Bolting Calculations in thermal cycling application

Hello,
I have an assembly with 4 bolts secured with nuts. The assembly is two aluminum pieces held together and an o-ring in between. The assembly has heaters inside of it which means the joints will be subject to thermal cycling. I'm looking to do some calculations to determine what size bolt I should use, how to factor in the thermal cycling, and recommending torque value.
So far I have found one formula that determines torque req. based on the size of the bolt and clamping force required.
The concern is the clamping force being diminished after cycling. Some ideas I've had so far is also employing lock washers and high temp loctite.
Phil

Hi Phil and welcome to the forum.

So the temperature must not be going much over 200F since you have an o-ring in there? Guessing even a silicone o-ring wouldn't appreciate getting beyond maybe 240F? So running on that...

Don't know the rest of your scoop far as size of these parts, pressure and resulting required clamping force etc. but given the likelihood of less than a 200 degree change in temperature I think this will be a non-issue so long as the bolt you select allows you to apply full seating torque to the bolts.

You don't want to pick a bolt that can't be tightened to the recommended seating torque (derived from it's material, size, and pitch,) without crushing your aluminum pieces. As long as you have enough clamping force and some stretch to your bolts it's not going to lose grip from thermal cycles of approx. 200F. If you go over size with your bolts and can't put a little stretch (seating torque) to your bolts then yes, they would work loose.

Look at aluminum heads on car engines... same thing only different.

Given that you're pulling your nut up on aluminum I'd personally steer clear of a conventional lock washer. Maybe look at a flat washer to protect the aluminum from galling at tightening and use a lock nut.

In order to assess your problem you must provide :

sketch of the assembly
temperature variation of the assembly
clamping requirements
environmental input like vibrations or shock to unit.

Hi Phil,

Some more information would be helpful. The temp spread between hottest and coldest. The thickness of the flange material. Grade of aluminum. Grade of bolt. Expansion and contraction has a linear-coefficient which means a thin plate will expand fewer thousandths of an inch than a thick one. There may not be enough expansion/contraction to worry about.

Threadlockers generally are not waterproof, so I wold be considering a mechanical cone-locking nut, or a nut and lock nut pair. Depending on vibration or movement a lock washer may worry it's way into the aluminum and loosen over time, so I would not be a fan of that approach.

Many more questions than answers.

:D

Great minds all thinking alike and at the same time too, it seems. :D

There were no replies when I clicked "Reply" and then three after I clicked "Submit."

[QUOTE=PHIL_RIVALDO;2300]Hello,
I have an assembly with 4 bolts secured with nuts. The assembly is two aluminum pieces held together and an o-ring in between. The assembly has heaters inside of it which means the joints will be subject to thermal cycling. I'm looking to do some calculations to determine what size bolt I should use, how to factor in the thermal cycling, and recommending torque value.
So far I have found one formula that determines torque req. based on the size of the bolt and clamping force required.
[COLOR=#ff8c00]The concern is the clamping force being diminished after cycling.[/COLOR] Some ideas I've had so far is also employing lock washers and high temp loctite.
Phil[/QUOTE]


You will be doing the nut and bolts up while its cold probably..........
so heating may increase the tension slightly and then return to the original tension when cold

Hello,

I've included this sketch to give an idea. I don't know the requirement yet for my clamping force. The temp cycle will be from 60 to 200F. My flange thickness is .25". My bolts are steel with zinc plating, M6 cap with socket heads. My materials are AL 6061-T6 and AL AA356.0F.

Thanks.

I assume the "o-ring" is a molded square thing?

Not knowing the operating pressure, I would be concerned about the difference in distance apart of the bolts. With pressurized sealing systems it is a good idea have all bolts at the same center-distance from each other or close too. I would worry about the1/4" plate bulging. With evenly spaced bolts the sealing plate gets evenly applied pressure. If that were my project I would be using six bolts.

As you know since you are applying the force on four joints all mechanical calculations are proximate and the force can not be very well propagated. Aluminum bolts are usually alloy aluminum to provide both strength and thermal resistance.

Depending upon the amount of external loading, placing a Belleville washer under each nut can be a good solution to maintaining a needed fastener tension without excessive installation torques.