Blatantly copied from the Dimeband Garage web site:
"The SAE 45 degree double flare usually has a male-threaded tube nut that
bears directly on the OD of the flared tube- so you need a double flare to
help control galling that can result in stress cracking right at the flare.
In short, you need "give" there. Problem is, the deformation that results
is kind of irreversible, so the next cycle or two will result in your
having to use astronomical torques to keep the flare from weeping. Worse is
trying to use a single flare in an SAE flare nut and seat, and worse still
if the seat is brass- the flared tube is sqaushed from both sides, even as
it is deformed by the nut galling on it. The brass seat deforms and work
hardens. It may seal once, with a ton of torque and some luck. It's not
recommended practice - it's not even a good idea.
Racers (and aircraft, which is where the system originated as the
"Army-Navy" or AN standard in WWII) use the single 37 degree flare. The AN
single flare is still a concave flare, but its 37deg angle seals by
stretching, not squashing. The tube is supported by a separate sleeve that
the female-threaded tube nut bears upon. This isolates the flare from the
torques imparted by the nut. So rather than trying to get a seal despite
the presence of rotating torques and the resulting galling, you press the
flare between precisely-machined (steel!) seat and precisely-machined
support sleeve. The sealing area under compression is at least double that
of the SAE flare. An additional bonus is that the OD of the nut is a lot
larger than the 3/8" of an SAE nut, which means you won't kill as many
trying to get the proper sealing torque. (Even so, you should always use a
proper flare nut wrench on any tube nut.) "
So you're right - the double flare helps reduce the damage caused by
galling. In the case of Girling "bubble" flares I think that the squashing
and deformation of the bubble is critical, and you can only squash it once.
brian
At 11:29 AM 7/27/01 -0400, you wrote:
>Note: normal people can delete this before reading -- abnormally intense
>brake pipers can read on.
>
>>From: Brian Evans <brian@uunet.ca>
>>What I do now is simply cut off the old girling or SAE flare from
>>the >bundy tube, and re-flare the existing tube in place
>
>Brian,
>
>Yes, this seems to be the concensus ... no one (yet) has mentioned a
>source for Girling-to-AN adapters (or Girling to anything else, for that
>matter), so the easiest thing to do is cut & re-flare the hard pipe.
>
>>One big bennie is that the AN flare is a single flare and almost
>>foolproof to make.
>
>This brings up an interesting question, for which I THINK I know the
>answer ... have been reading lots of stuff on types of flares for brake
>pipe, and nearly every "expert" says, somewhere, that one should NEVER use
>a single-thickness flare (i.e. they all recommend a double-thickness
>flare, like the SAE 45-deg inverted flare). However, obviously the AN
>system uses a single-thickness flare! And we all know that works.
>
>I suspect that the single thickness is OK on the AN system because the AN
>flare nut has a second piece -- the sleeve -- under the nut, so it doesn't
>transfer the tightening torque to the flare & therefre doesn't gall the
>flare. Other systems (notably SAE) do not have the sleeve under the nut,
>so tightening the nut tends to gall the flare, and I guess that's why they
>use a double-thickness flare. Comments?
>
>Regards,
>Mark
>
>_________________________________________________________________
>Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp
Brian Evans
Director, Canada
MCI Wholesale Internet Services
|