James Annan writes:
>> I have no doubt that your assessment of disengagement of a QR (or even nutted axle) wheel is
>> probable with a typical front disk brake located behind the fork having conventional "vertical"
>> dropouts. However, I disagree that this can unscrew QR's. I see no mechanism that would cause
>> rotation of the QR lever or the adjusting nut at the other end.
> The skewer tension multiplied by the thread pitch generates an unscrewing torque. This unscrewing
> seems to be an enirely standard failure mode of a bolted fastener when vibrational and slipping
> forces are present.
That's true for any threaded fastener but thread pitches, even coarse ones are in the friction safe
mode even when lubricated. What unscrews them is incremental torsional fretting, something that
probably occurs with one sided brake torque on the fork ends. Still, the radius of the pressure
faces of the skewerlever and nut would cause the skewer to twist and possibly to have its thread
move in the nut, the nut remaining rotationally fixed with the dropout.
> I did put up a quote from www.boltscience.com and also a single anecdote where the unscrewing is
> observed in some detail (there are many more similar examples).
They describe vibration loosening.
http://www.boltscience.com/pages/vibloose.htm
However, the ratio of bolt length to diameter and diameter of but face is nowhere near that of a QR.
The QR has worked all these years because it is such a good "strain bolt" In fact the over-center
feel of the closure motion is an indication of how much strain there is in the axle-skewer
combination.
> I also note that many other bolts connected with the disk calliper do frequently come loose. Do
> you object in principle to the idea of bolted fasteners loosening under vibration, or do you have
> a more particular objection to considering the QR as one such possible case?
This all goes back to the basic problem that there are insufficient engiennering skills in the
bicycle industry where competition is fierce for low cost/weight/fashion instead of druability and
reliability.
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From:
[email protected] (Jobst Brandt) Newsgroups: rec.bicycles.tech Subject: Re: What
is Loctite and were can I get it Date: 23 Jun 1998 17:54:08 GMT
Frank Krygowski writes:
>> Loctite on a new product is, with rare exception, a major design flaw. Properly designed screwed
>> connections allow enough elastic preload that the part in question that it will never unload the
>> thread in use. Therefore, the tread cannot unscrew. For this reason, critical joints used strain
>> bolts, or other elastic components, to maintain position. Screws that should hold, even if loose,
>> such as brake anchor bolts, should have elastic stop nuts. These have the disadvantage that they
>> resist turning on installation, but for such applications this tradeoff is acceptable.
> I disagree with the statement that Loctite use indicates a major design flaw.
> A successful design is one that achieves its objectives. If the objective is to fasten something
> in place and not have it loosen, what would be the advantage of a strain bolt over Loctite? One
> may be perhaps more elegant, in an engineering sense, but engineers typically don't go for
> elegance as much as for getting the job done.
Controlling the dosage, the cleanliness of the joint and that it gets into the assembly at all, are
major production costs and headaches. As I mentioned, in machine design, products are rated in
motions during assembly with multiplying factors. Low scores are better in assessment of "design for
assembly" and adding any glue, filler or paste carries a high penalty. For this reason, fasteners
that do the job without glue are greatly preferred.
> 50 years ago, Loctite didn't exist. People invented a wide, wide array of devices to prevent
> loosening of threaded fasteners. If every instance of the use of lockwashers, jam nuts, special
> thread forms, etc. indicated a major design flaw in your eyes, you have high standards indeed!
> Higher than almost anyone else, in fact.
Loctite was invented as a fix for otherwise unreliable designs. Its main role is in aftermarket
applications for faulty machinery. There are applications where it is used in lieu of precision
cartridge bearing mounting by having a clearance bore whose gap is filled with Loctite. Even these
are better designed to not be glued.
> While I admit to being something of a retro-grouch, I see Loctite as a useful technology that can
> simplify life both for the designer and for the end user.
You probably mean the lazy engineer and the person who has to use his design. The term end user
evokes Scott Tissue, 400 sheets per roll.
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Jobst Brandt
[email protected] Palo Alto CA
