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# Flat end caps on a hydraulic cylinder2

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## Flat end caps on a hydraulic cylinder

(OP)
Hi,
I am designing a hydraulic cylinder that will have a flat end cap and need to figure out the thickness it will need to be.  I was wondering if anyone could provide me with a formula?  The ID=6" and will be under a pressure of 3000psi.  TIA.

### RE: Flat end caps on a hydraulic cylinder

The formula is in the ASME Boiler Code. The old formula was t = d x (K p / s)^0.5
d bolt PCD inch
p pressure psi
s unit stress psi
K 0.162 for plates bolted rigidly to flanges.

That should give you a start.
You should check the Code for the latest data and qualifiers.

### RE: Flat end caps on a hydraulic cylinder

(OP)
What would K be if the plate was welded and not bolted.

### RE: Flat end caps on a hydraulic cylinder

K as given by johnp was anyway incorrect.
For a welded cap use the formula of johnp taking:
d=inner pipe diameter (cap should be inserted into the cylinder and the weld done on the extra cylinder length projecting beyond the cap, otherwise take the pipe outer diameter)
K=0.33
s=15000 psi (typical for low carbon steel)
If the cap may be welded also on inner side, the thickness could be slightly reduced (down to using K=0.2 if the cylinder wall is thicker than the minimum required) but this doesn't seem very important for such a small cap.
Of course you will require a heavy weld, with an effective depth equal to cap thickness.
Note that for your conditions (high pressure, small diameter) the code is quite conservative, especially for the weld.

prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design

### RE: Flat end caps on a hydraulic cylinder

(OP)
Thanks for all the information.  I do have one more question though.  Does the s relate to the factor of safety and the yield strength?  I guess I would like to know how you figure out s or where does it come from.  Thanks again for all the help.

### RE: Flat end caps on a hydraulic cylinder

The allowable stress per ASME VIII Div.1 is quite conservative, especially at room temperature.
As you don't want to use the code you might take 2/3 of the yield strength (except for very high strength steels).
For the weld you can calculate it in shear using an allowable shear stress equal to 70% of the allowable in tension.
All this is for building a prototype and testing it. Note that if you are designing an equipment that shall go to the market, you should hire someone to do a professional calculation.

prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design

### RE: Flat end caps on a hydraulic cylinder

I once got caught out with flat plate design.  I spent a lot of time studying the basic formula looking for escape.  However the case was settled out of court, much to my relief.  All the work I did I put on my website at www.gowelding.com in the section on welding calculations.  Its mind-blowing maths.  The thing to note is the maximum bending is at the edge of the plate and if the plate is welded it could bend the edge of the pipe.  BS5500 has a very complex formula for determining the stress at this point but ASME just uses spare thickness in the pipe.

What I don’t understand is why the calculated stress can’t go up to 1.5 x design stress as the stress comes from bending? But no code that I know will allow this.

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