Fatigue life of 200 Million cycles needed for compression spring 2

[markarnoldTN]

We have a customer requirement for a spring to last 200 million cycles. The current spring chosen by another engineer is Century Spring #70834S. The spring has the following specifications:
OD .300"
ID .240"
Free length .940"
Rate 7.600 lbf/in
Solid height .260"
Wire diamter .030"
Total coils 8.630
Material 302 Stainless Steel
Ends closed and ground
Installed height is .787"
Compressed height is .551"
The height is not super critical as the forces are not. However, the stroke must remain .246"

The spring is enclosed in an 8mm diameter housing made of hard anodized aluminum. Operating frequency is 800 cycles per minute.

Another supplier supplied this product with an uncoated steel spring. The spring fretted away some of the aluminum. This aluminum clung to the spring and caused dissimilar metal corrosion and ultimately failure. The other supplier change to zinc plated spring steel and is now undergoing retest.

Our engineer chose stainless as the material for the resistance to the dissimilar metal corrosion. My concern is that stainless will have no where near the fatigue life required.

I am currently in a sales position so it has been a while since I have calculated spring life. I want to be able to come up with an alternative if necessary and have data to back me up when I go to engineering.

With the small size of this spring, I do not think we can shot peen it to increase life. Aside from pre-setting the spring, are there any other ideas to help achieve the life cycle?

 

[sbozy25]

Well, there are companies out there that are capeable of peening a spring of that size. It is just very difficult is all. Now if you were to peen and press it, I show a calculated cycle life of over 250 million cycles in the range you specify.

Century is a catalog spring company, which is fine, but it makes it hard to make changes or try different processes. You might speak with Automatic spring in Chicago @ 773.539.5600 or with Capital Spring in Ohio @ 614.418.0250. They have the ability to peen small springs and can handle high volumes if needed. If you use them you can make changes to the part as needed, however with Century you won't.

 

[markarnoldTN]

sbozy25,
Is the life calculation you show of 250M with the stainless 302 or changing it to music wire and zinc plating? My understanding is with wire diameters under .040" shot peening may buckle the spring.

Thanks for the reply.

 

[sbozy25]

I did the calcs with 302 SST, small wire can be peened with glass beads.... real gentle cycling required.

 

[markarnoldTN]

Thanks again.

 

[israelkk]

sbozy25

How do you calculate 250M with certainty?
The normal S/N curve only goes to 10M and from there it is considered unlimited life cycle but real life number is a problem to predict.

What fatigue formulas are you using?

 

[sbozy25]

Used a formula that one of our old former associated spring engineers developed a few years back. He takes the second whall correction factor divided by 1.808 then raises it to the 1/-.13 power.

I don't really know where he got all his data, I think much of it was derived over the years. I don't typically deal with fatigue with the product lines I work with, but I was able to dig up his old program for standard compression springs to see what it would get.

 

[sreid]

I get a maximum shear stress of 87,500 PSI vs. a suggested fatigue shear stress of 48,000 PSI for 0.03 inch music wire. This is suggestive to me that 250 million cycles is unrealistic for this spring.

 

[israelkk]

markarnoldTN

.787-.551 = .236 and not .246

My calculations show that this spring must be presetted and life cycle will never reach 250M I am not even convined it will do 10M. You need to take into account the minimum wire properties as the spec allows.

 

[sbozy25]

Ok, forgive me if this is confusing... I don't use this "program very often" But I took a screen shot of the calculations according to the information above.... here you can see the stresses and fatigue life...

 

[markarnoldTN]

israelkk, .246 is the correct number. Installed height should be .797. Sorry for the typo.

My understanding of design stress for maximum life of a stainless spring should be <=20% of minimum tensile strength. I show 282 ksi for 302 SST. Therefore maximum stress should be <=28.2 ksi. The applied stress is 87.5 ksi.

If I change to music wire, mininum tensile stress is 330 ksi. Recommended design stress will change to 35% of this. That is equal to 115.5 ksi. Of course the rate will change to 9.0 lbf/in, but this is acceptable for this application. With music wire, I would be looking at zinc plating or other coating to inhibit corrosion.

sbozy25, I follow most of your spreadsheet, but I do not see how the fatigue life was calculated. Is there someway you could show that?

Thanks.

 

[sbozy25]

I'm not quite sure where some of the numbers were developed from. I know the engineer that developed it used some "factors" he had developed over the years that were based on Goodman Diagrams, and testing he performed.

Ummm... not quite sure where you are getting your information, but you are not quite correct on a few things.

Music wire tensile can range from 230ksi to 480 ksi depending on the diameter of wire you choose. I have a sheet from out material supplier with the tensile ranges of the wire. Also, 302 SST ranges from 225ksi to 350 ksi depending on the size.

Also, if you heat, peen, and press the parts, you can take to 55-60% tensile on sst, and 65-70% for music. That is according to the associated spring design manual.

I will have to go through the former engineer's notes, and see if I can piece back where he got his long term cycle life predictions from....

 

[markarnoldTN]

For the tensile strength numbers, Century Spring lists .030 in 302 SST as 282 ksi, .030 in Music Wire 330 ksi. The stress recommendations are out of their book as well.

 

[sbozy25]

Ok, I understand now. For some reason I thought you were speaking generally, yes those values are correct. Now as far as their stress recommendations go, you can ignore them. Since they are a catalogue company, they play things very convervitive. I am looking at their book now, and don't agree with them. You can push the parts much further than that and be find. Also, they probably aren't pressing or peening the parts. I will know for sure next month, I am going to be in LA on a customer visit, and plan on stopping in to visit them and see their operation. I work for the same corporation as them, but different division.

 

[israelkk]

sbozy25

I looked at the spreadsheet and I understand that KW1 is the full Wahl correction factor where KW2 is a reduced one.

To my best knowledge Wahl factor can be ignored only for static use when the spring were gone plastic process such as in a preset. But for fatigue you can not ignore Wahl factor which represent a stress concentration factor. More than that, wire surface finish is far less perfect than a polished standard rotating bending test sample. There are tool marking, dents, nicks etc., (even if not seen by the naked eye or X10 magnification. Therefore, the theoretical fatigue calculations are far too optimistic.

Therefore, the 88M cycles is probably a wishful thinking.

From my knowledge of fatigue analysis I haven't seen any formulations that goes beyond 10M. At 10M it is customary to consider the spring life as infinite but to say that it will go to a specified life cycle value I haven't seen any book, article etc. that claim such.

 

[sreid]

Remember that coil springs are stress in torsion and the stress numbers that apply are SHEAR stresses, not tensile stresses. As a first approximation, the maximum shear stress is half the tensile stress for steel.

 

[markarnoldTN]

Certainly we are going to do extensive life testing of this spring as we will be using over 30,000 a year. I guess what I need to do is utilize the strongest material (music wire) that is reasonable for fatigue purposes and apply the least stress possible and still obtain required system response. I will also recommend that we shot peen the spring if possible as well as pre-set it as well to extend life.

Thank you for all the help.

 

[sreid]

Let us know how the life test went with the zinc plated spring.

Also make certain that your hard anodized spring pocket isn't sanding your spring coils.

 

[unclesyd]

Here is a material Rocket Wire that we used in some springs that were in an extremely high cycle enviroment. I don't recall the number of cycles to failure but it was considerably higher than the OEM material by several orders of magnitude.

http://www.mjwire.com/mjw_rocket.html

 

[IggyD]

When you say fretted away some of the aluminum it sound to me as if the grind caused a sharpness to the edge of the spring ends. What would alleviate this problem would be to reduce the ends approximately 1/2 wire size so that they do not come into contact with the walls of the housing.