|
sed
May 5, 2008, 10:12 PM
Post #1 of 15
(7843 views)
Shortcut
Registered: Aug 3, 2003
Posts: 356
|
I always look forward to your test results. This one is kind of interesting but again the sample size limits interpretation. I would rather see 10 tested using the exact same conditions, or for tightening comparisons we need a bunch at each torque. The testing you did here just make suggestions about the affects of tightening I suppose. I am too lazy to look at your previous thread but how do the results here compare to the finger tight test results?
S
|
|
|
|
|
AlexCV
May 5, 2008, 10:15 PM
Post #2 of 15
(7841 views)
Shortcut
Registered: Jan 25, 2008
Posts: 283
|
That 400 in-lbf tightening torque figure is a bit over 45 n-M or overspect for a 20mm Peguet maillon... It seems way excessive in any case.
|
|
|
|
|
adatesman
May 6, 2008, 12:03 AM
Post #3 of 15
(7825 views)
Shortcut
Registered: Jul 13, 2005
Posts: 3479
|
|
|
|
|
|
bigo
May 6, 2008, 12:26 AM
Post #4 of 15
(7815 views)
Shortcut
Registered: Mar 11, 2002
Posts: 237
|
Ponder on "Preload for Bolts in Loaded Joints" on page 1400 of Machinery Handbook (25th ed) for why tightening the quicklinks did not decrease the strength.
|
|
|
|
|
jt512
May 6, 2008, 12:47 AM
Post #5 of 15
(7809 views)
Shortcut
Registered: Apr 12, 2001
Posts: 21904
|
I compared the results from the 8 wrench-tightened quick links that survived the tightening with the results from the 6 finger-tightened Chinese quick links tested earlier. The following figure shows side-by-side boxplots from the two samples.
The mean breaking strength of the wrench-tightened links was greater than that of the finger- tightened links (5660 lb vs 5278 lb). The difference was not statistically significant (p-value = 0.13), but the power of the test was limited due to the small sample sizes.
The wrench-tightened links had a standard deviation of 436 lb, virtually identical to that of the hand-tightened links (435 lb), providing further evidence of poor quality control.
Jay
(This post was edited by jt512 on May 6, 2008, 12:50 AM)
|
|
|
|
|
adatesman
May 6, 2008, 12:57 AM
Post #6 of 15
(7797 views)
Shortcut
Registered: Jul 13, 2005
Posts: 3479
|
|
|
|
|
|
bigo
May 6, 2008, 1:45 AM
Post #7 of 15
(7779 views)
Shortcut
Registered: Mar 11, 2002
Posts: 237
|
No worries aric 
|
|
|
|
|
adatesman
May 6, 2008, 1:46 AM
Post #8 of 15
(7778 views)
Shortcut
Registered: Jul 13, 2005
Posts: 3479
|
|
|
|
|
|
bigo
May 6, 2008, 2:02 AM
Post #9 of 15
(7770 views)
Shortcut
Registered: Mar 11, 2002
Posts: 237
|
The large standard deviation may not be a function of poor quality control, but rather poor engineering (if there was any). Because the failure mode is shearing of the threads, the governing geometric parameter for the failure is the thread area which is a function of thread height, major and minor pitch diameter, ..... It may well be that the observed spread in strength is within the predicted max/min strength based on the thread tolerance. The petzl quicklinks failed at in tension in the bend; this failure is governed by diameter of the quicklink. A tight tolerance on the diameter is easier to maintain than a tight thread tolerance.
The engineering solution for a 'better' cheap quicklink is to increase the length of engagement on the threads, so failure is forced out of the threads and to the thicker, easier to control body of the link.
So really, it is just a semantics argument, but i don't think you can put cause to quality control based on a large standard deviation until you have a predicted deviation. That said, I know almost nothing about statistics outside of the stat 200 series I took 10 years ago and I certainly have not done any strength checks on the quicklinks in question ....
Oh yeah, I'll whip all frickin day on those cheapo quicklinks 
-edit because I forgot how old I am
(This post was edited by bigo on May 6, 2008, 2:10 AM)
|
|
|
|
|
curt
May 9, 2008, 6:02 AM
Post #10 of 15
(7623 views)
Shortcut
Registered: Aug 27, 2002
Posts: 18275
|
adatesman wrote: Thanks for the analysis, Jay. That supports what Bigo's reference says will happen. Basically tightening the nut puts the joint in compression and the subsequent loading has to first overcome the compressive force before the joint goes into tension, thereby increasing its strength.
For what it's worth, that would be my guess too. It certainly isn't "work hardening."
