Calculating the Bend Radius of Different Materials 1

[BioMedicalEngineer30]

I need to know what will be the bend radius of a Stainless steel tube (different kinds of alloys)without kinking it of course.
The OD is 1mm and ID is 0.8mm, 20cm long.
Is there a way to calculate the different bend radiuses for different alloys using the material's mechanical properties (Young's module).
Is there any general calculation?
Regards,
Michael

 

[jbknudsen]

I dont have any hard numbers for you but I think that the bend rad will be a function of the apparatus that is used to bend the material as well as the properties of the material itself. Meaning, if you were to bend the tubing by hand (or by grabbing each end), you would most certainly place a kink in the tubing. There are tube benders that support the walls of the tubing, by supporting or capturing the walls, the tubing yields rather than kinks. There are some small dia tube benders on the market, the smallest I have seen is for 3/16" od tube. The radius is 1/2".

Good Luck,
Jay

 

[alexit]

Depending on your desired result, you can try FEA analysis of the bend process (there are sheet metal designed software to predict bend behavior, but I am unsure if they will predict the kink in tubing...)


Alex

 

[aviat]

Usually ss, and most other materials, have a min bend of ~ 2*OD but that is for tubing much larger that what you are using. You may have to make or purchase different size spools, similar to what bender mentioned above uses, and experiment to get proper min radius. Bear in mind that ss requires about twice the force to bend than carbon steel. If you are having problems you could look into heating the tubing before bending.

 

[griffengm]

BioMedicalEngineer30,
Several features control your choices.
1. Type of equipment- Try Tools for Bending site to get an appreciation of tooling and process issues.
2. Metal characteristics- This puts an ultimate on the amount of stretch available to you on the outer radius.
3. Use of final product- this may limit the amount of thinning allowed in your final product in that tissue thin walls may be obtainable but pressure requirments dictate a larger radius to keep the walls thick enough.

Griffy

 

[borjame]

That's a pretty small tube, it will difficult to find data on tubing that small and I'm not sure if equations for larger pipe will apply (

http://www.techsavvy.com/industry/file/zyyz/mich/08q9b/mec03.html,

L = 1.45 (OD2 - ID2) / (OD-ID))

At least SS is common enough so the mat'l won't be a problem.

 

[BioMedicalEngineer30]

Thank you all for your answers, BUT !
I believe that there must be an "ENGINEERED" way to CALCULATE the bend readius using the material and tube's properties (young's module, sigma yeild point, tube profile etc.)
I am a MECHANICAL ENGINEER and I want to know the answer not from catalogs and standards.

Regards

Michael

 

[diamondjim]

I do think it would be related to the
elongation properties, tensile, and
temperature. Try searching on bending
radii on google and you should get
many sites to look at. It does make
sense that it can be calculated knowing
all of the material properties and
temperature before bending. There must
be theoretical papers addressing same.
Keep looking!

 

[pmover]

BioMed,

fyi, IF you are a ME, you may look in the ME Handbook. you will find all the information you need in that textbook. your textbooks used in college certainly have that information as well.

also, you may find of interest the following website:

http://www.hollaender.com/pdf/handraildesign.pdf

good luck!
-pmover

 

[israelkk]

This is besically a geometry problem. When you bend a tube let say 90 degrees using a benging radius R_i the outside bending radius will be R_i+D_tube in the tube outside radius.

Therefore, assuming that the tube material elongate only on the outer radius while in the bending radius there is no strain the tube strain on the outer bending radius is:

(Pi X (R_i+D_tube)) / 2 - (Pi X R_i)/2 D_tube
----------------------------------------- = -----------
(Pi X R_i) / 2 Pi X R_i

comparing this to the maximum tensile strain of the tube you can solve for the minimum R_i.

 

[israelkk]

Sorry, the correct formula is:

(Pi X (R_i+D_tube)) / 2 - (Pi X R_i)/2 D_tube
----------------------------------------- = -----------
(Pi X R_i) / 2 R_i

 

[griffengm]

BioMedicalEngineer30,
Are you looking for a theoretical minimum or a practical method?
If theoretical, then Israelkk's method works up to the point that compression forces on the inner radius and sidewalls exceed the yield strength and the tube buckles.
If practical, then the radius at the tube centerline = approximately 3 X od.. In other words, the practical radius for you tube will be about 3.0mm. Bends smaller than 3x increase the cost of the process by orders of magnitude.
Some bending methods are able to obtain smaller radii by using mechanical means to control the deformation of the side walls or by allowing the inner radius to "wrinkle" for controlled shrinkage.
After several years of dealing with tube bending specs, I've come to the conclusion that Minimum Bend Radius Attainable = OD/(material factors * $ spent to overcome them).
Griffy

 

[flamby]

Michael,

I dont want to discourage you in your persuit, but please be aware that bending is not a elastic phenomenon. It means that there is no methematical formula which can be applied to "compute" a minimum bend radius that you are referring to. Kinks will always be there whether visible to eye or not. The best is to rely on "practical" bend radii which are usually based on experience and published by various manufacturers.
Regards.

PS: Compare a doughnut with an equivalent cylinder. Do they compare in terms of volume, area, or anything? Only when the bend radius is infinite, meaning till they both are CYLINDERS.