Spring Mechanical Properties Nomenclature

[TylerJ0]

Hello All,

I'm working on a project where we're having valve springs fail occasionally. I was tasked with measuring the stress levels on the coil of the spring and comparing them with the stated material limits from the supplier.

I have installed torsional strain gages (45 deg from center) on the coil of the spring and measured strain on a running engine...I used the young's modulus and poisson ratio to calculate the shear modulus, and since I only had room for a quarter bridge (one gage) on the wire I multiplied the strains by 2 to get the full value of torsional stress on the coil. Stop me now if anyone see's anything wrong with that post processing method.

Now that I have stress values I'm trying to compare them with the material limits I have. What I mainly want to ask is: in spring design, is it standard practice for the torsional fatigue limit to be called "% of tensile strength"??? What's throwing me is normally fatigue strengths of basic metals hangs around 50ish % of the ultimate tensile strength, and this spring spec sheet lists a "40% of Tensile" limit...pretty close to what I'm used to seeing in basic tension/compression fatigue data. So I'm confused as to whether that value in the spec is regular fatigue for the metal or specifically a torsional fatigue associated with the spring. Seems like they should just call it "torsional endurance limit" or something to make it clear but all I see is "% of tensile: 40%" which seems kinda vague...unless that's just what they call it in the spring industry.

Hopefully my question makes sense. Thanks a lot.

Tyler

 

[israelkk]

Do you have the spring drawing and working condition?

 

[dgallup]

Springs have well known stress calculation equations, how do they compare to your measurements? I don't know what a "torsional" strain gage is, but I've always seen spring stress calculations as a percentage of minimum tensile stress.

SAE HS 795 is a detailed design manual containing 10 standards on all aspects of helical and spiral springs, this publication covers:
Spring materials
Cold-wound helical and spiral springs
Hot-coiled helical springs
Design of helical springs

For helical compression springs, the standard Goodman diagram references the design stresses relative to a point at 67% of the tensile strength. The SAE standard reduces this to 56% of the tensile strength for non preset springs. The higher Goodman percentage results in increased slopes for fatigue lines and the estimated life would be significantly increased.

Also, do you know the natural frequency of the spring? If you are operating anywhere close to it you may be getting into into surge which can quickly destroy a spring.

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[TylerJ0]

Some info on the spring from the spec:

Beehive Compression Spring
Wire Diameter: 0.187"
OD: 1.194"
Installed Load: 105 lbf
Working Load: 280 lbf
Natural Freq: 532 Hz

Min Tensile: 296.5 ksi
% Tensile: 40.05% (Here's where I'm confused as to what this represents, is it torsional fatigue or material fatigue in tension?)
Fatigue Life: 2.41E+16

Torsional strain gages are just regular strain gages that have the wire grid angled at 45deg to measure the peak shear stress. My stress values are around 79-80ksi (when you include the preload from installing the spring). If this "% Tensile" property is a torsional value then I'm good, it's 118ksi and I'm well below it. But if it's something else, I'm not sure where I stand...we do have springs breaking but we're not sure of the cause...I'm trying to rule out fatigue.

I had read in the past that a safe ballpark estimate of a materials endurance limit is ~50% of UTS, and torsional fatigue is roughly another 50% of that...so torsional fatigue is in the ballpark 25% UTS, much lower than the 40% listed, and would put my stress values above the limit. The discrepancy makes me think it represents something else. That being said, compression springs wire is designed to survive torsion so it's possible it's torsional fatigue is higher than regular materials.

Long story short, "% of Tensile" seems to me to be an odd way of pointing out torsional fatigue limits, but I'm new to working with springs so maybe that's just how it's labelled in the industry and I wanted to see what the spring experts had to say about it.

Thanks again,

Tyler

 

[dgallup]

What does this mean: Fatigue Life: 2.41E+16

Most fatigue calcs I've seen give allowable stress at 10^5, 10^6 and 10^7. Our most extreme tests run to 10^9, 10^16 is way off the charts. Running at 100 Hz that would take almost 32 million years.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[TylerJ0]

It's listed as "Fatigue Life" but I have no idea where it comes from... I'm assuming it's some sort of calculated value since based of a line of best fit for the SN curve...because like you said they usually list it as a 10^6 type number. There are minimum and nominal fatigue lifes listed and they both go to the 10^16, so I'm guessing it's not a typo.

 

[israelkk]

Some data is missing in the spring info:
1. Number of total coils
2. Spring wire material
3. The spring deflection between the 105 lb load to the 280 lb load.
4. The spring height at 280 lb load
5. Solid height when all coils touch each other.
6. Top coil OD
If you can give the OD for each coil since beehive spring are basically some ling of a conical springs, will be better.

One more question: Is the spring rate is a straight line or this is a non-linear rate spring?

 

[israelkk]

Sorry or the typo error, should be "some kind of a conical spring"

 

[desertfox]

Hi TylerJo

I have spent many years designing springs and the 40% you mention rings a bell with me, I've seen it in spring design books like Berry etc and yes if you are working with dynamic stresses then keep below the 40% figure, my experience of doing this and keeping below I've never had a spring let me down.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein