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#4448097 - 07/03/17 08:30 AM Do bolts get weaker if loosened and tighten often?
VeeDubb Offline


Registered: 08/15/02
Posts: 2435
Loc: Indiana
So I had a bolt on my bike snap even though I was under-torquing it by about 0.7nm (max is 5.2nm and torque wrench was at 4.5nm). This was one of two stem pinch bolts. I usually tighten gradually since I try to balance the torque across the two opposing bolts. Usually, I have no problems getting them to 5.2nm. But today, one of the bolts snapped at 4.5nm. I often loosen and tighten these bolts to adjust the headset after long mountain bike rides. I'm just wondering if the frequent loosening and tightening might have weakened the bolts to the point that they snapped even at less than max torque.

Thanks
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#4448112 - 07/03/17 08:47 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
This is a metallurgical issue, and I'm going to venture a guess that the bolt was made from bad steel from it's origin.

Fatigue cycle is ZERO on many steels (you can go up to a certain % of its yield strength indefinitely and it will NEVER fatigue)

I don't think it was your fault - - replace with better quality hardware.
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#4448114 - 07/03/17 08:49 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
I suppose there is the rare (VERY rare) chance this was specifically a "torque to yield" style of bolt, then if that is the case it's definitely your fault. But I kind of doubt it.
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#4448137 - 07/03/17 09:07 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
Johnny2Bad Offline


Registered: 05/20/13
Posts: 1810
Loc: Saskatchewan, Canada
Yes. Torque applied by a fastener is based on stretching of the metal; for critical applications one does not measure bolt torque, but instead bolt stretch.

The materials used in the fastener play a role here ... high strength bolts don't like to re-stretch. Repeatedly re-usimg a fastener will result in bolt failure.
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#4448143 - 07/03/17 09:16 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
VeeDubb Offline


Registered: 08/15/02
Posts: 2435
Loc: Indiana
Good info. So it sounds like I should just go ahead and replace the bolts every few months if I am going to adjust the headset often.
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#4448152 - 07/03/17 09:29 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
KrisZ Offline


Registered: 12/23/06
Posts: 7469
Loc: Toronto, Canada
I believe you should look at calibrating your torque wrench first. There is a high possibility that you've been over torquing these bolts without knowing it. Or maybe there is some sort of lubricant or other contamination on the threads that alters the torque reading.

As mentioned before, steel bolts don't fatigue, unless they are torque to yield. Torque to yield takes the material form its elastic range (that's where it always comes back to the original length), to the plastic range (that's where the material stretches beyond its ability to spring back).
Once the bolt goes into the plastic range, the structural changes are irreversible and it is only a matter of time when the bolt fails.


Edited by KrisZ (07/03/17 09:30 AM)
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#4448162 - 07/03/17 09:39 AM Re: Do bolts get weaker if loosened and tighten often? [Re: KrisZ]
VeeDubb Offline


Registered: 08/15/02
Posts: 2435
Loc: Indiana
Thanks KrisZ. As for the torque wrench, it's fairly new. I've used it a few times on the handlebar stem bolts and a few other bolts using the correct torque settings. The wrench clicked without tightening the bolts further. That is, most of the other fasteners didn't need additional tightening with this wrench. Do you think this is good enough info to trust the wrench?
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#4448171 - 07/03/17 09:46 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
VeeDubb Offline


Registered: 08/15/02
Posts: 2435
Loc: Indiana
One more question for you guys. If the bolt fails, is that likely to compromise the threads as well? I'm worried that I may have to replace the stem.
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#4448182 - 07/03/17 10:01 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
KrisZ Offline


Registered: 12/23/06
Posts: 7469
Loc: Toronto, Canada
Clicker type wrenches will not tell you if a fastener is over torqued. They will click at their setting and that's about it. If there is a little bit of rotation before it clicks, then the fastener is at the set torque, but if the wrench clicks right away, then the fastener could either be at the correct setting or higher.
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#4448183 - 07/03/17 10:03 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
andyd Offline


Registered: 09/25/04
Posts: 7150
Loc: Marshfield , MA
If the new bolt tightens smoothly and solidly into place, then the threads are OK. My brother is a biker and does all his own work. I'll ask him, but I don't think he owns a torque wrench. His road bike is worth more than most of my cars.
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#4448258 - 07/03/17 11:20 AM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
Coprolite Offline


Registered: 07/03/11
Posts: 1224
Loc: Houston, TX
I wouldn't worry about proactive replacement. If I were in your shoes, I would simply have a spare on hand in case it breaks. This assumes that you use it often and can't wait for replacements to be available.

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#4448317 - 07/03/17 12:27 PM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
VeeDubb Offline


Registered: 08/15/02
Posts: 2435
Loc: Indiana
How would you know if the bolt is a "Torque to yield?" The best description I got on my stem bolts is: ``Uses M5 countersunk, corrosion-resistant steel bolts with 5.2Nm torque spec''
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#4448361 - 07/03/17 01:34 PM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
bdcardinal Offline


Registered: 06/03/05
Posts: 10992
Loc: Santa Barbara, CA
A torque to yield bolt would have a torque sequence that has you turn a specific number of degrees in addition to a Nm or Ft-lb setting.
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#4448373 - 07/03/17 01:49 PM Re: Do bolts get weaker if loosened and tighten often? [Re: Johnny2Bad]
L_Sludger Offline


Registered: 10/04/09
Posts: 3947
Loc: Ohio
Originally Posted By: Johnny2Bad
Yes. Torque applied by a fastener is based on stretching of the metal; for critical applications one does not measure bolt torque, but instead bolt stretch.

The materials used in the fastener play a role here ... high strength bolts don't like to re-stretch. Repeatedly re-usimg a fastener will result in bolt failure.
Well the cyclic fatigue of retightening a bolt should be absolutely minimal if the torque applied is a small enough proportion of the bolt's elastic region.

If you're stretching a bolt to the plastic region every time you tighten it down then of course it'll fail quickly after retorquing. But that's only going to be on critical engine components like head bolts.

I have a specific counter point to your last statement.
Many German cars use screw fasteners to attach wheels to their hubs. If these bolts were prone to fail after multiple uses then they'd need to be replaced often. Instead they are of a high enough grade that torquing it repeatedly to remove and reattach wheels does not put them on the path to failure.
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#4448376 - 07/03/17 01:52 PM Re: Do bolts get weaker if loosened and tighten often? [Re: VeeDubb]
Johnny2Bad Offline


Registered: 05/20/13
Posts: 1810
Loc: Saskatchewan, Canada
It's highly unlikely that a header bolt would be a "torque to yield" type.

What I do is install a nut on an over-length bolt, fully seat the bolt, then tighten the nut to secure the header. You could also use two nuts, the outer one secured against the inner one (hold the inner in place with a wrench).

In that way the bolt is never re-installed unless you are removing the header completely.
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