Effect of Preset on Fatigue Life
Effect of Preset on Fatigue Life
(OP)
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.
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.





RE: Effect of Preset on Fatigue Life
RE: Effect of Preset on Fatigue Life
I am currently trying to get the attention of some vendors, but I am also impatient to figure this out!
RE: Effect of Preset on Fatigue Life
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
RE: Effect of Preset on Fatigue Life
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.
RE: Effect of Preset on Fatigue Life
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
RE: Effect of Preset on Fatigue Life
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.
RE: Effect of Preset on Fatigue Life
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
RE: Effect of Preset on Fatigue Life
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.
RE: Effect of Preset on Fatigue Life
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.
RE: Effect of Preset on Fatigue Life
I guess we'll have to rethink the problem! Thanks for your comments.
RE: Effect of Preset on Fatigue Life
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%