Joint physics - hose/pipe connection

[jbenedet]

TL;DR

Pull-off force at vehicle joint

Hello,

Did a simple study at work recently and was confounded by the results. Hoping for a simple explanation….

Consider a pipe is inserted into a hose and a clamp is tightened over the connection.

The pipe has a groove and the hose has a bead. To complete the connection the hose bead is pushed into the pipe groove

We tightened the clamp in 3 different positions;

1)5 samples with the clamp tightened 5mm upstream from the bead/groove location. (Closest to pull stress).
2) 5 samples with the clamp tightened directly over the bead/groove location.
3) 5 samples with the clamp tightened 5 mm downstream from the bead groove location. (Furthest for pull tension)

What we found was the pull-off force was highest in 1, followed by 3, and worst is 2. This was the opposite of our hypothesis….

 

[Baluncore]

Welcome to PF.

TL;DR Summary: Pull-off force at vehicle joint

This was the opposite of our hypothesis….

It is often the case that you can more easily push a hose on, or off a pipe, but you cannot pull it off.

When you pull on a hose, the tension causes the hose becomes slightly longer, while the diameter of the hose is reduced. The surface area of the hose effectively remains constant.

With the clamp at the end of the hose, the hose between the clamp and the end of the pipe will 'shrink' onto the pipe when pulled. That will increase friction between hose and pipe, and do it over a longer length than with the clamp in any other position. The friction grips better, and so has a multiplying effect beyond that expected.

There is a similar multiplying effect with a rope wrapped around a bollard or capstan.
https://en.wikipedia.org/wiki/Capstan_equation

 

[jbenedet]

Thank you for the welcome and explanation.

Still don’t fully understand why the center location—clamp directly over the bead had the worst results. Thinking this is due to the force of the clamp being exerted unevenly (less surface area) over the hose and therefore resulting in less contact friction as the hose is pulled. If the pipe groove/bead formed a perfect mate, dimensionally this wouldn’t be the case, but we see slight variation in both dimensions across the circumference….

 

[jack action]

Here is my point of view:

When you crush the hose with the clamp, on each side of the clamp, the hose is lifted, making the hose bulge.

When pulling the pipe:

• the open-end side of the hose tends to lift even more because the clamp follows the pipe;
• the other side of the clamp has the opposite effect and tends to bring the hose down on the pipe.

So:

• Case 1: the bead tends to get into the groove. The higher the pull force, the more the bead-groove mechanical constraint is effective;
• Case 3: the bead tends to get out of the groove but the vertical friction force provided by the bead-groove counteracts that. The mechanical constraint of the bead-groove is effective but less than with case 1;
• Case 2: The bead-groove mechanical constraint is most likely ineffective as only the friction on each side of the groove is holding the pipe and the hose. As you said, with less surface area, it is easier to break free, and once it is moving, the mechanical constraint of the groove will [fail to] absorb more energy.

 

[Lnewqban]

a pipe is inserted into a hose...

The pipe has a groove and the hose has a bead. To complete the connection the hose bead is pushed into the pipe groove

Welcome!

Could you show us a diagram of this arrangement?
Sorry, it seems confusing to me.

 

[jbenedet]

Thank you, this was great. It completes what I was missing…