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Actuator Piston Thickness Calculation

11/04/2016 1:01 AM

Hi Everyone, Greetings!!!

How do we find out the required thickness of the Piston for pneumatic actuator? what are all the factors to be considered for the same?

we have cylinder area, actuator force, spring force, piston rod diameter, piston diameter and required dimension. we have considered the piston as a circular plate which carrying the uniform load on entire plate. the circular flat plate shall be considered as outer edge guided and inner fixed.

Which method will be preferable for finding the thickness of the piston? please assist me for this....

Thanks in Advance.

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Guru

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#1

Re: Actuator Piston Thickness Calculation

11/04/2016 1:13 AM

Determine the force needed...

https://en.wikipedia.org/wiki/Pneumatic_cylinder#Pressure.2C_radius.2C_area_and_force_relationships

Pick suitable material for piston...calculate strength of material, add safety margin, determine thickness required....

http://www.thefabricator.com/article/metalsmaterials/the-differences-between-stiffness-and-strength-in-metal

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#2

Re: Actuator Piston Thickness Calculation

11/04/2016 1:42 AM
  • Thickness, t = (P x Di )/(2σE -0.2P)

Where,

  • P = design pressure
  • Di = inside diameter
  • σ = allowable design stress, for stainless steel=1675 kg/cm²
  • E = weld joint factor = 0.85
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#3
In reply to #2

Re: Actuator Piston Thickness Calculation

11/04/2016 1:52 AM

Hi,

this is for THICKNESS OF SHELLS UNDER INTERNAL PRESSURE. i think, this formula is going to used for the cylinder/Vessels based on ASME Sec 8. apologize me please if i am wrong.

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#4
In reply to #3

Re: Actuator Piston Thickness Calculation

11/04/2016 2:05 AM

You are correct, but look for the formula for circular plate subject to differential pressure. (I don't have a Code copy.)

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#7
In reply to #2

Re: Actuator Piston Thickness Calculation

11/04/2016 4:24 AM

Hi,

If we go with ASME wall think formula, we found very minimum thickness.

How can we assure the load factors in this formula. can you please assist me for finding solution for this?

Thickness, t = (P x Di )/(2σE -0.2P)
Where,
P = design pressure
Di = inside diameter
σ = allowable design stress
E = weld joint factor = 0.85
Diameter, D:28.5in
Design Pressure, P:145psi
Allowable Stress, σ:20000psi
Thickness, t:0.121648in
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#5

Re: Actuator Piston Thickness Calculation

11/04/2016 3:45 AM

You are right.

If you find Thickness Calculation for a cylinder formula, the equation is almost same.

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#6

Re: Actuator Piston Thickness Calculation

11/04/2016 4:03 AM

hi,

how can we ensure that the load will be acted uniformly throughout the circular flat plate while go with shell thickness formula from ASME?

Thanks

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#8

Re: Actuator Piston Thickness Calculation

11/04/2016 4:24 AM

http://www.slideshare.net/yeswanthkumarreddy1/design-of-engine-cylinders-and-pistons

The cylinder formula is more simple.

HAVE A NICE DAY~~

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#9
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Re: Actuator Piston Thickness Calculation

11/04/2016 4:35 AM

Hi,

need to find out the piston thickness.....

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Guru

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#10
In reply to #9

Re: Actuator Piston Thickness Calculation

11/04/2016 10:55 AM

I think you already calculated your answer above. I see no need in doing further calculations.

Make several of a range of thicknesses leading from 0.5t to 1.5 t, and test them in the physical device. See which one fails when, and then add a safety margin to that.

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Guru

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#11
In reply to #9

Re: Actuator Piston Thickness Calculation

11/04/2016 11:24 AM

You need to flesh out your design a little more....

http://allenair.com/cylinders/cylinders/types-a-c-e-single-ended/

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#12

Re: Actuator Piston Thickness Calculation

11/04/2016 10:59 PM

The first rule of system or component design always holds true: What is the load. Be it pneumatic or hydraulic, everything centers around the load. The cylinder piston will see that load. You can determine thickness right out of a metal suppliers catalog of round stock predicated on thickness of the finished product. Is the piston material steel or aluminum, or perhaps composite? If you want to play it safe, multiply the load times three, and look at the charts on the material selected and see where the numbers are as it applies to piston deflection within the tube occurs. (at what load force). What type of seals do you want to use, Is stiction an issue? Is heat an issue? Is cold an issue? Is lubricity a concern? You can run yourself onto an endless treadmill, but common sense rules the day.

