Piston Rod buckling

[abrtek]

Guys,

How the piston rod is calculated in terms of buckling ? Is there any special way to calculate this since this is a horizontal laout or the piston rod is supported on ie. packing case?

Maybe some of you know any reliable source just for referrence. (I am not talking about the Mechanical Handbook or sth. like that)

Appreciate your help

Tahnks
Bart

 

[dcasto]

We call this RODLOAD. The piston rod has a mechanical limit of forces on it just like any steel structure. The forces on the rod are a function of the gas pressure acting on a piston When the gas pressur in the cylinder goes up, the psi (or kg/cm^2) acts on the area of the piston. so lets say we have a piston with 100 inch^2 area and the pressure is 100 pounds/inch^2, the net force is 100 * 100 = 10000 pounds. If the cylinder is double acting, the other side is seeing 30 psi gas pressure being acted on 90 inch^2 (the area on the back side has the same 100 in^2 less the area of the rod itself). so it has 30 * 90 or 2700. the net force is 10000 - 2700 or 8300 pounds.

When the cylinder reverses, the inside has 90 in^2 times 100 psi or 9000 pounds force and the other end has 30 psi times 100 in^2 or 3000 pounds net 600 pounds force.

Each compressor has a specified limit of the maximum forces. The number are typically 11000 pounds of force per 1 inch^2 area of the rod for a major industrial compressor. Some smaller air compressors may only allow 5000 pounds per inch^2.

Another way to buckle or break a rod is to allow liquids into the cylinder. The liquids cannot pass out of the check valve quickly enound and the piston trys to compress them and the unstopple force meets the unmoveable wall and something gives, the rod.

 

[abrtek]

Thanks for your reply but this is not an answer to my question . I am aware of the mechanics that occures inside the compressor ( rod load, non reversal etc.)

I just wanted to know if there is a "special" methodology of calculating the buckling or I can just simply use the common equations.

 

[dcasto]

I guess I don't understand buckling. do you mean deflection? Where the rod bends slighly as it put into compression load?

 

[BigInch]

I believe this is close to what you're looking for.
Its a modification of the Euler equation that's used here,

http://www.rocw.raifoundation.org/mechanical/BTec-Mec/machinedesign-II/lecture-notes/lecture-13.pdf

may be too much,

http://www.forging.org/FIERF/pdf/DynamicLoadAnalysis.pdf

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ivymike]

I'd have suggested a modified Euler approach as well... (see also Shigley)

 

[ivymike]

http://www.mame.mu.oz.au/eng_design/simled/SiMLED_cols/SiMLED_cols.html

from a quick internet search using "shigley euler buckling"

 

[tgmcg]

Upon liquid ingestion, sometimes the rod fails first...if you're lucky. Sometimes the cylinder fails...if you're not so lucky. Sometimes the conrod goes.

I've also seen the distance piece shear all the way through on a wet gas recovery compressor on an unusually cold day. Visualize the cylinder and attached pulsation bottles and piping heaving back and forth at 327 rpm. It was a single throw machine with large spoke-type flywheel. All the flywheel spokes sheared at the hub.

Operators swore it wasn't liquid ingestion. Yeah, right.

Small independent refineries scare me.

Best regards,

Tom McGuinness, PE
Turbosystems Engineering

http://www.turbosynthesis.com

 

[FeX32]

yes Shigley

Fe