Corrections for Art & Science of Practical Rigging

[RescueMan]

I just emailed ISA-ARBOR with some comments for improving their excellent book, The Art and Science of Practical Rigging. These included a couple of mistakes that need correcting.

The gist of my comments are below:

On page 49, there is an inconsistency between the text and the picture and an inconsistency between what is described and conventional language.

In the discussion of the "slip knot", the text instructs to take a counterclockwise turn while the picture shows a clockwise turn (though the knot will come out the same either way). More importantly, what is described as a "slip knot" is not what is commonly known as a slip knot, which is an overhand noose knot which will cinch around an object. What is described and pictured is a slipped overhand (which is stated later in the text). It would be helpful to differentiate this knot from what every kid learns as a slip-knot by using the term "slipped knot" rather than "slip knot" and by describing a possible function, such as an easily spilled stopper knot. The picture makes it appear that the knot is a loop knot, but the loop will spill as soon as it is loaded, causing a potential catastrophe.

On page 88, there is what I believe to be an error of fact and also a case of misapplied terminology.

In the middle of the page there is a statement that the input force, with one person pulling, is approximately equal to their body weight. I suspect that few arborists can climb a fixed rope hand over hand, thus lifting their body weight while gripping a rope.

Studies done by Rigging for Rescue of Canada on the grip strength of rope rescue personnel, demonstrated a wide range with an average one-gloved-hand grip strength on a ½" rope of 50 lbs. When I figure mechanical advantage systems, I calculate the number of available haulers times 50 lbs each (assuming they are using a hand-over-hand method with only one hand on the rope at a time). Few people are aware of how little force can be exerted on a rope by pulling and it would be helpful to clarify this common misunderstanding.

Also, in the second to last paragraph, the text states that "it would be possible to rig a system to disadvantage, where the anchor force is greater than the output force." The text earlier defined mechanical advantage correctly as "the output divided by the input," but then uses the term "disadvantage" inappropriately. While the system described would place more load on the anchor, there would still by a 4:1 increase in output. A mechanical disadvantage system would create less output for a given input input (a 1:4 system, e.g.).

- Robert

 

[murphy4trees]

Some good points to consider...
The question of how much pull can a man put on a rope and extended to how much pull can a truck put on a rope has been of interest to me... Grip strength is not the weak link in my thinking...
You don't need to do a pull up to grip equal to your body weight...all you need to do is be able to hang by the rope....
And it seems reasonable that the pull of a man is going to be some factor less than his body weight and that grip strength does play a role in that..
That's why I love those ugly gloves...
So I think the limiting factor is balance when standing on grass or pavement... So I always like to "get a foot on something"... That is stand behind a tree or the truck and get a foot up... My guess is that will add about 50% to pulling force...
It would be fairly easy to measure this with two men of similar weight..
Set a pulley on an over head anchor and a redirect pulley and see if one man can lift the other... Who wants to take that project on???

 

[knudeNoggin]

Originally posted by RescueMan
I just emailed ISA-ARBOR with some comments for improving their excellent book, The Art and Science of Practical Rigging.

Good show! I suspect that the reason many obvious errors remain in books
is because folks don't take this sort of helpful action.

And one can use forums like this as a sanity check on suspected errors.

... . More importantly, what is described as a "slip knot" is not what is commonly known as a slip knot, which is an overhand noose knot which will cinch around an object. What is described and pictured is a slipped overhand (which is stated later in the text). It would be helpful to differentiate this knot from what every kid learns ...

Well, not so fast. In knotting, Ashley is usually held as the "bible",
and "slip knot" for him--quite explicity--is a stopper, slipped,
NOT the noose. (In working on a knots book I had some trouble chasing after
all the uses & abuses of this, and ultimately lamented: ...
>> Slip Knot ...
>
> This is Slip Sliding Away, and I'm punting. The uses/descriptions
> are often for the noose, not the stopper, and sometimes
> even something else (as noted re the Chain Stitch). And
> sometimes it's a slipped noose or overhand. Argh. So, I punt.

