Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

In article EqbZf.88820$YX1.64885@dukeread06,
"Glenn Ashmore" wrote:

Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

IME, no. Different springs, tempered differently, behave differently.
Consider a slinky as one extreme case, and a truck coil spring as
another.

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.

Basic farmyard engineering. Try it. If you were building 100's or 1000's
it would be worth figuring it all out either theoretically and/or on a
prototype. Since you are building just one, build it, try it out, and if
the spring fails, try a different one - small ones are not so expensive.
You can either try it on the project, or simply yank it out to the
length you think it needs to go to and see if it snaps back.

--
Cats, coffee, chocolate...vices to live by

Glenn Ashmore wrote:
Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.

Wow. I thought my view cameras were heavy :-).

At 6", if it's still linear, then you're pulling on it with 72 pounds.

If it's a "stock spring" then the manufacturer probably makes a
recommendation about maximum extension. You can usually go a little
more than that, but going from a 3.5" max to 6" will probably deform
it.

Actual max extension depends on many parameters including material,
dimensions, preload, and heat treatment.

Good tutorial on the web that I HIGHLY recommend:

http://home.earthlink.net/~bazillion/springs.html

Tim.

Glenn Ashmore wrote:

Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

If it is an compression spring (at least designed for), you can compress
it until it is a block. That travel is OK for the reverse direction too.

HTH,
Nick
--
Motor Modelle // Engine Models
http://www.motor-manufaktur.de
DIY-DRO // Eigenbau-Digitalanzeige
http://www.yadro.de

Glenn Ashmore wrote:
Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.

Get one and try it. Most likely, it will take this overtravel, but
that will shorten its life somewhat. The 6" extension will be about
the limit, I'd guess. Once you exceed the absolute elastic limit, the
spring will no longer contract fully, so you will know that you've
found the limit.

Jon

In article EqbZf.88820$YX1.64885@dukeread06, says...
Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.



The rated load is usually set to keep the material below its fatigue
limit, i.e., if the spring must go thru many load/unload cycles without
failing, you should stay below the rated load. If you can find a number
for the fatigue limit for the spring material and compare to its yield,
you'd be able to make a SWAG on the load that would permanently stretch
the spring.

Or try one of the spring calcualtors here and analyze the spring or
design your own...
http://www.engineersedge.com/Calulators_Online.shtml

Ned Simmons

In article ,
says...
Glenn Ashmore wrote:

Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

If it is an compression spring (at least designed for), you can compress
it until it is a block. That travel is OK for the reverse direction too.


But many compression springs will fail prematurely if used this way in a
cyclical application.

Ned Simmons

Glenn Ashmore wrote:

Is there any rule of thumb about how far can stretch a spring before it is
"sprung". i.e. permanently deformed?

I have found almost the perfect spring for my camera stabilizer project but
it is only 4" long and rated for 42 lb at 3.5" and I need it to extend a bit
more than 6". I am worried that if it bottoms out once it will never come
back to its original initial tension.


Buy two of 'em and connect them together lengthwise?

GWE

Glenn Ashmore writes:

Is there any rule of thumb about how far can stretch a spring before
it is "sprung". i.e. permanently deformed?

Yes. You can calculate the material stress on a given spring using
formulas, and predict the cycle lifetime from that stress. I do it for
torsion springs on my Web page at http://www.truetex.com/garage.htm and
Machinery's Handbook has data for other types of springs.