Suspension Brakes Torque Setting...

Discussion in 'Tuning and Performance' started by Metalman, Mar 31, 2012.

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  1. Metalman

    Metalman Well-Known Member
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    #1 Metalman, Mar 31, 2012
    Last edited: Mar 31, 2012
    Right now I'm changing the springs on the rear suspension.
    The rear lower strut to trailing arm calls for 122 ft-lb (dry). This is the fitting that tends to corrode and can strip out the soft aluminum threads over time.
    I've coated the bolt with anti-seize. What is the rule of thumb for reducing the torque setting for a lubricated bolt? 10% less?

    It is a graphite, not moly based product.
     
  2. Crashton

    Crashton Club Coordinator

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    I don't know for sure Chuck, but 10% sounds like a good guess to me. Anti-seize is a very good idea.
     
  3. BRG_Paul

    BRG_Paul Active Member

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    #3 BRG_Paul, Mar 31, 2012
    Last edited: Mar 31, 2012
    Torque-tension equation for fasteners

    T=KDF
    Where:
    T=torque (inch-lbs)
    D=diameter of screw (inches)
    F=force (lbs) "tension in screw"
    K=Friction factor (not the coefficient of friction)

    The friction factor for dry screws is ~ .15 to .17. The factor for oiled screws is about .12 to .13. This results in a torque reduction of between 13% to 29% for oiled screws relative to a dry torque spec. If you only reduce the torque by 10% you risk stripping the threads or yielding the screw.

    Never Seize can reduce the K factor even further.
     
  4. Justa Jim

    Justa Jim Well-Known Member
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    Well, now you've gone and done it. You don't know what the torque should be and I'll bet you're torqued about it. :lol:

    Jim
     
  5. BRG_Paul

    BRG_Paul Active Member

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    If you are using graphite based thread lube then you should reduce the 122 ft-lb torque to no more than about 90 ft-lbs.
     
  6. Metalman

    Metalman Well-Known Member
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    I was holding off until I heard back from the M/A brain pool.
    I went with a 25% reduction on my torque setting.
    Molybdenum disulfide has a K=.11
    Graphite has a K=.16

    No stripped aluminum threads...

    Thanks Paul....:D
     
  7. Metalman

    Metalman Well-Known Member
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    Haha, I just read your answer.... Yup, I did the math correctly. Thanks again Paul...:D

    And Jim..... Your horses need feedin....pittouii....
     
  8. k-huevo

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    #8 k-huevo, Mar 31, 2012
    Last edited: Mar 31, 2012
    I don't think the reason this particular fastener is blamed for stripped threads on the trailing arm, is due to corrosion. It is a tapered self tapping bolt, and if it is not carefully threaded in, it can cut new grooves over the originals. I would suspect corroded bolt threads could gall on their way out and back in again, but once again I think attention to feel could minimize the affect.

    I've yet to find definitive guidance on lubrication with self tapping bolts and their torque values, but self tapping bolts are in a class of their own, just like applying lubrication alters torque specs. I primarily rely on Carroll Smith's book for answers, but he hates self tapping bolts & screws, and says little about them. What Mr. Smith says in general, it is better to over-torque than under-torque (keep in mind he builds race cars). So, I clean the threads, light lube, thread carefully, and torque to 122 ft-lb.
     
  9. Metalman

    Metalman Well-Known Member
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    Keith, mine had absolutely no corrosion. I know it's not a particular good idea to have dissimilar metals in contact with each other and water, and especially with the salt they throw on the roads here in Ohio, as this can start galvanic action. The graphite based anti seize was an attempt to prevent moisture wicking up into this particular fastener and aluminum connection. I was very careful and finger threaded the bolt into place and it was more than half way in before I put the wrench to it.
    I am particularly interested in the 122 ft lb setting. You don't feel it's necessary to adjust for the lubrication factor? And is it because it's a tapered bolt (by the way I didn't notice it being tapered but the threads seems very shallow to me), that precludes using a lower torque value?
     
