Mechanics of a lockout spring machine + Testing&Calibrat

Discussion in 'Stringing Techniques / Stringing Machines' started by Adas, Jul 11, 2004.

  1. Adas

    Adas New User

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    I understand that they lock out when the desired tension is reached.
    But does this mean they should only lock out at this tension? As we turn the crank and it moves further from the racquet there is a higher tension. My question is when I was testing the accuracy of the tensioner, I had 58lbs of weight strung through the racquet, with the weights resting on the ground on the opposite side of the tensioner. When I set the tension higher to say 60lbs should the lock ever come out? I would think not. But as we keep turning the crank are we not applying more tension?

    How hard/easy is it then to calibrate, is it as simple as turning a screw on the spring?

    Im getting closer to stringing my first racquet on this machine, but I am being rather pedantic about it, as I want to make sure everything is just right before I have a go.

    thank you,
    Adam
     
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  2. Gaines Hillix

    Gaines Hillix Hall of Fame

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    adas, it sounds like you are saying that you ran some string through a frame mounted in the machine with one end of the strings in the string gripper on the tension head and the other through some weights on the ground? If so, that is not going to provide a true reading of the tension being pulled. First, the string is much longer than normal and will stretch resulting in tension loss. Second, there will be friction between the string and the grommets and this will cause additional tension loss. You need a tension calibrator tool to do this properly. If there was some way to suspend the weight directly from the string gripper your method might work and your permise that the tension head would not lock out if set to 60 lbs. with 58 lbs of weight were hanging from it might work. If you were cranking very slowly the weights would be lifted off the floor before the head locked out. This is a very imprecise measurement though and very prone to errors in operator technique, IMO. The method used to adjust the tension usually involves turning a set screw on the tension head. It varies based on the type of machine.
     
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  3. Audiodude

    Audiodude Rookie

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    Just go ahead and string a racquet. You're making it sound as if you're preparing to land the space shuttle. On a difficulty scale of 1 to 10, stringing a racquet barely rates a 3. You're suffering from a major case of paralysis by analysis.
     
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  4. Adas

    Adas New User

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    yes I am analyzing to the extent of paralyzing. But I will eventually be putting in there some expensive racquets which I am emotionally attached to. haha :lol: . I found some information on adjusting the tension. I know that you can measure tension through calibrators, but wanted to do some testing in the interim.

    thanks
     
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  5. Audiodude

    Audiodude Rookie

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    You must be a pilot, an engineer, or a dentist. Anyone else would've strung a half dozen racquets by now.

    Mike
     
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  6. Adas

    Adas New User

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    No just a poor university student, who his racquets are one of his biggest investments.
     
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  7. Gaines Hillix

    Gaines Hillix Hall of Fame

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    Put it this way, I wouldn't want my racquets strung on a machine whose tension was calibrated using your method.
     
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  8. Power Game

    Power Game Professional

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    Hey same here, until I got my stringer my racquets were the 2nd biggest investment for me
     
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  9. Adas

    Adas New User

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    Well that is why I was being so pedantic about calibrating before stringing a racquet - still haven't. It was a second hand machine, and I want to know how accurate it was. Looks like I will have to wait even longer and have to buy a tension calibrator. But surely there is another way to measure the tension being pulled without the calibrator? - it all comes down to physics again - as nearly everything does! haha. The calibrator is just another spring afterall.

    thanks again guys
    Adam
     
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  10. Gaines Hillix

    Gaines Hillix Hall of Fame

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    adas, put it this way, I've been on this board for several years and have been a USRSA member for several years and I've never heard of a manual way to accurately measure the tension on a stringing machine. You can ck a spring calibrator with verified weights by hanging the weights directly from the calibrator and reading the weight on the scale on the tool. But, trying to imply the calibration is correct by pulling on the weights with the machine's tension head is not the same thing especially since the strings are going through the grommets in the racquet and there's a 90o angle between the tension head and the weights.
     
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  11. SW Stringer

    SW Stringer Semi-Pro

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    Adas, Yes there is a way to accurately check the tension of your machine without buying a calibrator - which you accurately state is just another spring. The Handbook of Chemistry and Physics lists a formula for Frequency of vibrating strings in the Definitions and Formulae section. Since you obviously have the intellectual curiosity to pursue this course I'll leave it as an excercise for the interested student to figure out the actual steps. If you have any questions don't hesitate to ask. Hint: You could do a search on this message board where I did actually list all the steps - but then again, when one starts to get technical - most people stop reading. That's probably why nobody ever heard of a way to do this. Good luck!!
     
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  12. Gaines Hillix

    Gaines Hillix Hall of Fame

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    A search doesn't show any earlier posts on this. However, I believe that the frequency of vibration of the string and the weight of the length of string being used have to be accurately measured to use the formula you are referring to?
     
