david.clement@verizon.net wrote:
: Just to make sure I wasn't going to get egg on my face I checked the
: McMaster-Carr catalog and the common brass compression fitting for 1/8 or 3/16
: tubing is the numbers I quoted below. We used to use a compression fitting that
: was brass with a plastic ferrel that was designed for plastic tubing that would
: seperate at 500psi in 1/4" size but I have never seen as small size fitting
: that was rated for well over 1000psi. Even 1/2" PVC water pipe is rated for
: 300psi.
Page 121: http://www.mcmaster.com/nav/enter.asp?pagenum=121
The general purpose brass compression tube fittings in 1/8 and 3/16
are both rated at only 400psi. While it is true that the "self-aligning"
type (with a built-in ferrule) are rated for 1900-2800psi, I have seldom
seen that type in a hardware store; if you ask for a compression fitting,
you'll get the type with the separate ferrule. Somebody who goes into
their local Ace Hardware or Autozone and thinks they are getting a fitting
rated for 2,000psi may be in for a nasty surprise.
: I have been using this type of compression fiting for 30 years for break line
: repairs and the brass ferrel easily deforms the carbon steel used in brakes
: lines and have never seen one fail that was put together properly (the most
: common mistake is the tube not being fully seated in the fitting resulting in
: the ferrel not deforming the tube an locking in). Also, keep in mind that a
: 3/16" brake line has about a .150 bore which works out to .0177 sqr inches of
: cross section, even at 2000psi operating pressure that is only 35.5 lbs of
: force trying to force the tube out of the fitting, the brass it self is much
: stronger than the because of the mass than the actual tubing.
Your calculations appear to be correct, but since the manufacturer of
the fitting has basically stated that it is effectively only rated for
7 lbs in that situation (400psi), I'll be a big wuss and stick to 45 degree
double flared fittings for my brake lines. :-)
: As for the quality of what you purchase at Autozone! What makes you think the
: steel line you get there is going to be any better?
I didn't word that very well; I wasn't meaning to imply that the
fitting is of inferior quality since it comes from Autozone, I was
referring to the fact that at hardware and auto parts stores, usually
these things are thrown into bins and not individually labeled.
Probably the only thing the store is going to be able to tell you is
the price. In such a situation, you're essentially buying "anonymous
fittings"; there is likely no way to determine who the manufacturer
even was, let alone the manufacturer's recommended pressure limits
for that particular item.
--
-Jon-
.-- Jon Steiger ---- jon@dakota-truck.net or jon@jonsteiger.com --.
| 1970 Barracuda - 1990 Dakota 'vert - 1992 Ram 4x4 - 1996 Dakota |
| 1996 Intruder 1400 - 1996 Kolb FireFly - 2001 Ram QC 3500 CTD |
`------------------------------------ http://www.jonsteiger.com --'
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