As a consequence, the confusion continues. And what "every kid learns"
can be affected by which book, which person, ... .

The picture makes it appear that the knot is a loop knot, but the loop will spill as soon as it is loaded, causing a potential catastrophe.

Now THAT is a frequent problem with images--no clear depiction
of what is the end, what's the standing part!
(And, nevertheless, BAD images get copied over & over!)

In the middle of the page there is a statement that the input force, with one person pulling, is approximately equal to their body weight. I suspect that few arborists can climb a fixed rope hand over hand, thus lifting their body weight while gripping a rope.

But what about using quads vice lats/biceps, and giving a surging
pull with a crouch? That's surely more than 50#!
As noted by another, if a large person leans back into a rope,
there's a large force (and the larger the person the
less likely the hand-over-hand climbing! )

Few people are aware of how little force can be exerted on a rope by pulling and it would be helpful to clarify this common misunderstanding.

Also, the effect of friction in a system is typically not
mentioned or understated--esp. with non-pulley sheaves
(i.e., rope on rope or over fixed meta--a carabiner).

Also, in the second to last paragraph, the text states that "it would be possible to rig a system to disadvantage, where the anchor force is greater than the output force." ... A mechanical disadvantage system would create less output for a given input input (a 1:4 system, e.g.).

You point to theoretical MA, but might the authors have in mind
effects of friction?

Thanks,
knudeNoggin

 

[RescueMan]

You point to theoretical MA, but might the authors have in mind effects of friction?

No, actually what the authors are referring to is rigging a set of fiddle blocks "backward" so that the pull is upward from the anchor and the bitter end of the rope is tied to the becket on the anchor block. This creates a 4:1 MA instead of the 5:1 if the system were reversed.

What they're correctly pointing out is that there is now 5X the pulling force on the anchor and only 4X the force on the load, but that is not an mechanical DISadvantage system. And, as they also point out, it might be necessary to reverse the blocks in order to place the workers in a safer position.

- Robert

 

[TheTreeSpyder]

My education is of the read what i can type, but thought that the proper term for gaining MA power in a system is Positive MA, and it's reciprocal of gaining speed/reducing power to be Fractional MA.

So i've kinda taken the 'disadvantaged' term to mean that the anchor is at a disadvantage in comparison to the load; whereby you must be more careful that the load didn't become the anchor/pivot of the successive pulls as more force was put on the anchor, that might become the load. The anchor is only an anchor if it doesn't move compared to the load; done screwed up enough to learn that! So, i just kinda made up that someone else had same experience as i read that!


Orrrrrrrrrr something like that!
:alien:

 

[ptar]

The TreeSpyder types:

"My education is of the read what i can type, but thought that the proper term for gaining MA power in a system is Positive MA, and it's reciprocal of gaining speed/reducing power to be Fractional MA."

Calling it positive is misleading since any
number greater or equal to zero is positive.

I prefer: "MA of 3 to 1" or simply "MA of 3".

 

[RescueMan]

thought that the proper term for gaining MA power in a system is Positive MA, and it's reciprocal of gaining speed/reducing power to be Fractional MA.

To call a 1:4 system "fractional mechanical advantage" is like calling what's going on in our economy a "jobless recovery". All mechanical advantage is positive, just as all economic recoveries are positive. But to the fraction that don't got no job, to say it's a "fractional recovery" ain't much help - they feel disadvantaged.

So i've kinda taken the 'disadvantaged' term to mean that the anchor is at a disadvantage in comparison to the load

If mechanical advantage means getting more force out of a system than what's put in, then mechanical disadvantage has to mean getting less force out than is put in. Any speed increasing system (like a transmission's overdrive or an alternator belt pulley system) is a mechanical disadvantage system. Any speed reduction system (like the other 4 gears in the transmission) is a mechanical advantage system.

