Results 1 to 10 of 10

Thread: Bolting Calculations in thermal cycling application

  1. #1
    Associate Engineer
    Join Date
    Mar 2012
    Posts
    2

    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

  2. #2
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Location
    Rochester Pennsylvania
    Posts
    396
    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.

  3. #3
    Principle Engineer
    Join Date
    Mar 2011
    Posts
    175
    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.

  4. #4
    Technical Fellow
    Join Date
    Feb 2011
    Posts
    1,043
    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.

  5. #5
    Technical Fellow
    Join Date
    Feb 2011
    Posts
    1,043


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

    There were no replies when I clicked "Reply" and then three after I clicked "Submit."
    Last edited by PinkertonD; 03-01-2012 at 01:45 PM.

  6. #6
    Android
    Guest
    Quote Originally Posted by PHIL_RIVALDO View Post
    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

    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
    Last edited by Android; 03-01-2012 at 04:19 PM.

  7. #7
    Associate Engineer
    Join Date
    Mar 2012
    Posts
    2
    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.
    Attached Files Attached Files

  8. #8
    Technical Fellow
    Join Date
    Feb 2011
    Posts
    1,043
    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.

  9. #9
    Associate Engineer
    Join Date
    Feb 2017
    Posts
    1
    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.

  10. #10
    Lead Engineer
    Join Date
    Aug 2013
    Location
    Houston TX USA
    Posts
    366
    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •