Effect of Preset on Fatigue Life

[SpringDingaling]

I'm having a problem with designing a spring/springs that fits my application.
Here are the design constraints I am currently working with:

Cycle Life Requirement: 50,000 cycles
Load Point 1: 450mm
Force1: 600N
Load Point2: 250mm (deflection required: 200mm)
Force2: 1000N
Min ID: 16mm
Max OD: 29mm

I am currently experiencing high stresses using two springs in parallel (>70%)and the fatigue life is not met. I am wondering if presetting can help my situation, and what the effects exactly of presetting are on design life.

 

[TVP]

Presetting is commonly performed on compression springs that require good fatigue life (dynamic cycling). This operation will cause the spring to become somewhat shorter when compared to springs that have only been wound, but it is very repeatable when good material and a controlled process is used. Fatigue life is improved because the small deformation that takes place induces residual compressive stresses. Shot peening should also be performed on springs requiring fatigue resistance. A good spring vendor can help you with both of these items.

 

[SpringDingaling]

I have also been told that someone has seen in a design handbook that presetting can reduce the resultant stresses by up to 1/3. Is this true? If so, does this hold true for low cycle fatigue or is this only good for static applications?
I am currently trying to get the attention of some vendors, but I am also impatient to figure this out!

 

[israelkk]

As TVP said preset increase the wire ability to withtand highr stresses.

However, quick checking of your data shows to my best knowledge and 30 years of experience that you can not achieve your goal even with a preset. Your design is confined by the loads and the rate. It appears that in the 29 mm diameter maximum OD and maximum length of 450 mm for the first load point of 600 N, there is no possibility to get the 2N/mm rate and at the same time the first and second loads.

http://israelkk.googlepages.com/home

 

[SpringDingaling]

What would be an acceptable level of design stress in music wire material for presetting to have enough effect for me to make my cycle life. For ex.
Two Springs:
Spring 1 is 29mm OD, 3.1mm wire diameter, 600N @ 250mm, 375N @ 450mm. Corrected Stress is 1562 MPa, or 84% of tensile strength.
Spring2 is 16mm ID, 2.5mm wire diameter, 400N @ 250mm, 215N @ 450mm. Corrected Stress is 1448 MPa, or 75% of tensile strength.
If I take these and fatigue analyze them, they're junk. however, if I apply that 1/3 rule that another engineer found in some old italian textbook (Julyenne, machine design??), then they become acceptable for 100,000 cycles life.

If you can, please answer the first question directly then refer to the example. I could use that information as design guidance when deciding where I will make concessions, as it seems I must.

 

[israelkk]

The wire in compression spring is loaded by shear stress not by tensile stress therefore, the ultimate shear strength is 0.577 to 0.6 of the tensile strength. Your calculated stress is far beyond the ultimate shear strength of the wire material.

I suspect that you have no experience in the spring design area if you refer to the tensile strength of the wire instead of the shear strength.

http://israelkk.googlepages.com/home

 

[SpringDingaling]

Thank you for your correction. You're right. I have no experience in spring design. I was copying what spring designer V5 is giving me. It states tensile in the top right hand corner. The corrected stress levels it gives corresond to 65% of the value given under "tensile", although yes I understand that shear is the failure stress. My bad.
So from what you say it seems the design is way out of reach. Two other suppliers are doing the job in this package, or at least stating that they can. They seem to feel they would have solved the problem, or are just not aware of it, which I doubt. I feel there should be a solution in this package, and perhaps processing is the answer, which is why I persist.
Can you please answer question #1.

 

[israelkk]

First of all it depends on the wire diameter every wire diameter has a different tensile/shear strength. Assuming the whole wire section was in the plastic zone as the result of the preset the theoretical equivalent new tensile strength will be 1.33 higher. However, in practical life it will be less.

Remember that for the spring design you should take the minimum tensile strength that the material spec (ASTM A-228 for Music wire for example) specifies. No one can guarantee that manufactured batches will have higher tensile strength.

Many designer takes the typical value and the find that in real life the spring fails.

For example you states that the stress for the 3.1 wire diameter is 1562 MPa which is 84% of the ultimate tensile strength. However, ASTM A-228 says that minimum tensile strength is 1800 MPa. Therefore, 1562 MPa is 86.8% and not 84%.

Do you aware that springs that long will buckle sideways and will rub on each other (if you use one inside the other) which will cause an erratic and inconsistent load readings in addition to reduced life from the wire wear.

Even if you use one spring it will buckle and will rub on the inside bar/outside cylinder and will give inconsistent loads.

Is there an option to change the spring dimensions/materials etc.?

http://israelkk.googlepages.com/home

 

[SpringDingaling]

Unfortunately I haven't been able to figure out a way to change the dimensions. I had thought about the buckling issue and had made our inner and outer tubes from plastic to reduce the wear on the spring.
How much load variation do you figure could happen? Also, is there a good way to estimate the hysteresis from friciton?
I can change materials, but am constrained by my outside diameter. The ID could change a small amount, but really not much. I could add another 50mm to the position 1&2 if i could get right on 1000 & 600N. Otherwise, I can let go of the 1000N, but unfortunately can't live with too much less than 600N @ P2.
It's an ugly problem.

 

[israelkk]

I do not have an answer for you questions about the friction. But I know that it will be a bad design if you have two spring one inside the other and they may buckle.

How did you come to the requirements from the spring/s. I do not see a way that you can achieve what your are after at the current dimensions.

 

[SpringDingaling]

We require a certain amount of counterbalance for a door that requires lifting. The package dimensions come from customer requirements.
I guess we'll have to rethink the problem! Thanks for your comments.

 

[SpringDingaling]

One more question.....for preset to take effect, does the preset have to be outside the working range?
e.g.....I have a spring that yields at 60%UTS, is solid at 75% UTS, and has a working point at 70% UTS.
Do I derive any benefit from the spring set between 60 and 70%, or is it only the preset from going solid to the working point, i.e. 5%