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  13. mr. stevo

    mr. stevo Rookie

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  14. Gaines Hillix

    Gaines Hillix Hall of Fame

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    stevo, double ck that; I don't think that was the topic he was referring to. I see no formula for calculating tension based on string vibration in that post.
     
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  15. Adas

    Adas New User

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    gaines - this is the link SW was referring to.
    http://tt.tennis-warehouse.com/viewtopic.php?t=10507&highlight=

    last point made by coach raises some questions. But then again there will be another formula that should be able to take care of that as well.

    thanks, Again
    I might compare it against a calibrator and see how accurate my readings are. I guess some of you guys could test it out as well, I would be interested in its accuracy.

    Adam
     
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  16. mr. stevo

    mr. stevo Rookie

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    oh, oops my bad.
     
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  17. Gaines Hillix

    Gaines Hillix Hall of Fame

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    Adas, I may be dense, but I don't see how that helps with calibration of a stringing machine, since it assumes you know what the reference tension is to begin with.

    "So for example if the racket measured C (above middle C) or 526 hertz originally at a 60 lb reference tension, and it sat overnight and you measured it at 507 hertz then the new tension is 60 times 507 times 507 divided by 526 divided by 526. (60*(507/526)^2) The new tension is then 60 times .929 or 55.7."


    What you are asking for is to use this method to calibrate the machine and in that case you are solving for T(tension). It has to be treated as an unknown in this case, right? Also, there are other variables that effect frequency of vibration of a string, such as the length and gauge. These string frequency measuring techiques can be used to measure stringbed stiffness in a strung racquet(such as the ERT700 device), but that is still not directly useful in calibrating the machine's tension, AFAIK.
     
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  18. Adas

    Adas New User

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    SW - which formula did you use to calculate frequency?
    was it F=1/2*L*(T/M)^0.5

    where F= fundamental frequency
    L = length of string
    M = mass per unit of length g/m or lb/inch - god the imperial system is so ugly, when will you get rid of it? :p
    T = tension in newtons =1600N=360lbs - easy to convert to pounds

    To measure harmonic frequency with a tuner must be different.
    Beacuse 60lbs and 60cm of string is giving me a freq of around 10hz. I have no idea how you were getting a reading of few hundred hz (and much more realistic) in your calculations.

    I feel in theory this could work quite well. We wont have to work with any angles and friction as in my first attempt with weights. Afterall if we implement it correctly and accurately - physics is the basis of everything!

    BTW calibrator is being ordered in @ 50 AUD a pop, quite steep!- which will in the end only be a few more bucks than had I imported them off the net, with shipping included. Only pays to buy expensive items on the net.

    Adam
     
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  19. Adas

    Adas New User

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    Gaines - we just took into acount those factors u mentioned - see the equation, by rearranging we can solve the tension?
    agreed?
     
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  20. Gaines Hillix

    Gaines Hillix Hall of Fame

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    adas, assuming that is the correct formula, I agree you should be able to rearrange it to solve for T, but seems like it is going to be more expensive to purchase the equipment to accurately measure the frequency, length and mass/weight of the string than it is to buy a tension gauge(unless you already have access to it)? And certainly harder to do. There is the practical side to consider also. :)
     
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  21. Adas

    Adas New User

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    yes, i was just being curious - I wanted to play around before my calibrator comes. Most of the equipment I have already, so I was going to see how the physics worked and see what kind of measurements I could get, compared to the calibrator. But I dont think my tuner has enough of a range to test at this point in time.

    Thanks for debating with me, I have learnt a lot already, from some of your other posts in other topics as well.

    Adam
     
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  22. Gaines Hillix

    Gaines Hillix Hall of Fame

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    Adam, I applaud your curiosity and willingness to try different techniques. We all learn from experimenting. Thanks for starting the discussion!
     
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  23. SW Stringer

    SW Stringer Semi-Pro

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    Adas, You asked - "SW - which formula did you use to calculate frequency?
    was it F=1/2*L*(T/M)^0.5

    where F= fundamental frequency
    L = length of string
    M = mass per unit of length g/m or lb/inch - god the imperial system is so ugly, when will you get rid of it?
    T = tension in newtons =1600N=360lbs - easy to convert to pounds .

    Answer: I used both equations to verify my methods. The above formula only requires that you accurately measure length and weight. The other formula:

    F = 1/(2*r*l)*(T/pi*d)^0.5 also requires accurately measuring the diameter of the string to get the density and radius.

    Since I already had a scale (~$10 USD) and calibration weights that could measure the weight of a piece of string, and a micrometer that reads in ten thousandths of an inch doing both methods was simple.

    I locked the turntable down, used a flying clamp to attach the test string to the far mounting post and put the other end in the tensioner. The free length of vibrating string was about 16 inches.

