Hydraulic Cylinder Buckling 5

[rpmiller]

I am trying to determine the buckling load for a custom built hydraulic cylinder. Of course my fist thought is to use Euler's equation but I'm thinking that only applies to cross sections that are constant.

Since it is custom, the manufacturer has been of no help.

Another engineer here at work claims that he has saw some equations developed by a manufacturer (possibly Parker?) but can't recall who or where.

Any help is appreciated.

 

[arto]

How about Roark Table "Formulas for Elastic Stability of plates & Shells" [Table 35, case15 in 5thed. is Thin-walled circ. tube under uniform long. comp. ]

 

[CESSNA1]

RPMILLER: I cmae across buckling of a hydraulic cylinder when I was in Machine Design-a long time ago. The text was "Machine Design by SPOTTS". I am sure some, if not most machine design books will have this. The assumption is that since the cylinder is larger than the rod, all the bending is aasumed to take place in the rod.

Regards
DAve Hall

 

[JStephen]

You might note that most hydraulic cylinders are short enought that they aren't going to buckle elastically. So yes, you may need a column equation, but it won't be Euler's.

Secondly, I suspect most hydraulic cylinders don't count as "thin walled" for buckling. Anyway, if it's pressurized, the cylinder wall isn't in compression.

 

[Kwan]

JStephen is most likely correct on the short column. Also the pressure will restrain the cylinder from crippling. But, due to the two different section properties (cylinder and rod) a specialize column stability method is required. I believe it's called Engesser columns. This will represent the two different section properties. The bending will be present in the cylinder and the rod. Also because the capability of the column is in the plastic range (most likely), you will have to use a tangent modulus and iterate to obtain the finaly allowable.

It can become a little complicated as most real world problems are.

 

[mrMikee]

Another thing that may be a factor are stop tubes that keep the cylinder from extending all the way. I don't have a way to calculate the stop tube length or what this effect is overall, but it could be important.

Just thought I'd mention it.

Regards,
-Mike

 

[lewtam]

Don't forget to consider other modes of failure if you're looking to rate a cylinder.

- Elastic bulging of cylinder causing blow-by.
- Yield of the cylinder wall.
- Clevis rupture.
- Pin shear.

FEA is your safest bet without a test rig.

 

[Kwan]

Lewtam,

FEA is only good if you validate the model. The best way is to do testing which costs a lot of money. A more cost efficient way to analyse the actuator is to use hand analysis. There are many many methods to substantiate the actuator. For example: lug analysis, beam column (there may be beam column effects due to lateral deflection at load), etc. A good number cruncher can finish this analysis in a fraction of the time it takes to complete a valid FEA.

FEA can very easily fool you. I've seen models with results that impart torsion just due to the mesh (a slightly canted mesh), contact that incorrectly transfers loads, non-linear models with way way to much deflection (the adjacent structure would have failed, but not noticed during the model runs), incorrect mesh densities that produce unconservative (and conservative) results. I can go on and on.

 

[lewtam]

Kwan - I have to take you up on that. A pen and paper analysis will only get you so far. Along the way the simplifications and assumptions necessary to reduce the system to calculable levels may not give accurate results. Sure anyone would be able to bang out a max load rating in short order, but at each step an assumption of worst case usually applies. So if this cylinder went into a specialised application in an over-rated state, it would be heavier and more expensive than it needed to be. In most situations that's fine, but I thought someone from an Aerospace background would appreciate the possibility of the need for a more refined touch.

Of course FEA has to be interpreted correctly, but that also goes for the results you get using any of the equations required to analyse the system by hand.

Perfect world - scribble, FEA, Test, repeat. Imperfect world - as many of those steps as you can.





LewTam Inc.
Petrophysicist, Head Stockman, Gun Welder, Gun Shearer, Ski Instructor, Drama Coach.

 

[Kwan]

A pen and paper analysis can be just as precise as a FEA. Also more cost effective. For example, a lug analysis method is based on test data. This provides a refined analysis, based on testing, for many parts. Using FEA for a lug analysis would require a validation for each model. One way to validate this is to use hand analysis (lug analysis) to verify the FEA answer.

I looked up some analysis of landing gear retract actuators for rpmiller. The column stability is analyzed using beam column methods. The column uses two diffent section properties. This method used two pages of analysis. The equivalent FEA would take much more time and effort. Remember, beam column analysis take into account the nonlinear aspect of loaded columns.

For many people, the analysis method of choice is FEA. I feel many people also trust the results without question. I believe this is dangerous (especially for aircraft analysis). Note that aircraft analysis uses FEA all the time (mostly couse grid models for developing loads or fine grid for complex parts). Fortunately the results are validated and correlated.

Rpmiller, hopefully I provided constructive input for you. I apologize for standing on the soapbox; however, I believe FEA is not the answer for your problem. If you are interested I can lay out the beam analysis method for you.

 

[djw2k3]

Usually the limiting buckling load of a standard cylinder occurs in the rod.

Bosch Rexroth have excellent literature on this for selecting a cylinder stroke. You may be able to adapt this to your cylinder.

 

[rpmiller]

Thanks for the offer Kwan but it won't be necessary. In my application, the tensile failure is more serious than a compressive failure.

On that note, thanks to all for their replies.

But back to Kwan's soapbox, I actually enjoyed the FEA discussion on the side. It's given me some real insight about how others think/feel about FEA.

Thanks again.