  10. BRG_Paul

    BRG_Paul Active Member

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    If anyone is interested, when I get back to work on Monday and can look over my reference books I'll publish the K factors for a lot of different lubes, greases and Loctite. (yea that changes the K too).
     
  11. Metalman

    Metalman Well-Known Member
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    Good idea Paul....:D
     
  12. k-huevo

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    I use lube, so the self tapping threads will "find the path of least Resistance" on their way into the aluminum trailing arm, I torque to original spec, so the bolt will fasten securely for sure. The lower strut bolt is subjected to great loads, I want the insert to handle the shear forces, to do that it can't be allowed to move. Carroll Smith's Nuts, Bolts, Fasteners, and Plumbing, handbook has numerous reports of failures, avoiding failure is my core reasoning.
     
  13. Metalman

    Metalman Well-Known Member
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    My MINI is sitting in the shop overnight, so I didn't drive it home tonight. Torquing to 122 ft lbs with a graphite base anti-seize shouldn't strip the soft aluminum threads?
     
  14. k-huevo

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  15. Metalman

    Metalman Well-Known Member
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    Thanks Keith, I'll be setting it to 122 tomorrow.
     
  16. grodenglaive

    grodenglaive New Member

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    #16 grodenglaive, Apr 1, 2012
    Last edited: Apr 1, 2012
    Interesting thread.
    I like the difference of opinion between the 2 schools: engineers and mechanics. The math of the k-values appeals to my sense of logic, but I'm also afraid to lower the torque by as much as it implies (20%). On the other hand, it's hard to argue against real world experience: I expect Keith has had those bolts in and out many times and would know if 122 ftlbs caused stripping.

    btw I managed to strip one of mine last year (there's a thread about it here somewhere). It was the third time reinstalling the shock. I was using anti-seize and and the torque wrench was set for 122. I can't say for sure I didn't cross thread it though - I forgot it was aluminum and I wasn't being super-careful. But anyway ymmv as they say.
     
  17. minimark

    minimark Well-Known Member

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    I consider Carroll Smith's Nuts and Bolts along with all his books a must read.....even more so for us DIY's.... His logic has not failed me yet. 2cents
     
  18. Nathan

    Nathan Founder

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    For those interested...

    [ame="http://www.amazon.com/Fasteners-Plumbing-Handbook-Motorbooks-Workshop/dp/0879384069"]Amazon.com: Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook (Motorbooks Workshop) (9780879384067): Carroll Smith: [email protected]@[email protected]@http://ecx.images-amazon.com/images/I/[email protected]@[email protected]@51wiaKeZAXL[/ame]
     
  19. BRG_Paul

    BRG_Paul Active Member

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    #19 BRG_Paul, Apr 1, 2012
    Last edited: Apr 1, 2012
    The dry torque is specified as 122 foot pounds ... If you apply graphite to the threads and then apply the dry torque value you are highly likely to: 1) snap the screw.. or 2) strip the threads in the aluminum. (Which is most likely how the threads striped out of the aluminum with the 122 ft-lbs torque while using anti seize compound in the post above.) If you want to prevent the corrosion, then there is another method. Apply Loctite 222 and reduce the torque by ~15%. It is a low strength thread locker. The Loctite effects the K factor so you'll need to adjust the torque appropriately. It will seal the threads to prevent corrosion but is low enough in strength that it will not be difficult to remove the screw.

    I supplied the info and equations above to help show that this is not "opinion", it's well established science.

    In deference to Mr. Smith, over torquing can be disastrous, often leading to sudden failure. While I agree that self tapping screws are problematic, there are ways to properly use them. A good reference is The Design and Behavior of the Bolted Joint, by John Bickford.

    No offense intense to anyone here. :(
     
  20. BRG_Paul

    BRG_Paul Active Member

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    Here is a "cheat sheet" for K factors .....

    Cheers... :smile5:
     

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