RE: [CR]Track ENDS/Nutted Fasteners

(Example: Component Manufacturers)

From: Don Ferris <ojv@earthlink.net>
To: "Sheldon Brown" <CaptBike@sheldonbrown.com>, <classicrendezvous@bikelist.org>
Subject: RE: [CR]Track ENDS/Nutted Fasteners
Date: Fri, 29 Mar 2002 11:16:08 -0700
In-Reply-To: <v0421017bb8ca3c19617e@[10.0.1.28]>


Sheldon wrote: "There are a couple of differences you're overlooking. The threads in a QR skewer are adjusted under no load, so there are no frictional heating issues weakening the threads. A thread can _hold_ a greater stress than it can generate by being tightened without damage."

I have a feeling I'm dangerously close to being scolded since this is "classic bike list" and not "tech list" but I'll take the risk. I agree there are things I overlooked, I did so intentionally since those variables are a wash, i.e., different designs of skewers and different designs of nuts and axles. I think you're misunderstanding my point on the QRs: regardless of how tight it is clamped or how strong the nut or the cam/rod interface is, and assuming those do not detract from a QR's maximum clamping force, the limiting factor will be the rod itself either reaching its yield or its ultimate strength in tension. Since that rod is known to be typically 5mm in diameter, it is easy to calculate its yield strength, in tension, if one knows it's mechanical properties. Since every QR rod I've ever seen has threading on the nut end, that threading results in a reduced cross section and on a 5mm skewer rod, that cross section is nominally 4.2mm (actually it's less than that, 4.2mm assumes 100% thread engagement). It is within the threaded portion of the rod not captured within the nut that a rod/bolt/screw normally fails due to tension, due to the reduced cross section. Again, I was not trying to imply that the skewer rod would fail by trying to turn the nut under load, I assumed that the QR was properly designed and being used properly, i.e., using the nut to provide length adjustment and the cam action of the QR to provide clamping pressure.

Sheldon wrote: "In the case of a nutted axle, however, the _threads_ are the weakest link, not the axle shaft, so the comparison quoted above is not relevant, since it ignores this fact."

Perhaps I could of used better wording, since the threading is on the axle shaft, I used "axle" to refer to the threaded portion of the shaft. I accounted for the reduced cross section of the threading (as noted in my post at 8.8mm nominal, again assuming 100% thread engagement). I really think we're into semantics here. If we assume a properly designed skewer, ala your using Shimano as an example, then we must assume a properly designed axle/nut. I ignored the fact that there are some very poorly designed QR's out there (USE, Ringle, etc., that rely on turn-of-the-nut clamping) so in my exercise I "assumed" the skewer was properly designed and the skewer rod itself was the weak link though this is not often the case. The assumptions I've made were non-discriminatory and make the comparison relevant, IMO.

In regards to the "huh?", a skewer, by virtue of using a common rod in tension to secure the axle to the dropouts, equalizes its load on both sides. A nutted axle only places the span of the axle between the nut and the lock nut/cone at each dropout in tension, therefore, tension is localized to that one dropout and not communicated to the other through the axle shaft.

Cheers! Don Ferris Littleton, Colorado

-----Original Message----- From: Sheldon Brown [mailto:CaptBike@sheldonbrown.com] Sent: Friday, March 29, 2002 8:59 AM To: Don Ferris; classicrendezvous@bikelist.org Subject: RE: [CR]Track ENDS/Nutted Fasteners

Don Ferris wrote:
> As far as the strength
>of a bolted connection, as with anything else, the strength of the assembly
>is only as great as its weakest link. If the case of nuts and bolts, the
>strength of the nut's threads in shear could/would/should equal or exceed
>the the tensile strength of the bolt/axle. Whether or not this is the case
>in all nuts and axles as it applies to all nutted hubs, I can't comment,
>I've never tried to tighten one to failure. You can make the case that not
>all hub nuts are properly designed, but the same is true for skewers. A
>wash. Obviously an axle that has flats on it, ala Sturmey's, etc., the
>threads in the nuts have a reduced area to purchase and would be far weaker
>unless the nut was threaded deeply.

There are a couple of differences you're overlooking. The threads in a QR skewer are adjusted under no load, so there are no frictional heating issues weakening the threads. A thread can _hold_ a greater stress than it can generate by being tightened without damage.

A previous posting seemed to be trying to make the case that a nutted system could clamp more than 4 times as hard as a QR:
>>"that gives a potential of yield strength on a standard steel
>>skewer of around 1,610-pounds based on the root dimension of the
>>threads (4.2mm nom.)...If we assume the same properties for a 4130
>>track axle and nut of standard dimensions, 10mm, then the potential
>>clamping pressure is limited by the root dimension of the axle
>>threads (8.8mm) and its ability to resist tightening stresses,
>>preload, and external load. That'd be about 7,000-pounds...

This skips over the issue of thread engagement. Typical axle nuts are not tall enough to provide sufficient thread engagement to break an axle by overtightening. Instead, the threads will strip. This is not the case with a skewer.

To summarize, indeed, one needs to look at the "weak link".

In the case of a skewer, the skewer shaft is, as noted the weakest link.

In the case of a nutted axle, however, the _threads_ are the weakest link, not the axle shaft, so the comparison quoted above is not relevant, since it ignores this fact.
>>...and since each nut clamps each dropout between itself and the
>>face of the axle, that's 7,000-pounds per dropout.

Huh?

Sheldon "Threads" Brown Newtonville, Massachusetts +------------------------------------------------------+ | It were not best that we should all think alike; | | it is difference of opinion that makes horse-races. | | -- Mark Twain | +------------------------------------------------------+
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