I am currently working on a valve design. The valve reqires a adjuster which is used to hold the spring in the chamber. The adjuster is tightened to increase the load on the spring so the pressure at which the valve open can be altered. However the adjuster is cross shaped with a hole at the center of it. I am trying to find the stress acting at the corners of the cross. However from the stress calculation I have got the maximum shear stress are well with the maximum shear stress of the material but I was testing the adjuster, it tendes to crack on the corner under a pressure of 50 bar. Is there any tip on how to claculate the stress so I can see where I am going wrong
Eng-Tips is the largest forum for Engineering Professionals on the Internet.
Members share and learn making Eng-Tips Forums the best source of engineering information on the Internet!
-
Congratulations dmapguru on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Calculating stress on a adjuster for a valve 1
- Thread starter Rishy007
- Start date
-
1
- #2
- Thread starter
- #3
I have attached a copy of the assembly of the valve. The 50 bar pressure enters at the bottom of the valve which forces the plunger to move up and compress the spring. This force then is translated on to the adjuster. The adjuster is screwed into the valve and a cap on top of it but the cap is not incontact with the adjuster.
- Thread starter
- #4
hi Rishy007
Okay I see the situation, so when the valve is closed the only force on the adjuster is exerted by the spring.
When the valve is opened you have the combined force of the spring and that due to the pressure.
In practice you probably have some bending on the adjuster and whether Roarks formula for stress and strain will cover your case I am not sure.
Firstly I would do your shear calculation based on the spring force and that due to the fluid pressure of 50 Bar.
Which side of the adjuster is cracking? my guess would be the farside of the adjuster ie the face without the spring.
the calculation will be conserative as the full 50bar pressure won't act on the adjuster because the fluid is now flowing.
desertfox
Okay I see the situation, so when the valve is closed the only force on the adjuster is exerted by the spring.
When the valve is opened you have the combined force of the spring and that due to the pressure.
In practice you probably have some bending on the adjuster and whether Roarks formula for stress and strain will cover your case I am not sure.
Firstly I would do your shear calculation based on the spring force and that due to the fluid pressure of 50 Bar.
Which side of the adjuster is cracking? my guess would be the farside of the adjuster ie the face without the spring.
the calculation will be conserative as the full 50bar pressure won't act on the adjuster because the fluid is now flowing.
desertfox
- Thread starter
- #6
I have modeled the adjuster as a beam with a hole in the middle and used the formula Sm=k(P/t^2)
where k= outside diameter of the spring/inside diameter of the spring
I am getting shear stress values of 255.635 N/mm^2 which seems right but I m not sure that this equation can be used and I dont think that this anywhere near accurate.
where k= outside diameter of the spring/inside diameter of the spring
I am getting shear stress values of 255.635 N/mm^2 which seems right but I m not sure that this equation can be used and I dont think that this anywhere near accurate.
Hi Rishy007
I don't recognise the equation your using were did you get it from? what is P and t in the equation?
Without knowing the ultimate tensile stress of the brass your using I cannot comment on the shear stress figure for sure but it looks very high to me.
I would increase the thickness of the adjuster.
desertfox
I don't recognise the equation your using were did you get it from? what is P and t in the equation?
Without knowing the ultimate tensile stress of the brass your using I cannot comment on the shear stress figure for sure but it looks very high to me.
I would increase the thickness of the adjuster.
desertfox
- Thread starter
- #8
hi
I got the equation from Mark's mechanical engineering handbook for flat plates. P is the load applied to the plate but I think this equation only applies if we model it as a flat plate with a hole in it. The proof stress of the material is 250N/mm^2 but the maximum shear stress is 425N/mm^2.
I got the equation from Mark's mechanical engineering handbook for flat plates. P is the load applied to the plate but I think this equation only applies if we model it as a flat plate with a hole in it. The proof stress of the material is 250N/mm^2 but the maximum shear stress is 425N/mm^2.
Hi
I think you might be mistaken with your shear stress figure
look at this site:-
Your way over stressing the brass I think.
By the way which side are you seeing the cracks on in the adjuster.
I haven't got a copy of Marks what is the formula finding if its for flat plates ie whats Sm the shear stress?
desertfox
I think you might be mistaken with your shear stress figure
look at this site:-
Your way over stressing the brass I think.
By the way which side are you seeing the cracks on in the adjuster.
I haven't got a copy of Marks what is the formula finding if its for flat plates ie whats Sm the shear stress?
desertfox
- Thread starter
- #10
Hi
Yes I also think that the shear stress is high but when I was testing the adjuster made from brass bar cz121 pb3 there was no cracks but it did have a very small deflection init. But we have recived some adjusters made in china, which they say is made from the same material but seems to be cracking. The crack starts at the corners at the top of the adjuster(not the spring resting side) and then it starts to expand down through the corners.
