 now!
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« Reply #9 on: January 31, 2012, 03:18:06 PM »

Dave!
I made lots of rubber testing in the last 2 weeks as a preperation for lost hills next month
what you say about 2009 -2011 rubber is generaly correct, but for the first time i encountered some
low energy stuff in the bottom of my july 2010 box. it is about 5-7% less energy than average
and is useless for F1B competition.
as for 1\8 or 1\16 rubber i  can't understand  why 1/16 gives more energy
i'v seen Andriukov climbing higer than anyone using 1/8...
Omri
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« Reply #10 on: February 09, 2012, 05:33:38 PM »

In the 60's/70's at least there was a strong thought in the indoor community that the higher the surface area the greater the energy output. So for example, some people tried to strip rubber to be as close to square in cross section as possible rather than rectangular in cross section (and then adjust their props to suit). This seems to have come from observations that a couple of the top fliers used narrow rubber strip and made better times than other people in the same conditions.

I recall that someone knowledgeable (can't remember who, the article was in INAV sometime in the 70's) did very careful specific energy tests of the same batch of rubber stripped to various widths. I believe they were wind/unwind tests IIRC.

The net result was that all samples recorded the same energy within the tolerances of the test method and normal rubber variance. Certainly nothing like the variances recorded above by Andre.

If you do an energy (stretch) test on 1/16 and 1/8 rubber from adjacent samples of rubber from the same box you will of course get the same energy (yes you have to strip the rubber to get the 1/16 samples). This is the only way to make sure that the samples are nearly the same. Of course in the real world we don't extract rubber energy by stretching alone but also by winding and this introduces a whole other set of complications. One of those is that the corners of the rubber strip probably contribute more to the formation of nicks and therefore broken strands. A 1/16 based motor has twice as many corners, can you wind it as reliably without broken strands?

I don't doubt Andres' results. However were the samples taken from the same box? This would be an important part of the test in my opinion. If was to do this test I would take a 1/8 motor, then take the next sample from the same box and strip it into two 1/16 pieces (yes there will be two knots), then the next 1/8 sample etc. If the samples came from different boxes the energy could vary significantly.

Also, the samples need to be made to the same lengths for any results in torque or unwinding time to have any relevance. The important thing here is rubber cross-section and you can only guarantee the same cross-section if the weight and length are identical.

In the indoor scenario it turned out that a couple of top fliers were just better at trimming than everyone else!

Perhaps the same could be said of Alex Andriukov (if he is indeed still using 1/8 rubber).
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« Reply #11 on: February 29, 2012, 12:02:02 PM »

I haven't had time to reply to this topic until now but Richard has said most of what I would have said.
I've never seen any accurate tests that show any significant energy storage difference between rubber of the same cross sectional area arranged into different strip widths.
As Richard commented, to do this sort of test properly you would be best to start with some 1/8" rubber (or wider) of a known batch, and perform tests to determine the specific energy available. Preferably by winding.
Then take the same rubber (from the adjacent strip in the same splice area) and strip it into narrower pieces, make it up to the equivalent cross sectional area as the first piece and perform the same test and compare. I'd wager that you would find no significant difference (within measurement error) if the rubber cross sectional area is maintained (the easiest method might be to maintain the same mass?).

My guess is that Andres has the same batch (July 2011) but his 1/16" and 1/8" strips are from different parts of the "factory" box (12 lb boxes divided into smaller lots for sale). Within every factory box of rubber are several (6 to 10 and it varies) splices where different lengths of rubber sheet were joined. The specific energy has been found to vary between splices often more than the average energy varies between different batches of Super Sport! You will find a variation in thickness too from splice to splice.

So comparing rubber from the same batch is difficult, unless you know where in the factory box the pieces were from (ie: which splice are they from).

For F1B we first divide our boxes of rubber (10 or 12 lbs at a time) into individual pieces (29.3 grams for example) and note the sequence that they were removed and which splice they were from. We test the energy at the beginning and end of each splice to be able to determine which areas of the rubber batch have the most energy.

