What is acceptable stress on an Extension Spring 2

[BP77]

Using a spring calculator, I determined that the stress on my spring will be 140%. I only need the spring to cycle 15K times and from what I understand, most spring calculations are for 100K.

I'm using ASTM A228 wire that is Ø1.7mm. The spring is being used like a torsion spring to wind up material.

I'm constrained by package space both Diamater and Length and I'm trying to achieve 100N of force when the material is fully deployed.

Is this stress level far to high? What other options do you suggest?

 

[israelkk]

140% of what?

 

[BP77]

Sorry, 140% of the Ultimate Strength. The calculator describs this as "actual max stress at full extension as percentage of SUT".

 

[rb1957]

"Is this stress level far to high?" you'd think so.

you point out that your fatigue life is smaller than that assumed. But not by much (IMHO), a factor < 10, so the impact on the design would be small. does your calculator tell you if the spring is being designed by static strength of fatigue requirements ?

 

[israelkk]

Extension spring are designed for shear stresses in the spring main body and for tension stresses in the spring ends. Therefore, the stresses in the body of the spring should never be beyond the ultimate shear strength which theoretically is 0.577 of the ultimate tensile strength.

The spring ends on the other hand can have stresses for no more of the minimum ultimate tensile strength of the wire.

It is clear that one can not design to the ultimate shear and tensile strength if the spring has to function in real life. Even for static use a safety factor is still needed to avoid disasters.

For cyclic use as in your case the stresses must be even smaller to assure the 15K load repetitions. Take into consideration that fatigue analysis is not accurate and a safety factor of 4 to 10 is necessary.

Wire ultimate strength always have a range between a minimum and maximum tensile strength specified under the governing specification (ASTM A 228 in this case). Therefore, to guaranty 15K cycles you have to show in you calculations and then prove by tests that the spring will hold at least 60K even for the weakest wire that the ASTM A 228 allows.

In addition to that, most of the time extension springs fail due to stress concentration factors caused by wire surface imperfection, scratches, dents, nicks and tool markings caused by the production process, especially at the transition radius between the spring main body and the spring ends. Therefore, you either need to use higher safety factor of strict inspection rules.

Now you can understand that there is no way to give a simple answer to what stress level your spring needs to be designed due to the lack of information. However, it is clear the 140% of ultimate tensile strength is out of the question.

 

[MiketheEngineer]

Right before it breaks!!!!!

 

[dhengr]

Israelkk:
First you got a star (your first post), and only God knows what for on that one; but then someone took it away. Your second post really deserves a star, very nicely done.

There really seems to be a horrible disconnect btwn. what some people are calling engineering these days, and any/some real engineering thought and knowledge which your second post shows, very nicely. It’s a wonder that some of these people who call themselves engineers these days know the difference btwn. a compression spring and an extension spring. And, then with so little understanding of the subject, they go to a spring calculator on the internet and wonder if 140% of something is o.k. for a stress in any situation; it is down right scary, when you consider that they are involved in the design of our cars and trucks. He’s asking about a tension spring, but using it like a torsion spring, and he didn’t even bother looking at an old text book to get a handle on the fundamentals of spring design or how they work and are stressed. He uses all the right words (fancy mechanical design words; he’s “constrained by package space” and “fully deployed”) and doesn’t know that 140% of ultimate strength might mean failure before 15k cycles of load application.

BP77:
You are certainly welcome to join us here at Eng-Tips, but get on the stick! 140% of ultimate is a FoS of .7, probably not safe even if you are the only one likely to be hurt by a failure. There are plenty of smart people here who are giving willingly and freely of their time to help others. But, you have to give a little good engineering thought to your own problem first, do a little homework on your own first, and they will help you fill in some of the details, or tell you what to watch out for. They are not here to teach you something you should have learned in your first Strength of Materials, or Machine Design classes in college. And, please don’t pretend to be doing real engineering, then come here or to an internet spring calculator for quick answers, when you have no business purporting to be doing that kind of design. Go to a senior engineer or your boss with these kinds of questions. They should know what you know and don’t know, if for no other reason than to keep you and your company out of trouble.

 

[TheTick]

A little thought experiment...

What happens to your spring once it reaches 100% UTS? How do you expect it to reach 101%?

 

[desertfox]

Hi BP77


The title of your post is confusing ie a "tension spring" however your torsion spring will fail due to bending stresses.
For cyclic loading of torsion springs I use an allowable stress of 40% of the tensile stress and I've never had a failure to date.
Look at this reference it gives values of about 50% of the tensile stress for cyclic loading:-

http://www.scribd.com/doc/29044778/Spring-Design-Handbook#outer_page_106

Look for table 9-2

desertfox

 

[zekeman]

I did this problem using Machinery handbook and found for a 1.7mm wire and 15K cycles that the "allowable working torsional stress" is 130,000.

Since the OP has a volume constraint problem, I would use (as in my experience) 1.5 to 2 as a factor of safety.

Using 2 , I get an allowable of 65,000 psi in torsion.

You can use the spring formulas to complete the design, or if you are using an off the shelf spring, then you can use your spring calculator to verify that

s<65000 psi

 

[BP77]

Thank you all for your reply's and I’m sorry for those of you who’s time I wasted as this was not my intent.

For what it's worth, I posed this question because my experience does not make me comfortable with such a design and I was looking for some alternative options...hence "What other options do you suggest?" I’ve used springs in the past that worked at 110% and spoken to some colleagues who have used these springs up to 120% without issue and therefore I thought maybe someone else had more expertise (seeing I have none).

I thought these forums were a place for people to share thoughts, experience, etc. I read posts by people like israelkk and desertfox and thought that maybe my eyes would be opened up to different materials, springs, technology, etc.

Being my first post I’ve learned that I shouldn’t rush my question and that I need to provide more details…. I was trying to get a quick answer with a quick question but as we all know… there are no shortcuts in proper design.

 

[zekeman]

" I've used springs in the past that worked at 110% and spoken to some colleagues who have used these springs up to 120% without issue and therefore I thought maybe someone else had more expertise (seeing I have none)."


110%, 120% of what?

Probably the allowable which has a substantial factor of safety built in.