Yes I also think that the shear stress is high but when I was testing the adjuster made from brass bar cz121 pb3 there was no cracks but it did have a very small deflection init. But we have recived some adjusters made in china, which they say is made from the same material but seems to be cracking. The crack starts at the corners at the top of the adjuster(not the spring resting side) and then it starts to expand down through the corners.
Hi Rishy007
Well from what you now describe that you have a small deflection in the adjuster that suggests the material is reaching the yield stress value and hence permanent deformation.
As stated earlier there is probably some bending involved in the failure also the corners of the adjuster probably have a stress concentration factor there to because of the change in section.
The site I gave you shows th CZ121 ultimate tensile figure and if we to say 70% of that, then that would be the maximum theoretical shear stress of the brass that works out at about 287N/mm^2.
The drawing you uploaded shows a material grade of CZ121 Pb4 but now you mention Pb3 is there any difference in the material properties?
Finally can you upload the page from the Marks book with the formula on that your using.
desertfox
Well from what you now describe that you have a small deflection in the adjuster that suggests the material is reaching the yield stress value and hence permanent deformation.
As stated earlier there is probably some bending involved in the failure also the corners of the adjuster probably have a stress concentration factor there to because of the change in section.
The site I gave you shows th CZ121 ultimate tensile figure and if we to say 70% of that, then that would be the maximum theoretical shear stress of the brass that works out at about 287N/mm^2.
The drawing you uploaded shows a material grade of CZ121 Pb4 but now you mention Pb3 is there any difference in the material properties?
Finally can you upload the page from the Marks book with the formula on that your using.
desertfox
Hi
Check this link brass Pb3 as a higher tensile stress then Pb4
Your drawing calls for CZ121 Pb4 which is not quite as good on its tensile strength as Pb3.
Should of mentioned before anything from China you should check out throughly because they say its the same material doesn't mean it is, I would have a lab check the material.
desertfox
Check this link brass Pb3 as a higher tensile stress then Pb4
Your drawing calls for CZ121 Pb4 which is not quite as good on its tensile strength as Pb3.
Should of mentioned before anything from China you should check out throughly because they say its the same material doesn't mean it is, I would have a lab check the material.
desertfox
- Thread starter
- #13
- Thread starter
- #14
"I have modeled the adjuster as a beam with a hole in the middle and used the formula Sm=k(P/t^2)" ... this confuses me (not difficult sometimes).
have you made an FE model of the part and you're using this stress equation to check it? or
have you "modelled" the part as a plate and you're applying the stress equation as the solution to your problem, and you're wondering if this is valid.
if the latter, the part is pretty complex in shape, i'd be surprised if a plate stress equation can capture the detail stresses with any accuracy.
have you made an FE model of the part and you're using this stress equation to check it? or
have you "modelled" the part as a plate and you're applying the stress equation as the solution to your problem, and you're wondering if this is valid.
if the latter, the part is pretty complex in shape, i'd be surprised if a plate stress equation can capture the detail stresses with any accuracy.
- Thread starter
- #16
I am planing to model this as FE but I have not got the software yet. Right now I am trying to calculate the stress on the adjuster and find out if it will be able to withstand the forces acting on it. I am not sure what equation to applt to this inorder to find an accurate stress value
Hi Rishy007
The formula you are using are for flat plates in bending and the stress figure you quoted earlier of 255 N/mm^2 is not a shear stress if you calculated it from that formula, it would be a tensile stress due to bending and it would exceed the proof stress you quoted for the material in a later post.
Also it is not really the correct formula to use as your adjuster is cross shaped and is not a complete circular plate.
What I would do in your position is make the adjuster as thick as possible and test it so that I know it is okay for the application or failing that you would need an finite element stress analysis on the adjuster to confirm it is okay.
The formula you are using are for flat plates in bending and the stress figure you quoted earlier of 255 N/mm^2 is not a shear stress if you calculated it from that formula, it would be a tensile stress due to bending and it would exceed the proof stress you quoted for the material in a later post.
Also it is not really the correct formula to use as your adjuster is cross shaped and is not a complete circular plate.
What I would do in your position is make the adjuster as thick as possible and test it so that I know it is okay for the application or failing that you would need an finite element stress analysis on the adjuster to confirm it is okay.
does the part have to be brass ?
i'd worry about the intersection of the arms.
points to consider ...
if you build the assembly without an FEA, plan an "exit strategy" in case things don't go as hoped.
how does plasticity affect the part ? if it yields will this open up more area, venting pressure ? if you vent pressure, how does this affect the performance of the system ??
is the part exposed to fatigue loads ? how close are the fatigue load cycles to your static load ?
i'd worry about the intersection of the arms.
points to consider ...
if you build the assembly without an FEA, plan an "exit strategy" in case things don't go as hoped.
how does plasticity affect the part ? if it yields will this open up more area, venting pressure ? if you vent pressure, how does this affect the performance of the system ??
is the part exposed to fatigue loads ? how close are the fatigue load cycles to your static load ?
- Status
Similar threads
- Locked
- Question
- Question
- Question
- Locked
- Question