Oh Macky - you are simply wrong here. I'm really sorry, i don't know how to explain it any better.
The scale will read 50 lbs, but the diagram doesn't show that the feet are also pushing back at 50 lbs. It isn't relevant either so it doesn't need to. The total exertion the man is using is 100 lbs, but the interest here is what is at the handles.
I do have a degree in physics and have worked as an engineer, maybe that makes me another lacky. I'm not paid by Lifeline, i just find this an interesting problem.
Like i said they are NOT measuring tension, they are measuring total force at the handles, which is quite smart because then the ratings do work for any cable situation, once you understand the idea. Its their choice to do it that way, its not wrong, its just the way they choose to do it. However the results will be consistent across the line.
In the case of their picture the force at the handles is ALSO the tension, because they don't count the force at the feet. In a strand pull that's the difference, so it winds up being 2x the tension, but the force at both handles is the same in either case.
I'm not contradicting anything i said before. I was earlier corrected by you on the matter of tension, when i claimed it was twice the pull of the weight. Now you are saying its twice the pull.
If i am holding the band with weight attached, i'm holding that weight. I'm exerting that much force against the weight to hold it. If i take the weight away, and use my free hand it has to push as much as the weight was, to the other hand it is still holding as much as before, so the hands together are now doing twice a much. Its Newton - for every force there is an equal and opposite force.
The only problem is why the degree of stretch is changing, from 2x to 2.5x to 3x.
My good friend, let's go back to the basics of my original assertion and firstly deal with 16" chest expander cables, ok ? This is my prime and only issue, as by now the other issues appear to be getting taken care of, with your able help I might add.
It's important that we have this interesting and friendly debate, and get things clear in your mind because you are dealing with Lifeline more closely now, and if you end up agreeing with their confused and erroneous Chest expander information, you will effectively give them an outside ( customer ) endorsement of their ratings, particularly since you have disclosed that you have a degree in physics, which they will undoubtedly seize on if they are monitoring these discussions on this Forum.
1. Anchor one end of the said 16" cable and using a scale, stretch out to twice its resting length. How that stretch is applied, or whether the other end is also being pulled instead of anchored has no relevance to the outcome. The instant the cable reaches 2X its resting length, the poundage registering on the scale is the rated pullweight, and that figure should be reflected on the Chest expander page as a bona fide, no BS, rating for that particular coloured cable. Are we agreed on this ? I have proven that even at full arm stretch, Lifeline's advertised pullweights are grossly exaggerated. That should have been the end of the matter, make some new measurements and post them up on the chest expander page.
2. However, since you and Lifeline both insist on making references to the methods and poundages contained in the FAQ page for their Long cables and relating them back to the Chest expander cables ( thereby utterly and completely confusing the issue ) let's get a few things straight right there and then.
First of all, the total exertion the man is using will not be 100lbs as you say. Otherwise the scale that he is standing on ( the other end of the pull ) would register 100lbs, apart from the addition of his own weight of course.
Have a look at the lower diagram on the FAQ page showing the cable stretched around a single pivot point instead of a foot scale. That reflects a truer picture of the stretch length of 3X than the "standing on the scale" pictures.
Taking that it is an Orange cable being measured, it is therefore 50lbs of tension registering on the scale because the scale is hooked around both handles.
That is Lifeline' rating for the Long Orange cable.
Because both handles are pulling in the same direction and held together by the scale, they are effectively one end of a straight line stretch, the pivot hook is the other end. 50lbs at each end. 50lbs TOTAL stretch for the TWO cable lengths. If it was not so, the scale would register something different.
If we obtain another scale and separate the handles, and measure them with each scale, there will be 25lbs of tension on each scale. There will still be 50lbs of tension on the pivot hook. I think that is well understood.
But the cable tension overall will not be actually 50lbs, as is rated, it will be ( and is ) 25lbs when stretched to 3X its resting length.
Therefore you are correct when you say Lifeline are not measuring cable tension, they are combining the total force at the handles at 25lbs each = 50lbs. Once this is clearly understood, then the diagrams with the guy standing on the scale being compared to the diagrams showing him lifting the same barbell weight, become straight forward and the ratings for the Long cables become clear.
Like I have often said in my posts, Lifeline's Long cable ratings may well be correct, for all I know. Now that I have been forced to spend time on the FAQ page determining what is true and what is not, they are in fact correct by way of the interpretation that has been applied.
This interpretation is not smart at all from a customer's point of view. The poundages have been listed next to the cable colours as a 3X stretch rating for that colour. Even the diagrams which become clear after some considerable thought do not help the situation, because as we both know, there wouldn't be all this debate and confusion if the page was more clearly presented. Most trainers brousing the Net for cable equipment will not spend time having to work out how Lifeline has determined its "clever" parameters. They will simply glance at the cable ratings list and take them as bona fide cable pullweights, basing their purchases and subsequent workouts on these ratings. There is plenty of evidence around this forum and on Youtube to support what I have just said.
Worse still, this confusion has spilled over to the Chest expander cables, which instead of actually being measured properly, as Lifeline asserts, have been given the same ratings as the Long cables, on the Chest expander page. This has resulted in confused and erroneous responses to my original assertion that Lifeline's Chest expander cables are advertised exaggerated by 300%.
Plain simple information which I have spent 17 months supplying to Lifeline has been completely ignored and the same old "party line" responses have been issued in answer to my ( and others ) test results for their Chest expander cables. What really amazes me is how long they can keep this corporate dogma up, while not even spending 30 minutes to check whether I am right or not.
The only reason why I am not taking them to the FTC at the moment is because I am waiting for their new Chest expander cable lengths to be tested and rated.
I would hope that they test them properly, and dont just do what they did with their current ones, transfer the Long cable ratings onto the Chest expander page.
As another respectful warning to you, I advised that with your Yellow near-new cable snapping and hitting the girl in the arm, you cease using Lifeline Chest expanders altogether until they have their act sorted out re cable batch quality.
Whether for monetary gain or not, you obviously take classes in PhysEd in a structured manner. You will know this better than I do, that the safety and well-being of your students/trainers are your first consideration.
If that girl had lost an eye instead of merely receiving a slap on the arm, and had decided to sue, her lawyer would have been advising her to not only sue Lifeline, but you as well. As the controller of the class it would be able to be proved that you knowingly and willingly exposed the girl ( under your care ) to equipment which you KNEW was erratic in quality and had many failures previously.
I'm certainly not presuming to tell you what or what not to do, I am simply conveying a warning to you as a friend, before you become the defendant in serious financial litigation.
The other option would be for your trainers to wear safety goggles.