Pneumatic Piston hard stop analysis

[Guncici]

I have a 1 time use pneumatic piston that will be operating at a high pressure. The piston will have a hard stop at the end of its stroke and I am trying to figure out the number, and size, of fasteners I need so the fasteners and wall will not shear. The piston must be pneumatic.

Thanks

—Joe

 

[Jboggs]

And your question is?

 

[Guncici]

Jboggs
Details.

The questions is: how do I calculate the amount of Impact a fastener can take before it shears.

 

[USMechE6]

Joe S,

If it's a standard NFPA-mounted cylinder, the bolts should already be sized to handle the maximum force the cylinder should produce based on its maximum operating pressure and bore (Force = Pressure x BoreArea).

The force from a sudden stop can be much larger though. You say it's a hard stop, but the piston will have a non-infinite peak deceleration value that will determine the force from this. This depends on the material of the piston, cylinder/housing, escape velocity of the air, etc. You can control this by metering out the flow. More information is needed for such a calculation.

 

[Jboggs]

Two more comments:
Your greatest force will probably come from the momentum of the piston at impact rather than the pressure behind it.

Also, you've mentioned "shear" twice. Is that really what you mean? Shear would only happen if the axis of the screw were perpendicular to the motion of the piston, a "sideways" or "slicing" action. But in most cylinder arrangements the retaining screw axes are parallel to the motion, which means their loading (and failure) is tensile, not shearing. Two very different things.

There are numerous sources where you can find the maximum tensile stress for specified screw materials, along with methods to calculate the tensile area of threaded parts. Get a copy of Machinery's Handbook.

 

[BrianE22]

I thought the following might be helpful:

http://hydraulicspneumatics.com/200/TechZone/Cylinders/Article/False/21523/TechZone-Cylinders

 

[BUGGAR]

If it's a one time use, what does it matter if it destroys itself?
Seriously, you need some deflection in the fasteners to absorb the energy or the forces are infinite. Figure fastener loads from the energy of the moving piston equal to the elastic and plastic energy of the deflecting connection. Put in a snubber and its deflection will lower the fastener loads.