# How to measure stress/tension on a rope?

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(a) Imagine an 5-100kg (we do not know exact weight) object is hanged with a
piece of string/rope/wire and swings randomly.
(b) We do not have access to either end of this string.
(c) How can we measure, electronically,em the stress/tension on the string?

Re: How to measure stress/tension on a rope?

Study catenary curves. Apply some high school physics and algebra.

Re: How to measure stress/tension on a rope?

Here's something that the OP may be interested in.
http://www.squid-labs.com/projects/erope/index.html

TROLL ALELRT

**  PISS   OFF   YOU  DOPEY   TROLL   !!

...........  Phil

Re: How to measure stress/tension on a rope?

I think you need to say if you are allowed to cut the rope in order to
insert a transducer (load cell etc).

Also the question appears a bit homework-like! So if it isn't, perhaps
you should tell us a bit more about the application so that people are

--

John Devereux

Re: How to measure stress/tension on a rope?

No, this is not homework, and I'm not a student.

Imagine a parachute. How can you measure stress/tension on a parachute line?
You can not access the either end of the string/line. (One end connectected
to parachute, the other is connected to carabiner.)

Re: How to measure stress/tension on a rope?

Can we contact the string/line at all?  Do we know what it is made of?

If so we could potentially measure stretch over a small sample if we can
access it before the load is applied and can previously know the stretch
characteristics.

Re: How to measure stress/tension on a rope?

finger to keyboard and composed:

Would it be acceptable to fit a sensor to the harness and assume the
load is equally distributed amongst the risers?

Otherwise, is this what you are looking for?

Collecting Parachute Test Drop Data:
http://www.industrologic.com/cptdd.htm

-- Franc Zabkar

Re: How to measure stress/tension on a rope?

You probably could make a simple gizmo to measure the strain
using inexpensive force sensors like below (search
www.digikey.com for force sensors). You would just slip the
parachute line in the gizmo sideways and when the string is
pulled tight the gizmo flexes pinching the sensor in preportion
to the pull on the line.

http://tinyurl.com/96srw

Re: How to measure stress/tension on a rope?

You might try calling the U.S. Navy NAWS, China Lake, Ca.
at 760-939-9011 and ask for the parachute department.  I
vaguely recall seeing something about the group being
disbanded but it may still be in business and I would
imagine they've instrumented a parachute at some time
or another.

Re: How to measure stress/tension on a rope?

Without access to the string, it can't be done in any way that could
justifiably be called "electronically".  You're talking about doing
some serious physics here.  Like: shoot a lot of x-ray intensity at it
and have an expert interpret the diffraction pattern for you to
determine the lattice length of the string, from that (assuming you at
least know the material) the deformation and from that, in turn, the
tension.  Or shoot acoustic energy at it over a wide spectrum and try
to find its resonance frequency.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: How to measure stress/tension on a rope?

"Hans-Bernhard Broeker"

** That is the key.

You need to know about the string/wire/rope  ( ie mass per unit length ) to
relate resonant frequency to tension.

Then just pluck the string and measure the frequency of vibration - many
ways to do that.

..........  Phil

Re: How to measure stress/tension on a rope?

Make a loop in the middle of the string and put an electronic scale or load
cell in there.

Mitch

Re: How to measure stress/tension on a rope?

You have described a "pendulum" : oscillation frequency is
proportional to pendulum length and weight on the end.

If you know the time for one oscillation, the gravitational
acceleration, length of the rope, you should be able to
solve for the mass of the pendulum (weight + rope).

Once you have the mass at the end and the velocity of it you
should be able to calculate the force exerted on the rope
through centrifugal force.

Sounds like a lot of mucking around.

Re: How to measure stress/tension on a rope?

Yes.

Umm, no.  But thanks for playing.

Hint: where is the "weight" in 2*pi*sqrt(L/g)?

Regards,

-=Dave
--
Change is inevitable, progress is not.

Re: How to measure stress/tension on a rope?

Unless the rope is infinitely rigid (wonderful first year physics assumption),
the period and the swing itself will vary with the mass, because the rope will
stretch with the angular acceleration. I don't want to do the math, even if I
could.

Not that this applies to this case, but just to be complete.

Re: How to measure stress/tension on a rope?

[...re:2*pi*sqrt(L/g)...]

Assume a spherical horse moving through a vacuum...     ;-)

Regards,

-=Dave
--
Change is inevitable, progress is not.

Re: How to measure stress/tension on a rope?

Try 2*pi*sqrt(L/g)= 2*pi*sqrt(m/k)

where m = mass

thanks for playing.

Regards,

-=Mark

Re: How to measure stress/tension on a rope?

put finger to keyboard and composed:

Just to be pedantic, that's the oscillation period, not frequency.

And "proportional" implies a linear relationship, not sqrt.

And the formula strictly only holds for a "simple" pendulum, ie one
where the oscillation angles are small.

See http://hyperphysics.phy-astr.gsu.edu/hbase/pendl.html

As for your interpretation of Hooke's Law, how long is a string when
no force is applied to it? ;-)

Hint #1:  F = k.dL,  not F = k.L.

See http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/SHM/HookesLaw.html

Hint #2: Would a 100kg mass swing with 10 times the period of a 1kg
mass, if released at the same angle?

-- Franc Zabkar

Re: How to measure stress/tension on a rope?

I haven't seen all of this post, but the answer seems obvious.

Deflect the rope with a roller, bewtween two other rollers. Measure the force on
the
deflecting roller perpendicular to the rope. This could be fitted to a rope
without
cutting it.

Andrew M