Tony
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Tapio Linkosalo
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« Reply #12 on: February 29, 2012, 02:44:40 PM »

I beg to disagree here. Of the about 440 F1B-motors that I have tested in the past 9 years (all the contest motors that I have used), typically the standard deviation of motors from one bax has varied from like 0.5 to 0.7%, which means that 95 motors out of 100 fall within +-1.5% in energy return. The consistency even remains from one test to another. Differences between batches are clear, however, so the reason is not that my test would give the same reading to all motors :-) There is one exception, I bought a batch of SuperSport secondhand, should have been from the same box but was split into 3 bags, and this is the sole one that gives different results for different sub-patches... The most obvious explanation would be that it actually was of different batches, but cannot tell for sure.

So I'm convinced that testing individual motors from one box would be useless in terms of energy return. However, I want to continue with this, as it gives me information of the maximum stretch of the motor, which I use as proxy for selecting motors for maximum turns. Also I like to collect data of the motors (differentiate between batches, find data about energy return vs. temperature, etc.) And, testing a motor takes quite a little time compared to breaking them in, and as I use the same rig also for the break in, testing comes as side-product...
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« Reply #13 on: February 29, 2012, 03:17:29 PM »

I'm sure you all know that I disagree with rubber being consistent within  a batch or a box or a splice or between two 1/16" strips cut from one 1/8" strip. But that's just me. Maybe F1B motors have so many strands that the differences are averaging out.
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« Reply #14 on: February 29, 2012, 03:30:31 PM »

I guess that means that Don will sell me his 5/99 now!

Hey Billy...I was wondering if anyone has tested their 5/99 recently. We all seem to compare current SS rubber energy with the 5/99 standard, but the energy numbers that we use for 5/99 were from tests done 10-12 years ago. My bet is that 5/99 has degraded enough to make the current SS very competitive.
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« Reply #15 on: February 29, 2012, 04:32:54 PM »

Tapio, I am familair with your feelings on this matter and have mentioned before that my own tests do not agree with yours. Also, I have talked about this with other rubber testing enthusiasts (Andriukov, Fred Pearce, Bob Piserchio etc.) and they have not found the consistency across the box that you have described. Either we are all wrong, or your method is somehow flawed. We can't both be right!

Tony

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« Reply #16 on: February 29, 2012, 04:48:09 PM »

I guess that means that Don will sell me his 5/99 now!

Hey Billy...I was wondering if anyone has tested their 5/99 recently. We all seem to compare current SS rubber energy with the 5/99 standard, but the energy numbers that we use for 5/99 were from tests done 10-12 years ago. My bet is that 5/99 has degraded enough to make the current SS very competitive.

I don't test for specific energy but my 5/99 is still awfully good.
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« Reply #17 on: March 01, 2012, 12:38:30 AM »

I have not broken in any 99/5 motors on the past 2 years or so, but until then my tests have showed a consistent energy return. And I have only stored my motors in cool, dark place, not in fridge or freezer. So I'd suspect the energy return of the batch should be still OK.

There are, however, reports that old Tan II is getting brittle. I had one 2002 motor explode last year in Norway world cup while addind another hand turn, it was not even at high torque then. So it looks like I better put the remaining Tan II that I have away for indoor use only (where no more turns are applied to the motor when it is loaded to the plane) and concentrate on SS for F1B use. It is a bad thing for your fly-off (flight time limited to 10 mins) if your motor breaks in the middle...

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« Reply #18 on: March 01, 2012, 12:57:57 AM »

Tapio, I am familair with your feelings on this matter and have mentioned before that my own tests do not agree with yours. Also, I have talked about this with other rubber testing enthusiasts (Andriukov, Fred Pearce, Bob Piserchio etc.) and they have not found the consistency across the box that you have described. Either we are all wrong, or your method is somehow flawed. We can't both be right!

