Impact Force 1

[loilfan]

I am designing a sampling rod that has a chain attached to it as a fail-safe in case the sampling rod's packing fails. My assumptions are that the rod is fully extended when the internal pressure of the vessel/pipe accelerates the rod backwards until the chain reaches the end of the length and absorbs all of the impact of the projectile rod.

Step 1etermine final velocity of rod
v2=SQRT(v1^2+2ad)

Step 2: Determine displacement of chain under loading
I treated the chain as a spring and used k=EA/L as my spring constant.
Kinetic Energy=Spring Energy
δ=√((L/(E*A))*(m*v2^2))

Step 3: Determine Chain Reaction Force
F=EAδ/L

The problem is that the force ends up being independent of length. So a rod that travels 1/4" ends up exerting the same force on the chain that travels 5'. The math seems right, but it doesn't make logical sense. Can anyone explain why?

 

[loilfan]

gruntguru, thanks for your input. Do you know of a technical reference that I can use for a justification of the high chain safety factor aside from the manufacturer?

 

[gruntguru]

Not sure if you should go down that path. Why not add some resilience to the tether design? Or some Friction?

Perhaps use rope made of high-extension material like nylon? You can still have a chain in parallel as backup.

je suis charlie

 

[loilfan]

Thanks for the input, everyone. We decided to remove any opportunity for the rod to accelerate by shortening the chain whenever the rod was inserted. This makes the chain resist a static load instead of a massive kinetic energy load. A SOP has been developed to make sure their is no slack in the chain.