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.
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.
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?
Dave Clement
99 SLT+ CC 4x4
In article <d6r2cd$sb9$1@bent.twistedbits.net>, jon@dakota-truck.net writes:
>
>
> 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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