Spring Creep issue

[siuhawks89]

I am struggling with trying to verify a new design. Here is most of the information:
I have a mineral insulated thermocouple cable that is coiled into a compression spring. It is being compressed slightly approx 2.25 lbs load.
The sheath material is Hastelloy X 0.063" dia. and it will be in an environment where the max temp could get up to 1200 F. Once installed the spring will be not be cycled.
I am just trying to figure out if the load/temperature will cause significant creep.
I want to complete this problem assuming a hollow tube as worst case scenario. I have calculated the stress in the spring but I have no information to calculate max allowable stress.
Although this should show if the spring is okay based on the load it does not take into account the temperature.
The temperature is what i am most worried about but cannot figure out a way to prove it will not cause creep in the spring.
Not sure if i am looking to deep into this but i need some kind of evidence/calculations to prove this out.

Any help would be appreciated.

Thank you
Ryan

 

[TVP]

You need to know what the stress is in the installed position. How long does this spring need to produce useful force? 1200 F is very hot for almost all high strength spring materials. Maximum use temperature is usually quoted as ~ 550 C (1020 F) for alloys like Nimonic 90 or Inconel 750. Inconel 178 can be used up to 700 C (1290 F). Very expensive, limited availability.

 

[siuhawks89]

1200 is the max temp it may see sometimes. probably a very small amount of time.
The big question is how long the spring needs to produce useful force. This is basically what i am struggling to figure out. We need the force to stay close to the initial load for this design to work.
I cant find any equation that takes into account stress and temperature to tell you how this force will react over time. I would like to know how to calculate this so i can come up with a graph or some number of hours/days the force will start to taper off.

 

[TVP]

This is an example of a multivariate problem where the "equation" is some type of differential equation, i.e., one that you cannot easily calculate. The way that high temperature springs are designed is by load loss or stress relaxation curves. Springs are manufactured using a process call heat setting so that the residual stresses are reduced or removed prior to end use. Take a look at the following links for more information:

http://www.lesjoforsab.com/technical-information/technical-report0901_id1174.pdf

http://www.amspringwire.com/valvetechbulletin.pdf

http://materion.com/~/media/Files/P...rs/AP0028 - Atlas of Stress Relaxation Curves

http://smihq.org/public/publications/07_47_04/_hot_setting.html

http://www.db-thueringen.de/servlets/DerivateServlet/Derivate-24673/ilm1-2011iwk-064.pdf

 

[tbuelna]

I do not know of any spring metal alloy that will not experience some amount of relaxation at sustained temperatures of 1200degF. Maintaining very small preload forces like 2.25lbf makes the situation even more difficult. The only solution I can suggest would be to use a mass weight and gravity to apply the axial force. As long as the operating conditions are static and friction in the system is constant, then a simple mass would be capable of applying a constant force regardless of temperature.