# Minimum bending radius for a tube

xJuggleboy

## Main Question or Discussion Point

How do you calculate the minimum bend radious for a tube?

Its a SS tube and we want to bend as small a radius as possible....

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FredGarvin
The standard I always follow is 4 times the OD. I'll see if I can't dig up a relationship that is a bit more accurate to include wall thickness.

BTW...what size tube? Is there any reason you can't use a 90° fitting?

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Q_Goest
Homework Helper
Gold Member
I have 3 Swagelok tube benders under my desk. The 1/4" and 1/2" are both the standard 3:1 radius, and the 3/8" is a standard also with a bend radius of just a bit more than 3:1. I believe the Parker ones are the same.

If you can't use an elbow as Fred suggests, there are weld fittings, or you might even consider making it a mitre joint, though that's very uncommon and generally not good practice.

xJuggleboy
The Tube is a 7/32 OD im not sure of the Wall thickness but the bend we need to make is about a .41 diamiter curve and it has to go 180Deg.

Bassiacly a U shape. the .41 is the id of the U.

But so far what I hear is the bend is way to small for the tube

Q_Goest
Homework Helper
Gold Member
Why not just make up a mandrel and try it? You don't have much to loose. I'd be a bit concerned about hoop stress because when you bend a tube, the ID will thicken but the OD will thin. If you're trying to meet a piping code, and especially if this is a mass produced item, it would be wise to apply minimum wall thickness to the thinned OD of the tube. That said, what pressure are you going to, what material is the tube made from and what are you flowing through it?

FredGarvin
Questions:

First of all...7/32? Couldn't you make this easy and make the tube Ø.250?
What's the set up like? Where is this tube going? If you can give us a general idea of what the plumbing is doing we may be able to find an alternate method.

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Danger
Gold Member
FredGarvin said:
we may bea ble to find an alternate method.
If it's a low-pressure, non-reactive environment, plastic might be easier to use... but it seems unlikely if the first choice was stainless.

Clausius2
Gold Member
Q_Goest said:
I have 3 Swagelok tube benders under my desk. The 1/4" and 1/2" are both the standard 3:1 radius, and the 3/8" is a standard also with a bend radius of just a bit more than 3:1. I believe the Parker ones are the same.

If you can't use an elbow as Fred suggests, there are weld fittings, or you might even consider making it a mitre joint, though that's very uncommon and generally not good practice.
What about taking a look at the Crane paper?? :rofl: :rofl: :rofl:

FredGarvin
Hardee-har-har. Go back to your tube now.

xJuggleboy
Well to answer some of the questions...

The size and material of the tubs can not be changed... Its for high pressure hydrogen.

I have been trying it. I made a few mandrels and the tube always kinks. Even with Annelling it.

Im pritty positive it cant be bent as tight as Im trying... I still have a few more things to try.

Now Im trying to fill teh tube with a low temp solder before I bend it. Ill see how that works out....

Q_Goest
Homework Helper
Gold Member
What about taking a look at the Crane paper??
LOL Maybe we need to perform an FEA analysis on it . . . <not>

Now Im trying to fill teh tube with a low temp solder before I bend it. Ill see how that works out....
I've also heard of people packing sand inside. That might be a bit more practical. But if this is for high pressure hydrogen, I'd suggest doing some burst testing (hydrostatic) to ensure you still have the proper strength after forming. Are you familiar with ASME B31.3? I'd assume that was the right code if you're in the US. There are some others depending on what kind of system it is. If you're in Europe, they have an equivalent code for pressure rating.

xJuggleboy
I tryed sand first... It didnt turn out so well...

And as far as coad goes... We make up our own coad! We dont need no stinking ASME! =-P

FredGarvin
I think you may be stuck having to use some fittings and doing a little rerouting. You could possibly use a small manifold...

xJuggleboy
FredGarvin said:
I think you may be stuck having to use some fittings and doing a little rerouting. You could possibly use a small manifold...

I keep trying to tell the head engener that :grumpy:

Danger
Gold Member
I have no idea what strength issues this will insult, but what about either machining or casting the bend and then welding it to the rest of the tube?

xJuggleboy
Danger said:
I have no idea what strength issues this will insult, but what about either machining or casting the bend and then welding it to the rest of the tube?

That would be a nice thing... But time and money wont allow for that =-(

I would like to thank you all for your input I think we have come up with a solution. We are filling the tube with a low tep solder then bending it to a slightly larger diamiter. It seems to be working!

Danger
Gold Member
xJuggleboy said:
It seems to be working!

xJuggleboy
Well after my 2 week vacation.... I decided that the best thing to do was bending the tube after filling it with solder.

The bends came out pritty good. It is just a pain in the ass to make. and I need to make a bunch of them. Thanks fer the help everyone! =-)

FredGarvin