I have given this a thought, many times. As I have written before, my results indicate a clear and obvious variation between motors from different batches, minimal variation between motors from the same batch. And this pattern persists between different measuring sessions (i.e. when I measure the same batch, say, 1 month apart, I get the same values for the same batch). Also, I routinely test different batches of rubber on the same session (for the obvious reason to test my testing rig), and I get the same result: different batches, different readings but within the same batch the same reading. So honestly, I have not figured any reasonable explanation where does the rig get the information that this motor is from this spesific batch, and should therefore get this spesific reading for the energy return.

As Sherlock Holmes has pointed out: when all the other possible explanations have been ruled out, the one, still remaining, must be the right one. As arrogant as this sounds, I have not figured out a flaw in my method, therefore the only conlusion is indeed that you all are wrong...

One point: is your method analog or digital? I started this stretch method by the tables I got from Bror Eimar: stretch the motor to tabulated values of stretch and force, then release and write down the force every 10cm (4 in). I soon realized that this is not accurate enough, finding the right combination of stretch and force takes time, and inaccuracies there give a big variability in the load you give to the rubber. Any indoor flier can tell you that the maximum load affects the whole unloading torque curve, so variations here repeat over the whole of the measurement. With my current method I have a computer software that measures the force and stretch, and calculates my goal of 90% breaking load automatically, so the software indicates me exactly how much to stretch, and when to start the unloading. Also the software takes force readings for every centimeter uf unloading (over 300 points), so I have pretty accurate data also of the force disctribution while unloading. Compared to the 20 to 30 datapoints recorded by the manual Eimar/Pearce method. 

Incidentally, I started testing some more motors for the ice-flying season this week, and as mr. Clapp had asked about the details of the test rig, took some pictures. Unfortunately taking good tecnical photos is pretty hard, I hope you can figure out something. The rig uses a computer mouse to digitize the measurements, there is a big spring inside whose stretch is converted to mevement with a pulley and a belt, and the other optofork of the mouse is re-built around the shaft of the boat winch used to stretch the motor.
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« Reply #19 on: March 01, 2012, 01:27:08 AM »

Tapio, that testing rig and you're testing technique could be an interesting article for the Symposium. Hint, hint, hint :-)
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« Reply #20 on: March 04, 2012, 09:52:13 AM »

Tapio,
I agree with Tony and others that have seen significant differences among motors from the same batch and even from the same segment between splices.
Last year I automated the test that Alex, Bob Piserchio, Tony and others are using to test rubber samples. My device uses a load cell and amplifier for force measurement, a ball screw and stepper motor for distance measurement, and a microcomputer for motion control and data acquisition. The accuracy of the system is at least an order of magnitude better than any human doing the test manually.
Samples are uniformly stressed to about 18,000 psi and I collect data at 100 points during relaxation. The output of the test is a number which represents the height to which the energy in the sample would lift the sample at 100% efficiency (area under the force/distance curve divided by the sample weight)
So far I've tested about 250 samples. My best results are around 1800 meters, the worst around 1600 meters, about a 12% variation. I find this range between batches but also within the same box. July 10 is a good example of this: top of box is great, bottom two segments are junk. The variation within a segment is much less but still 3 to 5%. These results are consistent those from both Alex and Bob P as I worked with both of them during the development of my machine.
I think you need to take a closer look at your test system.
For sure, if you made the assumption about uniformity within a box and flew a motor from the bottom of a box of July 10 in a contest, you wouldn't like the results.
Tom
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« Reply #21 on: March 04, 2012, 02:07:15 PM »

Tom,

have you tested the measuring noise in your system? One simple method is to repeatedly test the same sample, and compare the results. They should be the same - or the amount of variation between the repeats in measuring the same sample is the amount of noise produced by the measuring equipment.

Meanwhile, talking about my rig, I have given the system and the data a closer look over and over again, and have failed to find the flaw. Maybe you could help me out here, give some pointers where to look?

