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• # Permanent Set In Springs

Discussion in 'The main mechanical design forum' started by Msat, Mar 27, 2011.

1. ### MsatNew Member

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Mar 2011
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Hello,

I am planning to fix a compression spring in a fixture. The wire diameter is 0.4 mm, the free length is 7 mm, the minor diameter of spring is 3 mm.

I will compress the spring (by 2.5 mm) with a washer & nut.

So, i was concerned that will be spring undergo PERMANENT Set.
I asked this question because i am going to keep this spring (in compressed position) for the whole lifetime of the assembly (May be 15-20 years). I was scared that if the spring is been compressed for such long duration, it may result into PERMANENT SET of the Spring.

Need Help

Thk

2.
3. ### AndrewNewWell-Known Member

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Jul 2010
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You shouldn't see permanent set providing you keep the torsional stress in the spring below about 40% of the minimum tensile strength of the spring material. You can calculate torsional stress here http://www.efunda.com/designstandards/s ... r.cfm#calc. You shouldn't see any creep, even after 20 years, unless you operate at elevated temperatures, as creep rates for typical spring materials tend to be vanishingly small at room temperature.

4. ### MsatNew Member

Joined:
Mar 2011
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Thanks Andrew sir,

The Maximum Torsional stress is 768Mpa (Wire dia - 0.4mm, Outer Dia of spring - 3.5mm, Free length - 7mm).

But how shall I know that i have kept the stress below 40% ?

The max. displacement is 3.8 mm (calculated from efunda.com). So, shall i conclude that i should compress the spring by 40% of the max. displacement....It would be 3.8*.4 = 1.52 mm.

I hope that i was not too confusing.
Thanks
Msat

Joined:
Mar 2011
Posts:
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