Minimum Bend Radius 0.75" OD SS, .065" wall thickness 2

[imagineers]

Trying to find a minimum bend radius on a stainless steel pipe. I know the rule of thumb is 2xOD usually, but it would be nice to get a more accurate min bend radius. Also, I have not done this in a while and not sure if min bend radius is the centerline radius or the inside tube radius they are talking about when they talk about min bend radius??? Thanks

 

[marty007]

I've found that the 2xOD is a good rule of thumb. Going tighter than that becomes so dependent on material/thickness/diameter, that you really have to talk to the tube-bending companies on a case-by-case basis. If there is a shop that you send a lot of work to, I'd just give them a call and ask what they are comfortable with for your specific case. They should be able to provide some quick guidance.

Cheers,

 

[imagineers]

my second question is .....what radius are they referrinf to when they talk min bend radius? is it the centerline diameter or the inside diameter of the tube?

 

[imagineers]

my current centerline bend radius is 1.25" which isnt twice the OD at 0.75", but when I look at another website I can see they can do a 0.875" bend radii, but does not tell the wall thickness..

http://www.detroittubeproducts.com/die_inventory.html

 

[BigInch]

BTW it's not pipe, it's tubing.

 

[imagineers]

thanks, but the question remains

 

[marty007]

Typically they are talking center-line radius.

Another aspect to consider is the cost of going to smaller diameters. At a large bend diameter, you can get away with bending the tube with relatively simple mandrels. To achieve very tight radii without excessive flattening and wrinkling, the bending shop will have to start using more advanced techniques such as packing the tube with another material to resist flattening. These more advanced techniques all come at a cost.

Cheers,

 

[imagineers]

marty007, so do you think the 1.25 centerline radius is ok? or should I stick to 0.75 x 2 = 1.5 Centerline radius?

 

[racookpe1978]

You a the designer. Not us.

If YOU "have to" use the smaller radius for some particular (expensive!) but valid reason, require that special small radius in the dwg and bid and in the pricetag.

IF you do not absolutely-positively-have-to-use-it need that small a radius, USE THE BIGGEST (STANDARD) RADII YOU CAN. It is cheaper. Easier, more likely to be successful. Do NOT order a special size unless you really, really need.

 

[XL83NL]

Have you looked at the instructions of several tubing (fitting) suppliers, like e.g. Swagelok or Parker?
Their guidelines may provide some help. Swagelok for example has tables for bend radii depending on tube OD.

 

[Duwe6]

NOTE: due to the elongation and thinning of the extrados of a very tight bend, you will have to supply thicker tubing for that region [or accept the unavoidable thin spot]. This is the second major reason that you REALLY do not want to go below 1.5 x diameter bends. First is the physical difficulty of making these tight bends without excessive ovality.

Excessive thinning is the most common problem with 180° 'return' bend pipe fittings. Thus the mfr starts with a 'tube' much thicker than the pipe schedule, and bends the tube. And usually leaves the ID mismatch for the pipefitter to grind it down until a satisfactory fitup to the pipe can be obtained.

 

[imagineers]

from what I can gather, swagelok does not show min bend radius on a 3/4 tube, however the tubing it does list is using 3 x bend radius not 2. I have always heard 2 times, so maybe I will just use the 2 x to be safer than the 1.5 I am using now .

 

[moltenmetal]

A bend radius of 3 can be obtained cold, without difficulty, in an externally guided bender (i.e. for 3/4" tubing, we use a manual crank bender).

A bend radius of 2 or less will probably require INTERNAL guiding (there is tooling for this, if the bend is near one end), or another means (sand filling with plugged ends etc.) to reduce the tendency for the tubing to go oval or collapse inward at the bend. External guiding alone will be insufficient.

A bend radius of 1.5 could probably be done hot, or if internal guiding is possible.

 

[EdStainless]

We see ubends in feedwater heaters all of the time that are tighter than r=2d. And given the heavier tubing required (usually for the first four or five rows, both because of thinning and because of min order requirements) the impact on thermal performance is such that they would have been better off just canting the inside row and loosing one ubend.
Of course these sets are a bitch to box since row 1 is larger than row 3 in some cases.
When you go tighter than r=2d you usually have to relax both the wall thinning and ovality requirements. These can be ugly bends.
On ubends we usually use mandrels up through about r=3.5d.
And yes we mandrel bend tubes that start 120' long.

In high fatigue applications (aerospace) they require mandrel bends (ovality less than 3%) and in general they wont use a bend tighter than r=3d

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube