Ft-lbs to psi on a plate

[arokisbobcat]

I have a question regarding a calculation for finding the fastener torque required (in ft-lbs) to seal a gasket between two plates. More specifically, I am using 18 10-24 threaded bolts to hold two 1/2" aluminum plates together with a viton gasket in between. The diameter of the plates is 8 in DIA.

Based on gasket specs from the manufacturer, I would need about 800 psi to properly seal the gasket between these two plates. How can I translate the torque needed on each bolt to achieve an 800 psi clamping force on the gasket between the two plates.

Any suggestions would be greatly appreciated

Thanks,

Matt

 

[KENAT]

There have been a number of threads about bolt preload, you may be able to find one that gives a more complete or directly applicable solution than this FAQ but for starters:

faq725-536

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[rb1957]

and what area to use ? the gasket cross-section (=pi*D*t) ?

Quando Omni Flunkus Moritati

 

[Dougt115]

Torque = K * Diameter * force
K is a coefficient based on material, surface finish and lubrication. Often assumed to be 0.2-0.3
Diameter of the bolt or threaded object being turned
Force is the axial resultant force from the bolt
Torque is set on the torque wrench.

Convert your 800 psi to the force needed, divide by the area of a plate.
Divide the force found by the number of bolts.
Multiple the force by the diameter of the bolt time the 0.2. F*D*0.2 = Torque
Set your torque wrench.

 

[drawoh]

arokisbobcat,

How thick is your gasket?

Is it thick enough that you can measure the compression? Bolt torques do not provide a precise estimate of clamping force.

--
JHG

 

[dhengr]

Arokisbobcat:
I agree with Drawoh, torque alone is not a reliable means measuring the bolt forces. Furthermore, the bolt spacing and/or the thicknesses of the plates must be considered so that the plates don’t flex too much as the bolts are tightened, or the gasket won’t be compressed sufficiently btwn. the bolts.

 

[sreid]

Consider using an O-Ring instead of the gasket.

 

[racookpe1978]

"Two plates" .... And what does that mean?

See, a "standard" flange-to-flange gasket "connection" REQUIRES two standard flanges. The fastener tables, gasket tables for each diameter pipe and each type of gasket material and flange surface (flat-face, raised face, joint, etc) differ from each other BECAUSE the gasket designers know the sizes and characteristics of the two standard flanges.

So, here you have two 8 inch diameter "somethings" of an unknown material (aluminum of what alloy?) opposing an unknown force distributed over an unknown area between two connecting flanges that compress the gasket between two surfaces of unknown area using an completely unknown bolt pattern and bolt diameter. (18 bolts around a plate only 8 inches in diameter? What kind of material will nbr 10 (3/16 diameter) bolts will you use and what hole size do you think you are using?)

A minor point compared to the other above, but how is the pressurized liquid (or gas) getting in between the two flanges? Is there an central hole or inlet to the two Al flanges?

Can't figure out the result with that little information.

 

[arokisbobcat]

Thanks for all the input.

Sreid - O-rings are not an option because the plates contain a complex pattern with grooves that need to be separated.

Some additional information - the Aluminum plates are 6061. The Viton gasket is 1/16 thick. 12 of the 10-24 bolts are equally spaced on a 7 in. diameter and the other 6 10-24 bolts are equally spaced on a 4 in. diameter. The bolts used are stainless steel socket head cap screws. The two Aluminum plates have flat, machined surfaces and without the gasket sit flush with each other. The surface area of the gasket is 22 in^2.

Thanks for all the input and I will continue to look at some of the previous threads about preload and see if I can figure it out.

 

[Duwe6]

For a valid value of bolt tension vs. torque, you have to "Ask the Bolt and lube". Period*.

The coefficient of sliding friction on any fastener is so variable, that the range is too large to approximate in any useful calc. Using your specific lot of bolts, and using the assembly lube that be using; crank up the proposed torque with that bolt & lube in a Skidmore-Wilhelm device. It will give you the actual clamping force being developed.

Now the calc's are simple: (quantity of bolts x clamp force)/ area of gasket.

* been discussed many times in these fora; clamping force vs. torque, or 'Bolt Torque'. Look it up. Several good discussions.

 

[geesamand]

Don't forget that the load on the screws is not only the compression of the gasket, but also the compression of any pressure being retained by the cover plate.

 

[Tmoose]

the gasket compression specs I've seen are minimums.
I would not be trying to score a bullseye (hit a precises clamping value) , but a field goal ( working to exceed the minimum clamping value ).

 

[BrianE22]

Usually the preload is set by having a controlled gap in one or both of the pieces. The bolts then tighten the two pieces down to each tight to each other. That way, gasket preload is not controlled by the bolt force.

If you use the bolts to preload the gasket you run the risk of the bolts loosening over time. You also will have an extrusion gap for the seal.