The arms and legs of the human body are mechanical disadvantage systems. They're class III levers in which the force (muslce attachment) is between the fulcrum (joint) and the load (hand or foot), thus requiring more effort in than comes out. Makes you really appreciate the power of our muscles, though, doesn't it?

- Robert

Physics of Lever Systems

 

[TheTreeSpyder]

i guess i can accept that; and we should be on 'equal' terms.

But whether formally or here on the boards(can't remeber), i have seen leverage systems adjusted to their balanced point of no gain or loss of power(save from friction), and it refered to as an MA of 1 etc.; so i have taken that to indicate the MA as a multiplier, and thereby the fractional MA term slid in there somewhere in an output power/input power or in the pulley systems {output pulls/input pull(s)} x amount of pull.

But, whatever is write!

Nice link! i break the lever classes into just 2; center pivot or not; ie. first class or not. If the input arc is smaller than the output arc it is still a 1st class if pivot is not on end, or if the arc relationship are reversed; (power to speed). The only thing that defines a 2nd class from a 3rd class lever is that you gain power in a 2nd, and speed in a 3rd, such gain adjustments in 1st class lever does not alter it's classification. The real operative differance beyond power to speed of levers to me is that input and output move in opposite directions in a 1stclass lever, but in the same direction in non-1st class lever. Therefore a 1st class lever can offer self counterbalancing that a non1st class can't, but at a cost of space for 'MA' adjsuted; as in a non1st class lever system the input and output levers lay on top of each other as one coming off the pivot, conserving space.

Orrrrrr someething like that!
:alien:

 

[murphy4trees]

If mechanical advantage means getting more force out of a system than what's put in,

This will not happen..... force in = force out-friction....
Anything else violates the natural laws

 

[TheTreeSpyder]

i think once again it is just terms, here perhaps more power was meant rather than force.

Different people call things differently; this site aboutThe Mechanical Advantage of a Simple Machine uses the words:

"Mechanical advantage is the ratio of output force divided by input force.

If the output force is bigger than the input force, a machine has a mechanical advantage greater than one. "

So, i would say power instead of force as Murph(we went to different schools together as a matter of fact.....), but must acknowledge and adjust that some call it force; instead of a Mass or Power X Speed = Force.

As does this MA Calculator ; showing whole number and 'decmilac' MA outputs of the calculations. Either showing 1 MA as zero gain/oss of speed/power from the potential of the machine (less friction).


i like investigating it all, know matter how you lifted the log and that maid it rose up, it is as sweet by any other name to me; jest herd so many terms, i try to delineate the intent.

Orrrrrrrrrrr something like that!
:alien:

 

[RescueMan]

This will not happen..... force in = force out-friction....Anything else violates the natural laws

The terms are confusing, but you're thinking of the law of conservation of energy, which is different than force.

Energy, or work, is conserved (minus friction, which is energy converted to another form: heat). Work is force exerted over a distance. So what MA does is multiply force and reduce distance by the same factor so that the same amount of work is performed but a smaller force can move a more massive object.

Mechanical advantage is defined as the ratio of output force to input force. So a 3:1 MA means that 3 times as much force comes out of the system as went in, so that I can lift something 3x as heavy as what I can lift with my bare hands.

- Robert

 

[TheTreeSpyder]

Whar's VTMechEngineer?

i think the Law of Conservation of Energy should be consistant in terms; in that it is the confining property here?

Whereby; it rules that both sides of the equations that we are looking at are actually equal; that i can never get more out than i put in.

Then there can be no perpetual motion machine, that can keep itself going indefinitely by not eroding/losing power. For every machine there must be friction to erode the force down. If not for that magic bullet that killed Kennedy etc. would still be zinging around.

"So you have a mechanical lever provides what is called a fractional mechanical advantage of advantage which is really a mechanical disadvantage". from : http://www.tpub.com/content/engine/14037/css/14037_15.htm

Whatever terms ya like, whatever is consistant with the most etc.; so we can speak of the same things.