david.clement@verizon.net wrote:
: Jon,
: A common brass compression fitting for 1/8" tubing is rated for 3600psi and for
: 3/16" tubing it's rated at 2300psi, other are rated much higher. Even the
: plastic compression fittings in brake line sizes are rated up to 400psi
: depending on the plastic.
Hmmmm, I'm not sure where you're getting those numbers from; any of
the garden variety brass compression fittings I have seen range between
100-400psi or so. There are certainly different types of compression
fittings that can reach the pressures you specified (and much higher), but
all of the common, hardware store type brass compression fittings I have
seen max out around 400psi, depending on tube size. Check out some of the
on-line industrial supply catalogs for this info; MSC, McMaster-Carr, etc.
Here's another little snag: I believe that the garden variety brass
compression fittings are designed for "soft" lines such as plastics,
aluminum, copper, etc. If you try to use one of those on a steel brake
line, I highly suspect that the brass wouldn't be hard enough to deform
the steel enough to create a proper seal. This is probably why the high
pressure compression fittings are generally steel or stainless steel;
because that allows them to be used on steel lines. This is also likely
why 45 degree double flares are used on brake lines - because the steel
itself is responsible for the seal surface area. This is bound to create
a better seal than a brass ferrule being used as the sealing surface, on
a material which is harder than the ferrule itself. The ferrule needs
to bite into the tube somewhat, and using a brass ferrule on a steel
tube is likely to just distort the ferrule.
: Also, I think you also over estimate the amount of pressure in a brake system.
: I can leg press around 800lbs with both legs. Under panic braking it is
: reasonable to assume maybe 500lbs on the pedal with one leg. I would
: guesstamate the leverage ratio of the pedal is 2:1 so you are applying
: approximately 1000 lbs of pressure on the master cylinder. Most master cylinder
: bores are +/- a small amount around 1". Assuming 1" that is.785 in. sqrd or
: 785psi being applied by the master cylinder to the system. Well within the
: pressure rating of the of a 3/16" brass compression fitting of 2300psi. Even if
: the pressure on the master cylinder is 4 times it's still only 1570psi.
The master cylinder of course amplifies the physical pressure you
put on the pedal. Based on a cursory analysis, it would seem that most
(disc brake) vehicles are set up with caliper pressures of between 1000psi
and 1500psi, although it would seem to be not uncommon to reach 2,000psi.
Most brake line pressure testers have a range of 0-3,000psi.
: FWIW, the pressure rating on hose and tube fittings already have a safety
: factor built in. Standard production testing is done at 1.5 times the rating
: and design proof pressure testing is done at 4 times the rating (allowed to
: leak but not fail on this test). It is safe to operate these fittings righ at
: there maximum rating as long as you are still in the temperature range of the
: fitting.
Yep, almost anything will have an actual failure which is beyond
the published info for a safety factor, but that's sort've a moot
point in this case considering that the compression fittings we are
talking about are under-rated anywhere from 2.5 times (400psi fittings
in a 1000psi system) to 20 times (100psi fittings in a 2000psi system).
Its one thing to run at rated pressure, but to run almost 3 times
beyond spec (best case scenario) is begging for trouble. (Physical
and legal.)
Throw in the fact that who knows what sort of hardware you are getting
if you buy something out of a bin at a hardware store or Autozone, and
compound that with the fact that these pressure ratings are established
for new, properly installed components (who knows what effect ineptitude
and time will have on them) and in my book it all adds up to "please do
NOT use compression fittings in your brake system!" :-)
Who knows, we may have to agree to disagree, and maybe I'm being
a nervous ninny, but when lives are at stake, I'll gladly wear the
old lady hat and err on the side of caution.
--
-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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