Curt
|
|
|
|
|
gunkiemike
May 12, 2008, 12:16 AM
Post #11 of 15
(7537 views)
Shortcut
Registered: Oct 1, 2002
Posts: 2266
|
adatesman wrote: Thanks for the analysis, Jay. That supports what Bigo's reference says will happen. Basically tightening the nut puts the joint in compression and the subsequent loading has to first overcome the compressive force before the joint goes into tension, thereby increasing its strength. -aric.
But isn't the bolt in tension regardless?
What little I read of that tome suggests that joint failure is defined as fluid leakage rather than bolt failure. Not exactly a good model of what's going on with the links.
Back to the links - aren't the upper threads of the link body subjected to shear in the same direction by overtightening as in their ultimate failure? (admittedly I may not have understood the definition of "upper" and "lower" thread)
|
|
|
|
|
bigo
May 12, 2008, 4:45 PM
Post #12 of 15
(7506 views)
Shortcut
Registered: Mar 11, 2002
Posts: 237
|
In reply to: But isn't the bolt in tension regardless?
Yes, the bolt is in tension, but the joint interface is in compression. In an idealized system, the bolt will not take any additional tension until the compression in the joint is overcome.
example:
A plate with a hook is bolted to the ceiling with a bolt that has 100 lbs of preload. The bolt can be thought of as a spring, to stretch it one must apply a force.
What is the min weight that must be applied at the hook to stretch the bolt further? What is the load in the bolt when you apply the weight?
In reply to: What little I read of that tome suggests that joint failure is defined as fluid leakage rather than bolt failure. Not exactly a good model of what's going on with the links.
Read on to the next paragraph and it describes how preload in a bolt is useful for designing a fatigue resistant joint because the fastener is insulated against cyclic variation in the fastener load. This is due to the preload phenom that is described in the example.
In reply to: I may not have understood the definition of "upper" and "lower" thread
I think Aric was referring to the threads on either side of the opening in the link.
|
|
|
|
|
gunkiemike
May 12, 2008, 8:16 PM
Post #13 of 15
(7473 views)
Shortcut
Registered: Oct 1, 2002
Posts: 2266
|
bigo wrote: In reply to: But isn't the bolt in tension regardless? Yes, the bolt is in tension, but the joint interface is in compression. In an idealized system, the bolt will not take any additional tension until the compression in the joint is overcome. example: A plate with a hook is bolted to the ceiling with a bolt that has 100 lbs of preload. The bolt can be thought of as a spring, to stretch it one must apply a force. What is the min weight that must be applied at the hook to stretch the bolt further? What is the load in the bolt when you apply the weight? In reply to: What little I read of that tome suggests that joint failure is defined as fluid leakage rather than bolt failure. Not exactly a good model of what's going on with the links. Read on to the next paragraph and it describes how preload in a bolt is useful for designing a fatigue resistant joint because the fastener is insulated against cyclic variation in the fastener load. This is due to the preload phenom that is described in the example. In reply to: I may not have understood the definition of "upper" and "lower" thread I think Aric was referring to the threads on either side of the opening in the link.
Of course we're not talking about fatigue failure either.
A better simulation IMO is a bolt threaded into a hole that bottoms out. When the bolt hits the bottom, there is shear applied to the threads. Twist it hard enough and the threads will strip off. Now apply an outwards tension on the bolt. This applies shear to the bolt threads that is in the same direction as the torque that mashed the bolt in there. Now, I don't know if the forces are additive or not (such that the preload will compromise tensile strength), but the forces are directionally similar, are they not? So why is it wrong to see the forces involved in overtightening the collar on a quicklink as threatening the threads that have shown themselves to be the weakest part of the system? I welcome an engineer's take on this description.
|
|
|
|
|
bigo
May 13, 2008, 12:07 AM
Post #14 of 15
(7450 views)
Shortcut
Registered: Mar 11, 2002
Posts: 237
|
No we are not talking about fatigue failure  , but the idea explained in that paragraph still applies.
It is difficult to conceptualize with this scenario because the of the preload is in the thread interface and the lip/sleeve interface, but the basic idea of a bolted plate still applies. Until the compression between the sleeve and the lip/stop is relieved the shear load at the threads will remain relatively constant. The compression at the lip interface is equal to the preload at the threads minus the applied tension load; once the applied is greater that the preload, the compression at the sleeve/lip stop is relieved and the additional load is transferred through the threads.
Of course this is an idealization, but is close to reality.
(This post was edited by bigo on May 13, 2008, 12:08 AM)
|
|
|
|
|
|
|
|
|
