Double clamping doesn’t increase holding force!
PSA to all you aspiring irrigation techs. I see a lot of poly shenanigans on the job and in this sub.
If you go back to your high school physics textbooks you’ll notice the “coefficient of statistic friction” equation does not include surface area. This is because it doesn’t effect the friction force!
Double clamping, double barbs, etc. exert the same clamping force as single.
If a single clamp is insufficient, that means you are using the wrong type of fitting.
Posted May 2, 2022, 11:00 PM UTC
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You should go back to the physics text book. That’s not how it works. The clamps add force, not friction. Force against the fitting is creating friction between the 2 surfaces. A second clamp is adding more force.
This has a time and place, certainly not on all fittings but I know whenever we do anything 1.5” and up you better be doing 2 clamps, or you will be coming back to add another clamp when it fails the pressure test.
Great Scott! He's right!
I see where you’re coming from though. It’s a common assumption that the clamp provides the clamp force, which then stands to reason that more clamp force = more static friction.
However, it’s purpose is actually to stop the tube from expanding during a pressure surge and in our case to induce the tube flex around the barb tips.
The friction force is derived from the lateral movement of the ID of the tube and the tip of the barb, NOT THE CLAMP. The holding force increases as water the pressure increases.
If we analyse the cross-section of the barb. We notice that the clamp force is never applied to the same barb tip (clamps are placed in parallel, not stacked ontop of each other). Additionally, the nature of the tubes flexible body means that the force is localised. You don’t get the force radiating and adding to the clamps adjacent to the degree one would expect from something like a rigid body.
There is a time for double clamping. However, if you check your engineers instructions you’ll notice that it’s always for redundancy in contexts where a total circuit failure would be catastrophic.
You’ll notice that on double clamped jobs the clamping force of the single clamp is always greater than the failure load of the entire circuit.
Additionally, the barb fitting will be specifically designed to accommodate two clamps with adequate spacing between. And, the hose will be rubberised not poly.
Additionally, additionally, they don’t even make pressure rated barbs for high diameter HDPE. They use specialty compression fittings or electofusion.
RAID-1, 4, 5, 6, and parity backup (like Unraid) is also for redundancy - it's right in the name, and you'll fail to find a computer engineer who recommends against redundant backups. In the case of parity backup, some engineers will also recommend having two parity disks to act as redundancy for your redundant backup solution.
So I'll use three clamps :P