    To convert to mass don't forget the gravitational constant's units are ft/sec/sec - so the lengths should be in feet (or meters if using MKS). After I had figured out where all my calculation mistakes were both methods agreed within a percent or so of each other and the predicted frequency. The accuracy you can achieve with this method is only dependent on how accurately you can measure the different variables. I was able to attain about 1% accuracy with equipment I already had. So, for example, with the drop weight arm level, and the reference weight set at 50 pounds - the measured frequency aggreed with the predicted frequency within 2 hertz, so the actual applied tension was between 49.5 and 50.5 pounds. A calibrator may be able to give you a repeatable reference - but the accuracy is surely suspect - if the manufacterer even lists that in his spec sheet. I remember reading a thread last year by a guy who bought three calibrators and the readings varied by as much as 5 (maybe more?) pounds.

    My suggestion is to use a length of string short enough to get a usable frequency range (the two octaves above middle C works well) but long enough to allow an accurate length measurement with the tools you have. An error of one sixteenth of an inch in 16 inches is .39 percent accuracy.

    For frequency measurement there are many option: tuning fork, audio oscillator, DVM with freq measuring option, Korg chromatic tuner, piano, etc, etc. Your imagination is the limit. G'Day mate.
     
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  24. Gaines Hillix

    Gaines Hillix Hall of Fame

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    A spring based tension calibrator can be cked using known weights. I've found Olympic weights to be pretty reliable. My Wise tension head recalibrates itself when it's powered on. I hung 50 lbs of Olympic weights from my Gamma tension calibrator and it was off by several pounds. I adjusted it accordingly and then cked it on the Wise head and it was spot on.
     
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  25. Adas

    Adas New User

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    SW could you give me one worked example please, I feel I am miscalculating something. I need a frequency range of between 430 and 450 hz ideally.
    regards, Adam

    ps I have a Korg GA-10 guitar tuner.
     
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  26. SW Stringer

    SW Stringer Semi-Pro

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    Adas,
    I'll go dig up my old calculations and post them in the next day or so. In the meantime you need to work out a frequency chart for your Korg (I think anyway), my Korg just gives the note plus shows an indicator (like an analog meter) that points to center (0% off) or left or right of center up to 50% off - in other words, just like a VU meter - marked in percent. You pluck the string and the instrument locks on to the frequency and shows the "virtual" needle on the LCD display. From that you convert to actual frequency in Hz. I use the Korg with a reference of A=440Hz.
     
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  27. SW Stringer

    SW Stringer Semi-Pro

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    Adas, here it goes: using the equation freq = (T/m)^0.5/2*l , where l = length of string under test in feet, m = mass of 1 foot of string under test, T = tension applied to string under test in pounds. Using some PSGDF 18 gauge which weighs 8.2084 E-04 pounds per foot UNDER NO TENSION, you must divide that unit weight by the acceleration of gravity (ft/s^2) at your location (listed in the Handbook of Chem & Physics - I used 32.19) and crank the equation. For a 16.5 inch string length the prediction was 509 Hz and I was measuring 526 Hz. After I realized the string STRETCHES under tension, thus lowering the mass per foot, I measured the stretch @ 50 pounds and recalculated with the lower mass per foot and the new predicted frequency was now 520 Hz. That's it in a nutshell. Being as accurate as your instruments allow will give you the most confidence in your readings, and knowing the tolerance of your measurements allows you to calculate the tolerance of the tension setting that you're calibrating.

    I suggest using a stiff string, such as polyester (dacron) which has very little stretch, but also is more dense (about 15% more than nylon) so the frequencies will be a little lower, and of course you can always vary the length of string to raise or lower the frequency range to match your instrument needs.
     
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  28. fastdunn

    fastdunn Legend

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    The thing is you're not going to formulate "stringing" process
    in a closed form formular. The tension on final string bed is
    all that matter. Therfore, calibrating your reference tension
    has limited importance. Consider your stringing process
    as a black box and change input(reference tension) if the
    output(string bed tension) is not what you want.
    By the way, thank you guys for educating me about calculating
    tensions... :) |)
     
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  29. SW Stringer

    SW Stringer Semi-Pro

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    For me calibrating the reference tension is just a part of routine maintenance - like keeping the clamps, turntable, and gripper clean and well adjusted. On the more complicated machines (lock-out, electronic) the tensioning mechanism is more likely to get out of adjustment than on the simple dropweight devices, and hence needs more frequent checking. But if you assume your machine is functioning properly as it was designed then, yes, the actual reference tension, be it 1% off or 25% off doesn't matter as much as good consistent stringing practices. However, it's still nice to know the absolute accuracy of your pull, and it you track the accuracy, any changes can be an indication of something failing.
     
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  30. Adas

    Adas New User

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    thanks SW I will give it a whirl later this week.

    BTW I got my calibrator today - so I will measure this method against it :)
    My machine was was 4lbs off in the end.
     
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