The curious thing is that I give different readings for samples from different batches, and the same readings for samples from the same batch. If there is a flaw in my system, it means that the rig can somehow identify the batch where the rubber comes from, and adjust the outcome accordingly. I just cannot figure out, where the system could gather such information from...
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« Reply #22 on: March 04, 2012, 06:12:39 PM »

First, let me say that I am an indoor flier and have never made a motor with strands larger than 1/8" and have made many with strands well below 0.050".  I found Tomv's testing numbers interesting compared to the energy values publically given for various batches of Tan II and the new Tan SS and to my own tests.

I suspect that the 18000 psi loading is incorrect or that I do not understand something.  After reading this I quickly made a 3.0" motor, one single strand loop of 1/8" of 0.043" thickness rubber, from the January, 2010, batch.  I stretched the motor to breaking with a pull of 9.9 pounds, at a stretch of 30.5" .  This works out to 920 psi for the breaking stress (9.9lb / (2 x 0.125 x 0.043).  This breaking stress compares favorably to Tmat's 80lb breakin pull for a motor made from 26 strands of 1/8" rubber (570 psi).  I had never done a breaking stretch test before and only tested for breaking torque.

Tomv's specific energies work out to the range of 5250 ft-lb/lb to 5900 ft-lb/lb.  Indoor fliers usually quote a maximum specific energy of just under 5000 ft-lb/lb for the very best batches of Tan II rubber (5/99 and 3/02 for example) and the indoor fliers' measurements for tne new tan SS have yet to consistently exceed 4500 ft-lb/lb.  However, indoor fliers usually test using by winding so the hysteresis effects are certainly different between stretching and winding tests.

There are two factors that need to be considered when testing rubber samples.  Not only may the rubber differ within a batch and between batches, but the thickness of the rubber varies, too.  The new Tan SS seems more consistent than Tan II in this regard but my short 3" 1/10 test motor varied in thickness from 0.0415" to 0.430", or over 3%.  This variation, if not taken into account, can affect the energy measurements as the breaking load will vary along the strand length (because of the different cross sections).  The second factor is that one needs to get right up against the braking load before relaxation to get a measure of the maximum energy storage potential.  The total energy is sensitive to the last little bit of stretch (and maximum load).  Getting to the point of maximum load (or torque for winding tests), one fraction of inch (one or fraction of a wind) before breaking is not easy.  Since the breaking torque (for the same weight and length motors or same cross section) is different for various rubber batches, I suspect the breaking stress is different rubber batches, too.  I would expect there is a need to determine the breaking stress (rather than using 18000 or whatever psi for all batches) for each batch (just as we indoor fliers determine a breaking torque for each batch) before one can reliably stretch a sample to the very maximum load before breaking.

Just my two cents,

Leo, Bloomington IN
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« Reply #23 on: March 04, 2012, 07:28:28 PM »

Leo,
Your stress calculation didn't consider the reduced cross section of the sample when its stretched.
Since rubber isn't compressible if you know the weight and specific gravity about (.96) of the sample you can calculate the volume (L x W xH) of the sample. The length to which you pull the sample is divided into the volume which yields cross section. The measured force divided by the cross section at a lengths yields PSI loading on the sample.
Alex's spreadsheet makes the first part off this easy. A 250 mm sample is weighed. The length and weight is entered into his spreadsheet and an initial target tension for the sample is calculated. Typical tension for the first pull is around 9 Kg. My tester pulls to the target tension and then adjust 10 times as the sample cools. Immediately after the final adjustment is made the sample is relaxed and distance / force measurements are taken. Then the sample is cut at the jaws of the vise holding it and weighed and measured. The final calculations are made based on these parameters.
Tom
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« Reply #24 on: March 04, 2012, 09:08:22 PM »

Leo,
As Tom mentions, the maximum breaking pull force is much greater than the 9.9 lbs that you have quoted. Andriukov's method usually specifies a max pull (which is just under the maximum breaking stress) of 9 to 10 kg (19.8 to 22 lbs!). Or more than double the force you tried. To do this you have to eliminate the knot. So we grip the rubber in a special vice (with the jaws lined with rubber) that clamps the rubber very tightly. And as Tom mentioned, the cross sectional area at full stretch has changed dramatically.

Tony
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