Barbed hose fittings - don't use clamps?

[ChrisBennet]

I came across this post on a motorcylcle web site the other day when I was trying to find how to spell Oetiker (as in Oetiker clamps). It's non-intuitive but it makes sense.

BTW: QD in the post below refers to Quick Disconnect

Adventure Rider

How hose fittings work

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Have a coffee.

Here is something. The QDs sold by McMaster and Motio Pro are made by Colder Products. The hose barb on the Colder Products QD is designed for use without external clamps. The factory literature says "not to be used with".

That is the case for all multiple flute barbs. Sealing does not take place via external clamp pressure. The sealing takes place internally, at the pressure rise at the point of each barb flute. The resistance of the hose to expansion provides the pressure to make the designed seal.

External clamping creates spreading force lines and reduces the peak pressure at the point of each barb. And in the case of a thermoplastic barb, overstresses the material.

Another thing that the user often does, adds to the overstress. The clamp is tightened until it is 'seen' that the band has displaced some rubber. The 'visual' signal of 'tight enough' is substantially over tight, and will damage the hose on a multi-flute barb.

Here's the thing, by their design the multiple barbs have already displaced material and reshaped the hose from the inside, where the resistance of the hose to expansion provides the greatest pressure.

As the fitting is thermoplastic, we've seen that the material gives up to the constant strain of compression forces from both the hose and the clamp.

I know . . . "The clamp keeps the hose from coming off." Heh heh, try to pull a 5/16" FI hose off of a CPC multiple barb. You better be sure you have the hose lengths figured out right before pushing them onto the barbs. Because once you do, the hose isn't coming off without slicing through the fabric reinforcement with a razor.

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That is how sealing works with a multiple flute barb that has a longitudinal 'parting line' along the flutes.

Sealing for single 'smooth' ramp single flute barbs takes place along both the increasing diameter ramp surface and at the peak of the single flute. That type of fitting often has an area for an external clamp. And the instructions for those fittings will offer something like "Do not place clamp on top of the barb."

The clamp does not contribute to the seal. And if the clamp is tightened to the point of displacing hose material, or placed too close to the flute, the design performance of the fitting seal is compromised.

In addition, often making a seal isn't possible at the clamp area. On many single flute fittings, the band clamp area of the fitting has several longitudinal beads molded in to help keep the hose from rotating on the fitting.

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A third common automotive hose fitting is the 'bead' end form. Fuel filters and water pump and radiator fittings use a bead end form to make a seal on the inside of the hose, against the resistance of the hose to expansion.

Sealing from internal pressure created from the shape of the fitting. It's the common theme of all internal seals.

For a bead form to work well, a band is used. Not to make a seal, but instead as with the single flute barb, to be a non-expandable band.

Install the clamp on the hose until it is just snug. Now pull the hose, as if trying to pull it off the fitting, so that the internal bead is stopped against the external 'non-expandable band'. The properly installed clamp, functioning as a band, does not allow the reinforced hose to move beyond that point. It also prevents the bead-form from moving to different locations along the hose.

* * * * *

Many 1100/1150s have had fuel leaks from the bead fittings on the lines to the fuel pressure regulator, and blame the leak on the Ottiker constant-pressure band.

If there is a leak, the first thing tried is to push the hose further up the line. It is an intuitive action, but is can only make the leak worse. It works for a time because it moves the bead to a new section of hose.

But here's the thing. If the band isn't right against the bead form, side force on the hose works to pull the internal 'circular' diameter of the hose into an 'ellipse'. It is easy to see that the oval internal shape of the hose cannot seal against the circular external shape of the bead.

The properly installed clamp, functioning as a band and located against the low-pressure side of the bead form, maintains the seal not by clamping force, but by not allowing the sealing surface of the internal diameter of the hose to take on an elliptical shape.

Next, over tightening the band clamp is tried. That also makes the leak worse. The over tightened clamp displaces hose material, and the extra material has to go somewhere. Where does it go? Visualize the hose material flowing to either side of the clamp, and increasing the diameter of the hose adjacent to it. That description is technically inaccurate but contextually sound and functionally identical, the fitting leaks.

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Common to the above hose fittings is, the portion of hose used on a barb or bead fitting has a one-time-use restriction. If reinstalled on a fitting, that re-used portion of hose has reduced sealing capabilities and mechanical resistance.

* * * * *

The hose fitting makes a seal with internal pressure against the hose ID, not from external clamping force.

If there is a leak at the fitting, some compromise at the internal pressure point is the cause.

It is, that is, assuming no leaks caused by over pressurization, chemical incompatibility, mechanical strain on the hose, corrosion of the fitting, external damage to the hose, exceeding the design service live of the hose or the one-time-use restriction.

If the internal pressure point is leaking, and not caused from one of the above, the remaining causes are limited.

Either the band is in the wrong location relative to the pressure point, or the band is too tight.

External clamping force isn't creating the seal, and increasing the external clamping force does not repair the source of the leak.

Though the fitting seal against the hose ID may be otherwise compromised, enough clamping force can create a temporary seal elsewhere. And too much clamping force, or the band in the wrong location, creates it's own leak.

Over tightening a clamp is understandable. It is the same ahh . . . 'intuitiveness' . . . that over tightens a bolt.

- Jim


__________________
"The "revolutionary" is in the minds of the marketers......... "
-- Mully

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Last edited by Poolside : 09-02-2006 at 03:35 PM.

Thoughts?
-Chris

[ChrisBennet]

Oops, meant to post this to the 911 list. Moderator, could you move it for me?
-Chris

[Chuck Moreland]

Chris, there are some hose ends designed to be installed without any clamp as the quote points out. But most hose ends with hose barbs are designed to be secured with either a crimp-type collar or a threaded compression collar.

I'll refrain from saying "clamp" because most readers will think worm-drive clamp. In my experience these are not to be used with a proper hose barb. Worm-drive clamps are only appropriate for bead-formed tubes. Even then there are better style clamps.

The hose ends specifically designed to be used clamp-less do so only with a defined hose. I would only go this route per the manufacturer's recommendation. Never turn a crimp or compression collar type hose end into a clamp-less.