Vacuum Relief Compression Spring Calculation..spinning my wheels

[Big_Deddie]

I have been tasked with engineering a new vacuum relief valve for one of our products. I was given a vacuum relief valve, of a different design, and was told to match that relief vacuum (which is 15 in-Hg).

The "plunger" in the supplied vacuum relief valve is Ø1 3/8". Using a positive chamber test tool, I determined that it took 9 PSI to allow the relief to kick in. Calculated out (F = P x A), Force = 13.32 lbs.

As you know, all a vacuum relief, or pressure relief, valve has to do is start to open. It does not have to fully open - just has to crack open the seal.

I haven't worked with springs in a while. How would I go about determining the correct spring for this? How would you put a measured distance on "crack open"? My measured/calculated force is only based on moving the spring and not compressing it.

Secondly, how would I relate this information to selecting a spring from somewhere like McMaster-Carr? Their criteria doesn't include K-values and such.

Sorry, I just need some refreshing....

Thanks in advance for the help.

 

[drawoh]

Big_Deddie,

Buy your springs from somebody who publishes specifications for them. Associated Spring Raymond comes to mind. Lee Springs?

Work backwards. You want the system to open at 10lb. You work out the compression required to make the spring exert 10lb. You make your assembly compress it that much. Any force greater than 10lb will compress the spring more, and open the valve.

--
JHG

 

[BrianE22]

McMaster-Carr does publish K values. One consideration for the spring rate should be that the valve/spring doesn't have a natural frequency within the spectrum of the air pressure variations.

 

[btrueblood]

You also want a spring that won't buckle when it is deflected the required amount, so the plunger moves smoothly without binding or tilting. Having a spring with L/d and delta/L values below the "a" line in the attached chart should work.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1510854420/tips/newsharp_poa.com_20171116_104120_iqd2xe.pdf[/url]

 

[Big_Deddie]

@BrianE22

Yeah, I found the spring rate after I dug a little deeper. It would be nice of it was one of the criteria to search by.

After I was able to sleep on this question, I figured out what I needed to know. It took a few formulas to figure out I was going about it the wrong way...but trial and error prevailed