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#13

Re: Actuator Piston Thickness Calculation

11/05/2016 11:32 AM

I don't think your design basis ...."we have considered the piston as a circular plate which carrying the uniform load on entire plate. the circular flat plate shall be considered as outer edge guided and inner fixed" is correct. Inner fixed is OK, but the outer edge is not guided, it is free.

I have an old version of Roark, Issue 5 I believe, which gives formulas for stress and deflection. Table X, flat plate, constant thickness. Cases 11, 16 and 21 give same figure for deflection in the special case where inside radius b = 0. Max deflection ymax (at edge) from

where a = outside radius, w = pressure, E = elastic modulus, t = thickness, m = reciprocal of Poisson's ratio. Need to get the units right!

A bit more complicated for stress as it doesn't reduce to a simple formula when putting b = 0. Your case might be limited by stress or deflection. You wouldn't want too much deflection or the edge of the piston might interfere with the cylinder wall.

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#14
In reply to #13

Re: Actuator Piston Thickness Calculation

11/09/2016 12:05 AM

hi, Thanks...

The outer edge is guided in the cylinder wall.. we are also trying with roark's formulae. but the the required output thickness is more than 20in if i used roark's. anyway i will try to find through your guidance. let me see...

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#15
In reply to #14

Re: Actuator Piston Thickness Calculation

11/09/2016 4:35 AM

I think it's optimistic to expect the contact with the wall to guide the piston edge ie keep it parallel to the wall. Usually the piston itself does not make wall contact, only the seals.

20 inch thick seems very high, I did a check assuming steel 3" thick and got moderate deflection (if I remember right, I haven't turned the computer on yet today). If you give me an idea of material and allowable stress I'll try to estimate something for you. But I'm no expert on moving machinery, my guess is you need a generous safety factor for this.

BTW, where is the spring you mentioned in original post?

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#16
In reply to #15

Re: Actuator Piston Thickness Calculation

11/09/2016 4:49 AM

Thank You very much...

Material shall be carbon steel and the allowable stress for the CS is 20000psi.

Spring will be placed either top or bottom depends fail action of the actuator.

FYI, we too assumed wall thick as 3" and proceed further calculation for actuator design. meanwhile we have to find the exact needed thickness for the piston.

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#17
In reply to #16

Re: Actuator Piston Thickness Calculation

11/09/2016 10:18 AM

The Roark cases I mentioned all have outer edge free. Outside radius a.

Case 11 is solid plate, pressure over whole area, balanced by force over central area radius b.

Case 16 is annulus, pressure over actual area, pi*(a2 - b2). Inner edge simply supported.

Case 21 is as 16, but inner edge fixed and supported.

I would expect them all to be the same for special case when b = 0, and for deflection they are, but not for stress. Let's ignore Case 16, which with inner edge supported is more onerous than your case.

Using thickness 3" I get deflection 0.2mm.

Using piston rod dia 80mm, so b = 40mm, stress comes to

Case 11 - 14000psi

Case 21 - 17250psi

Not sure why the difference, but that looks like a reasonable starting point. You might need to put some thought into how the piston is connected to the rod, specially if the pressure can be applied in either direction.

28" dia is a big pneumatic actuator!

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#18
In reply to #16

Re: Actuator Piston Thickness Calculation

12/10/2016 1:59 AM

For the plate dimensions look at the book Timoschenko Shells end plates and you find all what you need.

Remark on the outer diameter is totally correct! It is not guided and further more the gap should consider the angle made by the plate at its outer diameter when under maximal load. If you do not do this the risk is to get the piston ream in contact with the cylinder, harm it and destroy your seals.

In general on the outer side there is a guide ring made of a low friction compound but its very low stiffness ( Young modulus is 1/100 of steel or even less) it can NOT be considered as an sliding guide with no angular deformation of plate its role is only in case of side loads on the piston and angle in the piston rod guide a metal to metal contact.

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