Finding strains and stresses on a hydraulic cylinder - NO external pressure?

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Discussion Overview

The discussion revolves around calculating strains and stresses in a hydraulic cylinder under internal pressure without any external pressure applied. Participants explore theoretical approaches and equations relevant to thick-walled cylindrical pressure vessels.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant describes a test simulation involving a hydraulic cylinder with specific dimensions and internal pressure, expressing uncertainty about the accuracy of strain gauge readings.
  • Another participant identifies the cylinder as a thick-walled cylindrical pressure vessel.
  • A participant confirms the material properties, noting the Young's modulus for standard steel as 200GPa.
  • One participant asserts that external pressure is not necessary for applying Lame's equations to the problem.
  • Another suggests using thick-walled pressure vessel equations to determine stresses and strains.
  • A participant questions the applicability of Lame's equations, recalling a misunderstanding about the need for external pressure.
  • One participant recommends consulting engineering resources for the relevant equations.
  • Another emphasizes the importance of knowing the boundary conditions, specifically stating that external pressure is zero and internal pressure is 60bar.

Areas of Agreement / Disagreement

Participants generally agree on the classification of the cylinder as a thick-walled pressure vessel and the parameters involved. However, there is some disagreement regarding the necessity of external pressure for Lame's equations, indicating that the discussion remains unresolved on this point.

Contextual Notes

Participants express uncertainty about the application of Lame's equations and the conditions under which they can be used, highlighting potential gaps in understanding or assumptions regarding the equations.

jrhickin
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I'm running a test simulation on a Festo rig, with a hydraulic cylinder measuring 45mm internal diameter and 54mm external diameter. The testing requires an internal pressure of 60Bar with no external pressure applied. The strain gauge and setup I'm running it on I suspect to be faulty, and I can't get any sensible principle stresses and strain answers out of it. Is there any way of working it out by hand? I want to see if it's the gauge that's at fault, the system, or if it just happens to be a strange figure.
Thinking back to my lectures, I recall Lane's equation, but as far as I know you need an external pressure to make that work. Is this the one to use? I haven't done it in ages, so any hints on how to work it out would be useful.
 
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This is just a thick walled cylindrical pressure vessel isn't it?
 
Yes, it is. It's standard steel, 200GPa for the Young's modulus if that's any help!
 
You don't need an external pressure to make the Lame equations work.
 
Then just use thick walled pressure vessel equations to find the stresses and strains.
 
You don't? I thought that was what a or b was in it?
The equation I have for Lame is a + b/r^2. How do you work it out? It's been a long while since I've had to use it, mind!
 
Check out engineering tool box (google it), they'll have the equtions.

They are also in Shigley - Mechanical Engineering Design if you have that book.
 
You should know the boundary conditions. External pressure = 0 and internal pressure = 60bar.
 

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