Square Tube Bending Rule of Thumb? 4

[Helepolis]

Hi,

Was wondering if there is a chart or simple calculation (as a rule of thumb, like the sheet metal bending charts) for estimating the feasibility of square tube bending.
For example if i have 30x30x1.5 ST-37 square tube, what is the minimum radius of the arc of the bend.

Thanks.

 

[MintJulep]

As a guide:

http://www.barnshaws.com/information/capacity-calculator/section-bending

 

[CWB1]

I would highly recommend discussing this with your suppliers as it will vary significantly between parts and equipment used.

 

[racookpe1978]

Three-Roller bending, or form bending around a fixed center post?

What tolerance can you accept for the inevitable wall kinking and deflection?

Cold bending only, right? You'll find hot bending will be near-impossible to control on anything but the smallest, thickest wall rectangular tube steel.

 

[Helepolis]

Three-Roller bending, or form bending around a fixed center post?

What tolerance can you accept for the inevitable wall kinking and deflection?

Cold bending only, right? You'll find hot bending will be near-impossible to control on anything but the smallest, thickest wall rectangular tube steel.

Three-Roller bending and cold bending, about the tolerance I'm not even sure (a low as possible), first time dealing with this kind of manufacturing.

 

[dhengr]

Helepolis:
I would talk with several structural steel fabricators who you have worked with in the past or who might be involved in this project. Ask them who they would job this bending work out to (sub this bending out to), and then talk with those people who specialize in this large structural shape and HSS bending. You will also see ads for this type of work in various structural steel mags. This type of bending work is quite a specialty in terms of the needed equipment, knowledge and experience, and what they can actually achieve with a given shape and size, without bending/buckling the hell out of the whole section. Some of these subs are better at handling one type of shape than another. We always routed the raw materials directly from the steel mills to our sub’s plant; they did the rough cutting and bending, etc. and then they sent that mat’l. on to our plant for final fab. This is really not something that lends itself well to a simple, industry wide rule-of-thumb, the entire process is just too dependent upon the equipment and methods used and individual experience, so that info. is help pretty close to the vest. But, they will tell you what they can do when you go to them will a well reasoned set of plans.

 

[racookpe1978]

Rect tube steel is hard to roll - much harder than simple round pipe or tube - because the two legs try to twist and compress at the same time the opposite (outside wall) web plate is pulling both of their top edges "in" while the compression forces from the inside wall push the bottom of the webs "out". but, those same compression forces are trying to buckle the inside wall plate. Round pipe distributes those forces over the wide curve of the whole half-tube when it is rolled, with no sharp bend at the wall plate-web plate intersection.

Your fabricator can try packing the rect tube steel with sand or clay, but then they have to fix the sand in position and close off both ends of the square tube during the bend. Very, very hard. More often, the fabricator just accepts the corrugations of the inner wall of the bend. (Look at plain aluminum (cheap) awnings frames, chairs, tables, etc. )

 

[berkshire]

Helepolis
If you want to bend square tubing, the better way is to pack the interior with greased metal shims stacked on the axis of the bend. after bending they can be removed one at a time starting with the center ones. Whilst this will keep some of the distortion down it will not eliminate all of it.
Or if you do not mind the distortion you can do as the lawn furniture people do and use a square die of the correct radius for the lower die and a half round die of the correct radius for the upper die , forcing the inner wall of the tube against the outer wall.
B.E.

You are judged not by what you know, but by what you can do.

 

[Helepolis]

Thanks!
Hoped this will be simpler process

 

[thaidavid40]

I have also see some rollers pack the interior of the tube with fine sand, to try and help minimize rolling distortion, then drain the sand to leave a clean bend.
Dave

Thaidavid

 

[JNieman]

Seems to me that stacks of shims would cause the outer wall to simply rupture and tear. The sand that's used is good because it allows the tube to distort and bow inward as required to reduce tension. Be stacking up shims inside, you're eliminating that possibility and requiring the outer wall to deform by stretching-alone.

Is this something that is used successfully and how does one prevent tearing/splitting of the exterior?

 

[mfgenggear]

searching my memory is that the tubes will have to be fully annealed, there is a process where the sq tubes are held at each end, with mandrels. stretched and bend over rolls concurrently.
heating the parts will also assist. the radii has to be generous.

 

[berkshire]

J neiman,
If the outer wall tears, using the shim method , you are trying to bend too tight a radius.
Alternate methods are Sand, which usually requires the ends to be sealed or low melting alloy i.e. Cerrobend which then requires that you have a melting tank to remove the material. Stretch bending with shims or Cerrobend as Mfgenggear says is also used.
B.E.

You are judged not by what you know, but by what you can do.